Audrey Magnin

Eric Pollet

Vincent Phalip

Luc Avérous
email: luc.averous@unistra.fr

Evaluation of biological degradation of polyurethanes

Keywords: Microbial degradation, enzymatic degradation, polyurethanes, technical review

Polyurethanes (PU) are a family of versatile synthetic polymers intended for diverse applications. Biological degradation of PU is a blooming research domain as it contributes to the design of ecofriendly materials sensitive to biodegradation phenomena and the development of green recycling processes. In this field, an increasing number of studies deal with the discovery and characterization of enzymes and microorganisms able to degrade PU chains. The synthesis of short lifespan PU material sensitive to biological degradation is also of growing interest. Measurement of PU degradation can be performed by a wide range of analytical tools depending on the architecture of the materials and the biological entities. Recent developments of these analytical techniques allowed for a better understanding of the mechanisms involved in PU biodegradation. Here, we reviewed the evaluation of biological PU degradation, including the required analytics. Advantages, drawbacks, specific uses, and results of these analytics are largely discussed to provide a critical overview and support future studies.

Introduction

In 2019, the International Union of Pure and Applied Chemistry (IUPAC) published a list of ten emerging technologies in Chemistry with the potential to make our planet more sustainable. Among them, technologies permitting the transformation of plastic material into monomers are highlighted. These recycling technologies will help reduce plastic waste and save fossil resources. Despite comfort and incomparable uses to our everyday life, plastic materials are a cause of global and increasing pollution resulting from inadequate behaviors of both producers and users. The massive production involves a polluting exploitation of fossil resources and their poor waste management induces uncontrolled dissemination in the environment. In 2010, more than 275 million metric tons of plastic waste were generated in almost 200 coastal countries, out of which an estimated 5 to 13 million metric tons reached the oceans where plastic waste accumulate [START_REF] Jambeck | Plastic waste inputs from land into the ocean[END_REF]. With the problematic of the nano-or microplastic debris, ocean garbage patches are one of the major environmental concerns of this century [START_REF] Cozar | Plastic debris in the open ocean[END_REF][START_REF] Eriksen | Plastic pollution in the South Pacific subtropical gyre[END_REF][START_REF] Law | Plastic accumulation in the North Atlantic subtropical gyre[END_REF]. Even if some very minor studies still question the actual impact of plastic waste [START_REF] Duis | Microplastics in the aquatic and terrestrial environment: sources (with a specific focus on personal care products), fate and effects[END_REF], most are warnings of irrevocable environmental damages [START_REF] Clukey | Persistent organic pollutants in fat of three species of Pacific pelagic longline caught sea turtles: Accumulation in relation to ingested plastic marine debris[END_REF][START_REF] Darmon | Risk assessment reveals high exposure of sea turtles to marine debris in French Mediterranean and metropolitan Atlantic waters[END_REF][START_REF] Galloway | Marine microplastics spell big problems for future generations[END_REF]. Furthermore, by entering the food chain, plastic materials finally attain human beings, thereby causing health concerns [START_REF] Barboza | Marine microplastic debris: An emerging issue for food security, food safety and human health[END_REF][START_REF] Bouwmeester | Potential health impact of environmentally released micro-and nanoplastics in the human food production chain: experiences from nanotoxicology[END_REF][START_REF] Chae | Effects of micro-and nanoplastics on aquatic ecosystems: Current research trends and perspectives[END_REF].

Among the vast families of resistant plastic materials, we can find the polyurethanes (PUs). Lowdensity and easily dispersible foams (soft to rigid) represent around 70% of the PU production. The presence of PUs as pollutants in marine ecosystems has been largely attested [START_REF] Frère | A semiautomated Raman micro-spectroscopy method for morphological and chemical characterizations of microplastic litter[END_REF][START_REF] Reddy | Description of the small plastics fragments in marine sediments along the Alang-Sosiya ship-breaking yard, India[END_REF]. In 2016, Turner et al. revealed that over the 70 foamed plastics fragments collected on a Britain beach, 39 were identified as PU (Figure 1), thus pointing out the significant role of PU in plastic pollution [START_REF] Turner | Elemental concentrations and bioaccessibilities in beached plastic foam litter, with particular reference to lead in polyurethane[END_REF]. [START_REF] Turner | Elemental concentrations and bioaccessibilities in beached plastic foam litter, with particular reference to lead in polyurethane[END_REF] First synthesized in the 1930s by the German chemist Otto Bayer, PU products were commercialized about 10 years later (Bayer, 1948). The use of PU spread during World War II, where it replaced natural rubber for elastomer production. Rapidly, other applications emerged in aviation and textile, which were flourishing markets at the time. Diversification of PU properties allowed to reach other numerous markets. In 2016, 18 million tons of PU were produced worldwide, representing 5.3% of global plastic production (Furtwengler et al., 2018a;PlasticsEurope, 2017). PU rank at the 6 th most produced synthetic polymers. About 22 million tons are expected to be produced in 2020 [START_REF] Akindoyo | Polyurethane types, synthesis and applications -a review[END_REF].

In 2014, the PU foam market was valued at $46.8 billion and is expected to reach $72.2 billion by 2020 [START_REF] Pillai | Metathesized palm oil polyol for the preparation of improved bio-based rigid and flexible polyurethane foams[END_REF]. Flexible and rigid foams represent respectively 32% and 36% of the global PU production. Flexible foams, renowned for the comfort they provide, are used for the cushioning of furniture, bedding or automotive seats. In the construction area, rigid foams are preferred for thermal insulation and their use increases in agreement with a growing demand for energetically efficient buildings. PU are also widely used as coatings, adhesives, sealants and elastomers (CASE). PU coatings provide a protection layer against weather, abrasion and corrosion. Elastomers are both elastic and flexible and can adopt any desired shape, such as wheels for rollerblades. Biocompatibility of certain types of polyurethanes make them polymers of choice for medical application, for instance, cardiovascular devices or orthopedic prosthesis [START_REF] Gunatillake | Biodegradable polyurethanes: design, synthesis, properties and potential applications[END_REF][START_REF] Zhou | The degradation and biocompatibility of pH-sensitive biodegradable polyurethanes for intracellular multifunctional antitumor drug delivery[END_REF]. The common thread between most of these PU materials is that they are mostly intended for long-term applications. They are mainly designed to resist environmental factors such as microbial degradation, abrasion, hydrolytic (moisture) or UV degradation.

Biodegradation is generally defined as the decomposition/degradation of materials by the means of biological entities such as microorganisms or enzymes. This process is used for numerous industrial applications such as waste water treatment [START_REF] Watanabe | Microorganisms relevant to bioremediation[END_REF] or depollution of contaminated site by, for instance, polycyclic aromatic hydrocarbons [START_REF] Shuttleworth | Environmental aspects of PAH biodegradation[END_REF]. Polymer degradation by microorganisms is performed through several steps (Figure 2). First, materials are fragmented into pieces thanks to abiotic and biotic factors such as UV, hydrolysis, abrasion or pressure exerted by filamentous microorganisms. Growth of filaments into polymer pores provokes cracks. Then, macromolecules are cleaved by enzymatic hydrolysis and/or oxidation, leading to the release of low molar mass molecules such as oligomers and monomers. These molecules are finally assimilated and mineralized by microorganisms to promote microbial growth [START_REF] Lucas | Polymer biodegradation: mechanisms and estimation techniques[END_REF]Shah et al., 2008b). A countless number of mineralization paths exists in nature. The step of enzymatic depolymerization or enzymatic degradation can be reproduced or mimicked in vitro, independently of the microbial degradation. Interestingly, the released molecules resulting from enzymatic depolymerization of polymers could be turned into building blocks and high value products for the chemistry market [START_REF] Wierckx | Plastic waste as a novel substrate for industrial biotechnology[END_REF]. PU are not biodegradable polymers [START_REF] Wierckx | Plastic Biodegradation: Challenges and Opportunities, Consequences of Microbial Interactions with Hydrocarbons, Oils, and Lipids: Biodegradation and Bioremediation[END_REF]. Even if some PU are partly sensitive to biological degradation, they do not answer, for instance, the requirements of the European norm EN 13432 defining biodegradable and compostable materials [START_REF] Avérous | Biodegradable polymers, Environmental silicate nano-biocomposites[END_REF][START_REF] Bastioli | Handbook of biodegradable polymers[END_REF]. This norm considers a material as biodegradable if the degradation reaches 90% after 6 months, under composting conditions. The non-toxicity of the degradation products is also a requirement of this norm to declare a material as biodegradable or compostable. Biodegradation of PU has been studied since the 1960s. The first scientific publications on this topic aimed to evaluate the microbial degradation susceptibility of PU formulations to promote the development of highly resistant materials [START_REF] Cooney | Effects of polyurethane foams on microbial growth in fuel-water systems[END_REF][START_REF] Darby | Fungal susceptibility of polyurethanes[END_REF][START_REF] Edmonds | Microbial growth in a fuel-water system containing polyesterurethane foam[END_REF][START_REF] Kanavel | Fungus Resistance of Millable Urethans[END_REF][START_REF] Kaplan | Microbial deterioration of polyurethane systems[END_REF]. Today, this approach is reversed to address the PU materials end-of-life issues. Due to increased environmental concerns, sensitivity to microbial degradation has become a desired feature to reduce the environmental footprint of PU materials, mainly at their ends of life [START_REF] Prieto | To be, or not to be biodegradable... that is the question for the bio-based plastics[END_REF]. Meanwhile, the development of bioresorbable PU materials for the biomedical industry raised interest [START_REF] Pavlova | Biocompatible and biodegradable polyurethane polymers[END_REF][START_REF] Špírková | The influence of the length of the degradable segment on the functional properties and hydrolytic stability of multi-component polyurethane elastomeric films[END_REF]. Currently, PU biodegradation assessments are focused on two main purposes: (i) biodegradation susceptibility of new eco-materials or materials intended for biomedical purposes, thus focusing on the polymer synthesis and (ii) bioremediation or biological recycling of PU, thus focusing on the biological entities capable of degradation.

Techniques used to evaluate the biodegradation of PU are diverse, depending on the type of PU and the degrading entity. Furthermore, conflicting results and conclusions are found in the scientific literature, often due to the difficulty in interpreting the analytical results. Nevertheless, significant advances have recently been made on the evaluation of PU biodegradation and consequently on the understanding of degradation mechanisms.

Review articles recently published on PU biodegradation have mainly focused on the degradation by microorganisms [START_REF] Mahajan | New insights into the microbial degradation of polyurethanes[END_REF], enzymatic degradation [START_REF] Loredo-Treviño | Microbial Enzymes Involved in Polyurethane Biodegradation: A Review[END_REF] or PU biodegradation for recycling [START_REF] Cregut | New insights into polyurethane biodegradation and realistic prospects for the development of a sustainable waste recycling process[END_REF]. Here, we reviewed comprehensively PU biodegradation including used analytics. For a full understanding of the different approaches, a first part is dedicated to the diversity of PU in term of compositions, architectures and corresponding waste management. Secondly, biological entities (microorganisms, enzymes) able to degrade PU and their degradation mechanisms, if known, are reviewed. Finally, analytical techniques used to assess PU biodegradation are gathered. Their applications, advantages and drawbacks are fully discussed.

3. Polyurethane structure and biological degradation susceptibility relationships 3.1. Diversity of the compositions and architectures of PU Chemical composition and macromolecular architectures are of prime importance for the biodegradation of polymers [START_REF] Kim | Effect of chemical structure on the biodegradation of polyurethanes under composting conditions[END_REF]. The nature of chemical bonds, crystallinity and molar mass are key parameters influencing the polymer susceptibility to biological attacks [START_REF] Zeng | Preparation and degradation mechanisms of biodegradable polymer: a review[END_REF]. PUs are characterized by the urethane or carbamate linkage, generally obtained by addition of an isocyanate to a hydroxyl group (Figure 3). For instance, in thermoplastic PU (TPU) synthesis, an excess of polyisocyanate reacts with a polyol, principally long polyester-or polyether-based polyols, with a controlled functionality close to 2. A linear prepolymer with isocyanate end groups is formed, followed by addition of a chain extender, usually a short diol, obtain high molar mass polymers. Molar mass is a parameter influencing the biological degradation susceptibility of polymers. It has been shown that for polymers with the same chemical structures and different molar mass, the higher the molar mass, the lower the biological degradation susceptibility [START_REF] Philip | Polyhydroxyalkanoates: biodegradable polymers with a range of applications[END_REF][START_REF] Zheng | A review of plastic waste biodegradation[END_REF]. The most frequently used isocyanates are bifunctional aromatic molecules such as 4,4'-methylene diphenyl isocyanate (4,4'-MDI) or toluene diisocyanate (2,4-TDI) that give, respectively, a rigid and a more flexible polymer backbone (Table 1) [START_REF] Delebecq | On the versatility of urethane/urea bonds: reversibility, blocked isocyanate, and non-isocyanate polyurethane[END_REF]. Due to its chemical structure presenting two aromatic rings, MDI is the most used isocyanate for rigid foams, one of the most prevalent PU-based product [START_REF] Sabbioni | Synthetic approaches to obtain amino acid adducts of 4,4'-methylenediphenyl diisocyanate[END_REF]. Aliphatic isocyanates are also of interest such as isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), lysine diisocyanate (LDI) or 4,4'-methylene dicyclohexyl diisocyanate (H12MDI) (Table 1). They are preferred for medical devices because of the mutagenicity of diamines derived from aromatic diisocyanate hydrolysis [START_REF] Darby | An evaluation of a polyurethane for use as a medical grade plastic[END_REF]. For the preparation of waterborne polyurethane dispersion, aliphatic isocyanates are chosen due to the high reactivity of aromatic isocyanates with water, making aromatic isocyanates hard to handle in these particular formulations [START_REF] Noble | Waterborne polyurethanes[END_REF]. Finally, in the frame of sustainable PUs development, non-isocyanate polyurethane (NIPU) is of growing interest. Indeed, isocyanates derived from phosgene are extremely toxic. NIPU can be obtained by reaction between amines and cyclic carbonates to form polyhydroxy-urethanes, rendering the process of PU synthesis more environment-friendly [START_REF] Carré | Original biobased nonisocyanate polyurethanes: solventand catalyst-free synthesis, thermal properties and rheological behaviour[END_REF][START_REF] Carré | Solvent-and catalyst-free synthesis of fully biobased nonisocyanate polyurethanes with different macromolecular architectures[END_REF].The high reactivity of isocyanates makes them unstable in water. Consequently, even if free isocyanates remain entrapped in the polymer after its synthesis, they will immediately react with water molecules from the aqueous media. Therefore, no isocyanates can be found either before or after degradation. Common polyols are polyether, polyester or, more rarely, polycarbonates. Higher flexibility of polyether polyols makes them more convenient for polyurethane production [START_REF] Krasowska | Degradation of polyurethanes in compost under natural conditions[END_REF]. A non-exhaustive list of polyols with their structures is available in Table 2. Polyols ordinarily used are fossil-based molecules, but an increasing number of studies deal with bio-based polyols, on agreement with green chemistry principles. Polyols from vegetal sources such as castor oil [START_REF] Hablot | Polyurethanes based on castor oil: kinetics, chemical, mechanical and thermal properties[END_REF][START_REF] Trovati | Characterization of polyurethane resins by FTIR, TGA, and XRD[END_REF], starch [START_REF] Duarah | Sustainable starch modified polyol based tough, biocompatible, hyperbranched polyurethane with a shape memory attribute[END_REF] or aromatic biopolymers such as tannins or lignin [START_REF] Ignat | Effects of flax lignin addition on enzymatic oxidation of poly(ethylene adipate) urethanes[END_REF][START_REF] Laurichesse | Original polyols based on organosolv lignin and fatty acids: new bio-based building blocks for segmented polyurethane synthesis[END_REF] are increasingly incorporated in PU formulations. Based on short diols such as 1,4-butanediol, ethylene glycol or 1,6-hexanediol, [START_REF] Akindoyo | Polyurethane types, synthesis and applications -a review[END_REF] chain extenders are used to obtain highmolar mass polymers. Low molar mass diamines can also be used, such as ethylene diamine [START_REF] Tang | Application of macromolecular additives to reduce the hydrolytic degradation of polyurethanes by lysosomal enzymes[END_REF], thus generating urea instead of urethane bonds. et al., 2004) 178

PUs can be thermoplastics or thermosets. Thermoplastics are linear or slightly crosslinked/branched structures. Isocyanates, polyols and chain extenders used for TPU synthesis have only two functional groups (diols or diisocyanates). Thermoplastics can be soluble in organic solvent and can be melted, or present a liquid-like behavior with increased temperature. TPUs are commonly described by two types of segments, hard and soft segments. The segments are generally organized with a specific micro-segregation which can lead to micro-crystalline phases.

The hard segment (HS) is a block segment with low mobility mainly formed by the isocyanate and the short-chain extender. By contrast, the soft segment (SS) is mainly based on the long polyol part (Figure 4a). TPUs are often semi-crystalline structures (Figure 4b). HS content and chemistry influenced the biodegradation susceptibility of a polycarbonate PU (Tang et al., 2001a, b).

Interactions between enzymes and mobile SS are higher than with the HS. Consequently, the higher the HS content, the lower the biological degradation susceptibility. Thermosets are highly cross-linked polymers with 3D structures brought by molecules with a functionality higher than 2. Due to their architecture, thermosets are not soluble, do not melt and are denatured at high temperature, and thus cannot be reshaped by heating, unlike thermoplastics. PU foams are thermoset materials. The alveolar structures of these complex and multicomponent systems are obtained thanks to physical or chemical blowing with air, carbon dioxide, hydrocarbons such as isopentane or other gaseous substances. Foams are structured by struts and walls defining cell cavities. Cells can be closed (closed-cell foams, mainly rigid e.g. thermal insulation) or open (open-cell foams, mainly soft e.g. damping) (Gautam et al., 2007a). Side reactions during foaming induce the formation of various reversible and irreversible bonds such as urea or isocyanurate moieties (Furtwengler and Avérous, 2018). Isocyanurate linkage is obtained by isocyanate trimerization. Polyisocyanurate foams are increasingly produced because of their better properties such as fire resistance and thermal stability [START_REF] Arbenz | Advanced biobased and rigid foams, based on urethane-modified isocyanurate from oxypropylated gambier tannin polyol[END_REF]Furtwengler et al., 2018b). It is then important to consider the bond nature when dealing with foam degradation. Because of the variety of bonds, it is difficult to anticipate and know if the urethane bond was degraded. Also, commercial foams are supplemented with several additives such as fire retardants, antioxidant, processing aids, and anti-microbial compounds (mainly fungicides) that can prevent biodegradation.

Due to the nanometric size of the particles and their hydrophilicity, waterborne PU dispersions (WPUDs) are particularly suitable for biological assays (Figure 5a). To provide hydrophilicity, WPUDs are synthesized using an emulsifier, often the 2,2-dimethylol propionic acid. This molecule contains two hydroxyl groups which react with the isocyanates to form urethane linkage (Figure 5b). The hydroxyl group of the carboxylic acid does not react with isocyanate because of steric hindrance and the lower reactivity [START_REF] Coutinho | Anionic waterborne polyurethane dispersions based on hydroxyl-terminated polybutadiene and poly (propylene glycol): Synthesis and characterization[END_REF]. Hydrophilic carboxylic acid then forms a stabilizing top-layer around the hydrophobic polymer [START_REF] Zhang | Highly branched polyurethane acrylates and their waterborne UV curing coating[END_REF]. Systems containing acrylic polyols are readily dispersed in water [START_REF] Ionescu | Chemistry and technology of polyols for polyurethanes[END_REF]. Acrylic polyols are thus widely used in WPUD formulations. These polyols provide strength and resistance to coatings [START_REF] Akindoyo | Polyurethane types, synthesis and applications -a review[END_REF]. Acrylic polyols are generally based on hydroxyethyl methacrylate or hydroxyethyl acrylate (Figure 5c). The hydroxyl groups of the lateral chains then react with isocyanate for urethane formation.

PU waste disposal

The diversity of PUs macromolecular structures and chemistry is a clear obstacle for efficient waste management. To appreciate the importance of PU biodegradation, it is necessary to analyze the current PU waste disposal. PU waste is made of post-consumer products as well as PU production waste, mostly from foam. Indeed, scrap from slabstock foam can reach up to 10% of the production [START_REF] Simon | Recycling of polyurethanes from laboratory to industry, a journey towards the sustainability[END_REF]. In France, scrap PUs were estimated at 13 kTons in 2011 while end-of-life PU volume is about 198 kTons [START_REF] Boujard | Panorama du marché du polyuréthane et état de l'art de ses techniques de recyclages[END_REF]. This source of PU waste mainly arises from construction, furniture, bedding, automotive, shoes and home appliances. Efficient collection and product dismantling are required to recycle these materials, thus limiting their valorization.

PU wastes are mainly treated by three different methods: landfilling, incineration (which can also be considered as quaternary recycling [START_REF] Ignatyev | Recycling of polymers: a review[END_REF]) and conventional recycling. Landfill discharge is often the main option but is gradually decreasing, especially in Europe, since it requires large land areas and no value is brought from the waste. European Union aims at reducing municipal waste landfilling to a maximum of 10% by 2030 [START_REF] Castillo-Gimenez | Performance and convergence in municipal waste treatment in the European Union[END_REF][START_REF] Makarichi | The evolution of waste-to-energy incineration: A review[END_REF]. Landfilling and the absence of waste management lead generally to pollution [START_REF] Jambeck | Plastic waste inputs from land into the ocean[END_REF].

Incineration presents the advantage of being a mature technique, practiced for several decades. Energy is recovered by burning waste and can totally or partly offset the energy spent in the heating process. Even if some CO2 emission exists and may cause greenhouse effect and contribute to global warming, new generations of plants are equipped with dry and wet air pollution control system to make this process as clean as possible [START_REF] Brunner | Waste to energy--key element for sustainable waste management[END_REF][START_REF] Makarichi | The evolution of waste-to-energy incineration: A review[END_REF]. Because of the low value recovered it is hardly considered as recycling. It is a proper solution to reduce landfill volumes, yet incineration is not a satisfying strategy since the richness from the chemical architecture is fully lost.

Depending on the nature (thermoplastics vs. thermosets), recycling processes differ. TPUs can be heated and remolded therefore making the physical recycling process easier. However, it is estimated that only 1% of PU are recycled thanks to physical methods [START_REF] Behrendt | The chemical recycling of polyurethanes[END_REF]. The recycling of PU foams is more challenging since foams cannot be remolded. The main path for foam recycling is regrinding. In 2002, more than 380 kTons were used for carpet underlay [START_REF] Zia | Methods for polyurethane and polyurethane composites, recycling and recovery: A review[END_REF].

Chemical recycling can address both thermoplastic and thermoset architectures [START_REF] Simón | Glycolysis of high resilience flexible polyurethane foams containing polyurethane dispersion polyol[END_REF][START_REF] Wang | Chemical degradation of thermoplastic polyurethane for recycling polyether polyol[END_REF]). Glycolysis appears as the most promising technique [START_REF] Simon | Recycling of polyurethanes from laboratory to industry, a journey towards the sustainability[END_REF]. Glycolysis is a transesterification reaction. The ester group of the urethane bond is interchanged by the hydroxyl group of a diol (glycol) added in large excess [START_REF] Simón | Glycolysis of flexible polyurethane wastes using stannous octoate as the catalyst: Study on the influence of reaction parameters[END_REF]. Simón et al., developed a glycolysis process allowing polyether polyol recovery from high resilience PU foams [START_REF] Simón | Glycolysis of high resilience flexible polyurethane foams containing polyurethane dispersion polyol[END_REF]. These polyols can then serve as building blocks for the synthesis of secondgeneration polymers. The major limits of chemical recycling are the processing temperature that leads to high energy consumption (in the example mentioned above, the glycolysis temperature is 190°C) and the side chemical reactions occurring on the urethane bond during the chemical reactions [START_REF] Gadhave | Recycling and Disposal Methods for Polyurethane Wastes: A Review[END_REF].

Biological recycling is a growing route with high potential that might answer the need for PU recycling in the coming years. This is a soft process that can be implemented at low temperatures (less than 70°C) [START_REF] Mueller | Biological degradation of synthetic polyesters-Enzymes as potential catalysts for polyester recycling[END_REF][START_REF] Valerio | Environmental impacts of post-consumer material managements: recycling, biological treatments, incineration[END_REF]. This process is catalyzed by biological entities, namely enzymes. The resulting degradation products may then be valorized. Efficient enzymatic depolymerization of poly(ethylene terephthalate) (PET) has been demonstrated at 60°C leading to the release of valuable building blocks such as terephthalic acid and mono(2-hydroxyethyl)terephthalate [START_REF] Gamerith | Enzymatic recovery of polyester building blocks from polymer blends[END_REF]. This result, based on PET, can be considered as the first benchmark for PU. For instance, the company Carbios (France) is starting the biological recycling of PET for the synthesis of second-generation polymers.

4. Actors of the PU biodegradation: Biological entities and associated mechanisms 4.1. Microorganisms

Biodegradation involving microorganisms can be performed by a microbial community or a single strain (Figure 6). Microorganisms can form biofilms on the polymer surface by adhesion [START_REF] Sivan | New perspectives in plastic biodegradation[END_REF]. Once colonized, the material constitutes a source of carbon and nitrogen thus promoting microbial growth.

. PU degradation by microbial communities

A microbial community is a group of microorganisms sharing a common living place. These microorganisms interact in different ways such as mutualism, predation or competition [START_REF] Faust | Microbial interactions: from networks to models[END_REF]. Mutualism, also called symbiosis, may occur during the microbial degradation of xenobiotic, such as synthetic polymers, leading to an improved degradation [START_REF] Tsoi | Emerging strategies for engineering microbial communities[END_REF][START_REF] Vaclavkova | Novel aspects of symbiotic (polyvinyl alcohol) biodegradation[END_REF]. Comparison of degradation skills of a strain of fungi (Aspergillus niger) and a strain of bacteria (Pseudomonas aeruginosa) revealed a slightly higher TPU degradation with the bacteria but, above all, an impressive synergistic effect was observed when the polymer was incubated with both strains [START_REF] Fernandes | Biobased Additives as Biodegradability Enhancers with Application in TPU-Based Footwear Components[END_REF]. Weight losses were approximately doubled for polyester TPU incubated with both microorganisms compared to single strain incubation. PU substrates degraded by communities range from simple structures such as thermoplastic polyester PU [START_REF] Genovese | Gas permeability, mechanical behaviour and compostability of fully-aliphatic bio-based multiblock poly(ester urethane)s[END_REF] to recalcitrant material such as polyether PU foam [START_REF] Ge | Biodegradable polyurethane materials from bark and starch. I. Highly resilient foams[END_REF]. As PU are recalcitrant material, degradation experiments by microbial communities are often performed on a long time scale going from 28 days [START_REF] Bentham | Rapid assessment of the microbial deterioration of polyurethanes[END_REF] to two years of incubation [START_REF] Seal | Microbiological testing of plastics: ongoing activities of IBRG plastics project group to improve standard test procedures[END_REF]. The predominant systems for studying PU degradation have been composting and soil burial (Table 3 -PU degradation by microbial communities and associated PU substrates. These ecosystems are rich in degrading microorganisms. Other strategies to obtain communities that are more acclimated to PU are also developed. For instance, Cregut et al., selected microbial communities from the soil of a PU foam industrial plant [START_REF] Cregut | Applying Raman spectroscopy to the assessment of the biodegradation of industrial polyurethanes wastes[END_REF]. Polyester PU foam 3 months [START_REF] Filip | Decomposition of polyurethane in a garbage landfill leakage water and by soil microorganisms[END_REF] Polyether PU foam 3 months [START_REF] Filip | Decomposition of polyurethane in a garbage landfill leakage water and by soil microorganisms[END_REF] Microbial communities from sewage water of a latex rubber factory

Polyester PU foam 60 days [START_REF] Rattanapan | Preparation and properties of bio-based polyurethane foams from natural rubber and polycaprolactone diol[END_REF] Microbial communities from soil of a PU foam industrial plant

Polyether PU foam 28 days [START_REF] Cregut | Applying Raman spectroscopy to the assessment of the biodegradation of industrial polyurethanes wastes[END_REF] Mixed culture of Aspergillus niger and

Pseudomonas aeruginosa

Thermoplastic polyester PU 30 days [START_REF] Fernandes | Biobased Additives as Biodegradability Enhancers with Application in TPU-Based Footwear Components[END_REF] Soil burial Polyester PU 383 days [START_REF] Aranguren | Biodegradation of a vegetable oil based polyurethane and wood flour composites[END_REF]) 24 months [START_REF] Seal | Microbiological testing of plastics: ongoing activities of IBRG plastics project group to improve standard test procedures[END_REF] Polyester PU foam 28 days [START_REF] Bentham | Rapid assessment of the microbial deterioration of polyurethanes[END_REF] 320 days [START_REF] Gómez | Biodegradability of crude glycerol-based polyurethane foams during composting, anaerobic digestion and soil incubation[END_REF] Polyether PU foam 6 months [START_REF] Ge | Biodegradable polyurethane materials from bark and starch. I. Highly resilient foams[END_REF] Polyether PU foam 12 months [START_REF] Zhang | A straightforward ninhydrin-based method for collagenase activity and inhibitor screening of collagenase using spectrophotometry[END_REF] PU coating 12 months [START_REF] Lu | Degradation of polyurethane coating materials from liquefied wheat straw for controlled release fertilizers[END_REF] Thermoplastic polyester PU 44 days [START_REF] Barratt | Fungi are the predominant micro-organisms responsible for degradation of soil-buried polyester polyurethane over a range of soil water holding capacities[END_REF] 12 weeks [START_REF] Zafar | Fungal communities associated with the biodegradation of polyester polyurethane buried under compost at different temperatures[END_REF] 16 weeks [START_REF] Huang | Improvement of biodegradability of UV-curable adhesives modified by a novel polyurethane acrylate[END_REF] 140 days (Umare and Chandure, 2008) 5 months [START_REF] Cosgrove | Fungal communities associated with degradation of polyester polyurethane in soil[END_REF] 6 months (Fernandes et

Single-strain degradation

Single species of bacteria and fungi can be isolated and identified using molecular tools. These isolated strains are then used alone for PU degradation assays [START_REF] Khan | Biodegradation of polyester polyurethane by Aspergillus tubingensis[END_REF][START_REF] Nair | Molecular characterization of a lipase-producing Bacillus pumilus strain (NMSN-1d) utilizing colloidal water-dispersible polyurethane[END_REF]. Another approach is to establish a collection of microorganisms and to screen it for the identification of PU-degrading strain. For instance, Russell et al. collected endophytic fungi from wood of the Ecuadorian Amazonian rainforest [START_REF] Russell | Biodegradation of polyester polyurethane by endophytic fungi[END_REF]. Equatorial are hot and humid environments, and above-all, well-known for the tremendous richness of their biodiversity. PU debris were sampled in e.g., dump-site for isolation of already adapted microorganisms [START_REF] Álvarez-Barragán | Biodegradative Activities of Selected Environmental Fungi on a Polyester Polyurethane Varnish and Polyether Polyurethane Foams[END_REF][START_REF] Oceguera-Cervantes | Characterization of the polyurethanolytic activity of two Alicycliphilus sp. strains able to degrade polyurethane and N-methylpyrrolidone[END_REF].

Bacteria

Bacteria are mainly studied for the degradation of TPU and coatings (Table 4). Only a few publications described the bacterial degradation of polyester-based PU foams, notably by Pseudomonas aeruginosa [START_REF] Cooney | Effects of polyurethane foams on microbial growth in fuel-water systems[END_REF]Gautam et al., 2007c;[START_REF] Hedrick | Effects of jet-fuel microbial isolates on a polyurethane foam[END_REF][START_REF] Kay | Bacterial degradation of polyester polyurethane[END_REF] or by a strain of Corynebacterium [START_REF] Kay | Bacterial degradation of polyester polyurethane[END_REF]. Pseudomonas is the most studied genus. The strain of P. aeruginosa ATCC 13388 is the only recommended strain by the ASTM for testing material resistance to bacterial degradation (ASTM code G22-76: Standard Practice for Determining Resistance of Plastics to Bacteria) [START_REF] Gu | Methods Currently Used in Testing Microbiological Degradation and Deterioration of a Wide Range of Polymeric Materials with Various Degree of Degradability: A Review[END_REF][START_REF] Kay | Bacterial degradation of polyester polyurethane[END_REF]. Other Pseudomonas species such as P. chlorographis or P. putida were described as PU-degrading entities (Gautam et al., 2007c;[START_REF] Peng | Degradation of polyurethane by bacterium isolated from soil and assessment of polyurethanolytic activity of a Pseudomonas putida strain[END_REF]. Furthermore, Pseudomonas strains are known to be of high interest in white biotechnology [START_REF] Wierckx | Plastic waste as a novel substrate for industrial biotechnology[END_REF].

Arthrobacter globiformis

Polyester PU coating [START_REF] El-Sayed | Biodegradation of polyurethane coatings by hydrocarbon-degrading bacteria[END_REF] Bacillus sp.

Polyester PU coating (Impranil) [START_REF] Ii | Adherence and growth of a Bacillus species on an insoluble polyester polyurethane[END_REF] Thermoplastic polyester PU (Shah et al., 2008a)

Bacillus amyloliquefaciens

Thermoplastic poly(ether urea) PU [START_REF] Rafiemanzelat | Study of Biological Degradation of New Poly(Ether-Urethane-Urea)s Containing Cyclopeptide Moiety and PEG by Bacillus amyloliquefaciens Isolated from Soil[END_REF] Bacillus pumilus NMSN-1d

Polyester PU coating (Impranil) [START_REF] Nair | Molecular characterization of a lipase-producing Bacillus pumilus strain (NMSN-1d) utilizing colloidal water-dispersible polyurethane[END_REF] Bacillus subtilis MZA-75 Thermoplastic polyester PU [START_REF] Shah | Degradation of polyester polyurethane by an indigenously developed consortium of Pseudomonas and Bacillus species isolated from soil[END_REF]) (Shah et al., 2013b) Chryseobacterium meningosepticum Polyester PU foam [START_REF] Cangemi | Biodegradation of polyurethane derived from castor oil[END_REF] Comamonas acidovorans TB-35 Thermoplastic polyester PU [START_REF] Akutsu | Purification and properties of a polyester polyurethane-degrading enzyme from Comamonas acidovorans TB-35[END_REF][START_REF] Nakajima-Kambe | Determination of the polyester polyurethane breakdown products and distribution of the polyurethane degrading enzyme of Comamonas acidovorans strain TB-35[END_REF][START_REF] Nakajima-Kambe | Isolation and characterization of a bacterium which utilizes polyester polyurethane as a sole carbon and nitrogen source[END_REF] Thermoplastic polyether PU [START_REF] Nakajima-Kambe | Isolation and characterization of a bacterium which utilizes polyester polyurethane as a sole carbon and nitrogen source[END_REF] Corynebacterium sp. Thermoplastic polyester PU [START_REF] Kay | Chemical and physical changes occurring in polyester polyurethane during biodegradation[END_REF]) (Shah et al., 2008a) Polyester PU foam [START_REF] Kay | Bacterial degradation of polyester polyurethane[END_REF] Escherichia coli Thermoplastic poly(ether urea) PU [START_REF] Rafiemanzelat | Synthesis of new poly(ether-urethaneurea)s based on amino acid cyclopeptide and PEG: study of their environmental degradation[END_REF] Micrococcus sp. Thermoplastic polyester PU (Shah et al., 2008a) Thermoplastic poly(ether urea) PU [START_REF] Rafiemanzelat | Synthesis of new poly(ether-urethaneurea)s based on amino acid cyclopeptide and PEG: study of their environmental degradation[END_REF] Pseudomonas sp.

Polyester PU foam [START_REF] Spontón | Biodegradation study by Pseudomonas sp. of flexible polyurethane foams derived from castor oil[END_REF] Thermoplastic polyester PU (Shah et al., 2008a)

Pseudomonas aeruginosa

Thermoset poly(amido amine) PU [START_REF] Gogoi | Bio-based high-performance waterborne hyperbranched polyurethane thermoset[END_REF] Thermoset polyester PU [START_REF] Duarah | Sustainable starch modified polyol based tough, biocompatible, hyperbranched polyurethane with a shape memory attribute[END_REF] Polyester PU coating (Impranil) [START_REF] Mukherjee | Isolation of a Pseudomonas aeruginosa strain from soil that can degrade polyurethane diol[END_REF] Thermoset poly(ester amide) PU [START_REF] Gogoi | Waterborne hyperbranched poly(ester amide urethane) thermoset: Mechanical, thermal and biodegradation behaviors[END_REF] Polyester PU foam [START_REF] Cooney | Effects of polyurethane foams on microbial growth in fuel-water systems[END_REF][START_REF] Edmonds | Microbial growth in a fuel-water system containing polyesterurethane foam[END_REF] Pseudomonas aeruginosa ATCC 13388 Polyester PU foam [START_REF] Kay | Bacterial degradation of polyester polyurethane[END_REF] Pseudomonas aeruginosa ATCC 9027 Thermoplastic polyester PU [START_REF] Fernandes | Biobased Additives as Biodegradability Enhancers with Application in TPU-Based Footwear Components[END_REF] Pseudomonas aeruginosa MTCC 7814 Thermoset polyester PU [START_REF] Bayan | Renewable resource modified polyol derived aliphatic hyperbranched polyurethane as a biodegradable and UV-resistant smart material[END_REF][START_REF] Gogoi | Biobased Biodegradable Waterborne Hyperbranched Polyurethane as an Ecofriendly Sustainable Material[END_REF]) Pseudomonas aeruginosa MZA-85 Thermoplastic polyester PU [START_REF] Shah | Degradation of polyester polyurethane by an indigenously developed consortium of Pseudomonas and Bacillus species isolated from soil[END_REF]Shah et al., 2013a) Pseudomonas aeruginosa NAV-6 Polyester PU coating [START_REF] El-Sayed | Biodegradation of polyurethane coatings by hydrocarbon-degrading bacteria[END_REF] Pseudomonas cepacia Polyester PU coating [START_REF] El-Sayed | Biodegradation of polyurethane coatings by hydrocarbon-degrading bacteria[END_REF] Pseudomonas chlororaphis ATCC 55729 Polyester PU foam (Gautam et al., 2007c)

Pseudomonas fluorescens

Polyether PU coating [START_REF] Crookes-Goodson | The impact of culture medium on the development and physiology of biofilms of Pseudomonas fluorescens formed on polyurethane paint[END_REF] Polyester PU coating (Impranil) [START_REF] Howard | Growth of Pseudomonas fluorescens on a polyesterpolyurethane and the purification and characterization of a polyurethanase-protease enzyme[END_REF][START_REF] Vega | Cloning and expression in Escherichia coli of apolyurethane-degrading enzyme from Pseudomonas fluorescens[END_REF] Pseudomonas protegens Pf-5 Polyester PU coating (Impranil) [START_REF] Biffinger | A direct quantitative agar-plate based assay for analysis of Pseudomonas protegens Pf-5 degradation of polyurethane films[END_REF] Thermoplastic polyether PU [START_REF] Barlow | The importance of correcting for variable probe-sample interactions in AFM-IR spectroscopy: AFM-IR of dried bacteria on a polyurethane film[END_REF] Pseudomonas putida Polyester PU coating (Impranil) [START_REF] Peng | Degradation of polyurethane by bacterium isolated from soil and assessment of polyurethanolytic activity of a Pseudomonas putida strain[END_REF] Pseudomonas putida ATCC 17484 Polyester PU coating [START_REF] El-Sayed | Biodegradation of polyurethane coatings by hydrocarbon-degrading bacteria[END_REF] Rhodococcus equi strain TB-60 Model urethane molecule (toluene-2,4dicarbamic acid dibutyl ester) [START_REF] Akutsu-Shigeno | Isolation of a bacterium that degrades urethane compounds and characterization of its urethane hydrolase[END_REF])

Staphylococcus aureus

Poly(ether urea) PU [START_REF] Rafiemanzelat | Synthesis of new poly(ether-urethaneurea)s based on amino acid cyclopeptide and PEG: study of their environmental degradation[END_REF] Undefined PU [START_REF] Curia | Beyond the biodestruction of polyurethane: S. aureus uptake of nanoparticles is a challenge for toxicology[END_REF] Staphylococcus epidermidis strain KH 11 Thermoplastic polyether PU [START_REF] Jansen | Evidence for degradation of synthetic polyurethanes by Staphylococcus epidermidis[END_REF]

Fungi

It is possible to appraise only the fungal activity of a consortium or isolate only fungal strain by adding antibacterial molecules which prevent bacterial growth. For instance, 50 µg/mL of chloramphenicol can be added to the medium to prevent bacterial growth and therefore perform analysis only on fungal strains [START_REF] Zafar | Fungal communities associated with the biodegradation of polyester polyurethane buried under compost at different temperatures[END_REF]. Microorganisms from the fungi kingdom described as PU degrading entities are almost only filamentous fungi. Strains belonging to the genus of Alternaria [START_REF] Magnin | Isolation and characterization of different promising fungi for biological waste management of polyurethanes[END_REF][START_REF] Matsumiya | Isolation and characterization of an ether-type polyurethane-degrading micro-organism and analysis of degradation mechanism by Alternaria sp[END_REF][START_REF] Oprea | Biodegradation of pyridine-based polyether polyurethanes by the Alternaria tenuissima fungus[END_REF], Aspergillus [START_REF] Khan | Biodegradation of polyester polyurethane by Aspergillus tubingensis[END_REF][START_REF] Magnin | Isolation and characterization of different promising fungi for biological waste management of polyurethanes[END_REF][START_REF] Mathur | Degradation of polyurethane by Aspergillus flavus (ITCC 6051) isolated from soil[END_REF][START_REF] Osman | Degradation of Polyester Polyurethane by Aspergillus sp. Strain S45 Isolated from Soil[END_REF] and Cladosporium (Álvarez- [START_REF] Álvarez-Barragán | Biodegradative Activities of Selected Environmental Fungi on a Polyester Polyurethane Varnish and Polyether Polyurethane Foams[END_REF] are frequently isolated for PU degradation (Table 5).

Only one study on yeast was found, describing the growth of Cryptococcus sp. MTCC 5455 on fish waste to produce a lipase with activity on PU [START_REF] Thirunavukarasu | Utilization of fish meal and fish oil for production of Cryptococcus sp. MTCC 5455 lipase and hydrolysis of polyurethane thereof[END_REF]. Five strains are recommended by the American Society for Testing and Materials (ASTM) to evaluate the fungal resistance of a material: Aspergillus niger ATCC 9642, Aureobasidium pullulans ATCC15233, Chaetomium globosum ATCC6205, Gliocladium virens ATCC9645, Penicillum pinophilum ATCC11797 (ASTM code: G21 -90 Standard Practice for Determining Resistance of Synthetic Polymetric Materials to Fungi) [START_REF] Gu | Methods Currently Used in Testing Microbiological Degradation and Deterioration of a Wide Range of Polymeric Materials with Various Degree of Degradability: A Review[END_REF]. Oprea et al. who developed new bio-based materials and tested their fungal degradation susceptibility with the strain of Chaetomium globosum [START_REF] Oprea | Dependence of fungal biodegradation of PEG/castor oil-based polyurethane elastomers on the hard-segment structure[END_REF][START_REF] Oprea | Biodegradation of polyurethane acrylate with acrylated epoxidized soybean oil blend elastomers by Chaetomium globosum[END_REF][START_REF] Oprea | Synthesis, properties, and fungal degradation of castor-oil-based polyurethane composites with different cellulose contents[END_REF]. Only a few studies are based on this norm and mentioned these strains. However, using such reference fungal strains would allow better reproducibility and comparison of results in-between studies.

A review on the biodegradation of fossil-based polymers interestingly shows the significant importance of the abiotic effect of fungal biodegradation [START_REF] Lucas | Polymer biodegradation: mechanisms and estimation techniques[END_REF]. The formation of filaments exerts physical pressure leading to polymer breaking. Filament apices penetrate in the material increasing the size of pores and provoking cracks. For instance, rifts under the form of a fungal filament network were observed by microscopy on the surface of a TPU incubated two months with a strain of Penicillium brasilianum [START_REF] Magnin | Isolation and characterization of different promising fungi for biological waste management of polyurethanes[END_REF]. Fungal degradation has been demonstrated on both polyester-and polyether-based PU, on TPU, foams and coatings. However, mechanisms of degradation have not been fully elucidated. The importance of each biotic and abiotic steps in fungal degradation still needs to be clarified.

Aspergillus niger

Polyether PU foam [START_REF] Filip | Polyurethane as the sole nutrient source for Aspergillus niger and Cladosporium herbarum[END_REF] Aspergillus niger ATCC 9642 Thermoplastic polyester PU [START_REF] Kanavel | Fungus Resistance of Millable Urethans[END_REF] Aspergillus section flavi Thermoplastic polyester PU [START_REF] Magnin | Isolation and characterization of different promising fungi for biological waste management of polyurethanes[END_REF] Aspergillus tubingensis Thermoplastic polyester PU [START_REF] Khan | Biodegradation of polyester polyurethane by Aspergillus tubingensis[END_REF] Aureobasidium pullulans Polyester PU coating (Impranil) [START_REF] Crabbe | Biodegradation of a colloidal ester-based polyurethane by soil fungi[END_REF])

Chaetomium globosum

Thermoset polyester PU [START_REF] Oprea | Biodegradation of polyurethane acrylate with acrylated epoxidized soybean oil blend elastomers by Chaetomium globosum[END_REF] Thermoplastic polyester PU [START_REF] Oprea | Synthesis, properties, and fungal degradation of castor-oil-based polyurethane composites with different cellulose contents[END_REF] Cladosporium sp.

Polyester PU coating (Impranil) [START_REF] Crabbe | Biodegradation of a colloidal ester-based polyurethane by soil fungi[END_REF] Polyether PU foam [START_REF] Cooney | Effects of polyurethane foams on microbial growth in fuel-water systems[END_REF]

Cryptococcus laurentii

Polyester PU coating (Impranil) [START_REF] Zicht | Detection and Analysis of Polyurethane Biodegradation due to Cryptococcus laurentii[END_REF] Curvularia senegalensis Polyester PU coating (Impranil) [START_REF] Crabbe | Biodegradation of a colloidal ester-based polyurethane by soil fungi[END_REF])

Exophiala jeanselmei

Model urethane molecule (Ntolylcarbamate) [START_REF] Owen | The Biodegradation of Low-molecular-weight Urethane Compounds by a Strain of Exophiala jeanselmei[END_REF] Fusarium solani Polyester PU coating (Impranil) [START_REF] Crabbe | Biodegradation of a colloidal ester-based polyurethane by soil fungi[END_REF])

Gliocladium roseum

Thermoplastic polyester PU [START_REF] Shuttleworth | A rapid technique for evaluating the biodeterioration potential of polyurethane elastomers[END_REF] Penicillium chrysogenum Polyester PU coating (Impranil) [START_REF] Álvarez-Barragán | Biodegradative Activities of Selected Environmental Fungi on a Polyester Polyurethane Varnish and Polyether Polyurethane Foams[END_REF] Polyether PU foam [START_REF] Álvarez-Barragán | Biodegradative Activities of Selected Environmental Fungi on a Polyester Polyurethane Varnish and Polyether Polyurethane Foams[END_REF] Penicillium section lanata-divaricata Thermoplastic polyester PU [START_REF] Magnin | Isolation and characterization of different promising fungi for biological waste management of polyurethanes[END_REF] Pestalotiopsis microspora Polyester PU coating (Impranil) [START_REF] Russell | Biodegradation of polyester polyurethane by endophytic fungi[END_REF]

Enzymes

Enzymes are biological catalysts (biocatalysts). Enzymes identified as PU degrading entities originate from microorganisms but also mammalian cells such as lipase porcine pancreas [START_REF] Ng | Preparation and modification of water-blown porous biodegradable polyurethane foams with palm oil-based polyester polyol[END_REF] or from plants such as papain from Carica papaya [START_REF] Ferris | Dithiothreitol-based polyurethanes. Synthesis and degradation studies[END_REF]. Enzymes used for PU degradation assays are either commercial enzymes or enzymes over-expressed in heterologous microorganisms. In this latter case, they correspond to enzymes identified in PUdegrading microorganisms for which the encoding genes were cloned into model organism.

Enzymes are then over-expressed and even purified in some cases. Enzymes are mainly described for the depolymerization of TPU or coatings (Table 6 -PU-degrading enzymes and associated PU substrates). As far as we know, only one publication has addressed the enzymatic degradation of foams [START_REF] Ng | Preparation and modification of water-blown porous biodegradable polyurethane foams with palm oil-based polyester polyol[END_REF] all other studies involved degradation by microorganisms. A set of different poly(ester ether) PU foam containing PCL, PEG and polyester from palm oil was studied by enzymatic degradation. A maximal weight loss of 70% was measured after 28 days of incubation with lipase from porcine pancreas for a polymer with 7.7% of PCL, 34.8% of PEG and 7.5% of polyester from palm oil. Pseudomonas cepacia lipase Thermoplastic poly(ester ether) PU [START_REF] Zhou | Synthesis and characterization of poly(ester ether urethane)s block copolymers based on biodegradable poly(butylene succinate) and Poly(ethylene glycol)[END_REF] Thermoset polyester PU [START_REF] Schöne | Polymer architecture versus chemical structure as adjusting tools for the enzymatic degradation of oligo(ε-caprolactone) based films at the air-water interface[END_REF] Pseudomonas sp. lipase Polyester PU (Impranil) [START_REF] Biffinger | The applicability of Impranil®DLN for gauging the biodegradation of polyurethanes[END_REF] Thermoplastic polyester PU [START_REF] Daemi | A robust super-tough biodegradable elastomer engineered by supramolecular ionic interactions[END_REF] PueA (Ps chlororaphis lipase) Polyester PU (Impranil) [START_REF] Howard | Effect of insertional mutations in the pueA and pueB genes encoding two polyurethanases in Pseudomonas chlororaphis contained within a gene cluster[END_REF]Hung et al., 2016;[START_REF] Langlois | A single glycine-rich repeat of Pseudomonas chlororaphis Polyurethanase A mediates secretion of a GST fusion protein in Escherichia coli[END_REF]Ruiz et al., 1999;[START_REF] Stern | The polyester polyurethanase gene (pueA) from Pseudomonas chlororaphis encodes a lipase[END_REF]) PueB (Pseudomonas chlororaphis lipase)

Polyester PU (Impranil) [START_REF] Howard | Effect of insertional mutations in the pueA and pueB genes encoding two polyurethanases in Pseudomonas chlororaphis contained within a gene cluster[END_REF][START_REF] Howard | Cloning, nucleotide sequencing and characterization of a polyurethanase gene (pueB) from Pseudomonas chlororaphis[END_REF][START_REF] Hung | Carbon Catabolite Repression and Impranil Polyurethane Degradation in Pseudomonas protegens Strain Pf-5[END_REF]Ruiz et al., 1999) Rhizopus arrhizus lipase Thermoplastic polyester PU Tokiwa [START_REF] Tokiwa | Two types of lipases in hydrolysis of polyester[END_REF] Rhizopus delemar lipase Thermoplastic polyester PU Tokiwa [START_REF] Tokiwa | Two types of lipases in hydrolysis of polyester[END_REF] Thermomyces lanuginosus lipase Thermoplastic poly(ester urea) PU [START_REF] Fang | Biodegradable poly(ester urethane)urea elastomers with variable amino content for subsequent functionalization with phosphorylcholine[END_REF] Thermoset polyester PU [START_REF] Wu | Electroactive biodegradable polyurethane significantly enhanced Schwann cells myelin gene expression and neurotrophin secretion for peripheral nerve tissue engineering[END_REF] EC 3.1.1.1 Esterase Thermoplastic polyether PU [START_REF] Smith | The biodegradation of poly (ether urethanes)[END_REF] Porcine liver esterase Thermoplastic polyester PU [START_REF] Li | Improvement of thermal and mechanical properties of poly (L-lactic acid) with 4, 4-methylene diphenyl diisocyanate[END_REF] EC 3.1.1.3 Candida antarctica lipase Thermoplastic polyester PU [START_REF] Takamoto | Lipase-catalyzed hydrolytic degradation of polyurethane in organic solvent[END_REF] Polyester PU coating [START_REF] Liu | Enzymatic waterborne polyurethane towards a robust and environmentally friendly anti-biofouling coating[END_REF] Candida cylindracea lipase Thermoplastic polyester PU [START_REF] Kim | Effect of chemical structure on the biodegradation of polyurethanes under composting conditions[END_REF] Candida rugosa lipase Thermoplastic polyester PU [START_REF] Li | Biobased Thermoplastic Poly(ester urethane) Elastomers Consisting of Poly(butylene succinate) and Poly(propylene succinate)[END_REF] Polyester PU (Impranil) (Gautam et al., 2007b) Porcine pancreas lipase Polyester PU foam [START_REF] Ng | Preparation and modification of water-blown porous biodegradable polyurethane foams with palm oil-based polyester polyol[END_REF] Thermoplastic poly(ester ether) PU [START_REF] Brzeska | Chemical and Enzymatic Hydrolysis of Polyurethane/Polylactide Blends[END_REF] Thermoplastic polyester PU [START_REF] Brzeska | Chemical and Enzymatic Hydrolysis of Polyurethane/Polylactide Blends[END_REF] Thermoplastic polyether PU [START_REF] Ferris | Dithiothreitol-based polyurethanes. Synthesis and degradation studies[END_REF] EC 3.1.1.13 Cholesterol esterase Thermoplastic poly(ester urea) PU [START_REF] Santerre | Enzyme-biomaterial interactions: effect of biosystems on degradation of polyurethanes[END_REF][START_REF] Santerre | Biodegradation evaluation of polyether and polyester-urethanes with oxidative and hydrolytic enzymes[END_REF]Wang et al., 1997a) Thermoplastic poly(ether urea) PU [START_REF] Santerre | Biodegradation evaluation of polyether and polyester-urethanes with oxidative and hydrolytic enzymes[END_REF] Thermoplastic polycarbonate PU [START_REF] Christenson | Enzymatic degradation of poly(ether urethane) and poly(carbonate urethane) by cholesterol esterase[END_REF]Tang et al., 2001a, b;[START_REF] Tang | Isolation of methylene dianiline and aqueous-soluble biodegradation products from polycarbonate-polyurethanes[END_REF]) Thermoplastic polyester PU [START_REF] Woo | Synthesis and characterization of a novel biodegradable antimicrobial polymer[END_REF] Thermoplastic polyether PU [START_REF] Christenson | Enzymatic degradation of poly(ether urethane) and poly(carbonate urethane) by cholesterol esterase[END_REF] EC 3. [START_REF] Ratner | Analysis of in vitro enzymatic and oxidative degradation of polyurethanes[END_REF] Thermoplastic polyether PU [START_REF] Ratner | Analysis of in vitro enzymatic and oxidative degradation of polyurethanes[END_REF] EC 3.4.14.1 Cathepsin C Thermoplastic polyether PU [START_REF] Smith | The biodegradation of poly (ether urethanes)[END_REF] EC 3.4.21.1 Chymotrypsin Thermoplastic poly(ester ether) PU [START_REF] Ciardelli | Segmented polyurethanes for medical applications: synthesis, characterization and in vitro enzymatic degradation studies[END_REF] Thermoplastic poly(ester urea) PU [START_REF] Elliott | Identification of biodegradation products formed by L-phenylalanine based segmented polyurethaneureas[END_REF] Thermoplastic polyester PU [START_REF] Ratner | Analysis of in vitro enzymatic and oxidative degradation of polyurethanes[END_REF][START_REF] Yamamoto | Enzymatic hydrolysis of lysine diisocyanate based polyurethanes and segmented polyurethane ureas by various proteases[END_REF]) Thermoplastic polyether PU [START_REF] Campinez | Study of the properties of the new biodegradable polyurethane PU (TEG-HMDI) as matrix forming excipient for controlled drug delivery[END_REF][START_REF] Ferris | Dithiothreitol-based polyurethanes. Synthesis and degradation studies[END_REF][START_REF] Ratner | Analysis of in vitro enzymatic and oxidative degradation of polyurethanes[END_REF]Smith et al., 1987) EC 3.4.21.36 Porcine pancreatic elastase Thermoplastic poly(ester urea) PU [START_REF] Guan | Elastase-sensitive elastomeric scaffolds with variable anisotropy for soft tissue engineering[END_REF][START_REF] Labow | Elastase-induced hydrolysis of synthetic solid substrates: poly (ester-urea-urethane) and poly (ether-urea-urethane)[END_REF] Thermoplastic poly(ether urea) PU [START_REF] Labow | Elastase-induced hydrolysis of synthetic solid substrates: poly (ester-urea-urethane) and poly (ether-urea-urethane)[END_REF] EC Papain Thermoplastic poly(ether urea) PU [START_REF] Zhao | Long term biodegradation in vitro of poly (ether urethane urea): a mechanical property study[END_REF] Thermoplastic polyester PU [START_REF] Ratner | Analysis of in vitro enzymatic and oxidative degradation of polyurethanes[END_REF][START_REF] Yamamoto | Enzymatic hydrolysis of lysine diisocyanate based polyurethanes and segmented polyurethane ureas by various proteases[END_REF]) Thermoplastic polyether PU [START_REF] Campinez | Study of the properties of the new biodegradable polyurethane PU (TEG-HMDI) as matrix forming excipient for controlled drug delivery[END_REF][START_REF] Ferris | Dithiothreitol-based polyurethanes. Synthesis and degradation studies[END_REF][START_REF] Phua | Biodegradation of a polyurethane in vitro[END_REF][START_REF] Ratner | Analysis of in vitro enzymatic and oxidative degradation of polyurethanes[END_REF][START_REF] Smith | The biodegradation of poly (ether urethanes)[END_REF]) poly(ether urethane urea) elastomer [START_REF] Marchant | Degradation of a poly (ether urethane urea) elastomer: infra-red and XPS studies[END_REF] EC 3.4.22.3 Ficin Thermoplastic polyester PU [START_REF] Yamamoto | Enzymatic hydrolysis of lysine diisocyanate based polyurethanes and segmented polyurethane ureas by various proteases[END_REF] Thermoplastic polyether PU [START_REF] Smith | The biodegradation of poly (ether urethanes)[END_REF] EC 3.4.4.24 Bromelain Thermoplastic polyester PU [START_REF] Smith | The biodegradation of poly (ether urethanes)[END_REF]Yamamoto et al., 2007) EC 3.4.21.37 Human neutrophil elastase Thermoplastic poly(ester urea) PU [START_REF] Labow | Elastase-induced hydrolysis of synthetic solid substrates: poly (ester-urea-urethane) and poly (ether-urea-urethane)[END_REF] Thermoplastic poly(ether urea) PU [START_REF] Labow | Elastase-induced hydrolysis of synthetic solid substrates: poly (ester-urea-urethane) and poly (ether-urea-urethane)[END_REF] EC 3.4.24 Collagenase Thermoplastic polyester PU [START_REF] Zhang | In vitro exposure of a novel polyesterurethane graft to enzymes: A study of the biostability of the Vascugraft® arterial prosthesis[END_REF] Thermoplastic polyether PU (Mendoza-Novelo et al., 2013) EC 3.5.1.4 E4143 (amidase) Thermoplastic polyester PU [START_REF] Magnin | Enzymatic recycling of thermoplastic polyurethanes: Synergistic effect of an esterase and an amidase and recovery of building blocks[END_REF] Nocardia farcinica polyamidase Thermoplastic polyester PU [START_REF] Gamerith | Improving enzymatic polyurethane hydrolysis by tuning enzyme sorption[END_REF] EC 3.5.1.5 Urease Thermoplastic polyether PU [START_REF] Phua | Biodegradation of a polyurethane in vitro[END_REF] No EC number Pancreatine (enzyme mixture) Thermoplastic polyester PU [START_REF] Zhang | In vitro exposure of a novel polyesterurethane graft to enzymes: A study of the biostability of the Vascugraft® arterial prosthesis[END_REF] Enzymatic degradation assays on PU using oxidase enzymes (Enzyme Classification 1, EC 1) e.g., fungal peroxidase (EC 1.11.1.7) and laccase (E.C. 1.10.3.2) have been performed with success on a polyester PU [START_REF] Ignat | Effects of flax lignin addition on enzymatic oxidation of poly(ethylene adipate) urethanes[END_REF]. All the other assays dealing with enzymatic degradation of PU relies on hydrolytic enzymes (EC 3). Several hydrolytic mechanisms have indeed been highlighted for PU degradation. The most common is the hydrolysis of the polyester moieties of polyesterbased PU by esterases (EC 3.1). Ester hydrolysis leads to the release of a carboxylic acid and an alcohol (Figure 7a). Other esterases such as lipases (EC 3.1.1) [START_REF] Fang | Biodegradable poly(ester urethane)urea elastomers with variable amino content for subsequent functionalization with phosphorylcholine[END_REF][START_REF] Schöne | Polymer architecture versus chemical structure as adjusting tools for the enzymatic degradation of oligo(ε-caprolactone) based films at the air-water interface[END_REF], cutinases (EC 3.1.1.74) [START_REF] Schmidt | Degradation of Polyester Polyurethane by Bacterial Polyester Hydrolases[END_REF][START_REF] Yang | A low molecular mass cutinase of Thielavia terrestris efficiently hydrolyzes poly(esters)[END_REF] or unspecific esterases (EC 3.1) [START_REF] Kang | A novel family VII esterase with industrial potential from compost metagenomic library[END_REF] have been described as polyester PU degrading enzymes. Esterase has also been described as hydrolyzing the urethane linkage. Some of these studies refer to a mechanism resulting in carbamic acid and alcohol chain-ends after hydrolysis [START_REF] Mahajan | New insights into the microbial degradation of polyurethanes[END_REF][START_REF] Wei | Microbial enzymes for the recycling of recalcitrant petroleumbased plastics: how far are we?[END_REF]. However, this mechanism does not seem conceivable because of the instability of the carbamic acid which immediately breaks down into an amine with the release of a molecule of carbon dioxide [START_REF] Ionescu | Chemistry and technology of polyols for polyurethanes[END_REF][START_REF] Ozaki | Recent advances in isocyanate chemistry[END_REF] (Figure 7b). Moreover, most of the assays involving esterases concern polyester-based PU and do not allow differentiation between ester and urethane bond hydrolysis. To evaluate the urethane bond hydrolysis by an esterase, assays must be performed on substrates that do not contain ester bonds. Publications describing slight esterase activity on polyether PU and showing the potential ability of esterase to hydrolyze the urethane bond are scarce [START_REF] Santerre | Biodegradation evaluation of polyether and polyester-urethanes with oxidative and hydrolytic enzymes[END_REF][START_REF] Smith | The biodegradation of poly (ether urethanes)[END_REF]. A cholesterol lipase was reported to display activity on a PU based on triethylene glycol and 1,4-di-S-benzyl-D,L-dithiothreitol [START_REF] Ferris | Dithiothreitol-based polyurethanes. Synthesis and degradation studies[END_REF]. Urease (EC 3.5.1.5) also showed activity on poly(ether urea) PU [START_REF] Phua | Biodegradation of a polyurethane in vitro[END_REF] but the degradation is mainly attributed to the urea bond hydrolysis (Figure 7c). Amidases (EC 3.5.1.4) and proteases hydrolyze amide or peptidic bonds leading to the release of a carboxylic acid and an amine (Figure 7d). These enzymes appeared to be also efficient for the hydrolysis of the urethane bond leading to the release of an amine, an alcohol and a carbon dioxide molecule (Figure 7e). Proteases such as papain (EC 3.4.22.2) [START_REF] Campinez | Study of the properties of the new biodegradable polyurethane PU (TEG-HMDI) as matrix forming excipient for controlled drug delivery[END_REF][START_REF] Ferris | Dithiothreitol-based polyurethanes. Synthesis and degradation studies[END_REF][START_REF] Marchant | Degradation of a poly (ether urethane urea) elastomer: infra-red and XPS studies[END_REF][START_REF] Yamamoto | Enzymatic hydrolysis of lysine diisocyanate based polyurethanes and segmented polyurethane ureas by various proteases[END_REF], bromelain (EC 3.4.22.32/33) [START_REF] Yamamoto | Enzymatic hydrolysis of lysine diisocyanate based polyurethanes and segmented polyurethane ureas by various proteases[END_REF], ficain (EC 3.4.22.3) [START_REF] Yamamoto | Enzymatic hydrolysis of lysine diisocyanate based polyurethanes and segmented polyurethane ureas by various proteases[END_REF] and chymotrypsin EC 3.4.21.1 [START_REF] Ciardelli | Segmented polyurethanes for medical applications: synthesis, characterization and in vitro enzymatic degradation studies[END_REF][START_REF] Elliott | Identification of biodegradation products formed by L-phenylalanine based segmented polyurethaneureas[END_REF][START_REF] Ferris | Dithiothreitol-based polyurethanes. Synthesis and degradation studies[END_REF] are also described for the degradation of PU. Recently, an amidase drew attention. This enzyme was isolated from Nocardia farcinica with the specificity of being able to hydrolyze polyamides [START_REF] Guo | Cloning, expression, and characterization of polyamidase from Nocardia farcinica and its application to polyamide modification[END_REF] but also both the ester and amide bonds of non-water soluble model substrates [START_REF] Heumann | A novel aryl acylamidase from Nocardia farcinica hydrolyses polyamide[END_REF] and polyester-based PUs [START_REF] Gamerith | Improving enzymatic polyurethane hydrolysis by tuning enzyme sorption[END_REF]. Even if esterase is the main class of enzyme describes for PU degradation, others such as amidase or oxidase are rising interest for the full degradation of PU material.

Figure 7 -Main mechanisms of enzymatic degradation of PU

An alternative strategy aiming at discovering efficient PU-degrading enzymes was recently published. Metagenomics tools allowing to screen bovine rumen microbiota were developed to select enzymes with activity towards carbamate insecticides and PUs [START_REF] Ufarte | Discovery of carbamate degrading enzymes by functional metagenomics[END_REF]. The main advantage of this strategy is the possibility of studying enzymes from uncultivable microorganisms that are predominant in microbial communities, and thus having potential access to new degrading enzymes.

To date, PU depolymerization using enzymes is not efficient enough for the development of recycling processes. Recent publications on this topic aimed to improve the efficiency of the depolymerization reactions. Complementary activities of enzymes presented above justify testing cocktail of enzymes with different activities. A mixture of an esterase and an amidase revealed a synergistic effect between these two enzymes for the degradation of a polyester PU.

Corresponding product analysis showed that hydrolysis of ester moieties from the SS by the esterase released low molar mass molecules containing urethane bonds that are then hydrolyzed by the amidase [START_REF] Magnin | Enzymatic recycling of thermoplastic polyurethanes: Synergistic effect of an esterase and an amidase and recovery of building blocks[END_REF]. Another strategy for enzymatic depolymerization improvement is to use protein engineering. This approach already proved itself on PET-degrading enzymes. As an example, decreased inhibitor sensitivity of a PET-degrading cutinase has been successfully achieved by amino-acid modification into the catalytic site [START_REF] Wei | Engineered bacterial polyester hydrolases efficiently degrade polyethylene terephthalate due to relieved product inhibition[END_REF]. Thermostability is also a key parameter for polymer degradation [START_REF] Kawai | A novel Ca 2+-activated, thermostabilized polyesterase capable of hydrolyzing polyethylene terephthalate from Saccharomonospora viridis AHK190[END_REF]. Ribitsch et al., fused a PET-degrading enzyme (a cutinase from Thermomyces cellullosylitica) with a binding module of a PHA depolymerase from Alcaligenes faecalis [START_REF] Ribitsch | Fusion of binding domains to Thermobifida cellulosilytica cutinase to tune sorption characteristics and enhancing PET hydrolysis[END_REF] to improve enzyme/polymer interactions. This binding domain was recently added to the amidase from Nocardia farcinica to improve the degradation of polyester PU pellets [START_REF] Gamerith | Improving enzymatic polyurethane hydrolysis by tuning enzyme sorption[END_REF]. These improvements will help reaching efficient depolymerization processes for PU biorecycling.

Analytical solutions for the measurement of polyurethane biodegradation

The great diversity of PU structures and biological entities gives rise to numerous analytical approaches to evaluate PU biological degradation. The methods have evolved lately with powerful tools for a better understanding of the mechanisms of PU biodegradation. This chapter offers an overview of the different techniques. The implementation of tools using urethane-based model molecules is first addressed. Then, the degradation of more complex substrates, such as TPU and PU foams, is presented.

Biological degradation of urethane-based model molecules

For the development of bioremediation or biological recycling processes, degradation of model molecules is generally the first step as it allows easy identification of efficient degrading entities. Urethane-based model molecules are readily hydrolysable and generally propose a simplified hydrolysis detection. These model molecules can be either low molar mass molecules or hydrophilic PU dispersion.

Analysis of low molar mass molecules

To cover the degradation of different PU structures, tracking the urethane bond hydrolysis appears as a relevant solution. Low molar mass molecules containing a single urethane linkage can be designed for this purpose. Urethane-based molecules are not soluble in water. Pre-dilution in organic solvents such as ethanol [START_REF] Akutsu-Shigeno | Isolation of a bacterium that degrades urethane compounds and characterization of its urethane hydrolase[END_REF] or DMSO [START_REF] Gamerith | Improving enzymatic polyurethane hydrolysis by tuning enzyme sorption[END_REF] is thus required.

Low molar mass N-tolylcarbamate molecules correspond to toluene with urethane linkage on one or two carbons of the aromatic ring bound to ethanol moieties [START_REF] Owen | The Biodegradation of Low-molecular-weight Urethane Compounds by a Strain of Exophiala jeanselmei[END_REF]. In Owen et al., aromatic amines resulting from the N-tolylcarbamate hydrolysis were extracted in chloroform and quantified by Gas Chromatography coupled with Mass Spectrometry (GC/MS). This assay revealed that the degrading activity of the Exophiala jeanselmei strain REN-11A depends on the position of the urethane(s) around the aromatic ring. Toluene-2,4-and -2,6-dicarbamic acid diethyl ester were the most readily biodegradable molecules (Figure 8). Akutsu-Shigeno et al., also described a set of molecules bearing a single urethane bond formed by reacting a di-isocyanate (2,4-TDI, 4,4'-MDI and HDI) with butanol [START_REF] Akutsu-Shigeno | Isolation of a bacterium that degrades urethane compounds and characterization of its urethane hydrolase[END_REF]. These compounds were degraded by both Rhodococcus equi strain TB-60 and a purified urethanedegrading enzyme secreted by this bacterium. Degradation products were extracted with ethyl acetate and analyzed by GC/MS except for the HDI-based model molecules which degradation products were extracted with toluene under alkaline conditions. Because of the difficulties to detect aliphatic amines in GC/MS, amines coming from HDI-based molecules hydrolysis were derivatized using heptafluorobutyric acid anhydride following the method of Skarping et al. [START_REF] Skarping | Trace analysis of airborne 1, 6hexamethylenediisocyanate and the related aminoisocyanate and diamine by glass capillary gas chromatography[END_REF].

Coupling a 96-wells microplate assay with HPLC analysis was proposed for the development of a medium-throughput screening [START_REF] Magnin | Enzymatic recycling of thermoplastic polyurethanes: Synergistic effect of an esterase and an amidase and recovery of building blocks[END_REF]. A model molecule based on p-Toluenesulfonyl isocyanate was synthesized for this microplate assay (Figure 9). Both the substrate and the degradation products were analyzed by HPLC. Finally, 55 enzymes were screened resulting in the identification of two amidases able to hydrolyze the urethane bond. [START_REF] Magnin | Enzymatic recycling of thermoplastic polyurethanes: Synergistic effect of an esterase and an amidase and recovery of building blocks[END_REF] To avoid the setup of complex analytical procedures such as GC/MS, Gamerith et al., proposed the synthesis of a model molecule based on 4-nitrophenol [START_REF] Gamerith | Improving enzymatic polyurethane hydrolysis by tuning enzyme sorption[END_REF]. Molecules based on this aromatic compound are well known as model substrates for enzymes such as esterase (4-nitrophenyl acetate) or amidase (4-nitroacetanilide). 1-methoxypropan-2-yl (4nitrophenyl) carbamate was synthesized (Figure 10) and the subsequent hydrolysis leads to the release of 4-nitroaniline that can be tracked and quantified by UV-vis absorbance measurements at 405 nm.

Figure 10 -Hydrolysis of the 1-methoxypropan-2-yl (4-nitrophenyl) carbamate leading to 4-nitroaniline and 1methoxy-2-propanol [START_REF] Gamerith | Improving enzymatic polyurethane hydrolysis by tuning enzyme sorption[END_REF] The use of low molar mass urethane substrates is a good way to identify efficient degrading entities. However, the low steric hindrance of these molecules makes them far from being representative of actual PU materials which often present organized and crystalline structures and are much more hydrophobic. Activity assays on real and complex polymers must then be performed with the identified degrading entities.

Waterborne polyurethane dispersions as a PU-based model

WPUD are particularly suitable to assess the degradation ability of enzymes and microorganisms thanks to the polymer particles nanometric size and homogeneity in water. Indeed, their specific surface is higher than for previously described polymers, thus maximizing interactions between the degrading entity and the polymer. Moreover, all biodegradation reactions occur in aqueous media. The most famous WPUD is the Impranil-DLN®, commercialized by Covestro (Germany) for textile coating applications. Particle size is estimated to range between 0.1 to 0.2 µm [START_REF] Biffinger | The applicability of Impranil®DLN for gauging the biodegradation of polyurethanes[END_REF]. This dispersion appears as a white, milky suspension containing 40% of polymer. The exact composition and structure of Impranil-DLN® are not precisely known. A tentative structure has been proposed by Biffinger et al. based on polyhexane/neopentyl adipate polyester and HDI (Figure 11). Diethylene glycol is also a component of Impranil-DLN® (Gautam et al., 2007b).

Figure 11 -Impranil-DLN® tentative structure adapted from [START_REF] Biffinger | The applicability of Impranil®DLN for gauging the biodegradation of polyurethanes[END_REF][START_REF] Biffinger | The applicability of Impranil®DLN for gauging the biodegradation of polyurethanes[END_REF] Impranil-DLN® has been widely studied as a PU-based model since it presents the particularity to become translucent when hydrolyzed as a result of water-soluble molecules being released in the medium. Impranil-DLN® was used as "polyurethane biodegradation benchmark" for the first time in 1994 [START_REF] Crabbe | Biodegradation of a colloidal ester-based polyurethane by soil fungi[END_REF]. Screening of soil fungi was performed on a plate where Impranil-DLN® was mixed with an agar medium. Fungi were allowed to grow on it and a transparent halo appeared after a few days when the microorganism was producing degrading enzymes. This agar plate technique has subsequently been used intensively [START_REF] Howard | Cloning, nucleotide sequencing and characterization of a polyurethanase gene (pueB) from Pseudomonas chlororaphis[END_REF][START_REF] Peng | Degradation of polyurethane by bacterium isolated from soil and assessment of polyurethanolytic activity of a Pseudomonas putida strain[END_REF][START_REF] Rowe | Growth of Bacillus subtilis on polyurethane and the purification and characterization of a polyurethanase-lipase enzyme[END_REF]Vega et al., 1999) (Figure 12a & b). Impranil-DLN® is also suitable for assays in liquid media for both microorganisms [START_REF] Álvarez-Barragán | Biodegradative Activities of Selected Environmental Fungi on a Polyester Polyurethane Varnish and Polyether Polyurethane Foams[END_REF][START_REF] Russell | Biodegradation of polyester polyurethane by endophytic fungi[END_REF] (Figure 12c) and enzymes (Gautam et al., 2007b;[START_REF] Schmidt | Degradation of Polyester Polyurethane by Bacterial Polyester Hydrolases[END_REF]. [START_REF] Russell | Biodegradation of polyester polyurethane by endophytic fungi[END_REF] As the exact polymer structure is unknown, it is difficult to appraise the mechanism of degradation. Biffinger et al., used NMR and FT-IR to offer quantitative analysis of alcohol and carboxylic acid release after the ester bond cleavage [START_REF] Biffinger | The applicability of Impranil®DLN for gauging the biodegradation of polyurethanes[END_REF]. They also highlighted that polymer aggregation can occur when incubated with enzymes without any measurable degradation. Observation of the polymer is thus not sufficient to conclude on enzymatic degradation activity. Ufarté et al., proposed to use Matrix Assisted Laser Desorption Ionisation -Time of Flight/Mass Spectrometry (MALDI-TOF/MS) to identify degradation by-products. Three peaks at m/z 682, 683 and 782 were specific of the bacterial degradation of Impranil-DLN®. All seemed to correspond to the formula C36H68O8N2, ionized with either Na + or I -. The peak at m/z 683 could correspond to the isotope form of the molecule at m/z 682. However, no specific chemical structure has been suggested. Alvarez-Barragan et al., presented the analysis of degradation by-products by GC/MS [START_REF] Álvarez-Barragán | Biodegradative Activities of Selected Environmental Fungi on a Polyester Polyurethane Varnish and Polyether Polyurethane Foams[END_REF]. Almost none of the products identified correspond to the Impranil putative structure shown above (Figure 11) suggesting that the structure is much more complex than expected. The only corresponding molecule is HDI. However, HDI cannot be a degradation product as isocyanates are not stable in water. A possible explanation would be the detection of either hexane diamine (HDA) or HDI-derivatives.

Despite some important limitations, Impranil-DLN® is thus a good model substrate to identify degrading entities that have a great chance to be efficient afterwards on polyester PUs. It provides a first approach that must then be confirmed. [START_REF] Hung | Carbon Catabolite Repression and Impranil Polyurethane Degradation in Pseudomonas protegens Strain Pf-5[END_REF] did not only present Impranil-DLN® as a model but also as a common coating which integrity can be affected by microbial degradation [START_REF] Hung | Carbon Catabolite Repression and Impranil Polyurethane Degradation in Pseudomonas protegens Strain Pf-5[END_REF]. Esterase and lipase are mainly involved in the enzymatic degradation of Impranil-DLN®. However, up to now, there is no clear evidence that these enzymes, improperly called "Polyurethanase" (Ruiz et al., 1999;[START_REF] Stern | The polyester polyurethanase gene (pueA) from Pseudomonas chlororaphis encodes a lipase[END_REF], effectively hydrolyze the urethane bond.

Few other WPUDs have been tested for degradation activities. Bayhydrol 110 (Covestro, Germany) is a polyester PU dispersion presenting the same clarification properties as Impranil-DLN®: translucent halo appeared when incubated with a strain of Pseudomonas chlorographis on an agar plate containing this WPUD [START_REF] Howard | Cloning, nucleotide sequencing and characterization of a polyurethanase gene (pueB) from Pseudomonas chlororaphis[END_REF]. Poly Lack (Sayer Lack Mexicana, Mexico), a polyether PU, was also tested on agar plate containing the polymer and minimal media [START_REF] Álvarez-Barragán | Biodegradative Activities of Selected Environmental Fungi on a Polyester Polyurethane Varnish and Polyether Polyurethane Foams[END_REF]. Strains able to grow on Poly Lack as sole carbon source were isolated, yet, no clarification zone could have been observed on this polymer.

Biological degradation of TPU and PU foams

The most mainstream PU systems, i.e. TPU and PU foams, have been tested in biodegradation assays. These products present a wide and varied range of chemical structures and groups. When the purpose is to evaluate the susceptibility to biological degradation of a material, the structure and formulation are precisely described in the study. In contrast, bioremediation and biological recycling studies on PU mainly involve commercial products of complex and often unknown chemical structure, additives and composition. The biological degradation experiments described in the literature are mainly partial, leading to the recovery of the degraded polymer and, possibly, soluble degradation products released in the aqueous media (Figure 13). Techniques developed to evaluate the biological degradation of polymers can thus be oriented towards the efficiency of the entire degradation system with the analysis of the degraded polymer products (soluble and insoluble).

Samples structures and preparation

Evaluation of PU foam degradation is more challenging than that of TPU, since foams are highly complex systems based on crosslinked architectures with different components and additives. Side reactions during synthesis and foaming steps can also lead to the formation of isocyanurate and other bonds. Foam analysis is limited by their insolubility in solvent.

PUs shape drives the specific surface and thus the bioavailability which is of importance for the biological degradation. Thanks to their alveolar structure, bioavailability is greater for foams, especially for open-cell foams, as microorganisms can easily circulate inside the material. Foams can also be ground to increase bioavailability [START_REF] Cregut | Applying Raman spectroscopy to the assessment of the biodegradation of industrial polyurethanes wastes[END_REF]. TPUs for degradation assays can be used as thin films obtained by coating after solubilizing in an appropriate solvent, pouring on glassware and solvent evaporation [START_REF] Chen | Poly(propylene carbonate) polyurethane selfpolishing coating for marine antifouling application[END_REF][START_REF] Woo | Synthesis and characterization of a novel biodegradable antimicrobial polymer[END_REF]. Thin films obtained can reach a few dozen micrometers [START_REF] Phua | Biodegradation of a polyurethane in vitro[END_REF]. TPU films can also be obtained by thermoforming [START_REF] Zhou | Synthesis and characterization of poly(ester ether urethane)s block copolymers based on biodegradable poly(butylene succinate) and Poly(ethylene glycol)[END_REF]. TPU cubes [START_REF] Nakajima-Kambe | Isolation and characterization of a bacterium which utilizes polyester polyurethane as a sole carbon and nitrogen source[END_REF], pellets [START_REF] Cosgrove | Fungal communities associated with degradation of polyester polyurethane in soil[END_REF] or sheets [START_REF] Ibrahim | Polyester-polyurethane Biodegradation by Alternaria Solani, Isolated from Northern Jordan[END_REF] are also used for PU degradation assays.

Preparation of samples

It is not recommended to sterilize TPUs by autoclaving for microbial experiments as most of the TPU becomes liquid-like or very soft at autoclaving temperatures (121°C). Degradation can occur e.g., a study comparing autoclaved and non-autoclaved poly(ether urea) PU material revealed that no weight loss was observed after autoclaving but a surface alteration appeared, leading to bias in degradation measurement [START_REF] Rafiemanzelat | Study of Biological Degradation of New Poly(Ether-Urethane-Urea)s Containing Cyclopeptide Moiety and PEG by Bacillus amyloliquefaciens Isolated from Soil[END_REF]. Alternatives such as rinsing with ethanol [START_REF] Cosgrove | Effect of biostimulation and bioaugmentation on degradation of polyurethane buried in soil[END_REF][START_REF] Mathur | Degradation of polyurethane by Aspergillus flavus (ITCC 6051) isolated from soil[END_REF], UV exposure [START_REF] Gogoi | Biobased Biodegradable Waterborne Hyperbranched Polyurethane as an Ecofriendly Sustainable Material[END_REF] or both [START_REF] Osman | Degradation of Polyester Polyurethane by Aspergillus sp. Strain S45 Isolated from Soil[END_REF] are thus frequently employed to sterilize the samples. Because of better thermal resistance of thermosetting materials, foams can be tested after autoclaving [START_REF] Álvarez-Barragán | Biodegradative Activities of Selected Environmental Fungi on a Polyester Polyurethane Varnish and Polyether Polyurethane Foams[END_REF].

Evaluation of the global biodegradation efficiency

The most widespread method to evaluate global biodegradation is the weight loss of the PU. Samples are weighted before and after degradation assay to evaluate the weight of degradation products released in the liquid media. It is a straightforward and easy method to implement, although some bias can be noticed. For instance, if the degradation is too superficial and too low to lead to detectable product release, the test will be considered as inefficient as no weight loss will be measured. Therefore, weight loss measurement must be associated with a surface analysis of the sample [START_REF] Rafiemanzelat | Study of Biological Degradation of New Poly(Ether-Urethane-Urea)s Containing Cyclopeptide Moiety and PEG by Bacillus amyloliquefaciens Isolated from Soil[END_REF] in case of a low degradation extent, or to analyze the early steps of the degradation. It is necessary to remove all the biological materials that can remain on the polymer surface. Hard washing with ethanol is generally performed [START_REF] Mathur | Degradation of polyurethane by Aspergillus flavus (ITCC 6051) isolated from soil[END_REF][START_REF] Urgun-Demirtas | Laboratory investigation of biodegradability of a polyurethane foam under anaerobic conditions[END_REF]. Non-ionic surfactants such as TritonX-100 (Polyethylene glycol tert-octylphenyl ether) (1% v/v) have also been suggested to remove reversibly bounded enzymes and cells [START_REF] Ciardelli | Segmented polyurethanes for medical applications: synthesis, characterization and in vitro enzymatic degradation studies[END_REF]. This cleaning step is particularly challenging for the fungal degradation of foams as filaments could be deeply trapped into the bulk structure of the samples. It was recently suggested to use 0.88% (wt/vol) sodium hypochlorite for 18 h to destroy and remove the remaining mycelium [START_REF] Álvarez-Barragán | Biodegradative Activities of Selected Environmental Fungi on a Polyester Polyurethane Varnish and Polyether Polyurethane Foams[END_REF]. When enzymatic degradations are performed, kinetic weight loss is generally set up [START_REF] Dogan | Thermally induced shape memory behavior, enzymatic degradation and biocompatibility of PLA/TPU blends: "Effects of compatibilization[END_REF][START_REF] Zhou | Synthesis and characterization of poly(ester ether urethane)s block copolymers based on biodegradable poly(butylene succinate) and Poly(ethylene glycol)[END_REF]. In such experiments, a loss of enzymatic activity is often observed with time. This loss of activity may be due to the thermal denaturation of enzymes [START_REF] Pastorino | Lipase-catalyzed degradation of poly(ε-caprolactone)[END_REF] or to the release of inhibiting compounds [START_REF] Barth | Effect of hydrolysis products on the enzymatic degradation of polyethylene terephthalate nanoparticles by a polyester hydrolase from Thermobifida fusca[END_REF]. To cope with this phenomenon, enzymatic solutions are frequently renewed at regular time intervals [START_REF] Phua | Biodegradation of a polyurethane in vitro[END_REF]. Between the removal step and the renewal step, polymer pieces are usually washed, dried and weighed.

As a parameter of global assay efficiency, the evaluation of the ability of a microorganism to grow on PUs when the polymer is used as the sole source of carbon (or carbon and nitrogen) is common practice [START_REF] Cooney | Effects of polyurethane foams on microbial growth in fuel-water systems[END_REF]. Indeed, microbial development means that microorganisms can depolymerize PUs and metabolize degradation products for growth. It is possible to quantitatively follow the bacterial growth through the McFarland method which estimates the number of bacteria thanks to turbidity measurement using UV-vis at 600 nm [START_REF] Bayan | Renewable resource modified polyol derived aliphatic hyperbranched polyurethane as a biodegradable and UV-resistant smart material[END_REF][START_REF] Fernandes | Biobased Additives as Biodegradability Enhancers with Application in TPU-Based Footwear Components[END_REF][START_REF] Gogoi | Biobased Biodegradable Waterborne Hyperbranched Polyurethane as an Ecofriendly Sustainable Material[END_REF]. Colony forming unit (CFU) is another method to count bacteria: after being incubated with polymers, bacteria are sampled, diluted and poured on an agar plate containing a rich medium [START_REF] Crookes-Goodson | The impact of culture medium on the development and physiology of biofilms of Pseudomonas fluorescens formed on polyurethane paint[END_REF][START_REF] Urgun-Demirtas | Laboratory investigation of biodegradability of a polyurethane foam under anaerobic conditions[END_REF]. Colonies are counted after overnight incubation. The dry or wet weight of the biomass corresponding to microorganisms growth can be measured [START_REF] Oceguera-Cervantes | Characterization of the polyurethanolytic activity of two Alicycliphilus sp. strains able to degrade polyurethane and N-methylpyrrolidone[END_REF]. This technique could be particularly suitable for bacteria that form aggregates or for filamentous fungi.

Mineralization of polymers by microorganisms in aerobic conditions leads to the production of CO2 with O2 consumption. Online sensors are used to measure both evolutions. These variations must be compared to a negative control made without polymers [START_REF] Cregut | Applying Raman spectroscopy to the assessment of the biodegradation of industrial polyurethanes wastes[END_REF] or with an 29 inert polymer such as low-density polyethylene [START_REF] Rattanapan | Preparation and properties of bio-based polyurethane foams from natural rubber and polycaprolactone diol[END_REF], and to a conventional biodegradable positive control such as cellulose [START_REF] Gómez | Biodegradability of crude glycerol-based polyurethane foams during composting, anaerobic digestion and soil incubation[END_REF] or sodium benzoate [START_REF] Rattanapan | Preparation and properties of bio-based polyurethane foams from natural rubber and polycaprolactone diol[END_REF] incubated with the same inoculum. For instance, low O2 consumption and CO2 release were measured during the 28 days of degradation of a ground polyether PU foam revealing low degradation by an acclimated microbial consortium [START_REF] Cregut | Applying Raman spectroscopy to the assessment of the biodegradation of industrial polyurethanes wastes[END_REF]. CO2 release during PU mineralization is associated with pressure increase. The pressure can be measured by a Sturm test (Standard OECD 301 B) [START_REF] Shah | Degradation of polyester polyurethane by an indigenously developed consortium of Pseudomonas and Bacillus species isolated from soil[END_REF]. This test is used for readily biodegradable materials and usually lasts 28 days. Rattanapan et al., used this assay to measure the biodegradation of a biobased polyester PU foam with long incubation time (60 days) [START_REF] Rattanapan | Preparation and properties of bio-based polyurethane foams from natural rubber and polycaprolactone diol[END_REF]. In fact, after 30 days, 7 to 11% of degradation was measured while a higher degradation rate occurred during the last 30 days leading to a maximal degradation yield of 46 wt%. Under anaerobic digestion, CH4 is produced proportionally to polymer consumption. Gomez et al., proposed to compare the biological degradation susceptibility of polyether PU foams under composting, soil burial and anaerobic digestion according to three ASTM standard methods based on CO2 or CH4 measurement [START_REF] Gómez | Biodegradability of crude glycerol-based polyurethane foams during composting, anaerobic digestion and soil incubation[END_REF] (International, 2003a) and the ASTM D5511-02 (Standard test method for determining anaerobic biodegradation of plastic materials under highsolid anaerobic-digestion conditions) [START_REF] International | ASTM D5511-02, Standard Test Method for Determining Anaerobic Biodegradation of Plastic Materials Under High-Solids Anaerobic-Digestion Conditions[END_REF]. The most pronounced degradation was observed for a bio-based PU foam after 320 days of soil burial. Methods involving CO2, O2 or CH4 measurement present a low throughput. Incubation time is superior to 28 days. Moreover, a pressure or gas monitor is required for each reaction which is generally performed in flasks from 2 to 5 liters (recommendation for OECD 301 series). These methods are mainly oriented towards the evaluation of the biological sensitivity or resistance of newly synthesized or commercial PUs, especially foams. These techniques are not suitable for screening of PU degrading entities.

Study of the degraded polymers samples and insoluble products

The first assessment of polymer degradation is a naked eye observation, sometimes sufficient to evaluate the degradation onset. Change in color, roughness or shape can be noticed (Pilch-Pitera, 2012). These observations can be completed by surface analysis such as spectroscopy and/or microscopy, polymer molar mass evolution by SEC or modifications of physical properties.

Spectroscopy techniques

FT-IR (Fourier-transform infrared spectroscopy) analysis relies on the fact that most molecules absorb in the infrared region. This absorption corresponds specifically to the vibration modes of the different bonds present in the analytes. Absorption spectra thus provide information on the chemical structure of the polymer as a fingerprint. FT-IR is particularly popular for PU degradation analysis because of its accuracy and rapidity and because FT-IR is a non-destructive method: the sample is recovered without damage after the analysis. Surface analysis can be easily performed to measure superficial biological degradation. Different issues can be encountered in the analysis and interpretation of a spectrum. For instance, partial similarities between bonds e.g., urethane and ester groups complicate the spectra interpretation for polyester PUs. Moreover, both hydroxyl (OH) moieties, resulting from ester and urethane degradation, and amine (NH) moieties, resulting from urethane degradation, absorb around 3400 cm -1 . For polyester PUs, the increase of this large band is generally attributed to ester-or both ester and urethane hydrolysis [START_REF] Oprea | Dependence of fungal biodegradation of PEG/castor oil-based polyurethane elastomers on the hard-segment structure[END_REF][START_REF] Spontón | Biodegradation study by Pseudomonas sp. of flexible polyurethane foams derived from castor oil[END_REF], but it has also already been interpreted exclusively as the cleavage of the urethane bond [START_REF] Umare | Synthesis, characterization and biodegradation studies of poly(ester urethane)s[END_REF].

Similarly, it is difficult to interpret changes in spectra presenting carbonyl bonds (C=O) that appear in ester (1750-1725 cm -1 ), urethane (1700 cm -1 ) and urea bonds (1630 cm -1 ). In polyester PUs, there is often a unique broad signal representing both urethane and ester carbonyl bonds. Its decrease is generally attributed to ester bond hydrolysis [START_REF] Schmidt | Degradation of Polyester Polyurethane by Bacterial Polyester Hydrolases[END_REF][START_REF] Shah | Degradation of polyester polyurethane by an indigenously developed consortium of Pseudomonas and Bacillus species isolated from soil[END_REF] but sometimes has been attributed only to urethane bond hydrolysis in polyester PUs [START_REF] Gómez | Biodegradability of crude glycerol-based polyurethane foams during composting, anaerobic digestion and soil incubation[END_REF][START_REF] Ozsagiroglu | Biodegradation and characterization studies of different kinds of polyurethanes with several enzyme solutions[END_REF]. The signal at 1530 cm -1 is generally attributed to the nitrogen of the urethane moieties. A comparison of the polyester PU based on PCL and the constitutive PCL polyester showed that the signal at 1530 cm -1 only appeared on the PU spectrum thus confirming that this signal is attributed to the urethane [START_REF] Magnin | Enzymatic recycling of thermoplastic polyurethanes: Synergistic effect of an esterase and an amidase and recovery of building blocks[END_REF]. Oprea et al. suggested that an increase of this signal is related to urethane bond hydrolysis [START_REF] Oprea | Dependence of fungal biodegradation of PEG/castor oil-based polyurethane elastomers on the hard-segment structure[END_REF] while others suggested that a decrease of this signal attests to urethane bond degradation [START_REF] Oceguera-Cervantes | Characterization of the polyurethanolytic activity of two Alicycliphilus sp. strains able to degrade polyurethane and N-methylpyrrolidone[END_REF][START_REF] Sarkar | Oxidative and enzymatic degradations of l-tyrosine based polyurethanes[END_REF]. It is also conceivable that an increase of this signal is correlated to the increase of the urethane proportion in the polymer after the biological hydrolysis of the soft segment. Concluding on the variation of this signal upon degradation appeared therefore complicated. Other peaks are sometimes considered as proof of PU degradation. For instance, the emergence of a peak at 2250 cm -1 after degradation has been attributed to isocyanate (NCO) release [START_REF] Shah | Degradation of polyester polyurethane by an indigenously developed consortium of Pseudomonas and Bacillus species isolated from soil[END_REF] (Figure 14b). However, isocyanates are unstable in water and cannot be released through biological degradation in aqueous media. This signal could correspond to atmospheric CO2 resulting from improperly done FT-IR background spectra [START_REF] Gerakines | The infrared band strengths of H2O, CO and CO2 in laboratory simulations of astrophysical ice mixtures[END_REF]. Since high variation on FT-IR spectra interpretations are observed through the literature, especially concerning polyester PUs, additional methods must be used to confirm urethane bond cleavage for PU degradation. Kay et al., suggested to consider the decrease of the ratio ester (C=O) /ether (1720 cm -1 /1125 cm - 1 ), the ratio urethane (NH)/ether (1630 cm -1 /1125cm -1 ) and the ratio aryl (C=C)/ether in order to provide a semi-quantitative analysis of the degradation of a polyether PU [START_REF] Kay | Chemical and physical changes occurring in polyester polyurethane during biodegradation[END_REF]. The ratio ester/ether decreases after degradation with a strain of Corynebacterium while the ratio urethane/ether and aryl/ether remain stable meaning that the ester bonds are affected by the bacterial degradation. Zhang et al., have used the band at 1463 cm -1 corresponding to CH2 moieties to normalize their results [START_REF] Zhang | In vitro exposure of a novel polyesterurethane graft to enzymes: A study of the biostability of the Vascugraft® arterial prosthesis[END_REF]. The decrease of the normalized signals of 1239 cm -1 /1463 cm -1 revealed an alteration of the polyester part of an arterial prosthesis made in polyester PU after 100 days of incubation with pancreatin and collagenase. However, no change of the normalized signal of 1695cm -1 /1463cm -1 showed the stability of the urethane moieties.

Recently, a Raman spectroscopy has been applied to monitor the biodegradation of a polyether polyurethane foam which is among the most recalcitrant PU [START_REF] Cregut | New insights into polyurethane biodegradation and realistic prospects for the development of a sustainable waste recycling process[END_REF]. This technique allowed conclusions to be drawn on the amorphous region degradation of the foam by a microbial consortium while the crystalline region remained unaffected.

Microscopy

For the assessment of morphological surface modification of PUs, microscopy, particularly scanning electron microscopy (SEM) is employed. SEM allows for a qualitative evaluation of the degradation on the surface after biological treatment by observation of cracks or holes on the degraded polymers. Enzymatic degradation generally leads to cracks (Figure 14a) or holes (Figure 14b) homogeneously spread at the TPU surface [START_REF] Ozsagiroglu | Biodegradation and characterization studies of different kinds of polyurethanes with several enzyme solutions[END_REF][START_REF] Schmidt | Degradation of Polyester Polyurethane by Bacterial Polyester Hydrolases[END_REF] while degradation with a microbial consortium leads to irregularities (Figure 14c) [START_REF] Das | Evaluation of biodegradability of green polyurethane/nanosilica composite synthesized from transesterified castor oil and palm oil based isocyanate[END_REF][START_REF] Thirunavukarasu | Utilization of fish meal and fish oil for production of Cryptococcus sp. MTCC 5455 lipase and hydrolysis of polyurethane thereof[END_REF][START_REF] Zafar | Fungal communities associated with the biodegradation of polyester polyurethane buried under compost at different temperatures[END_REF]. For instance, Das et al., showed the appearance of cracks at the surface of a polyester TPU degraded under composting conditions. Depth of the cracks, corresponding to fungal mycelium development, increased until the formation of holes [START_REF] Das | Evaluation of biodegradability of green polyurethane/nanosilica composite synthesized from transesterified castor oil and palm oil based isocyanate[END_REF].

SEM observations of the fungal mycelium propagation inside a PU foam highlights the higher biodegradability of open-cell foams compared to closed-cell foams. The strut of cells appeared distended, leading to the collapse of the alveolar structure (Figure 14d) [START_REF] Álvarez-Barragán | Biodegradative Activities of Selected Environmental Fungi on a Polyester Polyurethane Varnish and Polyether Polyurethane Foams[END_REF]. Degradation is efficient in a PDB medium (Potatoes Dextrose Broth, rich medium) but it is specified that no degradation was observed by either weight loss or microscopy in minimal media. Small holes appeared when foams were incubated with one of the three tested strains, confirming enzymatic action. Holes in the walls and struts of the foam structure were already described previously (Figure 14e) (Gautam et al., 2007c). [START_REF] Thirunavukarasu | Utilization of fish meal and fish oil for production of Cryptococcus sp. MTCC 5455 lipase and hydrolysis of polyurethane thereof[END_REF] Khan et al., 2017, (d) Alvarez-Barragan et al., 2016 and (e) Gautam et al., 2007 SEM is also used to evaluate microbial growth. For instance, Micrococcus biofilm formation on the surface of a poly(ether urea) PU was shown by microscopy [START_REF] Rafiemanzelat | Synthesis of new poly(ether-urethaneurea)s based on amino acid cyclopeptide and PEG: study of their environmental degradation[END_REF]. SEM can cope with the limitation of available assays to evaluate fungal growth on carbon depleted media containing polymers. Huang et al. indeed highlighted a higher fungal growth on the surface of an adhesive containing 70% of a polyester PU than the one containing 40% [START_REF] Huang | Improvement of biodegradability of UV-curable adhesives modified by a novel polyurethane acrylate[END_REF].

Size Exclusion Chromatography (SEC)

SEC analysis allows the determination of the molar mass distribution of polymers. This quantitative analysis is more powerful than weight loss because it can appraise the change of polymer mass distribution from the beginning of chains cut off even if soluble products are not released. This measurement relies on the separation of the polymer chains in a column according to their length. The polymer is solubilized in an organic solvent such as THF, chloroform, dimethylformamide (DMF), then the solution runs through a fixed column packed with porous beads (gel) with different sizes pores. Short chains pass through the pores while longer chains cannot enter and are eluted more rapidly, then the higher the retention time, the lower the molar mass is. Detection can be performed with a UV diode array detector and/or refractive index (RI) detector. UV-vis analysis is efficient for PU containing aromatic rings such as 4,4'-MDI-or TDI-based PUs (generally at 254 nm). Since the sample must be soluble in an organic solvent, this method is not suitable for PU foams. Analysis and comparison of chromatograms can be performed to evaluate PU degradation [START_REF] Christenson | Enzymatic degradation of poly(ether urethane) and poly(carbonate urethane) by cholesterol esterase[END_REF][START_REF] Rafiemanzelat | Synthesis of new poly(ether-urethaneurea)s based on amino acid cyclopeptide and PEG: study of their environmental degradation[END_REF]. Molar masses are usually determined with polystyrene standards. Because of its aromatic rings, this standard is adapted to both UV and RI detection. Three main parameters are usually considered with the number average molar mass (Mn), the weight average molar mass (Mw) and the dispersity (Đ) (ratio of Mw over Mn).

Polymer degradation leads to changes in the molar mass distribution. The most common observed variation on PU biodegradation studies is a decrease of the Mw whereas Mn remains unchanged, leading to a decreasing Đ value [START_REF] Schmidt | Degradation of Polyester Polyurethane by Bacterial Polyester Hydrolases[END_REF]. Mw being more sensitive to long polymer chains contribution, this is consistent with the cleavage of the long chains into lower molar mass molecules. In Ferris et al., only the Mw was found to decrease [START_REF] Ferris | Dithiothreitol-based polyurethanes. Synthesis and degradation studies[END_REF].

Changes in molar mass distribution reveal global degradation in the bulk material and not only what is occurring at the surface of the polymer (Shah et al., 2013a;Shah et al., 2013b). Rafiemanzelat et al. described a bi-modal SEC profile with a high and a low molar mass distribution after 4 months soil burial degradation of a poly(ether urea) PU [START_REF] Rafiemanzelat | Synthesis of new poly(ether-urethaneurea)s based on amino acid cyclopeptide and PEG: study of their environmental degradation[END_REF] resulting from the cleavage of the long polymer chains into shorter ones (Figure 15). The decrease of the peak area corresponding to the main polymer chain in favor of lower molar mass chains, after fungal degradation of two polyester PUs, has also been reported [START_REF] Magnin | Enzymatic recycling of thermoplastic polyurethanes: Synergistic effect of an esterase and an amidase and recovery of building blocks[END_REF].

Figure 15 -SEC chromatograms of a poly(ether urea) PU before (bottom), after 4 months (middle) and after 6 months (top) of soil burial [START_REF] Rafiemanzelat | Synthesis of new poly(ether-urethaneurea)s based on amino acid cyclopeptide and PEG: study of their environmental degradation[END_REF]. A et B correspond respectively to the long and the short polymers chains.

Throughput of SEC analysis is rather low since a run generally lasts around one hour. Only a few publications on PU biodegradation offered robust SEC data with several repetitions of the analysis [START_REF] Kuang | A latent crosslinkable PCL-based polyurethane: Synthesis, shape memory, and enzymatic degradation[END_REF][START_REF] Magnin | Enzymatic recycling of thermoplastic polyurethanes: Synergistic effect of an esterase and an amidase and recovery of building blocks[END_REF][START_REF] Schmidt | Degradation of Polyester Polyurethane by Bacterial Polyester Hydrolases[END_REF].

Modifications of the physical and physico-chemical properties of PU degraded materials

Observation of modifications in the physical and physicochemical properties of a sample can be an indirect method of assessing biodegradation since the polymer chain cleavage often affects some of these properties. These assays are thus used as complementary methods. Loss of mechanical properties such as uniaxial tensile strength is generally observed after significant biological degradation of a material. Tensile test measurement allows, for example, to evaluate material elasticity and the behavior at break [START_REF] Phua | Biodegradation of a polyurethane in vitro[END_REF]. For instance, after 24 months of composting, a decrease of the tensile strength from 20 to 10 MPa was measured for a poly(ether urea) TPU. In similar degradation conditions, samples of polyester PU were already broken down into pieces and cannot be evaluated by uniaxial tensile tests [START_REF] Krasowska | Degradation of polyurethanes in compost under natural conditions[END_REF]. This technique was also adapted for flexible PU foams. The tensile strength increased and the elongation at break decreased after 60 days of incubation with a strain of Pseudomonas for polyester PU foams [START_REF] Spontón | Biodegradation study by Pseudomonas sp. of flexible polyurethane foams derived from castor oil[END_REF] (Figure 17, PU-1 and PU-2). In contrast, no change in static mechanical properties was observed for the polyether PU foam before and after degradation (Figure 17, PU-3). The polyether PU is thus more stable than the polyester PU.

PUs thermal stability may also be affected by biological degradation. This property can be measured by thermogravimetric analysis (TGA) where the weight evolution of a sample is recorded while the temperature is increased in a furnace, under air (oxidative) or N2 (nonoxidative) environments. For most polymers, the analysis temperatures range from 0 to 600-800°C. Weight loss variations correspond to specific structure degradation and/or distinct mechanisms. The first derivative of the TGA curve (DTG) gives curve with different peaks, to determine different specific temperatures for each maximum in the case of a multistep degradation, for instance. The maximum thermal degradation temperature can be used to compare thermal stability between samples. For polyester TPUs, the first window from 100 to 300°C corresponds to the release of volatile compounds such as additives. Although urethane bonds present a reversibility at around 200°C [START_REF] Delebecq | On the versatility of urethane/urea bonds: reversibility, blocked isocyanate, and non-isocyanate polyurethane[END_REF], urethane bonds degradation induces a weight loss between 300 and 400°C while ester bond cleavage results in a weight loss between 400 and 500°C [START_REF] Cangemi | Study of the biodegradation of a polymer derived from castor oil by scanning electron microscopy, thermogravimetry and infrared spectroscopy[END_REF][START_REF] Mathur | Degradation of polyurethane by Aspergillus flavus (ITCC 6051) isolated from soil[END_REF]. A decrease in weight loss occurring between 400 and 500°C was observed after biological degradation [START_REF] Mathur | Degradation of polyurethane by Aspergillus flavus (ITCC 6051) isolated from soil[END_REF]. This revealed a decrease in the ester bond content per polymer and thus evidenced the biological hydrolysis of these linkages. Beyond the type of linkage affected, TGA may, therefore, provide information on the material part affected. For instance, a poly(ether urea) PU was incubated for one month with a strain of Bacillus. The observed changes in the material thermal stability attested for a higher proportion of hard segments and consequently a degradation occurring preferentially at the SS domains containing ether bonds [START_REF] Rafiemanzelat | Study of Biological Degradation of New Poly(Ether-Urethane-Urea)s Containing Cyclopeptide Moiety and PEG by Bacillus amyloliquefaciens Isolated from Soil[END_REF].TGA is perfectly appropriate to analyze cross-linked foams. In their study, Gomez et al., compared the composting of a fossil-based and a biobased PU foam [START_REF] Gómez | Biodegradability of crude glycerol-based polyurethane foams during composting, anaerobic digestion and soil incubation[END_REF]. TGA was combined with MS analysis to identify the gaseous products of thermal degradation. They found that a bio-based, aliphatic polyester PU is more sensitive to biodegradation than the fossilbased, polyester ether PU with aromatic rings.

Differential scanning calorimetry (DSC) is a thermal analysis which appraises the phase transitions of material. Glass transition temperature (Tg) is the temperature over which the amorphous region of a polymer transitions from a hard to a more mobile/viscous state. Working above the glass transition temperature (Tg) is an advantage for polymer degradation as it promotes chain mobility of the amorphous zone of semi-crystalline material. Temperature changes depending on the biological degrading-entities. Temperature of composting process can reach up to 58°C [START_REF] Genovese | Gas permeability, mechanical behaviour and compostability of fully-aliphatic bio-based multiblock poly(ester urethane)s[END_REF] while degradation with single strain of bacteria or fungi are generally performed between 25 and 37°C. Enzymatic reaction can be performed at 60 or 70°C [START_REF] Schmidt | Degradation of Polyester Polyurethane by Bacterial Polyester Hydrolases[END_REF]. The melting temperature (Tm) is defined as the temperature of transition from a solid to a liquid state, which corresponds to the fusion of the crystalline regions. In practice, the area of this fusion peak allows determining the crystallinity of the material. The crystallinity of a poly(ether-urea) PU was found to increase after 4 months of soil incubation because of the decrease in the amorphous region preferentially degraded [START_REF] Rafiemanzelat | Synthesis of new poly(ether-urethaneurea)s based on amino acid cyclopeptide and PEG: study of their environmental degradation[END_REF]. This modification was accompanied by an increase in the Tm (Figure 19). Similarly, Osman et al., observed a shift of Tm from 191 to 196°C after the fungal degradation of a polyester PU [START_REF] Osman | Degradation of Polyester Polyurethane by Aspergillus sp. Strain S45 Isolated from Soil[END_REF]. In addition to this change, a decrease of Tg was measured. Pilch-Pitera et al., also measured a lower Tg after PU degradation with an enzyme, associated with lower rigidity of the material (Pilch-Pitera, 2012).

Biological degradation can alter the hydrophobicity of a PU material. Degradation induces exposure of hydrophilic moieties on the polymer surface such as alcohol and carboxylic acid. Moreover, disruption (cracks, holes) observed on the material surface offers a higher surface area of access for water and thus higher hydrophilicity. To evaluate the hydrophobic/hydrophilic balance, a drop of known liquid, generally water, is deposited on the polymer surface and the contact angle is measured. With a hydrophilic material, the drop of water will collapse and the contact angle will be low. A shift from 90° to 63° has been measured after 320 days of soil burial of a polyester PU [START_REF] Aranguren | Biodegradation of a vegetable oil based polyurethane and wood flour composites[END_REF]. Therefore, this polyester PU is more hydrophilic after biological degradation.

Analysis of the soluble degradation products

The identification of soluble degradation products found in the liquid phase is the best route to understand PU degradation mechanisms. Enzymatic degradation is more relevant than microbial degradation in this case. Indeed, with microbial degradation, the quantifications are biased by the possible assimilation of some degradation products by the microorganisms. Consequently, the recovered products are those that cannot be metabolized by the microorganisms.

Quantification of these products is possible with the measurement of the Total Organic Carbon (TOC) of the soluble fraction [START_REF] Yamamoto | Enzymatic hydrolysis of lysine diisocyanate based polyurethanes and segmented polyurethane ureas by various proteases[END_REF]. This measurement can be performed only for enzymatic reactions as the amount of carbon brought by the enzyme remains stable in time contrary to microbial cultures which involve growth and thus an increase of the carbon content. Yamamoto et al., used this method to evaluate the degradation of several PUs based on LDI with various proteases. For instance, 518 ppm of carbon from degradation products were released after the degradation of a PU based on LDI and ethylene glycol, representing 44% of the theoretical carbon of the polymer. Another way to measure the release of soluble degradation products is the use of radiolabeled polymers. They were synthesized with 14 C molecules such as 14 C-TDI, 14 C ethylene diamine, 14 C 1,4-butanediol or 14 C 1,6-HDI [START_REF] Santerre | Biodegradation evaluation of polyether and polyester-urethanes with oxidative and hydrolytic enzymes[END_REF][START_REF] Woo | Synthesis and characterization of a novel biodegradable antimicrobial polymer[END_REF]. The increase of radiolabeled-based molecules in the supernatant is quantified in counts per minute (CPM). This is a very sensitive method, but the cost and the hazardous exposure to radioactivity for the experimenter limit its usage. Both TOC and radioactivity measurements allow precise quantification of the degradation products release, but these techniques do not give information about the chemical structure of the corresponding degradation products.

Another indirect way to assess PU biodegradation is to monitor the toxicity of the soluble fraction resulting from the release of toxic degradation by-products. This is the case of 4,4'-methylene dianiline (MDA), coming from the hydrolysis of MDI-based PU, which is known to be carcinogenic [START_REF] Mcqueen | Review of the genotoxicity and carcinogenicity of 4, 4′methylene-dianiline and 4, 4′-methylene-bis-2-chloroaniline[END_REF]. The toxicity of the soluble fraction containing degradation products can be evaluated using Microtox® bio-assay. This assay relies on a naturally luminescent bacterium, Photobacterium phosphoreum. The parameter consider is the IC50 which is, in this case, the volume of the soluble fraction that induces a decrease of the luminescence of 50% [START_REF] Spontón | Biodegradation study by Pseudomonas sp. of flexible polyurethane foams derived from castor oil[END_REF]. A liquid medium of a polyester PU foam incubated for 60 days with Pseudomonas sp, presented an IC50 of 13.29% (V/V) thus attesting the release of toxic products during the degradation, probably MDA or MDA-based molecules.

The efficient recovery of the degradation products for their analysis is an important issue. Indeed, the liquid fraction of a degradation assay is a mixture containing salts, enzymes, and eventually microbial cellular debris and degradation products which are molecules released from the polymer. Several strategies were suggested to recover only the degradation products from PU degradation assays. This recovery can be performed by solvent extraction using, for example, ethyl acetate [START_REF] Shah | Degradation of polyester polyurethane by an indigenously developed consortium of Pseudomonas and Bacillus species isolated from soil[END_REF], acetonitrile [START_REF] Tang | Isolation of methylene dianiline and aqueous-soluble biodegradation products from polycarbonate-polyurethanes[END_REF] or ethyl ether [START_REF] Spontón | Biodegradation study by Pseudomonas sp. of flexible polyurethane foams derived from castor oil[END_REF]. Instead of solvent extraction, selective recovery of degradation products can also be achieved by removing enzymes using filtration (Wang et al., 1997a). Gamerith et al., added one volume of methanol and acidified the supernatant to pH 3.5 so that proteins precipitated and could be removed by centrifugation [START_REF] Gamerith | Improving enzymatic polyurethane hydrolysis by tuning enzyme sorption[END_REF].

High Performance Liquid Chromatography (HPLC) with UV detection [START_REF] Thirunavukarasu | Utilization of fish meal and fish oil for production of Cryptococcus sp. MTCC 5455 lipase and hydrolysis of polyurethane thereof[END_REF] and mass spectrometry analyses associated with liquid chromatography (LC-MS) [START_REF] Elliott | Identification of biodegradation products formed by L-phenylalanine based segmented polyurethaneureas[END_REF]Wang et al., 1997b) or with gas chromatography (GC-MS) [START_REF] Pérez-Lara | Preliminary study on the biodegradation of adipate/phthalate polyester polyurethanes of commercial-type by Alicycliphilus sp. BQ8[END_REF] are methods of choice to identify the degradation products. These methods coupled a chromatographic, for the separation of the mix of degradation products and an analytical method such as the mass spectrometry. Detected degradation products highly depend on the initial structure of polymers. Some chemical structures of identified degradation products are shown in Figure 16. Thirunavukarasu et al., monitored the degradation of 50 mg of a poly(diethylene glycol adipate)-based PU by quantifying diethylene glycol and adipic acid with HPLC analysis. After 4 days of PU incubation with a Cryptococcus sp. lipase, about 25 mg of adipic acid and about 8 mg of diethylene glycol were released [START_REF] Thirunavukarasu | Utilization of fish meal and fish oil for production of Cryptococcus sp. MTCC 5455 lipase and hydrolysis of polyurethane thereof[END_REF]. The detection of specific amines appears as the best way to confirm the cleavage of urethane bonds. It is interesting to notice that the degradation of a PCL-based TPU (based on TDI as isocyanate starting material) with cholesterol esterase leads to the release of low molar mass urethane molecules containing TDIbased moieties but no TDA was detected (Wang et al., 1997a) (Figure 16). Then, the urethane bond has not been cleaved. MDA was the unique aromatic amine released from polycarbonatebased PUs synthesized with diverse diisocyanates (HDI, HMDI and MDI) after hydrolysis with cholesterol esterase [START_REF] Tang | Isolation of methylene dianiline and aqueous-soluble biodegradation products from polycarbonate-polyurethanes[END_REF]. This NMR analysis was used for the identification of degradation products from the enzymatic hydrolysis of a polyester PU based on PCL. 6-hydroxycaproic acid was predominantly identified showing the efficient depolymerization of the ester linkage of the SS into the constitutive building block by the esterase E3576 [START_REF] Magnin | Enzymatic recycling of thermoplastic polyurethanes: Synergistic effect of an esterase and an amidase and recovery of building blocks[END_REF]. PU are versatile polymers with high variability of structures, chemical compositions, formulations, morphologies, shapes, with a direct impact on the biodegradation mechanisms and kinetics. From the published literature, a large variety of potential or efficient biological degrading entities can be identified among fungi, bacteria or enzymes. Because of the diversity of substrates and analytical tools, a direct comparison of these degrading entities does not appear as an easy task. Microbial degradation of PUs remains a complex and rather obscure process. For instance, analysis of the set of enzymes produced by microorganisms during degradation often fails to understand the mechanisms involved in PU degradation.

To tackle the degradation of the widest range of PU, the urethane bond cleavage appears as the key parameter. However, only a few techniques provide undeniable proof of urethane bond cleavage. Detection of amines derived from isocyanates as degradation products seems to be the main direct way to prove urethane bond hydrolysis. For that purpose, the recent development of new techniques such as LC/ESI/TOF-MS helps going forward in the resolution of this issue. Once mastered, the efficient enzymatic hydrolysis of the urethane bond will undoubtedly pave the way for a biological recycling of PU. Indeed, besides giving information on the degradation mechanism, released molecules resulting from PU biological depolymerization can also be considered as valuable products and used as building blocks for second-generation polymer synthesis. In the review of Cregut et al., the economic value of major building blocks was evaluated, showing the interest of recovering molecules such as diethylene glycol, adipic acid or trimethylol propane, which are products often identified after PU biological degradation (Figure 16) [START_REF] Cregut | New insights into polyurethane biodegradation and realistic prospects for the development of a sustainable waste recycling process[END_REF].

Applicability of the biological recycling on mainstream PU waste still needs to be attested. As far as we know, only one study deals with the biodegradation of a real PU waste. Gautam et al., described the successful degradation of a waste polyester PU foam with a strain of Pseudomonas chlororaphis (Gautam et al., 2007c). Attempts to work with "real" PU waste (and no PU-based models) have recently been performed by Alvarez-Barragan et al. by studying the degradation of polyether-PU foam synthesized with and without the addition of a fire retardant tris(1,3-dichloro-2-propyl)phosphate (TDCPP) [START_REF] Álvarez-Barragán | Biodegradative Activities of Selected Environmental Fungi on a Polyester Polyurethane Varnish and Polyether Polyurethane Foams[END_REF]. The TDCPP-containing foam was found to be less sensitive to biodegradation thus highlighting the need for considering thoroughly the presence of additives in real PU biological degradation assessment.

Polluting waste management such as landfilling will no longer be suitable solution. Limitation of the pollution linked to PU waste is an outcome deeply needed and expected from recycling. An efficient biological recycling path for PUs will support the economic value of PU waste towards the development of a circular economy for plastic material.

Figure 1 -

 1 Figure 1 -Foamed plastic debris collected on a Britain beach, pieces a, b d, e and f are PU[START_REF] Turner | Elemental concentrations and bioaccessibilities in beached plastic foam litter, with particular reference to lead in polyurethane[END_REF]

Figure 2 -

 2 Figure 2 -Polymer biodegradation process

Figure 3 -

 3 Figure 3 -Polyaddition of an isocyanate and a hydroxyl group to form a urethane bond

Figure 4 -

 4 Figure 4 -(a) Schematic representation of a semi-crystalline polymer and (b) linear polyurethane structure

Figure 5 -

 5 Figure 5 -(a) Schematic structure of a polymer particle, (b) structure of the 2,2-dimethylol propionic acid and (c) structure of the hydroxyethyl methacrylate

Figure 6 -

 6 Figure 6 -Different paths for microbial PU degradation

Figure 8 -

 8 Figure 8 -Hydrolysis of (a) the toluene-2,6-dicarbamic acid diethyl ester into 2,6-toluene diamine and propanol and (b) the toluene-2,4-dicarbamic acid diethyl ester into 2,4-toluene diamine and propanol

Figure 9 -

 9 Figure9-Hydrolysis of p-Toluenesulfonyl isocyanate model substrate[START_REF] Magnin | Enzymatic recycling of thermoplastic polyurethanes: Synergistic effect of an esterase and an amidase and recovery of building blocks[END_REF]

Figure 12 -

 12 Figure 12 -Hydrolysis of Impranil-DLN®. Degradation assay in agar plate (a) negative control and (b) Pleosporales sp. strain E2705B after two weeks of incubation. (c) Assay in liquid media. Adapted from Russel et al.(Russell et al.,

Figure 13 -

 13 Figure 13 -Strategies and diversity of the analytical methods for TPU and PU foam biodegradation monitoring

Figure 14 -

 14 Figure 14 -SEM images showing the morphological modifications of (a, b and c) thermoplastics PU and (d and e) PU foams degraded by (a and b) enzymes, (c) microbial communities or (d and e) single strains. Photographs are adapted from (a)Schmidt et al., 2017, (b) [START_REF] Thirunavukarasu | Utilization of fish meal and fish oil for production of Cryptococcus sp. MTCC 5455 lipase and hydrolysis of polyurethane thereof[END_REF] Khan et al., 2017, (d) Alvarez-Barragan

  enzyme is thus able to cleave urethane linkages of MDA based PU but not HDI-and HMDI-based PU. Gamerith et al., used liquid chromatography/electrospray/time-of-flight mass spectrometry (LC/ESI/TOF-MS) and also detected MDA as well as MDA derivatives after the hydrolysis of a polyester PU incubated with a Nocardia farcinica polyamidase enzyme. Using the same procedure, Magnin et al., quantified MDA at 0.3 mg/L after incubation of a PCL-based TPU for 50 days with an amidase (Magnin et al., 2019).

Figure 16 -

 16 Figure 16 -Degradation products identified after PU biological degradation

Table 1 -

 1 Structure of the some common biobased and fossil-based isocyanates

	IUPAC name	Abbrev.	Type	Structure	Potentially biobased	Reference
	1-isocyanato-4-[(4-
	isocyanatophenyl)methyl]	4, 4'-MDI	Aromatic
	benzene					(Shah et
					No	al., 2016)
	2,4-diisocyanato-1-methyl-benzene	2, 4-TDI	Aromatic			(Spontón
						et al.,
					No	2013)
	1-Isocyanato-4-[(4-isocyanatocyclohexyl) methyl]cyclohexan	H12MDI	Aliphatic cyclic			(Brzeska et
					No	al., 2015)
	Ethyl Ester L-Lysine Diisocyanate	LDI	Aliphatic linear			(Zhou et
					Yes	al., 2012)
	5-isocyanato-1-(isocyanatomethyl)-1,3,3-trimethyl-cyclohexane	IPDI	Aliphatic cyclic			(Pereira et
					No	al., 2012)
	1,6-diisocyanatohexane	HDI	Aliphatic linear		No	(Tang et al., 2001a)
	Dimer fatty acid-based diisocyanate	DDI	Aliphatic cyclic			(Charlon
						et al.,
					Yes	2014)

Table 2 -

 2 Some biobased and fossil-based polyols used for PU synthesis

	177
				Potentially
	Polymer name	Abbrev.	Structure	biobased	Reference
	Polyester
		PCL			(Yeganeh and Hojati-Talemi,
	Poly(caprolactone)			No	2007)
					(Izadi-Vasafi
	Poly(lactic acid)	PLA		Yes	et al., 2017)
					(Debuissy et
	Poly(hydroxyalkanoates)	PHA		Yes	al., 2017)
					(Li et al.,
	Poly(butylene succinate)	PBS		Yes	2015)
					(Shah et al.,
	Poly(butylene adipate)	PBA		Yes	2013b)
	Polyether
					(Zhang et al.,
	Poly(ethylene glycol)	PEG		No	2013)
					(Chattopadhy
					ay et al.,
	Poly(propylene glycol)	PPG		No	2008)
					(Wiggins et
	Poly(tetramethylene glycol)	PTMEG		No	al., 2003)
	Polycarbonate
					(Chen et al.,
	Poly(propylene carbonate)	PPC		No	2016)

Poly(1,6-hexyl 1,2-ethyl carbonate) PHEC No

(Christenson

Table 3 -

 3 PU degradation by microbial communities and associated PU substrates

	Microbial communities	PU substrates	Time of incubation	Reference
	Composting	Polyester PU foam	50 days	(Gómez et al., 2014)
		Thermoplastic polyester PU	90 days	(Genovese et al.,
				2016; Kucharczyk et
				al., 2016)
			12 weeks	(Zafar et al., 2013)
			24 months	(Krasowska et al.,
				2012)
		Thermoset polyester PU	90 days	(Das et al., 2017)
	Microbial communities from Garbage
	Landfill Leakage Water

Table 4 -

 4

	Alicycliphilus sp. BQ1	Polyester PU coating	(Oceguera-Cervantes et al., 2007)
	Alicycliphilus sp. BQ8	Polyester PU coating	(Oceguera-Cervantes et al., 2007)
		Polyester PU foam	(Pérez-Lara et al., 2016)
	Arthrobacter sp.	Thermoplastic polyester PU	(Shah et al., 2008a)
	Arthrobacter calcoaceticus ATCC 31012	Polyester PU coating	(El-Sayed et al., 1996)
	Arthrobacter calcoaceticus NAV-2	Polyester PU coating	(El-Sayed et al., 1996)

PU-degrading bacteria and associated PU substrates

Species and/or strains PU substrates References

Acinetobacter gerneri P7 Polyester PU coating (Impranil)

[START_REF] Howard | Growth of Acinetobacter gerneri P7 on polyurethane and the purification and characterization of a polyurethanase enzyme[END_REF]

Table 5 -

 5 PU-degrading fungi and associated PU substrates

	363
	Species and/or strain	PU substrates	Reference
	Alternaria sp.	Thermoplastic polyester PU	(Magnin et al., 2018)
	Alternaria Solani Number Ss.1-3	Thermoplastic polyester PU	(Ibrahim N. Ibrahim, 2009)
	Alternaria sp. strain PURDK2	Polyether PU foam	(Matsumiya et al., 2010)
	Alternaria tenuissima	Thermoplastic polyether PU	(Oprea et al., 2018)
	Aspergillus sp.	Polyester PU foam	(Cangemi et al., 2006; Cangemi et
			al., 2008)
	Aspergillus flavus	Thermoplastic polyester PU	(Mathur and Prasad, 2012)
	Aspergillus fumigatus	Polyester PU coating (Impranil)	(Álvarez-Barragán et al., 2016)
		Polyether PU foam	(Álvarez-Barragán et al., 2016)
		Thermoplastic polyester PU	(Osman et al., 2018)

Table 6 -

 6 PU-degrading enzymes and associated PU substrates

	EC number	Enzyme	name	in	the	Type of PU	Reference
		publication
	EC 1.10.3.2	Laccase				Thermoplastic polyester PU	(Ignat et al., 2011)
	EC 1.11.1.7	Fungal peroxidase			Thermoplastic polyester PU	(Ignat et al., 2011)
	EC 3	Tcur0390 (Thermomonospora	Thermoplastic polyester PU	(Schmidt, J. et al., 2017)
		curvata DSM43183 hydrolase)
		Tcur1278 (Thermomonospora	Thermoplastic polyester PU	(Schmidt, J. et al., 2017)
		curvata DSM43183 hydrolase)
	EC 3.1	Bacillus subtilis esterase		Polyester PU (Impranil)	(Rowe and Howard, 2002)
		Comamonas acidovorans TB-	Polyester PU (Impranil)	(Allen et al., 1999)
		35 esterase
		Curvularia	senegalensis	Polyester PU (Impranil)	(Crabbe et al., 1994)
		esterase
		E3576 (esterase)			Polyester PU Impranil	(Magnin et al., 2019)
						Thermoplastic polyester PU	(Magnin et al., 2019)
		Pseudomonas	fluorescens	Polyester PU (Impranil)	(Biffinger et al., 2015)
		esterase
		PudA	(Comamonas	Thermoplastic polyester PU	(Akutsu et al., 1998; Nomura et
		acidovorans TB-35 esterase)		al., 1998)
		PulA	(Pseudomonas	Polyester PU (Impranil)	(Ruiz and Howard, 1999)
		fluorescens esterase)
	EC 3.1.1	Cryptococcus sp. MTCC 5455	Thermoplastic polyester PU	(Thirunavukarasu et al., 2015)
		lipase
		Lipase				Thermoplastic poly(ester ether) PU	(Feng et al., 2017)
		Lipase AK				Thermoplastic polyester PU	(Zhou et al., 2012)
						Thermoset poly(ester ether) PU	(Jiang et al., 2007)
		Lipase PS				Thermoplastic polycarbonate PU	(Chen et al., 2016)
						Thermoplastic polyester PU	(Xu et al., 2014)
		Lipolase 100L			Polyester PU coating	(Pilch-Pitera, 2012)
		Novozym 51,032			Polyester PU coating	(Pilch-Pitera, 2012)

  . These methods are ASTM D5988-03 (Standard Test Method for Determining Aerobic Biodegradation in Soil of Plastic Materials or Residual Plastic Materials After Composting) (International, 2003b), the ASTM D5338-98 (Standard Test Method for Determining Aerobic Biodegradation of Plastic Materials Under Controlled Composting Conditions)

Funding:

The work for this review was fully supported by the Fondation pour la Recherche en Chimie (the Frontier Research in Chemistry Foundation) with the project "Biocycling" (2019-2020), in the continuation of a previous H2020 project (P4SB).