Antoine Le Duigou
email: antoine.le-duigou@univ-ubs.fr

David Correa

Masahito Ueda

Ryosuke Matsuzaki

Mickael Castro

A review of 3D and 4D printing of natural fibre biocomposites

Keywords: natural fibres, composites, 3D printing, 4D printing, hygromorphing

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Additive manufacturing has been extensively investigated over the past decade due to its disruptive potential in the design of advanced materials and structures. Among the various additive manufacturing techniques, Fused Filament Fabrication (FFF) 3D printing is one of the most applied techniques to polymers and composites in the literature. The FFF method involves the layer-by-layer deposition of viscous thermoplastic material through a heated nozzle onto the platform or over previously printed layers. The advantages of FFF include shortening the design-manufacturing cycle [START_REF] Dizon | Mechanical characterization of 3D-printed polymers[END_REF], minimizing the cost for low production runs [START_REF] Ngo | Additive manufacturing (3D printing): A review of materials, methods, applications and challenges[END_REF], reducing the production waste [START_REF] Mitchell | Additive Manufacturing-A Review of 4D Printing and Future Applications[END_REF], being able to build intricate geometries [START_REF] Kabir | A critical review on 3D printed continuous fiber-reinforced composites: history, mechanism, materials and properties[END_REF], being able to tailor microstructure and properties in each layer [START_REF] Blok | An investigation into 3D printing of fi bre reinforced thermoplastic composites[END_REF], and providing high value to research applications [START_REF] Woern | Distributed Manufacturing of Flexible Products: Technical Feasibility and Economic Viability[END_REF]. Originally targeting prototyping applications, the development of FFF has gradually moved towards mechanical performance controlled by slicing and printing parameters. Slicing parameters include the raster angle (in-plane angle), the layer height, the interfilament distance, nozzle diameter, the filling pattern (e.g. honeycomb, hexagonal, triangular), the filament orientation, and the build orientation (out-of-plane) [START_REF] Kabir | A critical review on 3D printed continuous fiber-reinforced composites: history, mechanism, materials and properties[END_REF]. Nozzle geometry, printing speed, printing trajectory, nozzle temperature, bed temperature, and calibration are categorized as printing parameters [START_REF] Kabir | A critical review on 3D printed continuous fiber-reinforced composites: history, mechanism, materials and properties[END_REF].

The development of fibre-reinforced composites using FFF has enhanced the mechanical properties of 3D printed polymeric parts [START_REF] Parandoush | Review on additive manufacturing of polymer-fiber composites[END_REF]. However, major work is still required to enable the 3D printed parts to meet the structural application specifications. The main obstacles for printed composites are inherent extrusion-induced defects, such as porosity, which are caused by poor interfacial bonding between the fibres and the polymer [START_REF] Duigou | Hygromorph BioComposites: Effect of fibre content and interfacial strength on the actuation performances[END_REF] and between the printed beads or the printed layers [START_REF] Cole | Interfacial Mechanical Behavior of 3D Printed ABS[END_REF]. It is therefore important to distinguish between the properties of the materials and the properties of the printed part [START_REF] Brites | Cork Plastic Composite Optimization for 3D Printing Applications[END_REF]. In addition, reinforcing fibres, mainly glass and carbon, influence the printability of composites due to their orientation, geometry, and content [START_REF] Parandoush | Review on additive manufacturing of polymer-fiber composites[END_REF] [START_REF] Wang | 3D printing of polymer matrix composites: A review and prospective[END_REF]. Numerous review articles have been published, describing the advantages and disadvantages of 3D printing of composite materials with synthetic fibres, such as the limited mechanical properties, the narrow range of materials available, and their poor surface finish [START_REF] Ngo | Additive manufacturing (3D printing): A review of materials, methods, applications and challenges[END_REF] [START_REF] Khoo | 3D printing of smart materials: A review on recent progresses in 4D printing[END_REF] [START_REF] Wang | 3D printing of polymer matrix composites: A review and prospective[END_REF].

Natural fibres and especially bast fibres are used as composite reinforcement to substitute for glass fibres because of their mechanical [START_REF] Boumaud | Towards the design of high performance biocomposites[END_REF], acoustic [START_REF] Sekar | Additive Manufacturing: A Novel Method for Developing an Acoustic Panel Made of Natural Fiber-Reinforced Composites with Enhanced Mechanical and Acoustical Properties[END_REF], or even morphing properties [START_REF] Duigou | Natural fibres actuators for smart bio-inspired hygromorph biocomposites[END_REF] in combination with their low density, reduced environmental footprint, and alternative end-of-life management [START_REF] Faruk | Biocomposites reinforced with natural fibers: 2000-2010[END_REF]. However, their application as composite reinforcement cannot be conducted in the same way as their synthetic counterparts. Specific knowledge of their microstructure and compositionrelated properties, which vary from one type of natural fibre to another, is required to select them correctly [START_REF] Boumaud | Towards the design of high performance biocomposites[END_REF]. In addition, it is necessary to take into account their sensitivity to heat and humidity, complex polysaccharidic composition, inherent defects, and natural variability to achieve high performance [START_REF] Baley | Specific features of flax fibres used to manufacture composite materials[END_REF]. Today, most processing routes can be applied to these natural fibre biocomposites with a thermoplastic matrix including extrusion, injection moulding, thermoforming, film stacking, and vacuum bag moulding [START_REF] Faruk | Biocomposites reinforced with natural fibers: 2000-2010[END_REF][18] [START_REF] Pickering | A review of recent developments in natural fibre composites and their mechanical performance[END_REF]. Although biocomposites based on natural fibres were proposed in 1937 (i.e. Gordon aerolite [START_REF] De Bruyne | Plastic progress -Some Further Developments in the Manufacture and Use of Synthetic Materials for Aircraft Construction[END_REF]), their development is relatively recent in contrast to their synthetic counterparts. The use of natural fibres has encountered challenges related to a lack of preform availability, technical processing knowledge, lifespan information, and limited understanding of their mechanical behaviour. Recently, additive manufacturing has emerged as an ideal opportunity for biocomposites to fill the gap with synthetics and begin to bring biocomposites to a similar technological maturity. Thus, natural fibres are steadily gaining interest as fillers or composite reinforcements for FFF applications. However, most review articles on 3D printing deal with synthetic composites [START_REF] Ngo | Additive manufacturing (3D printing): A review of materials, methods, applications and challenges[END_REF][4] [START_REF] Blok | An investigation into 3D printing of fi bre reinforced thermoplastic composites[END_REF] [START_REF] Wang | 3D printing of polymer matrix composites: A review and prospective[END_REF] and do not mention the current trend of research on biocomposites and their unique characteristics. A complete state-of-the-art review of the work carried out to date that also highlights recent trends on 3D printed biocomposites is thus justified.

Starting in 2013, 4D printing is a relatively recent trend to develop 3D printed structures that can change their shape or properties over time [START_REF] Tibitts | The emergence of 4D printing[END_REF] [START_REF] Ge | Active materials by four-dimension printing[END_REF] [START_REF]4D Printing: Dawn of an Emerging Technology Cycle[END_REF]. 4D printing research aims to achieve a predictable and predefined time-dependent change in functionality (shape, property, self-assembly, or self-repair) that the 3D printed structure undergoes when in contact with an external stimulus (e.g. temperature, ultraviolet light, humidity, electric and magnetic field) [START_REF] Tibitts | The emergence of 4D printing[END_REF]. 4D printing involves the development of the raw printing materials and the design of the mechanism and multi-layer architecture of the printed structures that directly incorporate a pre-programmed transformation [START_REF] Miao | 4D Printing of Polymeric Materials for Tissue and Organ Regeneration[END_REF].

A wide range of materials and printing methods are being developed for this purpose, such as Shape Memory Polymer (SMP), Electro-Active Polymer (EAP), and Hygromorph Composites (moistureinduced morphing). The latter can be based on hydrogel [START_REF] Gladman | Biomimetic 4D printing[END_REF] or natural fibre biocomposites [START_REF] Duigou | Natural fibres actuators for smart bio-inspired hygromorph biocomposites[END_REF]. The moisture-induced anisotropic swelling of natural fibres has been used as a driver of actuation in the development of hygromorph biocomposites via 3D printing [START_REF] Duigou | Hygromorph BioComposites: Effect of fibre content and interfacial strength on the actuation performances[END_REF][15] [START_REF] Duigou | Moisture-induced self-shaping flax-reinforced polypropylene biocomposite actuator[END_REF][27] [START_REF] Correa | 3D Printed Hygroscopic Programmable Material Systems[END_REF][29] [START_REF] Correa | 3D-Printed Wood: Programming Hygroscopic Material Transformations[END_REF]. However, existing review articles on 4D printing provide a very general overview of these smart materials, and offer very limited specificity on the distinctive characteristics of hygromorph biocomposites [START_REF] Miao | 4D Printing of Polymeric Materials for Tissue and Organ Regeneration[END_REF][3] [START_REF] Lee | Two-Way 4D Printing: A Review on the Reversibility of 3D-Printed Shape Memory Materials[END_REF][32] [START_REF] Vazquez | Shape-Changing Architectural Skins a Review on Materials, Design and Fabrication Strategies and Performance Analysis[END_REF] [START_REF] Zhang | Developments in 4D-Printing: A Review on Current Smart Materials, Technologies, and Applications[END_REF].

The aim of this article is to propose an exhaustive and comprehensive literature review of natural fibre biocomposites manufactured by 3D printing (FFF) and 4D printing with mechanical and actuation features respectively. It should be noted that man-made cellulose and nanocrystalline cellulose are excluded from the scope of the work presented.

First, a general description of natural fibres and their specificities (e.g. microstructure, composition) is given to better understand the challenges for 3D and 4D printing of natural fibre biocomposites. Next, the present review article focuses on the 3D printing of biocomposites from filament production to the current state of their mechanical properties, including a description of the challenges, limitations, and future trends of the process. Finally, the application of natural fibre biocomposites to 4D printing for shape-changing features is reviewed. This includes an overview of 4D printing principles applied to hygromorph biocomposites, their actuation performance, and the future prospects at different levels, from the material to the architectured structure (metamaterial).

Specificities of natural fibre biocomposites for 3D/4D printing

The properties of 3D and 4D printed composite materials are affected by the type of fibres used, their stiffness, strength, and capacity for interfacial bonding with the polymer matrix. In an ideal scenario, the microstructure of the composite material would be homogeneous, with well-distributed fibres with a high aspect ratio (length/diameter), and the interfacial surface would be maximized to ensure load transfer. To meet this goal, defects or porosity in the matrix and interfacial area should be reduced as much as possible. However, for natural fibres, the mechanical properties (stiffness and strength) are anisotropic and depend on their hierarchical microstructure (Fig 1a ) and their biochemical composition (cellulose, hemicelluloses, pectins, lignin, and water-Table 1). Table 1 indicates the tensile properties, biochemical composition, and microstructure of various natural fibres, features that are needed for the analysis presented in the two sections dedicated to 3D and 4D printing. The relationship between their microstructure, composition, and properties is typical of naturally optimized biological materials. For example, the bast fibres that provide stability to the plant (e.g. hemp, flax) have greater stiffness and strength than fruit fibres dedicated to energy dissipation such as coir fibres [START_REF] Defoirdt | Assessment of the tensile properties of coir, bamboo and jute fibre[END_REF]. Their secondary cellwall controls the overall hygro-mechanical properties of the natural fibres owing to the angle of the cellulose microfibrils (MFA) [START_REF] Fratzl | Cellulose fibrils direct plant organ movements[END_REF] [START_REF] Joffre | Modelling of the hygroelastic behaviour of normal and compression wood tracheids[END_REF], the interactions between the components, and their overall composition. The multi-scale microstructure of natural fibres and their internal interactions between components and cell walls also involve complex non-linear tensile behaviour [START_REF] Baley | Analysis of the flax fibres tensile behaviour and analysis of the tensile stiffness increase[END_REF] [START_REF] Placet | Nonlinear tensile behaviour of elementary hemp fibres. Part I: Investigation of the possible origins using repeated progressive loading with in situ microscopic observations[END_REF]. Unidirectional natural fibre composites also exhibit this non-linear behaviour [START_REF] Shah | Damage in biocomposites: Stiffness evolution of aligned plant fibre composites during monotonic and cyclic fatigue loading[END_REF], which makes it difficult to assess their stiffness. Shah et al. [START_REF] Shah | Damage in biocomposites: Stiffness evolution of aligned plant fibre composites during monotonic and cyclic fatigue loading[END_REF] recommend evaluating the tensile modulus in a range of deformations from 0.05 to 0.1% and even more conservatively, i.e., after 0.4% deformation when the stiffness of the composite variation is low. Selecting the right natural fibres according to the part specification (e.g. stiffness, toughness) is the first step in developing a high-performance material for 3D printing.

Next, stress transfer at the interface needs to be considered, as the composite properties depend on fibre length, diameter, tensile properties, and the interfacial shear strength. The surface properties of natural fibres, which control the interfacial shear strength, are more complex than their synthetic counterparts, as can be observed in glass or carbon fibres. For instance, natural fibres are often assembled into a bundle of single fibres that is held together by a pectic cement called middle lamellae (Fig 1c : diameter of bundle ≈ 200-400µm and diameter of single fibre ≈ 20µm). This bundling is the result of the original material architecture of the plant. The scale of the fibre bundle is generally referred to as "technical fibres" in the literature. Retting is an important step for fibre production that involves a natural enzymatic degradation of the middle lamellae in order to facilitate the extraction of the fibres [START_REF] Martin | Influence of the degree of retting of flax fibers on the tensile properties of single fibers and short fiber/polypropylene composites[END_REF]. The retting process also influences the surface roughness of the fibres (Fig 2a, b andc) and the overall splitting state of the bundles, such as the aspect ratio (reinforcement length to diameter ratio) [START_REF] Bismarck | Surface characterization of flax, hemp and cellulose fibers; Surface properties and the water uptake behavior[END_REF] within the composite. Scaling a single fibre is achieved after splitting the bundle during retting and mechanical processing (e.g. extrusion, injection [START_REF] Gallos | Lignocellulosic fibers: A critical review of extrusion process for enhancement of the properties of Natural Fiber Composites[END_REF] and see Fig 3a), which means that residues of middle lamellae could be deposited on the surface of the fibre and radically change the composition of the surface of the fibre (Fig 2b andc) [START_REF] Duigou | Interfacial properties of flax fibre-epoxy resin systems: Existence of a complex interphase[END_REF]. Unlike hemp fibres, the roughness of flax fibres, mainly composed of low molecular weight pectins, does not participate in load transfer and appears as a potential surface defect [START_REF] Le Duigou | Improving the interfacial properties between flax fibres and PLLA by a water fibre treatment and drying cycle[END_REF]. Cleaning the fibres with water has been shown to be an effective means of removing low weight pectin and improving practical adhesion with the PolyLactic Acid (PLA) matrix [START_REF] Le Duigou | Improving the interfacial properties between flax fibres and PLLA by a water fibre treatment and drying cycle[END_REF]. The fibre surface modification has been extensively examined in the literature, and chemical treatments are often proposed. However, the appropriate stage at which the surface of the fibres is modified should be considered. For example, if the treatments are applied to fibre bundles, it is clear that the efficiency of the treatment will be limited, especially if bundle splitting occurs during high shear rate processing [START_REF] Moigne | Study of the interface in natural fibres reinforced poly(lactic acid) biocomposites modified by optimized organosilane treatments[END_REF] (e.g. extrusion that occurs prior to 3D printing) (Fig 3a).

In addition, unlike conventional fibre-reinforced polymer composites (e.g. glass or carbon fibre- It can be assumed that the surface of a natural fibre corresponds to its outer layer, i.e. the primary cell wall (Fig 1a). For a single flax fibre [START_REF] Bourmaud | Relationships between micro-fibrillar angle, mechanical properties and biochemical composition of flax fibers[END_REF][51] [START_REF] Morvan | Building flax fibres: more than one brick in the walls[END_REF], the primary cell wall with thickness ranging from 200 to 500 nm is heterogeneously composed of low-crystallized cellulose, hemicelluloses, and pectin.

Although it is one of the most biochemically described fibres in the literature, the exact distribution and quantity in this cell wall and thus the exact surface composition and interactions with the matrix are still unknown. Only the total biochemical composition is currently known (Table 1).

The initial natural fibre bundle length introduced in short fibre biocomposites manufactured by extrusion, injection, or compression moulding varies from 0.1 to several mm [START_REF] Beaugrand | Lignocellulosic fiber reinforced composites: Influence of compounding conditions on defibrization and mechanical properties[END_REF], but the process drastically affects the reinforcement geometry. For example, one extrusion reduces the length of flax bundles within a PLA matrix from 4 ± 0.6 to 2.3 ± 1.2 mm and the diameter from 182 ± 76 to 133 ± 60 µm [START_REF] Duigou | Effect of recycling on mechanical behaviour of biocompostable flax / poly(L-lactide ) composites[END_REF]. This leads to variable length, diameter, and aspect ratio (L/d) that affect the formation of interfacial porosity [START_REF] Madsen | Plant fibre composites -porosity and volumetric interaction[END_REF], the load transfer, and the corresponding mechanical properties [START_REF] Coroller | Effect of flax fibres individualisation on tensile failure of flax/epoxy unidirectional composite[END_REF]. Above a

(a) (b)
critical fibre length proposed by Kelly-Tyson [START_REF] Kelly | Tensile properties of fibre-reinforced metals: Copper/tungsten and copper/molybdenum[END_REF], fibres act as reinforcement and efficient load transfer at the fibre/matrix interface. Below this critical fibre length, fibres are debonded, and the load cannot be transferred properly to the fibres. These geometrical parameters are significant for 3D printing as they also influence the processability of the material [START_REF] Badouard | Exploring mechanical properties of fully compostable flax reinforced composite filaments for 3D printing applications[END_REF]. 3D printing via FFF is carried out with thermoplastic polymers. The fabrication process requires a thermal cycle of heating in order to reach the melting temperature of the polymer, followed by extrusion and subsequent cooling. Each of these processes directly influences the polymer microstructure and the physical properties of the printed component. Most current polymers or biopolymers that are used with natural fibres are amorphous or semi-crystalline polymers. For example, PLA [START_REF] Kariz | Effect of wood content in FDM filament on properties of 3D printed parts[END_REF][75] [START_REF] Xie | Plasticizer combinations and performance of wood flour-poly(lactic acid) 3D printing filaments[END_REF], Acrylonitrile butadiene styrene (ABS) [START_REF] Kariz | Effect of wood content in FDM filament on properties of 3D printed parts[END_REF][77], Polyhydroxyalkanoates (PHA) /PLA blend [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF], recycled Polypropylene (rPP) [START_REF] Stoof | Sustainable composite fused deposition modeling filament using recycled pre-consumer polypropylene[END_REF], biobased Polyethylene (bioPe) [START_REF] Filgueira | 3D Printable Filaments Made of Biobased Polyethylene Biocomposites[END_REF],

Polycaprolactone (PCL) [START_REF] Tran | Cocoa Shell Waste Biofilaments for 3D Printing Applications[END_REF], and thermoplastic Polyurethane (TPU) are used and require a temperature higher than 200°C to be printed: 210-230 °C for PLA, 210°C for bioPE, 230-275 °C for ABS, 210°C for PLA/PHA, 230°C for rPP, 120°C for PCL, and 190°C for TPU [START_REF] Bi | Fabrication of flexible wood flour/thermoplastic polyurethane elastomer composites using fused deposition molding[END_REF]. Despite the reinforcing effect of natural fibres, their addition could disrupt the printing process as their polysaccharide components are very sensitive to temperature and moisture. High temperatures (above 150°C) and long exposure time (above a few minutes) can have a dramatic effect on the mechanical properties of natural fibres. Reduction in their longitudinal and transverse modulus as well as their longitudinal strength [82][83] can occur due to the thermal decomposition of polysaccharides, evaporation of moisture, and differential shrinkage within the cell wall. Yang et al. [START_REF] Yang | Effect of extrusion temperature on the physico-mechanical properties of unidirectional wood fiber reinforced polylactic acid composite (WFRPC) components using fused deposition modeling[END_REF] have demonstrated that increasing the printing temperature for PLA/wood from 200 to 230°C reduces the tensile strength by 10% and modulus by 5%, flexural strength by 7% and modulus by 20%. However, due to the printing speed (15-30 mm/s [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF][79] [START_REF] Yang | Effect of extrusion temperature on the physico-mechanical properties of unidirectional wood fiber reinforced polylactic acid composite (WFRPC) components using fused deposition modeling[END_REF]), the short printing distance, and the cooling temperature control (bed temperature, convection depending on a printing chamber), a high temperature exposure is usually short (i.e. a few seconds).

The manufacturing temperature also has an effect on the properties of natural fibres, their surface quality, and the resulting fibre/matrix interface. For example, a drying of 14 h at 105 °C reduces the interfacial bond strength of flax fibre with PLA by 20% [START_REF] Le Duigou | Improving the interfacial properties between flax fibres and PLLA by a water fibre treatment and drying cycle[END_REF] due to embrittlement of the internal interfaces and changes in surface wettability due to the surface migration of low molecular weight components and the reorientation of the surface molecules [85][86].

Moreover, significant residual stresses accumulate as a result of the layer-by-layer process, the inplane and out-of-plane temperature gradient, the difference in the thermal expansion coefficient between each component (matrix and fibre), the difference in thermal expansion between the plies, and the inhomogeneous cooling or crystallization temperature field [87][88]. Their magnitude depends on the printing speed, the printing angle, and the type of material [START_REF] Zhang | Characterization of residual stress and deformation in additively manufactured ABS polymer and composite specimens[END_REF]. Stoof et al. [START_REF] Stoof | Sustainable composite fused deposition modeling filament using recycled pre-consumer polypropylene[END_REF] have shown that the addition of an increasing natural fibre content in the rPP matrix reduces its shrinkage to 84% for a sample loaded with 30% of hemp fibre (Fig 4).

Figure 4 Pure rPP and 30% hemp/rPP made with 3D printing. Their curvature enables the evaluation of the residual stress and shows the role of hemp fibres to reduce thermal stresses. Adapted from [START_REF] Stoof | Sustainable composite fused deposition modeling filament using recycled pre-consumer polypropylene[END_REF] with permission from Elsevier.

Xiao et al. [START_REF] Xiao | Polylactide/hemp hurd biocomposites as sustainable 3D printing feedstock[END_REF] confirm this observation of Hemp hurds/PLA biocomposites. In contrast to conventional synthetic fibres (glass or carbon), natural fibres such as flax have a high anisotropic Coefficient of Thermal Expansion (CTE), closer to that of conventional polymers (e.g.

Polypropylene), which reduces the overall residual thermal strains [START_REF] Thomason | Why natural fibres failing to deliver on composite performance[END_REF].

Table 2 Coefficient of thermal and hygroscopic expansion of different fibres and polymers However, natural fibres and flax have a very high Coefficient of Hygroscopic Expansion (CHE) due to their sensitivity to moisture and their microstructure (low MFA value) [START_REF] Fratzl | Cellulose fibrils direct plant organ movements[END_REF]. Therefore, during a heat cycle, flax fibres shrink mainly due to the evaporation of water rather than expand thermally; then, the molten polymer matrix fills all the interstices of the biocomposite (see the black dotted line that expresses the difference of position between initial and dry state in Fig 5). The magnitude of hygroscopic shrinkage is higher than the thermal expansion of the fibre or polymer (Table 2). proportional to the magnitude of stress. During heating, the hygroscopic shrinkage of the natural fibre occurs with higher magnitude than the thermal expansion of the fibre or polymer. The dotted line represents the initial position of the fibre prior to heating/drying. During cooling, the thermal shrinkage of the fibre and matrix occurs, and during storage, the hygroexpansion of the natural fibre induces large hygroscopic compression stresses at the fibre/matrix interface. Adapted from [START_REF] Le Duigou | Hygroscopic expansion: A key point to describe natural fibre/polymer matrix interface bond strength[END_REF] with permission from Elsevier.

During the cooling stage, the fibres and matrix shrink to different degrees, depending on their coefficient of thermal expansion. Thermal and hygroscopic stresses are generated at the fibre/matrix interface. When stored at 50% RH, flax fibres swell more than the matrix. A hygroscopic stress is produced (Fig 5). Therefore, the change in humidity during processing as in during storage causes the sample to curve, which is detrimental to mechanical applications. However, this shape transformation has opened up new design windows for 4D printing where moisture is used to trigger actuation (see section 4) [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF].

3 Natural fibre biocomposites for 3D printing

Filament production

In composite materials, the manufacturing stages of semi-finished products influence the performance characteristics by inducing defects (e.g. porosity, misalignments), mechanical or hygro-thermal degradation, and residual stresses. Filaments are seen as semi-finished products. For the FFF, the filament is produced by a common extrusion process where fibres and polymer are blended together.

Further work on natural fibre biocomposite extrusion can be found elsewhere [START_REF] Gallos | Lignocellulosic fibers: A critical review of extrusion process for enhancement of the properties of Natural Fiber Composites[END_REF] [START_REF] Stoof | Fused Deposition Modeling of Natural Fibre/Polylactic Acid Composites[END_REF], and waste macadamia nut shell [START_REF] Girdis | Rethinking Timber: Investigation into the Use of Waste Macadamia Nut Shells for Additive Manufacturing[END_REF]. Even though these can all be considered natural fibres, their composition, microstructure, and tensile properties are highly variable (see Table 1),

Thermal expansion polymer which leads to a variety of potential functionality ranging from mechanical reinforcement and acoustical dampening to shape changing.

Although most of the existing data on composite fibre filaments for FFF come from suppliers and data sheets, a growing number of research papers are currently dedicated to customized production of filaments for 3D printing. This has prompted a generalshift of 3D printing from a technological to a scientific paradigm, leading to deeper insights into the properties related to the microstructure of the composite fibre filaments. The fraction of natural fibre within the filaments has a direct effect on their surface roughness. An increase in fibre content leads to a rougher filament with visible clustered fibres on the surface (Fig 6 [START_REF] Tran | Cocoa Shell Waste Biofilaments for 3D Printing Applications[END_REF]).

Figure 6 Evolution of filament surface roughness with fibre content with PCL/Cocoa Shell Waste (CSW). Increasing fibre content induces filaments with a rough surface. Adapted from [START_REF] Tran | Cocoa Shell Waste Biofilaments for 3D Printing Applications[END_REF] with permission from Wiley.

The increase in fibre content also leads to higher porosity [74][79]. For example, the porosity content is between 27.1% and 47% for a wood fibre content varying from 10% to 20% by weight in a BioPE matrix [START_REF] Filgueira | 3D Printable Filaments Made of Biobased Polyethylene Biocomposites[END_REF].

The chemical treatments of the fibres and the selection of polymers of different viscosities could significantly reduce the porosity content and thickness variations of the filaments [START_REF] Filgueira | 3D Printable Filaments Made of Biobased Polyethylene Biocomposites[END_REF]. Filgueira et al. [START_REF] Filgueira | 3D Printable Filaments Made of Biobased Polyethylene Biocomposites[END_REF] have proposed a subsequent extrusion to reduce porosity, fibre agglomeration (fibre size), and

Pure PCL PCL90-CSW10 PCL80-CSW20 PCL70-CSW30 PCL60-CSW40 PCL50-CSW50
roughness due to an improvement of the compounding process (Fig 7) and reduced fibre geometry.

Furthermore, several papers on the recycling ability of natural fibre composites have shown that an increasing number of extrusions were correlated to a reduction of natural fibre length, fibre bundle division, and better homogenization of the blend [112][113]. Depuydt et al. [START_REF] Depuydt | Production and characterization of bamboo and flax fiber reinforced polylactic acid filaments for fused deposition modeling (FDM)[END_REF] have produced filaments with low porosity (around 0%). They have shown that the application of vacuum at the end of the compounding process (i.e. at the end of the PLA/Flax or PLA/Bamboo metering zone) lowers the porosity content to the limit of measurement (below 2µm). In addition, drying in an air dryer also results in a reduction in porosity.

The extrusion process during filament production influences the microstructure of the material and thus the properties of the 3D printed part. For instance, a skin/core effect has been observed [START_REF] Depuydt | Production and characterization of bamboo and flax fiber reinforced polylactic acid filaments for fused deposition modeling (FDM)[END_REF] in some samples with a relatively high fibre aspect ratio, while dust-like bamboo or curved flax fibre counterparts exhibit a random 2D orientation. showing the skin/core effect. Adapted From [START_REF] Depuydt | Production and characterization of bamboo and flax fiber reinforced polylactic acid filaments for fused deposition modeling (FDM)[END_REF] with permission from Elsevier.

Fibre length, diameter, and aspect ratio (Length/diameter) are also key parameters that control the performance of composite filaments. To produce filaments with a diameter between 1.75 and 3 mm, natural fibres are commonly used with a very low length/diameter (L/d) ratio (i.e. in powder form) to manage the viscosity of the biocomposite and reduce fibre agglomeration [80][114]. For example, Zhao et al. [START_REF] Zhao | Poplar as biofiber reinforcement in composites for large-scale 3D printing[END_REF] have shown that the complex viscosity of Wood/PLA composites is reduced from 1.683 Pa.s to 1.549 Pa.s when the poplar fibre length decreases from 850-2360 μm to 180 μm and below. The authors state that the rheological behaviour plays a significant role in the 3D printing process as low viscosities at high shear rates are required for easy and low energy extrusion, but high zero-shear viscosities are also required so that the extrudate retains its shape once it is out of the nozzle and deposited. An optimum viscosity for an extrusion-based hydrogel 3D printing system is reported to be approximately 100-10,000 Pa.s [START_REF] Kyle | Printability" of Candidate Biomaterials for Extrusion Based 3D Printing: State-of-the-Art[END_REF]. The available literature has indicated that natural fibres have a low dimension with spheric cork particles between 272 and 733 µm [START_REF] Daver | Cork-PLA composite filaments for fused deposition modelling[END_REF], rice flour from 100 to 150 µm [START_REF] Osman | Investigation of ABS-rice straw composite feedstock filament for FDM[END_REF], hemp powder from 75 to 180 µm [START_REF] Coppola | Investigation on the use of PLA/hemp composites for the fused deposition modelling (FDM) 3D printing[END_REF] and aspect ratio from 1 to 7 [START_REF] Xiao | Polylactide/hemp hurd biocomposites as sustainable 3D printing feedstock[END_REF],

bamboo fibres with a length from 627 µm to 827 µm, diameter from 134 to 189 µm and L/d between 4.12 and 4.72 [START_REF] Depuydt | Production and characterization of bamboo and flax fiber reinforced polylactic acid filaments for fused deposition modeling (FDM)[END_REF], flax fibres with a length of 355µm, diameter of 42 µm and L/d of 8.68 [START_REF] Depuydt | Production and characterization of bamboo and flax fiber reinforced polylactic acid filaments for fused deposition modeling (FDM)[END_REF], TMP fibres with a 400 µm length and 38µm diameter [START_REF] Filgueira | Enzymatic-Assisted Modification of Thermomechanical Pulp Fibers To Improve the Interfacial Adhesion with Poly(lactic acid) for 3D Printing. 5(10), 9338[END_REF], Cocoa shell with a mean diameter of 50 µm [START_REF] Tran | Cocoa Shell Waste Biofilaments for 3D Printing Applications[END_REF], wood saw dust of 14 µm-237 µm [START_REF] Kariz | Effect of wood content in FDM filament on properties of 3D printed parts[END_REF][107] [START_REF] Bi | Fabrication of flexible wood flour/thermoplastic polyurethane elastomer composites using fused deposition molding[END_REF], and flax shives with a median diameter of 162 µm and 20-650 µm for flax fibres [START_REF] Badouard | Exploring mechanical properties of fully compostable flax reinforced composite filaments for 3D printing applications[END_REF].

The effect of fibre size on 3D printed biocomposite porosity content is still rare. Powder-like bamboo fibres yield a porosity amount of 3.6% as against 0.3% for fibre-like geometry [START_REF] Depuydt | Production and characterization of bamboo and flax fiber reinforced polylactic acid filaments for fused deposition modeling (FDM)[END_REF]. However, these results need to be confirmed as a change of PLA plasticizer can also affect the trend with no distinguishable effect of the fibre geometry on the porosity level.

In general, the tensile modulus of reinforced filaments is higher than that of pure polymer, while the rupture properties are lower due to insufficient fibre L/d and weak dispersion. For example, Depuydt et al. [START_REF] Depuydt | Production and characterization of bamboo and flax fiber reinforced polylactic acid filaments for fused deposition modeling (FDM)[END_REF] have shown an increase in tensile modulus from 1.3 to 2.4 GPa by adding 15% flax or bamboo fibres at the same time as the strength is reduced from 31 MPa to 23-30 MPa. A L/d ratio lower than the critical L/d in the sense of Kelly-Tyson's equations [START_REF] Kelly | Tensile properties of fibre-reinforced metals: Copper/tungsten and copper/molybdenum[END_REF] does not allow the stress to be transmitted efficiently to the fibre. For Bamboo fibre-reinforced PLA biocomposites, the authors found a critical L/d of 35.3 that was drastically above the real aspect ratio. From this perspective, fibres could be considered defects that introduce stress concentrations. The low interfacial shear strength between fibre and matrix and the high porosity of the filament significantly reduce the properties of the filament. Filgueira et al. [START_REF] Filgueira | Enzymatic-Assisted Modification of Thermomechanical Pulp Fibers To Improve the Interfacial Adhesion with Poly(lactic acid) for 3D Printing. 5(10), 9338[END_REF] have shown that enzymatic modification of TMP fibres slightly increases the tensile strength of PLA composites filaments.

Filament production is a key step in the additive manufacturing cycle; any defects that occur at this stage (e.g. porosity) are transferred to the printed samples [START_REF] Mazzanti | FDM 3D Printing of Polymers Containing Natural Fillers: A Review of their Mechanical Properties[END_REF]. Coppola et al. [START_REF] Coppola | Investigation on the use of PLA/hemp composites for the fused deposition modelling (FDM) 3D printing[END_REF] claim that the tensile properties of filaments and 3D-printed parts are similar, particularly in terms of stiffness. This is confirmed by the work of Le Duigou et al. [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF] where the stiffness is similar between tests on filaments vs. tests on 3D-printed samples of wood/PLA/PHA biocomposites. In addition, when printed in the 0° direction (i.e. in the direction of the filament), the tensile strengths are similar. Printing at 90°

to the longitudinal direction evidenced a 25% reduction in strength, which is indicative of the anisotropy of the filaments and the overall anisotropy of the sample. Thus, in wood fibre/PLA/PHA biocomposites, the fibre orientation follows the printing orientation of the filament.

Milosevic et al. [START_REF] Milosevic | Characterizing the Mechanical Properties of Fused Deposition Modeling Natural Fiber Recycled Polypropylene Composites[END_REF], on Hemp and Harakeke recycled PP, showed a reduction in stiffness (-6 to -35%) and strength (around -60% for hemp and -30% for Harekeke) of 3D-printed tensile samples compared to filaments. This is due to the microstructure of the print showing poor bonding between the layers and the presence of stress concentration around the fibres.

Mechanical properties of natural fibre-based 3D printed biocomposites

The literature indicates that the mechanical characterization of 3D printed biocomposites has not been conducted in a systematic manner. Most articles have evaluated the capacity to manufacture bespoke filaments and have assessed printability by generating demonstrators [START_REF] Guo | Effect of toughening agents on the properties of poplar wood flour/poly(lactic acid) composites fabricated with fused deposition modeling[END_REF]. The present article proposes an overview of the literature on the tensile properties of natural discontinuous and continuous fibre-reinforced biocomposites manufactured using FFF (Fig 9a). Among the natural fibres examined are hemp fibres [START_REF] Stoof | Sustainable composite fused deposition modeling filament using recycled pre-consumer polypropylene[END_REF][96] [START_REF] Milosevic | Characterizing the Mechanical Properties of Fused Deposition Modeling Natural Fiber Recycled Polypropylene Composites[END_REF] [98], hemp hurds [START_REF] Xiao | Polylactide/hemp hurd biocomposites as sustainable 3D printing feedstock[END_REF], flax fibres [START_REF] Duigou | 3D printing of continuous flax fibre reinforced biocomposite for structural applications[END_REF] and shives [START_REF] Badouard | Exploring mechanical properties of fully compostable flax reinforced composite filaments for 3D printing applications[END_REF], jute fibres [START_REF] Perez | Fracture surface anlysis of 3D printed tensiles specimens of novel ABS based materials[END_REF][120], wood fibres [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF][74] [START_REF] Guo | Effect of toughening agents on the properties of poplar wood flour/poly(lactic acid) composites fabricated with fused deposition modeling[END_REF][121], wood flour [START_REF] Bi | Fabrication of flexible wood flour/thermoplastic polyurethane elastomer composites using fused deposition molding[END_REF][102] [START_REF] Zander | Recycled Cellulose Polypropylene Composite Feedstocks for Material Extrusion Additive Manufacturing[END_REF], waste macadamia nut shell [START_REF] Girdis | Rethinking Timber: Investigation into the Use of Waste Macadamia Nut Shells for Additive Manufacturing[END_REF], Harekeke fibres [START_REF] Coppola | Investigation on the use of PLA/hemp composites for the fused deposition modelling (FDM) 3D printing[END_REF][97] [START_REF] Stoof | Fused Deposition Modeling of Natural Fibre/Polylactic Acid Composites[END_REF], rice straw [START_REF] Osman | Investigation of ABS-rice straw composite feedstock filament for FDM[END_REF], coconut fibres [START_REF] Safka | Use of composite materials for FDM 3D print technology[END_REF] and bamboo flour [START_REF] Pop | Structural changes during 3D printing of bioderived and synthetic thermoplastic materials[END_REF]. In addition, it should be noted that there are currently no specific standards for the design of 3D printed mechanical samples. Even a standardized sample geometry influences the performance measured on a pure polymer [START_REF] Torrado | Failure Analysis and Anisotropy Evaluation of 3D-Printed Tensile Test Specimens of Different Geometries and Print Raster Patterns[END_REF] or a continuous fibre composite [START_REF] Pyl | Exploration of specimens' geometry and tab configuration for tensile testing exploiting the potential of 3D printing freeform shape continuous carbon fibre-reinforced nylon matrix composites[END_REF]. Safka et al. [START_REF] Safka | Use of composite materials for FDM 3D print technology[END_REF] have shown that ABS/Coconut biocomposites exhibit a slight increase in tensile modulus (+ 6%) and a drastic reduction in strength (-50%) and strain (-15%) compared to a neat matrix. Dong et al. [START_REF] Dong | Additive manufacturing of mechanical testing samples based on virgin poly (lactic acid) (PLA) and PLA/wood fibre composites[END_REF] have found a reduction of about 60% in tensile strength, about 55% in Charpy impact strength, and about 60% in flexural strength. The addition of cork powder to PLA also reduces the tensile modulus, strength, and impact strength of printed PLA biocomposites [START_REF] Daver | Cork-PLA composite filaments for fused deposition modelling[END_REF].

Zander et al. [START_REF] Zander | Recycled Cellulose Polypropylene Composite Feedstocks for Material Extrusion Additive Manufacturing[END_REF] have found an increase in tensile modulus of 60% for PP/10% wood flour and 20%

for PP/cardboard compared to pure PP, respectively. They also observed a decrease in tensile strength of 7 to 20% for PP/ paper waste fibres and on PP/cardboard fibres. Xiao et al. [START_REF] Xiao | Polylactide/hemp hurd biocomposites as sustainable 3D printing feedstock[END_REF] observed a reduction in tensile strength of about 20% while mixing hemp hurds in a PLA matrix. Guessasma et al.

[122] [START_REF] Guessasma | Microstructure and Mechanical Performance of 3D Printed Wood-PLA/PHA Using Fused Deposition Modelling: Effect of Printing Temperature[END_REF] have reported a reduction in stiffness (7-10%) and strength (10-20%) for PLA/hemp and PLA/wood compared to pure PLA. They also point out that the wide dispersion of results, within a range of 18%, could be attributed to the natural variability of hemp or it could also be due to an inhomogeneous distribution of fibres within the filament.

In conclusion, the available data indicate that there are only rare cases where the addition of natural fibres shows an improvement in all mechanical properties of the biocomposites. One such exception can be found in the work of Tarrès et al. [START_REF] Tarrés | Bio-polyethylene reinforced with thermomechanical pulp fibers: Mechanical and micromechanical characterization and its application in 3Dprinting by fused deposition modelling[END_REF], on a compatibilized bioPE reinforced with thermomechanical pulp (TMP) fibres, which showed an increase in stiffness and strength with the addition of 10 to 20% of fibres. It is assumed that this exception is due to the combination of a better bond strength of the fibre/matrix interface and a relatively high interfacial surface area (the initial length of the fibres is 1.5mm, and their diameter is 33µm).

The role of natural fibres as a potential reinforcement should therefore be examined. For instance, natural fibres could be better positioned as "fillers" used to valorise biobased feedstock and biological waste by reducing the content of polymer and potentially reducing cost [START_REF] Osman | Investigation of ABS-rice straw composite feedstock filament for FDM[END_REF]. Natural fibres can also increase the biobased content in targeted applications, while their aesthetic properties can be applied to bring original qualities to the printed design (colour, roughness). The porous characteristics can be similarly used to reduce the density of the composite [START_REF] Daver | Cork-PLA composite filaments for fused deposition modelling[END_REF]. However, what is increasingly evident is that due to their mechanical properties, it is difficult to apply natural fibres to a semi-structural application.

Printing and slicing parameters have been demonstrated to play an important role in the mechanical performance of 3D printed parts using FFF [START_REF] Fernandez-Vicente | Effect of Infill Parameters on Tensile Mechanical Behavior in Desktop 3D Printing[END_REF] and consequently, they have a great impact in the production of biocomposites. These parameters include infill percentage, printing pattern, raster angle, printing height, printing orientation, printing/bed temperature, and printing speed.

In terms of temperature, Guessasma et al. [START_REF] Guessasmaa | Understanding the microstructural role of bio-sourced 3D printed structures on the tensile performance[END_REF], have shown that wood-based biocomposites exhibit stable tensile properties over a range of 210 to 250°C, while Hemp/PLA biocomposites have shown an increase in performance over the same temperature range (210 to 250°C) [START_REF] Guessasma | Microstructure and Mechanical Performance of 3D Printed Wood-PLA/PHA Using Fused Deposition Modelling: Effect of Printing Temperature[END_REF]. Yang et al. [START_REF] Yang | Effect of extrusion temperature on the physico-mechanical properties of unidirectional wood fiber reinforced polylactic acid composite (WFRPC) components using fused deposition modeling[END_REF] have shown a slight decrease in tensile and flexural properties when the extrusion temperature is between 200-230°C, but an increase in compressive strength (+ 15.1%) and internal bond strength (+24.3%). The higher extrusion temperature has shown better compatibility at the fibre/PLA interfaces and good adhesion between the extruded filament segments, while at the low printing temperatures, the loss of tensile performance (approx. 20%) is similarly explained by a lack of cohesion between the filaments.

To our knowledge, no studies have been performed on the effect of printing speed on the performance of biocomposites based on natural fibre. Based on their review work, Mazzanti et al. [START_REF] Mazzanti | FDM 3D Printing of Polymers Containing Natural Fillers: A Review of their Mechanical Properties[END_REF] have argued that the reduction in high-temperature residence time implied by faster printing speed may prevent the degradation of natural fibres.

In regard to printing pattern and infill, research by Vigneshwaran et al. [START_REF] Vigneshwaran | Statistical analysis of mechanical properties of wood-PLA composites prepared via additive manufacturing[END_REF] shows that changing the filling percentage from 30 to 90% multiplies the tensile strength and stiffness by a factor of 2 and 1.5 respectively for wood/PLA biocomposites. The printing pattern, whether rectangular, triangular, or hexagonal, does not seem to influence the final properties of biocomposites [START_REF] Vigneshwaran | Statistical analysis of mechanical properties of wood-PLA composites prepared via additive manufacturing[END_REF].

Independent of the printing pattern, reducing the layer height generally results in better cohesion and tensile properties of the printed biocomposite. For instance, by decreasing the layer height from 0.3 to 0.05 mm, Ayrilmis et al. [START_REF] Ayrilmis | Effect of printing layer thickness on water absorption and mechanical properties of 3D-printed wood/PLA composite materials[END_REF] have shown an improvement in tensile strength from 20.5 to 35.5 MPa and stiffness from 2567 to 3542 MPa. This finding confirms similar work by Le Duigou et al. [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF] and Dong et al. [START_REF] Dong | Additive manufacturing of mechanical testing samples based on virgin poly (lactic acid) (PLA) and PLA/wood fibre composites[END_REF] on comparable wood/PLA/PHA biocomposites. The latter have also tested the role of shell number and found that 2, 4, and 6 shells slightly increased the tensile strength of Wood/PLA/PHA biocomposites because the filaments printed on the inside of the shell are oriented along the perimeter of the specimen and therefore in the direction of the tensile load.

Liu et al. [START_REF] Liu | Three-dimensional printing of poly(lactic acid) bio-based composites with sugarcane bagasse fiber: Effect of printing orientation on tensile performance[END_REF] have examined the effect of printing orientation (0, 90, ±45 and 0/90°) on the tensile strength of sugarcane bagasse fibre/PLA biocomposites. Longitudinal printing at 0° was the best solution, with a 7% improvement compared to the cross solution (± 45 and 0/90°) and 20% improvement compared to transverse printing at 90°. This trend continued to be observed when the fibre content was increased. Le Duigou et al. [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF] have observed more drastic changes between 0 and 90°, which also depend on the distance between the printed filament. Thus, printing at 0° could exhibit two (2x) or three times (3x) higher strength than printing at 90°. Interestingly, Torrado-Perez et al. [START_REF] Perez | Fracture surface anlysis of 3D printed tensiles specimens of novel ABS based materials[END_REF] have shown that depending on the test direction (XYZ or ZXY plane), the effect of the natural fibre is different with a greater decrease in the out-of-plane direction compared to the in-plane direction. Due to the high porosity of the Jute/ABS filament, a rupture of the trans-filament is observed (Fig 10 [123] demonstrated that Thermomechanical Pulp (TMP)/BioPE biocomposites have lower properties than their injection-moulded counterparts with similar fibre content (10-20%). This is indicated by a reduction of about 40% in modulus (even for neat polymer) and about 45% in strength with a fibre content of 10% and, interestingly, about 10% with a fibre content of 20%. This significant difference is attributed to the microstructure of the 3D printed parts with poor adhesion between the printed layers resulting in a significant increase in the porosity content [START_REF] Tarrés | Bio-polyethylene reinforced with thermomechanical pulp fibers: Mechanical and micromechanical characterization and its application in 3Dprinting by fused deposition modelling[END_REF].

Figure 10 Schematic view of the fracture plane for jute fibre-reinforced ABS compared to neat ABS [START_REF] Perez | Fracture surface anlysis of 3D printed tensiles specimens of novel ABS based materials[END_REF]. A cohesive fracture within one layer and more specifically within jute fibre bundle is observed for jute/ABS. The fracture occurs in the interlaminar area for neat ABS.

Comparison of 3D printing of flax fibres or shives/ PLA, PLA/PBS or PBS with injection-moulded samples shows a reduction in the stiffness of the composite between 5 and 25% depending on the matrix and a reduction of between 10 to 55% for the strength [START_REF] Badouard | Exploring mechanical properties of fully compostable flax reinforced composite filaments for 3D printing applications[END_REF]. However, the work of Daver et al. [START_REF] Daver | Cork-PLA composite filaments for fused deposition modelling[END_REF] shows that PLA/cork with 3D printing is only slightly lower than the compression-moulded counterparts. Unfortunately, no clear explanation for this results was presented in the article although it is speculated that the low cork content could be the culprit.

The comparison with short synthetic fibre composites shown in Figure 9a, b, and c demonstrates that there is a global trend towards lower stiffness and tensile strength for natural fibre composites compared to synthetic fibres, especially when carbon fibres are used [128][131]. This is logical and can be explained by the intrinsic difference in properties between types of reinforcement. Based on the current state of the art, short glass fibre-reinforced composites, which are often cited as the target for the mechanical properties of biocomposites [START_REF] Bodros | Could biopolymers reinforced by randomly scattered flax fibre be used in structural applications?[END_REF][56], have similar stiffness but slightly higher strength (compared to a similar fibre content) [START_REF] Stoof | Sustainable composite fused deposition modeling filament using recycled pre-consumer polypropylene[END_REF][98] [START_REF] Carneiro | Fused deposition modeling with polypropylene[END_REF]. However, from a mechanical point of view, the comparisons made in the literature are questionable because the weight fraction of the fibres is mainly considered due to technical issues (e.g. viscosity) whereas reinforcement is a volume effect.

When analysing the current literature, the moderate tensile properties of 3D printed natural fibre composites can be explained by several parameters:

-Low fibre content to maintain the viscosity of the composite at a relatively low level (generally less than 30w%). Filaments with a higher natural fibre content do not flow uniformly through the nozzle and generate poorly printed parts that are not evenly filled with material [START_REF] Kariz | Effect of wood content in FDM filament on properties of 3D printed parts[END_REF]. In addition, an increase in melt viscosity would require higher power for extrusion through the nozzle [START_REF] Mazzanti | FDM 3D Printing of Polymers Containing Natural Fillers: A Review of their Mechanical Properties[END_REF]. Most of the published work deals with a fibre content between 5 and 20%. Printing of the composites through a 1mm die showed that from a fibre content of 20wt %, sporadic semi-blockages of the die occur, leading to slip-stick extrusion [START_REF] Stoof | Fused Deposition Modeling of Natural Fibre/Polylactic Acid Composites[END_REF]. Semi blockage could be reduced by using a larger extrusion die, but it also reduces the definition of the part [START_REF] Stoof | 3D Printing of Natural Fibre Reinforced Recycled Polypropylene[END_REF]. The design of an appropriate die is therefore of great importance. Since the 3D printing process cannot provide a high shear rate, the selection of a suitable polymer matrix with adequate viscosity is essential to control the printability of the material. Hence, biocomposite filaments made from the bioPE polymer were not suitable for 3D printing [START_REF] Tarrés | Bio-polyethylene reinforced with thermomechanical pulp fibers: Mechanical and micromechanical characterization and its application in 3Dprinting by fused deposition modelling[END_REF].

Conversely, a low viscosity promotes warping and shrinkage problems during the 3D printing [START_REF] Filgueira | 3D Printable Filaments Made of Biobased Polyethylene Biocomposites[END_REF].

-A low aspect ratio (L/d <10) results in a low interfacial surface area. Initially assembled in bundles in the plant (Fig 1), the natural fibres exhibit an aspect ratio that evolves during the process with a high shear rate like extrusion [START_REF] Ausias | Study of the fibre morphology stability in polypropylene-flax composites[END_REF], but the change in L/d has not yet been demonstrated for 3D printing. A low aspect ratio (L/d) of the reinforcement, lower than the critical aspect ratio as evoked by Kelly-Tyson's equations [START_REF] Kelly | Tensile properties of fibre-reinforced metals: Copper/tungsten and copper/molybdenum[END_REF], implies that the load transfer at the fibre/matrix interface is not efficient. This leads to debonding rather than fibre breakage. For example, Depuydt et al. [START_REF] Depuydt | Production and characterization of bamboo and flax fiber reinforced polylactic acid filaments for fused deposition modeling (FDM)[END_REF] have calculated the average stress in the flax fibre of 75 MPA assuming interfacial shear strength of 9 MPa, with an L/d ratio of 8. This stress is theoretically only 0.14% of the maximum stress applicable to the fibres in a continuous fibre composite and therefore confirms interface debonding rather than effective stress transfer. Milosevic et al. [START_REF] Milosevic | Characterizing the Mechanical Properties of Fused Deposition Modeling Natural Fiber Recycled Polypropylene Composites[END_REF] have shown that Harekeke fibres with a higher L/d ratio than hemp fibres have a better reinforcement efficiency. The low aspect ratio is often chosen in accordance with the diameter of the nozzle, which limits the size of the reinforcement. If larger particles were used, there would be more clogging [START_REF] Kariz | Effect of wood content in FDM filament on properties of 3D printed parts[END_REF]. Again, further developments of the printing device are needed to meet the requirements of high-performance composites. An integrated extruder, bringing similar advantages of high strain rates, should be preferred in the near future.

-The moderate mechanical properties of the selected natural fibres generate a low reinforcing effect.

As shown in Table 1, natural fibres have different mechanical properties depending on their microstructure and biochemical composition. Bast fibres such as hemp or flax are the result of a dedicated harvesting and yield a higher potential for composite reinforcement. In the current literature, wastes from industry or agriculture, such as wood sawdust, rice or coconut husks, are mainly used.

Although they offer a cost-effective solution, their mechanical performance is inferior (Table 1), while their environmental benefit should be confirmed by a standardized life-cycle analysis.

-The high porosity content of the printed samples is attributed to the combined effect of the low pressure exerted between the layers during the printing process and the heterogeneous temperature in the different regions of a printed part. First, the porosity of printed biocomposites is higher than that of pure filaments and increases with fibre content, which indicates the need for further work on natural fibre/polymer rheology, fibre/matrix compatibility, and process-induced compression [START_REF] Filgueira | 3D Printable Filaments Made of Biobased Polyethylene Biocomposites[END_REF][123][127] [START_REF] Xiao | Polylactide/hemp hurd biocomposites as sustainable 3D printing feedstock[END_REF]. Porosity could be reduced by vacuum drying the natural fibre before printing [START_REF] Depuydt | Production and characterization of bamboo and flax fiber reinforced polylactic acid filaments for fused deposition modeling (FDM)[END_REF] or by controlling the temperature and humidity during printing [START_REF] Milosevic | Characterizing the Mechanical Properties of Fused Deposition Modeling Natural Fiber Recycled Polypropylene Composites[END_REF].

On printed samples, Guessasma et al. [START_REF] Guessasmaa | Understanding the microstructural role of bio-sourced 3D printed structures on the tensile performance[END_REF] showed on Hemp/PLA biocomposites that during printing, the temperature difference between the peak and the ground can reach 95°C at a printing temperature of 250°C due to convective and conductive heat dissipation. This temperature difference gives porosity located between the filaments (size ≈ 1mm), inside the filaments (size ≈ 100µm), and in the zone of abrupt change of trajectory (size ≈ 1mm) [START_REF] Guessasmaa | Understanding the microstructural role of bio-sourced 3D printed structures on the tensile performance[END_REF]. Porosities are generally organized in a network that represents the majority of voids within the biocomposite (Fig 11a). It could be suggested that printing chambers heated by active convection might be able to maintain more consistent temperatures throughout the part. In another study, the authors also stated that an increase in porosity from 3.15% to 8.7% was observed in the temperature range of 210 to 240°C due to the change in the filament cross section that is not compensated for the print height and filament build-up at the nozzle Layer height also influences the microstructure and porosity level of biocomposites. Le Duigou et al.

[29] and Ayrilmis et al. [START_REF] Ayrilmis | Effect of printing layer thickness on water absorption and mechanical properties of 3D-printed wood/PLA composite materials[END_REF], have demonstrated the effect of layer height on the porosity content of the wood/PLA composite. An increase in layer height from 0.05 to 3 mm induces a greater amount of porosity (Fig 12), an increase in water absorption, and a reduction in tensile properties (30% modulus and 40% strength) [START_REF] Ayrilmis | Effect of printing layer thickness on water absorption and mechanical properties of 3D-printed wood/PLA composite materials[END_REF].

Generally, the natural short fibre-reinforced biocomposites currently used in FFF yield to moderate mechanical properties that prevent them from being applied in semi-structural applications. Therefore,

(a) (b)
further research is needed to overcome the weaknesses identified in both processing methods and material formulation.

Future trends in 3D printing of natural fibre biocomposites

A promising recent strategy is to print continuous natural fibre composites by in-nozzle impregnation [START_REF] Matsuzaki | Three-dimensional printing of continuous-fiber composites by in-nozzle impregnation[END_REF] or using pre-impregnated filaments [START_REF] Duigou | 3D printing of continuous flax fibre reinforced biocomposite for structural applications[END_REF] using modified printers. The difference between these results relied on i) the lower linear density of flax with 68 tex compared to 500 tex for jute yarn favouring a homogeneous distribution of the reinforcement within the composite, impregnation during the filament manufacturing and printing processes, reducing printing

(a) (b) (c)
defects (Fig 13c ); ii) Flax fibres have much better mechanical performance than jute fibres (see Table 1); and iii) a higher fibre content can be achieved with pre-impregnated flax yarns compared to innozzle impregnation (30% vs. 6%). In addition, 3D printed continuous flax/PLA biocomposites present competitive tensile stiffness and strength as biocomposites of similar fibre content manufactured by thermocompression (Fig 9a, b, andc).

Compared Future strategies to achieve higher performance for biocomposites printed with a continuous natural fibre may include the following avenues. At the material scale, optimized filament production using a low twisted yarn combined with low linear weight is of great promise. Baets et al. [START_REF] Baets | Determination of the optimal flax fibre preparation for use in unidirectional flax-epoxy composites[END_REF] have shown that the overall performance of composites decreases when the yarn undergoes strong twisting due to load mis-orientation and lack of impregnation. In addition, commingled natural fibre/polymer yarns should also be proposed in order to improve their impregnation during the production of filaments and their printability. Finally, the selection of the reinforcing quality is of great importance to produce high quality printed biocomposites that should target structural or semi-structural applications. This may also include hybrid fibre formulations (natural and synthetic) and adapting fibre selection to the intended application.

A potential improvement in the performance of 3D-printed natural fibre composites could occur through the development of customized and cost-effective processing devices. This includes, for example, a modified nozzle with an integrated extrusion system with pelletized biocomposites that increases fibre content, printing speed, and may also reduce processing costs [START_REF] Zhao | Poplar as biofiber reinforcement in composites for large-scale 3D printing[END_REF]. Large-scale additive manufacturing for wood/PLA biocomposites (Fig 14) has recently been developed by Zhao et al. [START_REF] Zhao | Poplar as biofiber reinforcement in composites for large-scale 3D printing[END_REF]. The main advantages of this approach are a high deposition rate up to 50 kg/h and a high building volume (~ 27 m 3 ), which reduces treatment time and cost. In addition, instead of 3-axis printing (Cartesian printing), they have implemented a 6-axis printing (polar printing method), which makes it possible to print even more complex shapes. Beyond the additional degrees of freedom of movement of the 6-axis printing configuration, automated fibre placement (AFP) with a 6-axis industrial manipulator (robot arm) could also be achieved [START_REF] Baley | Flax/PP manufacture by automated fibre placement (AFP)[END_REF], but the additional cost would have to be questioned. Currently, this complex implementation requires significant costs related to the integration of a tailor-made system; it may even require additional technical steps compared to a gantry style 3-axis printer (e.g. additional compression). manufacture of high-performance bio-composites. For example, the 37%vol. PP/Flax biocomposite made with AFP followed by thermal compression has a tensile modulus (E1) of 28.9 ±1.9 MPa and a tensile strength of 183 ± 8 MPa. These values are similar to those of thermocompressed biocomposites, but this new process provides the ability to accurately control fibre orientation (and fraction), the lay-up thickness, and the overall architecture.

4 Natural fibre biocomposites for 4D printing

Principle

4D printing is defined as the ability of 3D printed materials to actuate when an external stimulus (e.g. temperature, moisture, pressure, electricity, pH, light) is applied [START_REF] Tibitts | The emergence of 4D printing[END_REF]. For shape-transformation materials, two subclasses are discussed in the literature: shape-changing materials and shape-memory materials [32][157]. Shape memory refers to the ability of materials to "remember" and recover their original shape. This implies that the original shape can be deformed and fixed into a secondary form by an external physical force. This new shape is retained until a specific stimulus is applied that triggers the transformation of matter and its return to its original shape [3][32]. Shape Memory Polymers (SMP) achieve this via a dual segment system (elastic segment and transition segment [START_REF] Sun | Mechanisms of the Multi-Shape Memory Effect and Temperature Memory Effect in Shape Memory Polymers[END_REF],

which allows them to be physically deformed but to return to their original shape under the effect of an appropriate thermal stimulus [START_REF] Sun | Stimulus-responsive shape memory materials: A review[END_REF]. Shape-changing materials operate differently because it is the external stimulus that triggers the transformation from their original shape; then, the transformation is reversed when the stimulus is removed. Therefore, one can consider that the material oscillates between two states of equilibrium without the need for an external force, allowing multiple transformation cycles. However, the direction and amplitude of the movement are pre-programmed into the material structure [START_REF] Zhou | Reversible Shape-Shifting in Polymeric Materials[END_REF]. The effect of humidity as a stimulus for shape-changing materials is a growing area of research for 4D printing systems [START_REF] Mulakkal | Responsive cellulose-hydrogel composite ink for 4D printing[END_REF][161] [START_REF] Gladman | Biomimetic 4D printing[END_REF]. Currently, hydrogels are the main active polymers used in 4D printing using moisture-triggered actuation. Hydrogels and SMP are often integrated with a non-swelling polymer or filament to produce moisture and/or temperature-sensitive materials. However, the stiffness of both hydrogels and SMPs is relatively low [START_REF] Khoo | 3D printing of smart materials: A review on recent progresses in 4D printing[END_REF], which reduces their actuation authority.

This limitation can be overcome by a composite strategy, in which the soft gel or polymer is combined with a reinforcement. This includes composites using localized regions of two different polymers (one SMP and one non-reactive) [START_REF] Yuan | 3D Printed Active Origami with Complicated Folding Patterns[END_REF][163] [START_REF] Bodaghi | Self-expanding/shrinking structures by 4D printing[END_REF], the use of shear-oriented cellulose fibres [START_REF] Gladman | Biomimetic 4D printing[END_REF] or magnetically oriented stiff particles [START_REF] Kokkinis | Multimaterial magnetically assisted 3D printing of composite materials[END_REF] to manipulate anisotropic swelling in hydrogels [START_REF] Gladman | Biomimetic 4D printing[END_REF], or the creation of thermally activated shape-change bilayers using continuous carbon fibre and PLA or PEEK polymers [START_REF] Yang | Modelling and characterisation for the responsive performance of CF/PLA and CF/PEEK smart materials fabricated by 4D printing[END_REF].

Natural fibres are anisotropic and sensitive to moisture, and this is one of their main drawbacks when used for structural applications [START_REF] Faruk | Biocomposites reinforced with natural fibers: 2000-2010[END_REF]. However, actuation in response to a stimulus by pre-programmed hierarchical structures, such as nastic fibrous motors [START_REF] Stahlberg | What Can We Learn from Nastic Plant Structures? The Phytomimetic Potentiality of Nastic Structures[END_REF], can provide a bio-inspired model useful for the functional gradation of natural fibres to develop hygroscopically induced morphing, namely hygromorphing. The hygroscopic actuation of pine cones [START_REF] Duigou | Moisture-induced self-shaping flax-reinforced polypropylene biocomposite actuator[END_REF][168] (Fig 15), wheat awns [START_REF] Burgert | Actuation systems in plants as prototypes for bioinspired devices[END_REF], and other seed dispersal mechanisms [START_REF] Elbaum | Insights into the microstructures of hygroscopic movement in plant seed dispersal[END_REF] can provide valuable insights for biomimicry and developing novel material functionalities. Their principle is based on a hierarchical bilayer microstructure composed of sclerenchyma and sclereids. Each of these tissues is organized as a bundle of single fibres where each fibre is

(a) (b) (c)
architectured as a concentric composite cylinder of different cell walls (similar to Fig 1). The secondary cell wall, which is mainly responsible for the hygromechanical properties of the single fibre, is composed of oriented stiff cellulose microfibrils embedded in the hygroscopic hemicellulose/pectin matrix. Thus, the response (bending) of the pine cone is provided at the cell wall that gives way to tissue swelling. The bilayer assembly leads to a differential swelling between two layers connected to each other by a gradual interphase (Fig 15 and [START_REF] Dunlop | Artful interfaces within biological materials[END_REF]).

Currently, work has been conducted on wood [START_REF] Rüggeberg | Bio-Inspired Wooden Actuators for Large Scale Applications[END_REF] Their design is currently based on adapted Bimetallic theory for bilayers, derived from the hygroelastic properties of each layer [START_REF] Reyssat | Hygromorph: from pine cone to biomimetic bilayers[END_REF]. In this case, the variation of curvature between the initial and the final state of actuation (also named responsiveness) is

∆κ = ∆ .∆ . ( , )
Equation 1

( , ) = ( ) ( ) ( ) Equation 2
With f(m,n) a function depending on several variables such as = , with tp and ta are the passive layers and the active layer thicknesses, respectively. The active layer is considered as the swelling phase, while the passive layer is the non-swelling phase that is used to bring stiffness to the assembly.

Then = where Ep and Ea are the longitudinal tensile moduli of the wet passive and active layers, respectively. ∆β is the differential hygroscopic expansion coefficient between the active and passive layers. ∆! is the moisture content difference within the material between the testing environment and the storage state. Finally, t is the total thickness of the sample (active and passive layers).

Actuation properties of natural fibre-based 4D printed biocomposites

4D printing of hygromorph biocomposites is generally limited to off-the-shelf materials with principally wood floor/biopolymer filaments [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF][183] [START_REF] Correa | 3D Printed Hygroscopic Programmable Material Systems[END_REF] provided by Laywoo® or Colorfabb®.

Wood-derived cellulose fibres are also used in combination with a polyacrilamide matrix [START_REF] Gladman | Biomimetic 4D printing[END_REF],

although they cannot be considered as true natural fibres due to their extraction process.

Like biological structures, anisotropic hygro-expansion is targeted in hygromorph biocomposites. At the microscopic level, fibre orientation dictates the direction of expansion, and within the polymer composite, the fibres are oriented primarily during filament production rather than printing [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF].

However, printing trajectories and the orientation of the raster pattern create oriented mesostructures in the printed structure that are of great importance in promoting complex actuation. Generating actuation with 4D printed hygromorph biocomposites is currently carried out with two design strategies: monomaterial printing and multi-material printing deposition (Fig 16a, b, c, d, e, andf).

Figure 16 Printing methodologies for hygromorph biocomposites, (a-d) Monomaterial option with wood/biopolymer and nanocellulose/hydrogel biocomposites. Adapted from [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF] with permission of Elsevier, from [START_REF] Correa | 3D-Printed Wood: Programming Hygroscopic Material Transformations[END_REF] and from [START_REF] Gladman | Biomimetic 4D printing[END_REF]. (e-f) Multimaterial option with wood biopolymer and ABS. Adapted from [START_REF] Correa | 3D-Printed Wood: Programming Hygroscopic Material Transformations[END_REF].

The monomaterial approach is based on the anisotropy of the material induced by the orientation of the fibres within the filament and the printing process itself. It presumes that the same printed material possesses different mechanical properties and different coefficients of expansion in different directions. In general, the stacking sequence follows an asymmetric lay-up, i.e. a bilayer microstructure. Differential swelling between layers, which leads to out-of-plane displacement, is the

(a) (b) (c) (d) (f) (e)
target of this approach. Among monomaterials, two sub-categories can be found: i) full material filling (Fig 16a) [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF], and ii) partial filling on both layers with variable distance between the printed filaments (Fig 16b, c, andd) [START_REF] Correa | 3D printing wood :Programming hygroscopic material transformations[END_REF]. i) For full material filling, actuation is influenced by the differentiated orientation between the printed layers (Fig 17a), the overlap of the filaments that controls the porosity content, the layer height, the order in which the active and the passive layers are printed in the bilayer, the total number of printed layers and the length to width ratio [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF] [START_REF] Vazquez | Designing for shape change : A Case study on 3D Printing Composite Materials for Responsive Architectures[END_REF], and the orientation of the raster pattern printed.

Printing a raster pattern in the hygroscopic layer of 0 or 45° in relation to the main longitudinal direction in a bilayer strip generates either curvature or twisting response of the hygromorph. Thus, the hygromorph biocomposite actuates in the orthogonal direction according to the printing angle of the active layer [START_REF] Correa | 3D printing wood :Programming hygroscopic material transformations[END_REF].

Figure 17 (a) Effect of printing orientation on the response to different RH on the wood/biopolymer [START_REF] Correa | 3D-Printed Wood: Programming Hygroscopic Material Transformations[END_REF] and (b) Responsiveness evolution as a function of reactivity (derivative of the curvature vs. time plot) of wood/biopolymers printed in 3D for different printed configurations compared to thermocompression. [START_REF] Zhao | Study on the Preparation of Bamboo Plastic Composite Intend for Additive Manufacturing[END_REF][START_REF] Tibbits | 4D printing: multi-material shape change[END_REF], and 300% are slicing parameters that adjust the filament width and the porosity level of the composite). Adapted from [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF] with permission of Elsevier.

Changing from positive to negative overlap during printing modifies the porosity content of a wood/PLA/PHA biocomposite from 8.4 ± 1.7 to 14 ± 1.2 % [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF]. It implies lower swelling strains and responsiveness, i.e. Kmax curvature (Fig 17b), due to a low interlayer bond strength that cannot transfer load properly. Hygroscopic strains are the driving force behind the responsiveness of hygromorph materials and must therefore be maximized. Higher porosity leads to faster water uptake with faster actuation illustrated by a higher dK/dt ratio (Fig 17b). dK/dt is the derivative of the curvature vs. time

(b) (a)
plot that describes the reactivity of the sample to a stimulus. Moisture diffusion and the relationship between microstructure and diffusion are the key to rapid actuation. Rapid actuation with a porous structure and high responsiveness seem paradoxical if the porosity is not optimally located in the material; a solution can be found in nature by taking inspiration from biological hydraulic actuators that have a structurally "programmed", passive nastic actuation, such as pine cone [START_REF] Reyssat | Hygromorph: from pine cone to biomimetic bilayers[END_REF][168], wheat awn [START_REF] Elbaum | Insights into the microstructures of hygroscopic movement in plant seed dispersal[END_REF], wild carrot [START_REF] Zhang | Evaluating the hierarchical, hygroscopic deformation of the Daucus carota umbel through structural characterization and mechanical analysis[END_REF]. Moisture transport in biological materials is enhanced by hierarchically porous structures with nanopores in the cell wall that trigger a capillary effect [START_REF] Van Opdenbosch | Moisture-Driven Ceramic Bilayer Actuators from a Biotemplating Approach[END_REF]. Therefore, the control of the dispersion of porosity, size, and volume may be essential to control the responsiveness of the hygromorph actuator [START_REF] Zhao | An instant multi-responsive porous polymer actuator driven by solvent molecule sorption[END_REF]. Porous structures should be built in such a way that pore networks are considered to be moisture pathways rather than mechanical defects [START_REF] Duigou | Natural fibres actuators for smart bio-inspired hygromorph biocomposites[END_REF]. For example, Van Opdenbosch et al. [START_REF] Van Opdenbosch | Moisture-Driven Ceramic Bilayer Actuators from a Biotemplating Approach[END_REF] have proposed that pores have strongly anisotropic shapes and perpendicular orientations within the two layers of a bilayer, helping reinforce the difference in directional expansion. Vailati et al. [START_REF] Vailati | Upscaling of wood bilayers: design principles for controlling shape change and increasing moisture change rate[END_REF] have successfully tested the effect of milled-in grooves on wood bilayers. As the diffusion paths are shorter along the direction of the fibres, the moisture content rate and the responsiveness are increased. This principle could be transferred to 4D printing by printing gaps between several layers, which can form artificial pores, and by the strategic selection of natural fibre composites. A study from Le Duigou et al. [START_REF] Duigou | Natural fibres actuators for smart bio-inspired hygromorph biocomposites[END_REF] was devoted to hygromorph biocomposites manufactured by thermocompression with different natural fibres possessing different internal lumen sizes (internal porosity). Flax, jute, kenaf, and coir fibres have an internal porosity of 0.10 ± 0.02%, 6.40 ± 0.17%, 13.80 ± 0.23%, and 39.10 ± 0.70% respectively. The higher the internal porosity, the faster the uptake of moisture, while the hygroscopic expansion is influenced by the biochemical composition of the fibres and the angle of the microfibrils. Pectin-rich fibres such as flax fibres swell faster [START_REF] Duigou | Natural fibres actuators for smart bio-inspired hygromorph biocomposites[END_REF], which makes them suitable candidates for future 4D printing research. Correa et al. [START_REF] Correa | 3D printing wood :Programming hygroscopic material transformations[END_REF] have proposed that because of the unbalanced contribution to actuation between moisture-responsive natural fibres and moisture-insensitive polymers, other stimuli should be tested to help amplify the transformation.

ii) The second strategy, also based on a bilayer architecture, is a pattern of partial filling of hygromorph biocomposites that relies on custom deposition where filament anisotropy, interfilament spacing, bonding, and surface-to-volume ratio play an important role (Fig 16b). For example, Vazquez et al. [START_REF] Vazquez | Designing for shape change : A Case study on 3D Printing Composite Materials for Responsive Architectures[END_REF] have proposed that increasing the printing path spacing from 1 to 1.5 mm, described in the literature as the road distance [START_REF] Elbaum | Insights into the microstructures of hygroscopic movement in plant seed dispersal[END_REF] or the distance between bead centres [START_REF] Vazquez | Formalizing Shape-Change: Three-Dimensional Printed Shapes and Hygroscopic Material Transformations[END_REF], improves both the reactivity and responsiveness of a wood/biopolymer hygromorph. Gladman et al. [START_REF] Gladman | Biomimetic 4D printing[END_REF], working on cellulose/hydrogel architectures, claim that a large spacing between the filaments promotes rapid water uptake and faster actuation due to the increased surface/volume ratio. These findings could be drawn in parallel with the previous section on porous structures.

Rather than focusing on materials properties, another objective is to add a structural effect through additive manufacturing. Recent advances in digital manufacturing, i.e. the combination of additive manufacturing and numerical simulation, are needed to build complex architectures and actuation.

The second design principle concerns multimaterial printing by using a very hygroscopic material (natural fibre composite for instance) and a more or less inert polymer: ABS, PLA, Polyamide (PA) [START_REF] Correa | 3D printing wood :Programming hygroscopic material transformations[END_REF]. The selection of materials is made on the basis of Timoshenko's equations (Eq 1 and 2) that state that an optimal curvature (responsiveness) is obtained with a higher difference of hygroscopic strains between layers, and with an optimal ratio of passive to active thickness (n ratio) depending on the tensile modulus in the two planar directions, longitudinal (L) and transversal (T) of each layer (m ratio). Compatibility between these two materials must be checked to reduce the risk of delamination compared to monomaterials. In most cases, a dual extruder printer is used [START_REF] Duigou | Evaluation of force generation mechanisms in natural, passive hydraulic actuators[END_REF]. Similarly, according to the first design principle, the primary raster pattern is the hygroscopic material, where all the lines of the print path are oriented in a direction perpendicular to the desired angle of expansion. The constraint material, on the other hand, which consists of the inert polymer, is oriented parallel to the desired angle of expansion. Both types of materials could be placed locally to create an interwoven pattern (Fig 16e) [START_REF] Correa | 3D printing wood :Programming hygroscopic material transformations[END_REF]. As with a pine cone scale model, the woven pattern is a matrix of multi-layer material where the main inert constraints are enclosed between the upper and lower raster layers, creating a mechanical interlock between the two layers to limit delamination of the bilayer structure [START_REF] Correa | 4D Pine Scale: Biomimetic 4D Printed Autonomous Scale and Flap Structures Capable of Multi-Phase Movement[END_REF]. Building on Timoshenko's theory, as previously established for hygroscopic bilayers [START_REF] Reyssat | Hygromorph: from pine cone to biomimetic bilayers[END_REF], for both the woven pattern or the simple bonding model, the thickness of the active layer should be greater than the thickness of the constraint layer [START_REF] Duigou | Hygromorph BioComposites: Effect of fibre content and interfacial strength on the actuation performances[END_REF][172] [START_REF] Holstov | Hygromorphic materials for sustainable responsive architecture[END_REF]. Reducing the thickness of the constraint layer coupled with increasing the porosity of the hygroscopic layer is an effective strategy to reduce the negative effect of high bending stiffness on actuation while maximizing moisture exchange and therefore responsiveness.

The use of isotropic, non-sensitive polymers creates greater differential hygroscopic swelling between the layers and, as a result, better control of actuation direction, precise angles, and response speed can be achieved. However, the use of anisotropic properties in both active and passive layers could lead to an interesting bidirectional actuation as observed in biological systems such as the pine cone scale [START_REF] Lin | On the shape transformation of cone scales[END_REF] [START_REF] Poppinga | New insight into the passive motions of pine cone and false indusia in ferns[END_REF].

Multimaterial printing combined with a localized asymmetrical structure enables to create adaptive hinges (Fig 16f) involving a shift from curving actuation to folding [START_REF] Liu | 2D or not 2D: shape programming polymer sheets[END_REF]. The wider the width of the hinge, the smaller and faster the angle of curvature [START_REF] Correa | 3D Printed Hygroscopic Programmable Material Systems[END_REF] [START_REF] Correa | 3D-Printed Wood: Programming Hygroscopic Material Transformations[END_REF]. Today, most of the available work dedicated to hygromorph biocomposites for 4D printing is oriented towards design methodology, proof of concepts and prototypes, but information on actuation, material properties, decay, or mechanical damage remains limited.

Table 3 shows the actuation performance (reactivity and responsiveness) of different 4D printed hygromorph biocomposites based on natural fibres according to their microstructure induced by different printing strategies (full or partial monomaterial filling and multi-materials) and by external environmental conditions. Based on Timoshenko's equations, the specific actuation (actuation*thickness) for reactivity and responsiveness is presented to facilitate comparison between materials. Actuation is triggered by changes in moisture content and moisture transport in the material.

Most of the time, a 1D diffusion mode can be reasonably applied because of the small thickness in relation to the length and width of the samples. Where the geometry is not given in the original article, it has been estimated by image analysis. To extend the literature review, the values are compared with those of natural fibre-based hygromorph biocomposites manufactured by other processes such as thermocompression, wood bilayer glued by adhesive bonding or cellulose fibre composites. The latter should be taken with care because their operating principle is opposed to that of hygromophic biocomposites. In composites based on cellulose fibre, the matrix swells, whereas in hygromorph biocomposites, it is the fibres that provide hygroscopic induced swelling.

The first point in evidence is the major role of the architecture of the material on the performance of the response. For monomaterials, the spacing of the filaments with space grid or ridges enhances both the responsiveness and reactivity of materials with respect to the complete filling. This is due to the maximisation of the surface exchange with moisture while reducing bending stiffness. Regardless of the architecture, printed monomaterials evidence an asymmetrical trend in terms of actuation speed, with a faster response to desorption than to sorption. For hygromorph biocomposites based on natural fibres manufactured by thermocompression, the sorption and desorption behaviour tends to be slightly asymmetrical (Table 3). The actuation speed is not linear in time with a rapid increase for a short time and then a deceleration effect before the saturation regime is reached [29][188]. For multi-materials, the mechanisms induced by desorption tend to be slower than those occurring during sorption. This is due to the very low sensitivity of the passive layer, which may protect the active layer from air-drying by convection.

The multi-material printing method demonstrates a higher actuation responsiveness explained by the higher anisotropic ratio and the difference in swelling between layers in contrast with monomaterials.

In this sense, the selection of a polymer for the passive layer with different customized thermo-hygro properties introduces a change of actuation. For example, the switch from ABS to PA results in a reverse curvature [START_REF] Correa | 3D-Printed Wood: Programming Hygroscopic Material Transformations[END_REF]. Fibre selection is also important. The type of fibres, their related microstructure in the S2 Layer (Microfibril Angle), and their biochemical composition influence the actuation of the hygromorphs. For example, coir fibres have a lower hygromorph actuation potential due to higher MFA value and a higher lignin content compared to flax and jute fibres [START_REF] Duigou | Natural fibres actuators for smart bio-inspired hygromorph biocomposites[END_REF]. Fibre content is a wellknown parameter that influences the hygro-expansion of composites [START_REF] Duigou | Evaluation of force generation mechanisms in natural, passive hydraulic actuators[END_REF]. As the natural fibre content increases, hygroexpansion increases and consequently hygromorph reactivity and responsiveness. As explained in a previous section, fibre content is a crucial parameter that controls the mechanical properties of 3D printed parts. However, in the current state of technology, the fibre content is often limited due to technical concerns, as discussed earlier. Considerations regarding the high interfacial bonding ability of the fibre to the polymer and its potential impact on the hygroscopic actuation also require further investigation. This emphasizes the importance of the printing path orientation to trigger and control actuation.

The reliability of actuation with a 4D printed biocomposite hygromorph is rarely discussed in the literature in terms of the relationship between process parameters and actuation. Empirical tests on Wood/co-polyester/PA or Wood/co-polyester with a multi-material architecture subjected to a high relative humidity environment seem to show more than 30 actuation cycles without any significant degradation [START_REF] Correa | 3D-Printed Wood: Programming Hygroscopic Material Transformations[END_REF]. More rigorous testing has shown that the visible delamination between the wood/copolyester and the ABS constraint layers starts around 13 cycles [START_REF] Correa | 4D Pine Scale: Biomimetic 4D Printed Autonomous Scale and Flap Structures Capable of Multi-Phase Movement[END_REF]. Similarly, studies on monomaterials subjected to water immersion and air-drying cycles show a 50% reduction in responsiveness after the first cycle [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF]. The use of multi-material assembly, i.e. Wood/copolyester/PA, does not allow the initial properties to be retained. Correa et al. [START_REF] Correa | 3D-Printed Wood: Programming Hygroscopic Material Transformations[END_REF] have evidenced a 70% reduction in responsiveness after 4 immersion/natural drying cycles and a 50% reduction when heat radiation up to 70°C is used for active drying.

To the best of our knowledge, no in-depth investigation has yet established the relationship between material/printing process and the reliability of actuation in a variant environment. It is worth noting that the various environmental conditions imply a wide variety of hygromorph biocomposite responses. Immersion in water results in a faster response to relative humidity due to the higher activity of the water, which leads to a faster transport of moisture and a faster expansion of the material. A similar material subjected to high relative humidity or immersion creates a difference in reactivity of a factor ranging from 2.5 on wood/composites to 5 on flax/ maleic anhydride grafted polypropylene (MAPP) hygromorphs [174][191][195]. This could also be due to the change of elastic properties of the layers between these experiments [176][195].

Future trends in 4D printing of natural fibre biocomposites

In this section, an extrapolation to cellulose-based composites and hydrogel-based materials is made to open a more in-depth discussion of the potential future trends for several applications. The global 4D

printing market is expected to grow at USD ~162 Million by 2022, at ~39% of CAGR between 2019 and 2022 in the fields of aerospace and defence, healthcare, automotive industry, construction, clothing, and packaging [198] [START_REF] Sullivan | Advances in 4D printing-next paradigm in manufacturing[END_REF]. As mentioned by Tibbits [START_REF] Tibbits | 4D printing: multi-material shape change[END_REF], 4D printing can integrate sensing and actuation features directly into a programmed material. Thus, external electromechanical systems are not necessary. This feature would decrease the number of parts in a structure, assembly time, material and energy costs, and the number of failure-prone devices, as usually utilized in current electromechanical systems. 4D printed biocomposites offer a novel approach to using available local resources to produce tailored autonomous and self-sufficient shape-morphing structures including impregnation of fibres and yarns during filament processing, improve adhesion between layers during printing, and reduce the amount of overall porosity.

As with biological systems, the anisotropy and stiffness of materials and their distribution should be well controlled to generate directional and complex actuation. This could be achieved using a combination of an oriented biocomposite with controlled swelling and a non-swelling matrix, customized control of fibre orientation within the composite, and precisely designed print path strategies across multiple layers. Customized control of fibre orientation could be achieved with precision by continuous fibres or yarns [START_REF] Baley | Flax/PP manufacture by automated fibre placement (AFP)[END_REF][166] [START_REF] Wang | Programmable morphing composites with embedded continuous fibers by 4D printing[END_REF]. In this sense, the continuous fibre composite appears to be an encouraging solution to producing a biocomposite with programmable morphing. The technical challenge should involve the production of the filaments and their printability.

Beyond moisture-induced actuation, natural fibre composites are also sensitive to temperature.

Temperature can influence the moisture content sorption and diffusion within natural fibre composites.

In a wet environment, an increase of temperature accelerates the drying of biocomposites while in immersion, it speeds up the water diffusion [START_REF] Deroiné | Accelerated ageing of polylactide in aqueous environments: Comparative study between distilled water and seawater[END_REF]. Correa et al. [START_REF] Correa | 3D-Printed Wood: Programming Hygroscopic Material Transformations[END_REF] have used heat ranging from 70 °C to 90 °C to help the drying process of wood/biopolymer materials and enhance shape transformation. This process opens the design space to thermo-hygromorph composites with a combination of multiple stimuli. Recently, Le Duigou et al. [START_REF] Duigou | Bioinspired ElectroTherma-Hygro Reversible Shape Changing Materials by 4D Printing[END_REF] have developed a 4D printed carbon fibre-reinforced PolyAmide hygromorph where the temperature rise is controlled by Joule effects. An increase in electrical stimulation leads to an augmentation of the temperature, a reduction of the moisture content, and ultimately the control of shape morphing. The concept is named electro-thermohygromorph or electrical pine cone and could be applied to hygromorph biocomposites with fibre modification.

At the structural level

Beyond the material performance, 4D printed materials could bring about complex active structures with original actuation performance. These complex structures with original properties are often composed of periodic or non-periodic cell patterns and can be referred to as metamaterials. A strict definition of a metamaterial is a material whose properties are due to both cellular architecture and chemical composition and go beyond what is found in nature by behaving in a way not found in nature [START_REF] Lincoln | Multifunctional composites: a metamaterial perspective[END_REF]. For example, Gladman et al. [START_REF] Gladman | Biomimetic 4D printing[END_REF] have developed a shape-changing bioinspired structure with a complex architecture, with localized interfilament spacing to develop a gradual log-curved microstructure (Fig 16c). Localized anisotropic swelling due to precise filament placement results in a multi-curvature actuation (Fig 16d).

Bio-inspired mechanisms such as a multi-stage double curvature following the pine cone [START_REF] Correa | Fused filament fabrication for multi kinetic state climate resonsive aperture. Fabr Chapter 3 Rethink Addit Startegies[END_REF], the edge growth-driven opening of the lily flower, snap-through elastic instability based on the venus flytrap, or origami-like curved-folding kinematic amplification based on the carnivorous waterwheel plant [START_REF] Poppinga | Plant Movements as Concept Generators for the Development of Biomimetic Compliant Mechanisms[END_REF] have been implemented using wood-based hygromorph biocomposites. The strength of the printing methodologies using biocomposites relies on their capacity to choreograph the direction of curvature based on the multi-layer difference between the dominant printing angle of each layer and its corresponding direction of hygro-expansion. Geometrical features such as shape, aspect ratio, or a rigid boundary can be further integrated to manipulate strain gradients across the sample [START_REF] Poppinga | Plant Movements as Concept Generators for the Development of Biomimetic Compliant Mechanisms[END_REF]. For example, for one bending curvature, all the expansion due to the anisotropic orientation of the fibre is directed along one axis. In order to obtain a double curvature (Fig 18a Turcaud et al. [START_REF] Turcaud | An excursion into the design space of biomimetic architectured biphasic actuator[END_REF] have developed the concepts of "benders" or "twisters", i.e. hygroscopic architectured materials produced by monitoring the symmetry and distribution of swellable materials in their undeformed geometry. Figure 18a presents an extrudable cross-section with different actuation patterns provided by the simulation with active and passive material distribution. Figure 19b shows the effect of material distribution on the control of the actuation of a slender beam. (c) Edge growth causing a strain gradient in the Lilium 'Casa Blanca' drives the flower opening mechanism, a principle that is translated into a 4D printed mechanism with a wood-based hygromorph composite edge. Adapted from [START_REF] Poppinga | Plant Movements as Concept Generators for the Development of Biomimetic Compliant Mechanisms[END_REF].

Another example of localized actuation is observed through the development of a hinge effect. Using a bilayer microstructure with multimaterial printing (nylon and wood biocomposites) on a dedicated area with a constant thickness ratio (tp/ta =1/5) and various widths (9.5-27mm) makes it possible to

(a) (c)
amplify the responsiveness from 98 to 161° due to a higher bending moment [START_REF] Correa | 3D-Printed Wood: Programming Hygroscopic Material Transformations[END_REF]. Unlike free displacement, the use of a hygromorph biocomposite as a hinge requires being specifically designed with appropriate stiffness and strength to generate a bending moment that lifts adjacent passive layers.

As with a hinge, a fold strategy can be used. Hygromorph biocomposites can also be used to trigger actuation in a polymer system with curved crease (Fig 20a). Origami-type structures could be used to amplify the displacement. In addition, recent work by Baker et al. [START_REF] Baker | 4D printing with robust thermoplastic polyurethane hydrogel-elastomer trilayers[END_REF] on the polyurethane hydrogel/elastomer trilayer has defined the hinge zone as a "mountain" or "valley", paving the way for origami-inspired shape-changing structures (Fig 20b andc). Similarly, kirigami geometries have also been tested to amplify 3D-printed multi-material structures

Conclusion

Due to environmental and economic considerations, natural fibre-reinforced composites have been the subject of growing research interest for about 20 years, which is however a short period of time when compared to research on synthetic composites (e.g. glass or carbon fibre-reinforced composites).

Despite the 20-year period, the development of biocomposites is still in its infancy in terms of technological maturity but also in terms of basic understanding. The advent of 3D and 4D printing represents a great opportunity for biocomposites to develop for the first time on the same time scale as their synthetic counterparts.

This article aimed to provide a baseline regarding the specificities of natural fibres to bolster the analysis of the mechanical and actuation properties of 3D and 4D printed biocomposites. To this end, an exhaustive literature review focusing on the printing process/microstructure and properties of natural fibre biocomposites has been achieved. Comparison has been made with synthetics materials and conventionally manufactured biocomposites. The main technical and scientific issues have been described and discussed, and new trends have been identified.

Biocomposites are used for 3D printing mainly in the form of discontinuous fibres or powder-like reinforced polymer. The current tensile properties are moderate compared to those of an established process such as injection or compression moulding. In general, it has been observed that adding natural fibres improves the stiffness of the composite but does not improve its tensile strength. 3D

printed biocomposites often have a low fibre content (< 30 wf%) with a very low aspect ratio (L/d) to reduce the overall viscosity and to improve printability, which accounts for their moderate properties.

In addition, discontinuous or short fibre-reinforced biocomposites exhibit a high porosity content due to i) the porosity within the filament prior printing and ii) the low pressure applied during printing.

(a) (b)

This porosity induces printed samples that are likely to absorb a large amount of water, which may reduce their lifetime in a humid environment.

It is worth noting that synthetic counterparts such as discontinuous glass or carbon fibre-reinforced composites suffer from similar technical limitations (low fibre content and aspect ratio, high porosity content) with overall moderate properties compared to similar specimens prepared by conventional processes.

Future trends in 3D printing are expected to relate to higher mechanical properties with improved material selection. Thus, continuous natural fibre biocomposites offer a relevant alternative to discontinuous biocomposites, bringing drastically higher mechanical performance due to the higher fibre content and better control of anisotropy by fibre orientation. For this purpose, high quality natural fibre yarns (in terms of fibre variety, low twist, and low linear weight) should be industrially available in the market, and filament production with high fibre content should be investigated. The evolution of printer desktops through a controlled environment (T and RH) and customized feeding and nozzle are essential to take benefit of the full potential of natural fibres (e.g. moisture sensitivity, surface topology, tensile properties). The current machines (e.g. Markforged printers) that are able to print continuous fibre composites cannot use other materials than those they have supplied. In the near future, full characterization of the effect of printing and slicing parameters on the properties and the geometrical limitations is expected so that reliable predictive models may be developed. Complex architectured composite structures, supported by disruptive design tools such as biomimicry and topological optimization, are intended to support the development of advanced metamaterials with an enhanced and original performance.

The implementation of printing methods using 6-axis industrial robot-arm extrusion effectors is designed to facilitate the development of larger components with novel multi-axis trajectories.

Moreover, the larger payload capacity of industrial manipulators is likely to facilitate the development of integrated fibre placement (both continuous and discontinuous fibres) and specialized extrusion

effectors or to open the spectrum for a more diverse range of multi-material printing processes. The challenges of this development involve the issue of cost, as the material and equipment is likely to be more expensive, and complementary specifications to the Advanced Function Presentation (AFP) such as the ability of to place the tape with a low curvature (steering effect).

As natural fibre biocomposites have been proven to be a new class of smart materials, namely hygromorph biocomposites, they can be used in 4D printing to develop specialized shape-changing mechanisms and structures. These biocomposites, owing to their bilayer microstructure inspired by biological hydraulic actuators (e.g. pine cone), can actuate with a change in humidity or with a combination of temperature/moisture stimuli. Additive manufacturing via 4D printing is a great opportunity for hygromorph biocomposites because it facilitates the design of complex material architectures through precise manipulation of fibre orientation and distribution. Different material and design strategies using monomaterials and multimaterials can amplify the difference in hygroexpansion between the layers. However, significant challenges remain for these technologies to develop fully. For instance, actuation performance (responsiveness and reactivity) remains slightly lower than with conventional manufacturing due to a lower fibre content. This is a critical dimension for further research since fibres act as the main actuating agent.

Further challenges exist regarding the material availability and the actuation performance and durability of the 4D printed structures. As with 3D printing, significant advances should be made regarding the quality of the materials through filament development with a wide range of polymers and fibres. The quality and the control of the microstructure are crucial to control actuation, and adapted printers must be proposed. For actuation of the morphing performance, the future trends could occur on two levels: material and structure. At the material level, the aim is to produce biocomposites with enhanced hygroexpansion (kinetic and amplitude) but durable against degradation. In addition, passive stimulus response could be combined with active electromechanical systems to enhance multifunctionality and the range of applications.

At the structure level, the target is to geometrically amplify the actuation of the unit cells by designing metamaterials. Thus, hygromorph biocomposites will be used as a building element towards a more complex structure whose morphing properties depend on both the material and geometric considerations. Finally, the most exciting challenge in the field of the 3D/4D printing of natural fibre

Figure 1

 1 Figure 1 Description of the different scales required to analyse the natural fibre composites' (a) internal fibre cell-wall interface, (b) fibre/matrix interface, and (c) fibre/fibre interface within the bundles. Adapted from [38][39][40] with permission from Elsevier.

Figure 2

 2 Figure 2 (a) SEM micrograph of single flax fibre surface with correct (left) and incorrect (right) retting. Correct retting leads to a homogeneous smooth surface whereas incorrect retting induces low adherent polysaccharidic residues (b) AFM image of a flax fibre surface with incorrect retting producing a rough surface. Adapted from [39] with permission from Elsevier.

  reinforced polymer), natural fibre composites have multiple hierarchical interfaces, ranging from cell wall components (Fig 1a), cell wall layers (Fig 1a), and fibre/matrix (Fig 1b), to fibre/fibre interfaces (Fig 1c). Thus, cell-wall peeling occurs during loading of the composite [49]. This mechanism, particularly in evidence in the micromechanical tests (Fig 3b), is also observed at the laminate level [49][START_REF] Moigne | Study of the interface in natural fibres reinforced poly(lactic acid) biocomposites modified by optimized organosilane treatments[END_REF]. Its effect on stress transfer is not yet clearly understood but suggests that even if a fibre/matrix interface is chemically or physically optimized, the internal interfaces between cell walls may ultimately be the weakest link.

Figure 3 (

 3 Figure3(a) Example of fibre bundle division into single fibres during processing, highlighting the reduction in the effectiveness of surface treatments[START_REF] Moigne | Study of the interface in natural fibres reinforced poly(lactic acid) biocomposites modified by optimized organosilane treatments[END_REF], (b) Detail of a flax cell wall peeling during microbond testing[START_REF] Le Duigou | Improving the interfacial properties between flax fibres and PLLA by a water fibre treatment and drying cycle[END_REF], which indicates that the weakest interface may be the cell wall interface compared to the fibre/matrix interface. With permission from Elsevier.

Figure 7

 7 Figure 7 Example of the microstructure of (a) extruded filaments with TMP fibre of 400µm length and 38 µm width (b) and two subsequent extrusions of TMP/bioPE biocomposite. The subsequent extrusion leads to a shorter fibre, better homogenization, and lower porosity content. Adapted from [79].

Figure 8 (

 8 Figure 8(a) Microstructure of Bamboo/PLA filament showing skin and centre region due to a distribution of the orientation of the bamboo fibres. Bamboo fibres with L= 713 µm, d= 153 µm and L/d = 4.67. (b) Distribution of the bamboo fibre angleshowing the skin/core effect. Adapted From[START_REF] Depuydt | Production and characterization of bamboo and flax fiber reinforced polylactic acid filaments for fused deposition modeling (FDM)[END_REF] with permission from Elsevier.

Figure

  Figure 9a, b, and c capture the data regarding tensile stiffness and strength provided by about 110articles on printed natural discontinuous and continuous fibre composites, their synthetic counterparts, and natural discontinuous and continuous fibre composites manufactured by conventional processes (i.e. injection and compression moulding). The results of all the collected data, as presented in Figure9aand 9b, indicate a very scattered distribution profile. This is due to differences in the choice of materials and formulation (polymer, fibre type, fibre content -among others) but also to different processing parameters and printers.

  Compared to pure polymers (Fig 9a, b, and c), the printing of natural fibre-reinforced biocomposites results in a slight increase in the stiffness of the biocomposites combined with a general decrease in their strength [77][80][90][96][97][104][105][121][116].

[ 124 ]

 124 (Fig 11b).

Figure 11 (

 11 Figure 11 (a) Perspective view obtained by microtomography showing the formation of void in a form of network in the printed structure and (b) the pore size distribution in Hemp/PLA biocomposites for different printing temperature. Increasing the temperature from 210 to 240°C induces a higher frequency of low size pores within Hemp/PLA biocomposites. Adapted from [122] with permission from Elsevier.

Figure 12 a

 12 Figure 12 a) Effect of layer height on wood, 0.3, 0,2 and 0.1 mm [126]. Higher layer height set by the slicer generates a microstructure with higher porosity content. b) SEM micrography of printed wood/PLA biocomposites with raw filament cross-section, compressed printed samples and various filament thickness. Adapted from [29][126] with permission of Elsevier.

Figure 13 a

 13 photograph of a broken area of a unidirectional printed sample of continuous jute fibre yarns with innozzle impregnation of PLA and a unidirectional printed sample of continuous flax fibre yarns/PLA

Figure 13 (

 13 Figure 13 (a) Fractured continuous unidirectional jute fibres/PLA sample printed with in-nozzle impregnation. Lack of jute yarn impregnation is observed with numerous fibre debonding. (b) Fractured continuous unidirectional flax fibre/PLA sample printed with pre-impregnated filaments. Typical rupture mode is observed with transverse crack followed by propagation along the tensile axis. (c) SEM micrography cross section of Flax/PLA composite microstructure. Adapted from [119][120].

Figure 14 (

 14 Figure 14 (a) Large scale printing process with poplar/PLA composites. Adapted from Zhao X, Tekinalp H, Meng X, Ker D, Benson B, Pu Y, et al. Poplar as biofibre reinforcement in composites for large-scale 3D printing. ACS Appl Bio Mater 2019. doi:DOI: 10.1021/acsabm.9b00675. Copyright (2019) American Chemical Society. (b) chitosan/wood flour/cellulose composites. Adapted from [156], (c) Picture of ± 45° PP/Flax laminate composite manufactured with Automated Fibre Placement (AFP). Each tape width is 6.35 mm. Adapted from [145] with permission of Elsevier.

Figure 15

 15 Figure 15 Synoptic of biomimicry approach applied to develop novel features for natural fibres based on their moisture sensitivity and microstructure: hygromorphing. The source of inspiration is the microstructure of a pine cone scale from the gradual transition zone between two types of tissues (red circle and dash point line) (a) to the asymmetric or bilayer lay-up (b) where each layer possess different elastic and hygro-expansion properties. (c) The bio-inspired solution i.e. hygromorph biocomposites are based on the anisotropic hygroexpansion of natural fibre and the asymmetric lay-up similarly to pine cone. Adapted from [26][168] with permission of Elsevier.

Figure 18 (

 18 Figure 18 (a) Three viewing perspectives of wood fibre/Biopolymer composite-responsive flap under various RH showing longitudinal and transverse curvature. (b) Example of a localized print path within the sample to induce a double curvature and multistep actuation. Adapted from[START_REF] Correa | Fused filament fabrication for multi kinetic state climate resonsive aperture. Fabr Chapter 3 Rethink Addit Startegies[END_REF]

Figure 19 (

 19 Figure 19 (a) Simulated actuation patterns for several cross-sections with a passive/active area ratio of 50:50. Adapted from [181] Copyright© (2010) Carl Hanser Verlag GmbH & Co.KG, Muenchen. (b) Helical bending obtained by rotating the bending plane along the rod as a result of the distribution of passive (green) and active (white) regions. Adapted from [206].(c) Edge growth causing a strain gradient in the Lilium 'Casa Blanca' drives the flower opening mechanism, a principle that is translated into a 4D printed mechanism with a wood-based hygromorph composite edge. Adapted from[START_REF] Poppinga | Plant Movements as Concept Generators for the Development of Biomimetic Compliant Mechanisms[END_REF].

Figure 20 (

 20 Figure 20 (a) Moisture-induced deployable structure based on curved-line folding inspired from aldrovanda, adapted from [15] (b) Polyurethane hydrogel/elastomer trilayer upon dehydration showing mountain and valley folding pattern and (c) Actuation of Miura-ori origami fold pattern with the same trilayer. Adapted from [161].

  The wood/paper bilayers are assembled into a single unit cell (Fig21a), and then each unit cell is assembled into a honeycomb structure (Fig21b). There is a 20-30% difference in responsiveness between a simple bilayer and the same bilayer integrated into an alveolar structure due to geometric constraints. It is worth noting that the small strain within the active layer is converted into a large displacement (≈ 500%) of the structure (Fig21b).

Figure 21 (

 21 Figure 21 (a) Actuated Wood/paper bilayer and single unit cell, (b) actuation of the metamaterial made by bilayer unit cell from 0, 2, 4 and 16 hours at 95% RH. Adapted from[START_REF] Guiducci | Honeycomb Actuators Inspired by the Unfolding of Ice Plant Seed Capsules[END_REF]

[ 189 ]

 189 . Multi-material 3D printing is used with the inclusion of swellable polyvinyl in rigid polyurethane cell wall resins (Fig 21a). The isotropic volume change of the highly swellable material induced by the solvent isopropanol leads to an anisotropic volume change (+50%) of the customdesigned cell (Fig 22a). The periodic hierarchical microstructure allows the anisotropic deformation of individual cells, resulting in unidirectional expansion/contraction of the cell structure (Fig 22b). The thickness of the cell wall, the rigidity ratio between the cell wall and the inclusion and topology modulate the responsiveness [207].

Figure 22 (

 22 Figure22(a) Schematic view of a diamond shaped unit cell inspired by the microstructure of the ice plant keel's tissue with a width-to-height ratio of 4 for stiff walls (1.13 mm thickness) and round soft inclusions. All dimensions are in mm. (b) Swelling of a honeycomb made with repetition of the diamond unit cell. The honeycombs were printed as rectangular arrays of 5-by-10 cells with stiff walls (verowhite) and soft (Tangoplus) inclusion. Snapshots cover 4 days of swelling in isopropanol at room temperature (T = 22°C). (L.H.W = 50.5.5); Adapted from[START_REF] Guiducci | Honeycomb Actuators Inspired by the Unfolding of Ice Plant Seed Capsules[END_REF]

Table 1

 1 Mechanical characteristics (Tensile modulus and Stress at break) and biochemical composition (Cellulose,

Hemicelluloses, Lignin, Pectins content, and microfibrillar angle (MFA)) of various natural fibres used in 3D and 4D printing

  Principle of residual stress generation at the natural fibre/polymer matrix interface. The thickness of arrows is

	Figure 5
		Carbon fibre	Glass fibre	Flax fibre	Polypropylene
	Radial thermal expansion coefficient αf,T (10 -6 /°C)	18 [92]	5 [92]	78 [82]	120 [93]
	Radial hygroscopic expansion
	coefficient βf,R (ε/Δm)	-	-	1.14 [94]	-
	Heating	Cooling		Storage at RH =50%
	Hygroscopic shrinkage flax	Thermal shrinkage Polymer	Hygroscopic expansion flax
	Thermal expansion flax

  ).

	Compared to conventional manufacturing (injection or compression moulding), 3D printed short
	natural fibre biocomposites have generally lower tensile properties (Fig 9 a, b, and c) [108]. Le
	Duigou et al. [29] experienced a 44% reduction in stiffness and a 30% reduction in strength compared
	to compression moulding for wood/PLA/PHA. Safka et al. [105] have found a 50% reduction in
	stiffness and strength of ABS/coconut fibres compared to injection moulded counterparts. Tarrès et al.

Table 3

 3 Actuation properties of natural fibre-based hygromorph biocomposites (* thermocompression, ** bonded). Note: MAPP stands for Maleic Anhydride grafted PolyPropylene

	Materials and pattern		Environmental conditions	Normalized time to reach stability (h/mm)	Normalized K.t (-) responsivness	Ref
	Monomaterial Copolyester-wood	Space grid One face smooth and the other with ridges	Immersion-various RH Immersion-various RH	Sorption :5-6 Desorption : 0.10-1 Sorption :3.3-5 Desorption : 2-4	0.028 0.0285	[183]
	Monomaterial Copolyester-wood	Full filling with different printing width	50%RH-immersion-50%RH	Sorption :6.25-7.5 Desorption : 0.625	0.010	[29]
	Monomaterial Cellulose/hydrogel	Space grid	Immersion in water	Sorption : <0.07	0.21-0.25	[25]
	Multimaterial		Immersion-drying
	Copolyester-wood	Interwoven	with heating radiation	Desorption : 0.021-0.0030 0.014-0.096
	and PA Multimaterial Copolyester-wood	Interwoven	(70°C) Relative humidity	Sorption :12-24	0.065	[183]
	and ABS
	Monomaterial* MAPP/flax		50%RH-immersion-50%RH	Sorption :3.3 Desorption :3.3	0.037	[15][8]
	Monomaterial* MAPP/flax		0-90%RH-0%RH	Sorption :22.2 Desorption :22	0.02	[195]
	Monomaterial* MAPP/jute		50%RH-immersion-50%RH	Sorption :5 Desorption :4.4	0.034
	Monomaterial* MAPP/kenaf		50%RH-immersion-50%RH	-	0.040	[15]
	Monomaterial* MAPP/coir		50%RH-immersion-50%RH	-	0.013
	Monomaterial** Wood		85-35%RH	Desorption : 5-10.5	0.009-0.043	[188][1 96][19 7][172]
	Multimaterials* PP-PP/flax		50%RH-immersion-50%RH	Sorption : 11 Desorption : 26	0.022	[26]
	Multimaterials** Wood/glass/epoxy composite	44-87%RH-44%RH	Sorption :8.3 Desorption :10	0.024	[191]
	Multimaterials** Wood/glass/epoxy composite	44-water sprayed-44%RH	Sorption :3.3 Desorption :12.5	0.0324-0.056	[174]
	Multimaterials** Wood/paper		50-95%RH 50%RH-sprayed	x x	0.0256 0.032	[176]
	Multimaterials** Paper/polymer		98%RH	x	0.021	[182]

adaptive underwater systems for maritime engineering (e.g. artificial reefs) [START_REF] Duigou | Natural fibres actuators for smart bio-inspired hygromorph biocomposites[END_REF], architectural skin systems [START_REF] Vazquez | Shape-Changing Architectural Skins a Review on Materials, Design and Fabrication Strategies and Performance Analysis[END_REF], shading systems and solar tracking systems for architecture [START_REF] Correa | 4D Pine Scale: Biomimetic 4D Printed Autonomous Scale and Flap Structures Capable of Multi-Phase Movement[END_REF] [START_REF] Rüggeberg | Bio-Inspired Wooden Actuators for Large Scale Applications[END_REF] in various environments where autonomy may be a requirement but that are not limited to extreme environments (e.g. war zones) [START_REF] Momeni | A review of 4D printing[END_REF]).

At the material level

The next steps in 4D printing with natural fibre composites should target materials with improved hygroscopic properties, higher anisotropy, customized stiffness, and a high bonding ability between plies. Focusing on the rigorous selection of materials and their formulation should optimise these properties. The hygroscopic properties (sorption and swelling coefficient) of biocomposites are influenced by the type of fibre involved, i.e. their microstructure (MFA) and composition. As described earlier, the use of different species of wood or coir, jute, flax or kenaf generates a dramatic effect on the potential responsiveness of the actuator [START_REF] Duigou | Natural fibres actuators for smart bio-inspired hygromorph biocomposites[END_REF]. Natural fibres act as an actuating agent through swelling. Therefore, hygromorph biocomposites should be developed with the highest possible volume fraction, especially when a moisture-insensitive polymer is used. A high natural fibre content promotes faster and greater swelling [START_REF] Duigou | Evaluation of force generation mechanisms in natural, passive hydraulic actuators[END_REF] and therefore improves both reactivity and actuator responsiveness [START_REF] Duigou | Natural fibres actuators for smart bio-inspired hygromorph biocomposites[END_REF]. However, a high discontinuous fibre content increases the viscosity of the mixture and thus leads to clogging or could also prevent printing. An optimal formulation including continuous natural fibres should be investigated.

Second, the selection of a moisture-sensitive polymer matrix could also promote faster actuation.

From this perspective, the best candidate may be hydrogel polymers such as those already used by Gladman et al. [START_REF] Gladman | Biomimetic 4D printing[END_REF] and Mulakkal et al. [START_REF] Mulakkal | Responsive cellulose-hydrogel composite ink for 4D printing[END_REF] with cellulose fibres. However, their moderate mechanical properties against moisture may prevent their use for technical applications in outdoor environments. Selecting a moisture-sensitive thermoplastic polymer may be an alternative. As far as the thermoplastics matrix is concerned, a low melting temperature of the polymer is to be preferred, as processing reduces the thermal degradation of the natural fibres and thus helps maintain their hygroscopic expansion [START_REF] Pucci | Capillary wicking in flax fabrics -Effects of swelling in water[END_REF] and actuation capacity. A lower viscosity of the polymer would promote biocomposites is probably to overcome the current material design habits by developing real multidisciplinary work involving eco-design, biomimicry, mechanics, materials sciences, technology, and other disciplines.

Acknowledgements

The authors are grateful to the CNRS AAP "Biomimétisme" for its financial support and Nicole Geslin for post-editing the English style.

Data availability

The raw/processed data needed to replicate these results cannot be shared at this time as they are part of an ongoing study.