Arthur Stricher

Dr Renaud G Rinaldi

Dr Guilherme Machado

Grégory Chagnon

Denis Favier

Laurent Chazeau

Dr François Ganachaud
email: francois.ganachaud@insa-lyon.fr

Gregory Chagnon

Dr A Stricher

Prof L Chazeau

Dr G Chagnon

Prof. D Favier

Light-Induced Bulk Architecturation of PDMS Membranes

Keywords: elastomer, materials by design, PDMS, UV-light

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

ability to sustain large deformation. Aside from mammal implants, silicones are used in many other applications, such as blood pumps, maxillofacial prosthesis, replacement esophagus, or artificial skin. [3] A singular property of most natural soft tissues is their mechanical anisotropy, [4,[START_REF] Abé | Data Book on Mechanical Properties of Living Cells, Tissues and Organs[END_REF] often related to the orientation of collagen fibers. [START_REF] Martin | [END_REF] To mimic this feature, anisotropic rubbers can be made by the introduction of oriented fibers, [7,[START_REF] Courtney | [END_REF] the controlled diffusion of reinforcing fillers [9] or species. [10][11][12] More straightforward routes include thickness variation [13,14] and oriented damaging of the network. [15,16] The limitations are however multiple: in the case of fibers, the incorporation of interfaces (and therefore potential defects) becomes an issue. As far as "diffusion" is concerned, fine control of the process is lacking, while thickness control requires specially crafted molds and highly uneven surfaces. Thus, locally controlling the cross-linking density of the network is a legit option, since it suppresses interfaces problems and allows for any mechanical pattern. Some studies investigated the addition of photo-cross-linking initiators [10] or inhibitors, [17][18][19] which allows a great versatility, considering UV rays responsible for cross-linking (or its inhibition) can be readily localized. However, they require the addition of harmful molecules and involve chemical reactions besides One major challenge of biomaterial engineering is to mimic the mechanical properties of anisotropic, multifunctional natural soft tissues. Existing solutions toward controlled anisotropy include the use of oriented reinforcing fillers, with complicated interface issues, or UV-curing processing through patterned masks, that makes use of harmful photosensitive molecules. Here, a versatile process to manufacture biocompatible silicone elastomer membranes by light degradation of the platinum catalyst prior to thermal cross-linking is presented. The spatial control of network density is demonstrated by experimental and theoretical characterizations of the mechanical responses of patterned cross-linked membranes, with a view to mimic advanced implantable materials.

Introduction

Elastomeric materials are of great use for biomedical applications, [1] providing properties close to the ones of natural soft tissues. More precisely, silicones are widespread, thanks to their biocompatibility, ease of sterilization, [2] and their regular silicone cross-linking, which may involve unwanted byproducts and/or side reactions.

Here, we introduce a new method allowing for the spatial control of the cross-linking density of any additioncured silicone elastomers (e.g., liquid silicone rubbers, or LSR), without either additional chemical reaction or macro molecular network degradation. Curing of these elastomers results from the hydrosilylation reaction of long vinyl telechelic polydimethylsiloxane (PDMS) chains on hydride-bearing siloxane cross-linkers (Figure 1 a). [START_REF] Marciniec | Comprehensive Handbook on Hydrosilylation[END_REF] The reaction takes place in temperature in the presence of a catalyst, the most widespread being the Karstedt's catalyst. [START_REF] Karstedt | [END_REF] The process benefi ts from the photoinstability [START_REF] Hitchcock | [END_REF] of this platinum complex: when irradiated in the presence of an UV-absorbing solvent, it gradually deteriorates into inert colloidal Pt(0). Therefore, the ultraviolet (UV) irradiation of a un-cross-linked LSR blend in presence of an aromatic solvent (here xylenes, vide infra) leads to the degradation of the Karstedt's catalyst, and hence to a reduction of the silicone cross-linking rate. The UV irradiation can be localized via the use of masks to block the rays, or laser devices to focus them. [ 23 ] By doing so, the cross-linking kinetics can be adjusted spatially, and the reaction stopped before its completion by cooling and/or end-capping the remaining reactive groups, as depicted in Figure 1 b. The resulting mechanical properties will only depend on the UV dose received, and consecutive hot cure parameters (temperature and duration).

. Results and Discussion

Based on the aforementioned technique, the manufacturing of mechanically patterned elastomer membranes is obtained by the method depicted in Figure 1 c. A mixture of two-part silicone (Silbione 4360AB from Bluestar Silicone) and xylenes is casted in a mold, irradiated, degassed, and fi nally hot cured. The curing time and temperature are chosen so that the non-irradiated samples are fully cross-linked. The irradiation dose is tuned to trigger the mechanical behavior of the irradiated samples/ zones, as illustrated in Figure 2 a where the nominal stressstrain uniaxial tension responses with varying irradiation time are displayed. The Young's modulus, stress, and strain at break decrease with increasing irradiation dose (from 0 to 4.5 J cm -2 ). R / 100 ref 100 UV σ σ = is then calculated to quantify the difference in stress magnitude at a 100% strain value between the irradiated and the non-irradiated (reference) samples. R is here as high as 15, which is the highest value reported in the literature for silicone elastomers without varying prepolymer length nor composition, [ 10,11,19 ] membrane thickness, [ 12,24 ] nor adding any photo-activated molecules. [ 10,18,25 ] Since the determination of the elastic modulus from tensile test is not trivial in elastomeric materials, [ 26 ] stress-strain plots were fi tted with the neo-Hookean model in order to estimate the decrease of mechanical properties with respect to the UV dose. Interestingly, the decrease of the fi tting constant is almost linear with an increasing dose, as shown in Section S1 (Supporting Information).

The observed drop in the deformation at break, yet remaining larger than 200% whatever the irradiation dose assigned, results from the fact that the macromolecular network of the irradiated silicone is mixed with a large amount of free chains (the amount of extractable material as a function of the UV-dose can be found in Section S2, Supporting Information) which decreases its resis tance to tear and to high strains.

To ensure that the observed effect was not due to poly mer photodegradation, polysiloxanes contained in the used LSR formulation were studied by nuclear magnetic resonance (NMR) spectroscopy and size exclusion chromatography (SEC), before and after irradiation (Section S3, Supporting Information). The results showed that the irradiation did not lead to any relevant change in molar mass or in the content of reactive moieties whether or not an aromatic solvent was present during the irradiation.

Silicone materials were then manufactured after irradiating only one of the two parts of the LSR formulation in the presence of xylenes. Both parts contain silica and vinyl functionalized oil while only one encloses the catalyst. [ 27 ] As shown in Section S4 (Supporting Information), when the part which does not contain any catalyst is irradiated, the resulting material reaches the macroscopic properties of the fully un-irradiated one. Conversely, the material obtained from the irradiation of the part comprising the catalyst exhibits mechanical properties equivalent to the fully irradiated sample, confi rming the key role of the catalyst degradation.

Cross-linking kinetics of irradiated LSR was therefore studied during an isotherm using a plane/plane rheometer (Figure 2 b,c). Upon curing, the silicone oil transforms into a cross-linked elastomer, which is accompanied by an increase of its storage modulus by several orders of magnitude. The cross-linking of the pristine LSR takes place in a few minutes at 90 °C, while it requires several hours when previously irradiated in the presence of xylenes. Interestingly enough, when irradiation was performed in presence of a non-aromatic solvent, such as methylcyclohexane, no effect on the reaction kinetics was noticed (Section S5, Supporting Information). This shows the important role of a UV-absorbing solvent that transfers UV energy to the catalyst, leading to the degradation of its labile ligands and platinum aggregation. Similar effect has been reported on other platinum or metallic complexes. [28,29] The effect of the quantity of xylenes during the irradiation has been investigated as shown in Figure 2c: the irradiation of the silicone formulation alone leads to the same cross-linking behavior as the nonirradiated one, and the presence of a quantity as low as 1% (in mass) of xylenes during irradiation is sufficient to deeply affect the cross-linking kinetics.

To further study the photo-degradation mechanism of the catalyst, which has not been clearly reported in the literature, Karstedt's catalyst (in an excess of divinyl tetramethyl disiloxane, which is inert to UV) was irradiated at a wavelength λ = 254 nm in the presence of an aromatic solvent. Throughout irradiation, the solution color turned from transparent to brown attributed to colloidal platinum formation. [28] Colloids were indeed observed by imaging this mixture in transmission electron microscope (TEM), as seen in Section S6 (Supporting Information). Bibliography indicates that such colloids are also formed during the regular hydrosilylation reaction, becoming an inert, transformed state of the catalyst. [30,31] When photoirradiated prior to curing, Karstedt's catalyst most likely degrades into platinum colloids and therefore cannot properly catalyze the cross-linking reaction of LSRs. The reduction of the cross-linking kinetics of irradiated sample was also demonstrated with the Speier's catalyst and a carbene-platinum-based catalyst (Section S7, Supporting Information). These important results suggest that the presented process can be adapted to a wide range of silicone formulations to produce biocompatible membranes without the use of photo-initiators.

Affecting the cross-linking kinetics rather than blocking it completely means that ultimately, provided that few active catalyst remains, almost full cross-linking of irradiated zones can still be achieved by bringing enough time and heat to the material. Chemical reactions can therefore be performed on the remaining crosslinking sites to functionalize, or to stabilize the material after curing, as depicted in Figure 1a,c. Mechanical stabilization of a membrane for 30 d was achieved by endcapping cross-linking sites with monofunctional vinyl silanes with remaining hydride groups of the cross-linker. High-resolution magic angle spinning nuclear magnetic resonance (HR-MAS NMR) spectroscopy, suitable for gels and lightly cured elastomers, allowed to monitor the reaction and evidenced the consuming of SiH while SiVi moieties remained unchanged (Section S8, Supporting Information). When samples were aged for a longer time (six months), they displayed a slight increase of mechanical properties. Full mechanical stabilization might still be achieved either by forcing the reaction with vinyltrimethylsilane or VTMS (longer post curing times) or by quenching the remaining active platinum with sulfur compounds. [32] The elastic recovery of stretched materials produced by our method was then studied in order to evaluate the influence of the reduced cross-linking on their elasticity, which is crucial when targeting biological applications. To do so, samples were stretched up to 100% strain, then released, and the elastic return was measured 10 s after the release. Regardless of the cross-linking state, samples displayed less than 3% of residual strain at the first stretch, and less than 1% for the following cycles, pictured in Figure 2d. Irradiated samples seemed to take longer time to relax and an attempt to quantify the difference of relaxation is described in Section S9 (Supporting Information).

Dynamical mechanical analysis provides the evolution of the storage modulus and damping factor versus frequency (for a given temperature) of the manufactured elastomers as displayed in Section S10 (Supporting Information). For the case of perfectly (entropic) hyperplastic materials, these properties are supposed to be insensitive to the loading rates. Nevertheless, the modulus and damping factor of the present silicones increase with increasing frequency, evidencing some viscoelastic contribution. More importantly, the damping is significantly greater in the irradiated sample (the damping factor is multiplied by 3 with R = 6), a consistent result since its network comprises a larger amount of unreacted and dangling chains. This difference of viscoelastic behavior can be used as an advantage, for instance, for applications where damping has to be localized in prescribed regions.

The patterning/architecturation of a membrane can be finally obtained through the use of a mask positioned between the UV source and the LSR mix, resulting in a spatial control of the irradiation. Practically, the non afocal UV source and a gap between the polymer and the mask suggest that a transition zone, or an interphase, may exist between the hard and soft phases of patterned elastomers where the modulus is likely to vary gradually. The characterization of such interphase is not easy, yet important. Indeed, a sharp interphase, i.e., a brutal change in the material mechanical properties, would result in stress concentration leading to early breakage of the sample. Moreover, the variation in material properties may provoke complex stress and strain fields in the transition zone even for simple (uniaxial) loading conditions. In order to control the strain field and thus proceed to the interphase macroscopic characterization, a bulge test was set. [33] The tested sample displays a circular under-cross-linked (r = 5 mm) disk within a normally cross-linked matrix, as depicted in Figure 3a, and Movie S1 (Supporting Information). The membrane is clamped so that the center of the irradiated disk is centered with the setup axis of symmetry. Unlike a simple uniaxial tension test, the present setting prevents lateral contraction. Hence, the strain state is well-defi ned, axisymmetric, and dependent on the radial coordinate. Finally, the determination of the neo-Hookean parameter ( C 10 ) was performed following the protocol described by Machado et al. [ 33 ] The evolution of the C 10 parameter, which is in good accordance with the one found in bulk materials, allowed for the identifi cation of the interphase's width (Figure 3 a). Presently, with our irradiation setup, the interphase is about 3 mm wide. This interphase width can be easily tuned by adapting the membrane processing condition, for instance, by using focused UV sources, or by diminishing the space between the masks and the irradiated polymer.

Natural soft tissues such as mussel byssus, [ 34 ] mammal sinews, [ 35 ] or fi nger extensor apparatus [ 36 ] exhibit a macroscopic spatial variation of their mechanical properties. Thanks to this local control of its cross-linking density, the macroscopic properties of the silicone membrane prepared here can be easily tailored. The developed method allows the easy processing of such graded structures, by varying the dose received all along the membrane, as represented in Figure 3 b, where the membrane's strain increases tenfold depending on its position along the Y -axis. Such variation, though on different stress and strain levels, is comparable to the one reported for the anterior cruciate ligament-bone insertion site, for instance. [ 37 ] Anisotropic membranes can be obtained using a simple striped pattern, as depicted in Figure 3 c, and Movies S2 and S3 (Supporting Information). With a stiffness ratio R of 10 between hard and soft zones and a 1 to 1 volumetric ratio, stripe-patterned membrane displays an anisotropic factor of E // / E + = 2.3, close to 3 theoretically expected. Here, E + and E // correspond to the Reuss (lower bound) and Voigt (upper bound) models, strain profi le at a 5% macroscopic strain. For comparison, the mechanical properties of human ligament-bone insertion site is provided on the right hand side (adapted from ref. [ 37 ] ). b) (Left schematic) Stripped membrane with controlled anisotropy, and (center fi gure) its behavior in uniaxial tension along the X (Lower-bound confi guration) and Y -axis (upper-bound confi guration). A qualitative comparison with heart tissue is provided in the right fi gure (modifi ed from ref. [START_REF] Young | Introduction to Polymers[END_REF] ). c) (Left fi gure) Experimental setup used for interface characterization through bulge test (center fi gure) strain fi eld determination (right fi gure) membrane modulus variation with respect to the distance from the axisymmetric axis.

respectively, that are based on the rule of mixture and neglect the contribution of the interfaces. [START_REF] Young | Introduction to Polymers[END_REF] Experimentally, the two moduli were reduced from the macroscopic loads and displacements measured during the test. It is worth noting that the ratio obtained is remarkable for a bulky, monophasic material, and is close to many natural tissues such as heart myocardium [START_REF] Jang | [END_REF] or blood vessels. [ 40,[START_REF] Humphrey | Cardiovascular Solid Mechanics: Cells, Tissues, and Organs[END_REF] The fact that irradiated samples are constituted of a lightly cross-linked network blended with a large amount of free chains allows promising developments. For instance, healable silicone can be obtained by joining two parts of lesser cross-linked silicone membranes, and then heating them, as displayed in Figure 4 a and Movie S4 (Supporting Information). The two networks interpenetrate one another, and the additional thermal curing step fi nishes cross-linking the silicone, forming a cohesive interface between them. In the case of bio-implantable devices, temperature healable and "patchwork" tailoring of prosthesis can therefore be performed easily, without any additives.

This process also offers the ability to fully block the cross-linking in selected areas, through high irradiation doses and careful curing conditions. The polymer is hence constituted only of free chains, which remains soluble and washable by a solvent within the irradiated zones, while it cross-links normally elsewhere (Figure 4 b). This allows for the topographical patterning of thin layers of silicone, which could easily fi nd applications in the fi eld of microfl uidic engineering.

. Conclusions

To conclude, we have explored in this study the potential of the photo-instability of a cross-linking catalyst to process biocompatible, transparent silicone membranes with local control of their mechanical and topological properties. The simple, rapid, and versatile approach is adaptable to any commercial silicone elastomer cross-linked by polyaddition, allowing for the design of advanced architectured PDMS membranes based on a unique macro molecular network. The UV-driven mechanism was explored thoroughly so that many applications were identifi ed (gluing, patterning, and grading). The process also gives the possibility to functionalize under-cross-linked zone prior to an additional post-cure of the material, for instance, to craft membranes with different physical properties (surface wetting properties, double cross-linking systems). Focusing on the "structural" performance, difference in modulus as high as 15, along with an infi nite choice of architectures can be used to meet the requirements/needs of numerous fi elds of applications, such as prosthesis engineering or microfl uidic.

Figure 1 .

 1 Figure 1 . Control of cross-linking strategy: a) UV-degradation of the cross-linking density. b) Post-functionalization of irradiated zones, comprising hydride moieties available. Viability is demonstrated by the mechanical stabilization of under cross-linked membranes, and by 1 H HR-MAS NMR study of the treated samples (Section S6, Supporting Information). c) Manufacturing of silicone membranes with local control of the mechanical properties.

Figure 2 .

 2 Figure 2 . Characterization of fully irradiated materials: a) Mechanical tensile behavior versus UV dose. b) Cross-linking kinetics of a pristine and two irradiated samples with or without xylenes. c) Effect of the quantity of solvent during irradiation on the cross-linking kinetics. d) Negligible residual strain after a 100% stretch ( D = 4 J cm -2 ).

Figure 3 .

 3 Figure 3 . Local control of mechanical properties: a) (Left schematic) Patterned membrane with a 10 ratio macroscopic gradient and (right fi gure) strain profi le at a 5% macroscopic strain. For comparison, the mechanical properties of human ligament-bone insertion site is provided on the right hand side (adapted from ref.[ 37 ] ). b) (Left schematic) Stripped membrane with controlled anisotropy, and (center fi gure) its behavior in uniaxial tension along the X (Lower-bound confi guration) and Y -axis (upper-bound confi guration). A qualitative comparison with heart tissue is provided in the right fi gure (modifi ed from ref.[START_REF] Young | Introduction to Polymers[END_REF] ). c) (Left fi gure) Experimental setup used for interface characterization through bulge test (center fi gure) strain fi eld determination (right fi gure) membrane modulus variation with respect to the distance from the axisymmetric axis.

Acknowledgements : This work was supported by the ANR under the project SAMBA (ANR-12-BS09-000). Many thanks to Cécile Chamignon for HR-MAS NMR advice and experiments, to the interns Quentin Barnes and Elodie Henrat for their general help, and to Jean-Pierre Pascault for helpful discussions. Dr. Renaud Rinaldi also thanks Prof. F. W. Zok from UCSB for access to his testing lab, and Kirk Fields for assistance.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.


Five-dimensional imaging of freezing emulsions with solute effects

Dmytro Dedovets 1 , Cécile Monteux, 2 Sylvain Deville The interaction of objects with a moving solidification front is a common feature of many industrial and natural processes such as metal processing, the growth of single-crystals, the cryopreservation of cells, or the formation of sea ice. Solidification fronts interact with objects with different outcomes, from the total rejection to their complete engulfment. We image the freezing of emulsions in 5D (space, time, and solute concentration) with confocal microscopy. We show the solute induces long-range interactions that determine the solidification microstructure. The local increase of solute concentration enhances premelting, which controls the engulfment of droplets by the front and the evolution of grain boundaries. Freezing emulsions may be a good analogue of many solidification systems where objects interact with a solidification interface.

Solidification fronts interact with hard or soft objects such as particles, bubbles, droplets, or cells, in three different ways: objects can be engulfed, rejected for some time and then encapsulated, or rejected for an extended period of time (1). Different outcomes are desired, depending 1 on the context. Particle-reinforced metal alloys require homogeneous particles distribution in the matrix and immediate engulfment is therefore preferred (2). In contrast, for single-crystal growth, a complete rejection of impurities is essential (3). The rejection of dissolved hydrogen can form gas bubbles which are eventually trapped by solidification fronts, becoming a major source of defect in many metals and alloys (4). Reproductive and red blood cells can be physically damaged when engulfed between growing ice crystals during cryopreservation procedures (5,6). The processing of porous materials by ice-templating relies on the complete segregation of particles by the solidification front (7). Our ability to control the solidification microstructure is essential in all these domains.

The dynamics of interfaces and objects are an inherent feature of these phenomena, but are extremely difficult to observe because of the time-scale (solidification fronts moving rapidly), space-scale (submicronic objects), and sometimes high-temperature. In addition, the role of solute is often important in these systems, from the gas and impurities in metals to the additives used in cryopreservation procedures or food engineering. Being able to follow in situ the concentration of the various phases in presence of objects is crucial and yet extremely challenging.

Direct photometric measurements of the solute concentration in thin layers of solution during solidification have been reported (8), but only in 2D, and not in presence of particles. Confocal Raman microspectroscopy was used to measure the static concentration of a cryoprotective agent (9) in frozen cells encapsulated in an ice matrix.

Progress in solidification research has thus largely relied on the studies of analogues: transparent materials that solidify near room temperature, using optical microscopy (10). However, 3D imaging cannot be performed and chemistry cannot be resolved. Thanks to recent progresses in X-ray computed tomography, we can obtain 3D, time-lapse images of the microstructure evolution in situ during the solidification of metals (11). The volume investigated is nevertheless limited, particles can barely be resolved, the temporal resolution is not good enough to capture the interaction events (12), and quantifying the solute concentration in the presence of objects is still not possible. It is thus fundamentally difficult to track in situ the development of 3D solidification microstructures where objects interact with a solid/liquid interface [START_REF] Deville | Colloids Obs. Princ. Control. Use[END_REF], even more so in presence of solute effects.

We studied the solidification process by confocal microscopy, translating a Hele-Shaw cell filled with an oil-in-water emulsions at constant velocity V through a fixed temperature gradient ∆T (Fig. 1A and movie S1). This way the solidification front propagates with a velocity V through the sample. A typical confocal image of the freezing emulsion is shown in Fig. 1B.

The sulforhodamine B fluorescence served as a local proxy for the surfactant concentration (Fig. S1).

We used the droplets trajectories to measure their velocity in the observation frame, U and deduce their velocity in the sample frame, U =V -U (Fig. S2). An example of the variation of the droplet velocity U with time (Fig. 2A) shows that it goes through a maximum as the droplet approaches the solidification front. The droplets start being repelled from the front at a distance L v of the order of 100µm, accelerate up to a maximal velocity U max of 1.2µm/s, and rapidly decelerate as they cross the solidification front (Fig. 2B). The droplets are eventually completely engulfed and stop moving. The time during which the droplets are repelled by the front before being engulfed is called the interaction time, plotted as a function of interface velocity in Fig. 2D. During the interaction time, droplets are displaced by a large distance, from 2µm to more than 100µm (Fig. 2E).

To understand the origin of the droplet displacement, we measure the solute concentration profile as a function of the distance from the solidification front. The solute concentration (Fig. 2C) is constant far from the front and then progressively increases. The concentration abruptly drops as we cross the interface because of the low solubility of solute in ice. The droplet starts being repelled by the interface at a distance L v close to L c -the distance at which the concentration of solute increases. This is more apparent when we plot L v as a function of L c for several front velocities (Fig. 2F), which shows that L v increases linearly with L c .

The presence of a surfactant in the system results in driving forces that can displace droplets over up to hundreds of microns from the solidification front, on time-scale of tens to hundreds of seconds. These results depart from the behavior observed and predicted by most physical models of the interaction of a particle with a solidification front (1) which, in absence of solute, assumed a dominant role of the thermomolecular forces in the interactions between the object and the front (5). However, such interactions are only effective on very short distances (nanometers). The distances measured here are several orders of magnitude larger. In the presence of a solute such as a surfactant, the thermomolecular forces seem to play a minor role.

We suggest that the driving force for the motion of the droplets is diffusiophoresis or solutal Marangoni effect, which are known to induce micron scale motion of droplets or particles in gradients of solute concentration [START_REF] Anderson | [END_REF]15). The droplet velocity increases with the gradient of surfactant concentration along the x axis (Fig. 2G). Diffusiophoresis is due to a gradient of osmotic pressure, while Marangoni flow is due to a gradient of surface tension along the droplet surface. In our case, the bulk surfactant concentration is two orders of magnitude above the critical micelle concentration (CMC) and rises by a factor of 3 to 10 near the solidification front. Therefore, we expect a weak variation of the surfactant monomer concentration and of the surface tension along the x axis but a significant variation of the concentration of micelles, which may lead to a diffusiophoretic motion.

The long range interactions between the droplets and the front have a strong influence on the solidification microstructure. When the droplets displacement (Fig. 2E) is larger than the inter-droplet distance, the droplets form clusters as they get engulfed by the front (top of Fig. 2H, obtained for a front velocity of 1µm/s). At 2µm/s, the interaction time is shorter and almost no aggregates are formed. For a front velocity of 3µm/s, the droplets displacement is significantly smaller than mean inter-droplet distance, which results in the preservation of the original spatial distribution of droplets upon solidification. In all these cases, the droplets are engulfed by the growing ice, but the microstructure of the solidified sample differs, depending on the solidification front velocity.

The majority of physical models developed for the past 60 years to describe the interaction of particles with a solidification front focused on the criterion of critical particle size for a given velocity (or conversely the critical velocity for a given size). The results shown here demonstrate how important the dynamics of interactions with the solidification front can be. In situations where the distribution of objects should be retained in the solidified microstructure, such as particle reinforcements in a metal matrix, the displacement of objects by the solidification front is a much more relevant parameter to understand and control the solidification microstructures.

We investigated the solute redistribution in the premelted film at the ice grains boundaries and around the droplets after their engulfment. Premelting describes the existence of liquid films at solid surfaces at temperatures below the bulk freezing temperature, a phenomenon common to many solids, including ice. Premelting has consequences in a wide range of biological, geophysical, and technological processes (16), such as the heaving of frozen ground (17), glacier motion, weathering, material transport through ice, using ice core as climate proxies (18), or the mobility of particles in solid materials. The driving force for melting is a reduction of interfacial energy between the particles and the matrix. The concentration of impurities or solute that depress the freezing point of the melt can greatly enhance the interfacial premelting, which is then called solute premelting. Because of their small dimensions, imaging premelted films remains a challenge.

Confocal microscopy allows in situ, 3D imaging of the solute redistribution in the solidifying sample with a semi-quantitative estimation of its concentration. In the solidified part of the sample, the solute is concentrated in the premelted film at the ice crystals grain bound-aries (Fig. 3A, region 1) and around the engulfed oil droplets (Fig. 3A, region 2). Because the concentrated solute broadens the thickness of these wetting films, they can be observed.

At low solidification front velocities (1 and 2 µm/s), the interface is planar. Increase of the interface velocity to 3 µm/s (Fig. 3B) results in the formation of grain boundaries-periodical liquid films of concentrated solute. Their number and spacing increase with the interface velocity (Fig. S1).

The lateral growth of ice crystals results in a progressive thinning of the premelted films at the grain boundaries, eventually leading their breakup into spherical droplets by an interfacial instability (Fig. 3B). These microstructures are analogue to those developed in metal alloys with a miscibility gap (19) (Fig. 3C,D). The transition from planar to a cellular front and the destabilization of the premelted film into droplets depend on both the temperature gradient and the solidification velocity (Fig. 3E), similar to that in metals (20).

We can capture the instability initiation and propagation dynamics (Fig. 3F). The growing geometrical instabilities results in the formation of side-branches that elongate and break up into droplets. This process is quite slow, on the time-scale of tens of seconds, which is a result of low interfacial tension between the solid and liquid phase. The development of these instabilities in 3D (Fig. S4) reveal that they initiate where the liquid film is the thinnest. This instability is analogous to the pinch off in dendrites observed in metal alloy solidification, which is driven by capillarity (21).

The premelted film also plays a critical role in the droplet engulfment by the solidification front (Fig. 4A). At the approach of the droplet, the solidification front bends away from it due to the formation of a pocket of concentrated solute which colligatively depresses the freezing point of water. Analytical models predict a deflection of the solidification front towards an object if the thermal conductivity of the latter is lower than that of the melt (22). The thermal conductivity of oil (propyl benzoate) is smaller than that of water (0.141W m -1 K -1 versus 0.569W m -1 K -1 ) and thus the formation of a convex interface is expected, which is what we observe in the absence of solute (Fig. S5). However, in the presence of solute, we observe only concave interfaces. The solute is thus able to reverse the bending of the front, which indicates the dominant role of the solute in droplet/interface interaction.

The ice continues to grow over and around the droplet, until the latter is completely engulfed. 3D imaging (Fig. 4B) provides a complete picture of the evolution of the premelted film around the droplet. As the solidification proceeds, the solute-rich liquid pocket, initially located in front of the oil droplet, passes on the opposite side of it-away from growing ice. From the fluorescence intensity, we obtain semi-quantitative information of the evolution of the solute concentration in the premelted film around the oil droplet (Fig. 4C andS6). We then correlate the solute concentration with the thickness of the liquid film (Fig. 4D), to reveal the dynamics of the process, and see how the premelted film evolves towards its equilibrium thickness (5µm in this case).

For the droplet size investigated, comet tail-shaped segregation pattern were systematically observed (Fig. 4E). These observations may be a good analogue for industrial processes where pores and particles can have an impact on the segregation pattern (Fig. 4F-H). Understanding the solute redistribution mechanisms could thus be an important step towards a better control of the solidification microstructures.

Solute also plays a critical role in the cryopreservation of red blood cells, platelets, fibroblasts, and reproductive cells (23). Cryoprotection agents are now systematically used to improve the survival rate. The crystal growth pattern and concentration of these agents between the growing crystals, along with the cells, impact their survival rates (5,24,25). Quantitative studies of solute concentration during freezing (but without cells) were done in thin films, by photometric measurements (8). However, when the integration of concentration is done through the depth of the sample, the peak solute concentration may be underestimated if the solidifica-tion microstructure is not homogeneous. Our results show that, because of the complex solute concentration and redistribution pathways, 5D imaging is required to better estimate the solute location and concentration. Our approach may help better understand the physical damages inflicted to cells during the cryopreservation procedures and optimize such procedures.

The features observed during the freezing of emulsions seems thus to be good analogues of many solidification systems. The rapid 3D confocal microscopy imaging, along with the ability to map the solute concentration, provides new insights into the dynamics and mechanisms of solidification in the presence of foreign hard (particles) or soft (droplets, bubbles, cells) objects.

The approach proposed here is versatile and can be easily adapted to a variety of solidification systems, from geophysics to food engineering or biology. We expect confocal microscopy to be a welcome addition to the toolbox available for the studies of interfacial evolution problems in three dimensions (26).

Figure 1 :

 1 Figure 1: Cooling stage and typical confocal image of a freezing emulsion. A: Stage for the directional solidification experiment with a Hele-Shaw cell, placed under the objective of the confocal microscope. B: Typical confocal image of the emulsion during freezing (front velocity of 3µm/s, temperature gradient of 5 • C/mm). The ice is shown in black, the liquid in magenta, and the oil droplets in cyan.

Figure 2 :

 2 Figure 2: Droplets dynamics and solidification microstructures. A: Droplet velocity vs. relative time. Time zero is when the front edge of the droplet is at the solidification front position. B, C: Droplet velocity (B) and solute concentration (C) vs. distance to interface, for a solidification front velocity of 3µm/s. D, E: Interaction time (D) and droplet displacement (E) vs interface velocity. The error bars do not exceed the size of the circles. F: L c vs. L v . G: Droplet velocity vs. concentration gradient of solute, for a solidification front velocity of 3µm/s. H: Impact of the droplet dynamics and interaction time on the development of solidification microstructure (temperature gradient of 5 • C/mm). The dashed line indicates the same groups of droplets before and after passing the interface.

Figure 3 :

 3 Figure 3: Solute premelting around the droplet and at the grain boundaries. A: Typical 3D view of the emulsion during freezing. Premelting plays a role on the destabilization of thin liquid film (concentrated solute) between ice crystals (grain boundaries, region 1), and on the evolution of the droplet/ice interface (region 2). B: String-of-pearls microstructure (solidification front velocity of 3µm/s, temperature gradient of 15 • C/mm) similar to that developed in zinc-bismuth (27) (C) and aluminium-bismuth alloys (19) (D). E: Stability diagram of the solidification front as a function of the temperature gradient and interface velocity. F: Time-lapse sequence of the dynamics of a premelted film instability at a grain boundary during freezing. The white arrow indicates the location where a second instability is initiated. Credits: (C) Reprinted from Scripta Materialia, 54, Ratke, L. & Müller, A., On the destabilisation of fibrous growth in monotectic alloys, 1217-1220, Copyright (2006), with permission from Elsevier (D) Reprinted by permission from Springer Nature: Springer Nature, Metallurgical and Materials Transactions A (19), Copyright (1996).

Figure 4 :

 4 Figure 4: Dynamics of premelted films around the droplets and impact on the solidification microstructure. A: Typical interaction between a droplet and the solidification front, that shows the accumulation of solute and its redistribution around the droplet. Horizontal cross-section. Sample velocity: 2 µm/s, temperature gradient: 5 • C/mm. B: Same sequence, but in 3D, and in vertical cross-section. The meniscus formed by the solidification front can clearly be observed, as well as the premelted film dynamics. Most of the solute is eventually pushed below the crystal. C: Time-lapse evolution of solute concentration ahead and behind the droplet. D: Timelapse evolution of solute concentration vs. film thickness. E: Solute-rich, comet-tail shaped region behind the droplet and corresponding solute concentration profile across the droplet in the frozen region. A similar solute segregation has been found in metal alloys (F), such as gold segregated around a porosity (28). The plot shows the gold concentration across the picture. A solute-rich, comet-tail shaped region is observed behind the pore. Reprinted by permission from Springer Nature: Metall. Mater. Trans. A (28), copyright (2004) G: Agglomerates of droplets. The solute-rich regions around the droplets are the last regions to solidify. H: An analogous behaviour is observed in metals, where this segregation can lead to the formation of a secondary phase around objects, such as the θ(CuAl 2 ) phase in an Al-4.5wt.% Cu alloy formed around alumina fibers. Micrograph courtesy of Pavel Bystricky and Andreas Mortensen (29).

  1 * 1 LSFC, UMR 3080 CNRS/Saint-Gobain CREE, Cavaillon, France

2 SIMM, UMR 7615 CNRS-ESPCI-Université Pierre Et Marie Curie, ESPCI, Paris, France * To whom correspondence should be addressed; E-mail: sylvain.deville@saint-gobain.com.

Supplementary materials

References [START_REF] Marcellini | [END_REF][START_REF] Schindelin | ) for his advices on microfluidics, and Félix Ginot for his assistance for some of the Python coding[END_REF]


Sylvain Deville

The lure of ice-templating: recent trends and opportunities for porous materials

Keywords: Ice-templating, Porous materials, Ceramics, Metals and alloys, Solidification

Ice-templating is a simple materials processing and shaping route where the growth of crystals from a solvent template the porosity. Although the principles of ice-templating were established a long time ago, it has lured considerable attention over the past 5 years to become a well-established processing route for porous materials of all kinds. In this viewpoint, I summarize the current status and recent trends of ice-templating studies. I then review the unique assets of ice-templating and propose a roadmap of the most promising or pressing questions and opportunities in this domain.

Introduction

When a suspension of particles is frozen, the growing crystals can repel and concentrate the particles in the space between them, until the ice eventually invades the inter-particle space [START_REF] Deville | Freezing Colloids: Observations, Principles, Control, and Use, Engineering Materials and Processes[END_REF]. After elimination of the crystals, the arrangement of concentrated particles in the frozen structure is maintained and the pores are a replica of the crystals. The process is simple and versatile. It has thus been tested with all kinds of materials, from ceramics to metals, polymers, carbon materials such as graphene or nanotubes, and composites thereof. During the past 10 years, the status of ice-templating has evolved from a new processing route to an established one. With hundreds of ice-templating papers published, we have now a good overview of what can be achieved and what makes ice-templating unique, in terms of both process and materials. This viewpoint thus aims to discuss the future of ice-templating and highlight areas that probably deserve a particular attention in the future.

Where do we stand today ?

The interest for ice-templating was mostly driven by the anisotropy of the structure and sometimes of the properties. The templating mechanisms by the ice crystals are versatile and have been demonstrated for all Email address: sylvain.deville@saint-gobain.com (Sylvain Deville) classes of materials. Matter, whereas found as particles, monomers, or more generally objects, is segregated by the growing crystals. All ice-templated materials have thus a similar morphology, independently of their nature.

The objective of ice-templating has been, for a long time, to develop macroporosity. The templated pores are usually macropores, with a variety of morphologies. With the usual cooling rates reported (1 to 10K/min), macropores in the 5-50 µm range are obtained [START_REF] Deville | [END_REF]. However, many studies also demonstrated how to obtain micro-and mesoporosity [START_REF] Tamon | APCChE 2015 Congr. Inc. Chemeca 2015[END_REF]. Such techniques can be combined with the usual ice-templating routes to obtain hierarchical porosity.

The porosity content can be anywhere in the 0-100% range. On the extreme porosity content side, the main limitation is the strength of the material that we can handle. Such extreme porosity can be a limitation for ceramics or metals, but has been an advantage for polymeric and carbon-based materials, with the possibility discussed below of ice-templating aerogels with extreme porosity content. In the low porosity content range, the development of macroporosity is limited by the ability of ice crystals to repel particles in a highlyconcentrated suspension. Above a solid loading threshold around 55-60 vol.%, the crystals grow directly in the interparticle space, no macroporosity is thus obtained.

Many solvents have been successfully tested, including camphene, tertbutyl-alcohol (TBA), dioxane, naphtalene, cyclohexane, DMSO, and a few others [START_REF] Deville | Freezing Colloids: Observations, Principles, Control, and Use, Engineering Materials and Processes[END_REF] of these solvents were used for polymers which are not water-soluble. However, the pore morphology that is sought can also dictate the choice of solvents.

We also have now a good idea of the typical processing time and size limitations. Because of practical and physical constraints, we do not have a total freedom in terms of sample dimensions/pore size combinations. With freeze-front velocity typically in the 5-50 µm/s range, the time required to freeze samples can vary from a few seconds (for thin films or microparticles) to several hours (for thick samples). Besides patience, the control of the freeze front velocity and morphology over long periods (hours) can be problematic. In most situations, one can freeze 1 cm samples within 10 min or so.

Ice-templating is mostly material-agnostic, all kinds of materials have thus been ice-templated. If the structural properties were the main interest for a long time, the last few years have seen many studies focused on other physical and functional properties (Tab. 1). The range of properties and applications that can be targeted is extremely broad.

Finally, we have a fair understanding of the fundamentals of the process, albeit mostly experimental. For the most part, we know which levers of the process should be pulled to control the microstructure, architecture, and properties of ice-templated materials. However, it is still difficult to predict the precise behavior of the system. A first screening of parameters (solid loading, cooling rate, etc. . . ) must first be done on any new system (material, powder).

Recent trends

Three important trends have been observed in the last 2-3 years: (1) ice-templated metals (2) ice-templated aerogels, and (3) the use of ice-templating to trigger self-organization or self-assembly.

Ice-templated metals

Metals are the latest class of ice-templated materials. This delay is related to a few specificities of metals that make ice-templating more difficult:

• the density of metals is usually higher than that of polymers and ceramics. Settling issues must be fixed [START_REF] Driscoll | [END_REF]5].

• the availability of metal powders of suitable particle size (submicronic), which at the same time exhibit a limited reactivity for the solvent. The almost instantaneous oxidation of metals particles in water makes their use difficile. Metal oxide powders, reduced before or during sintering, can thus be used [6]. Alternatively, metal precursors (such as T iH 2 ) have also been ice-templated [7], followed by a reducing treatment.

Many metals have now been ice-templated (Fig. 1): Cu [8], Au [9], Fe [10], Mo [11], Ni [12], Sn [13], Ag [14], Ti [15], W [16], or stainless steel [START_REF] Driscoll | [END_REF]. Icetemplating is a great alternative to the current routes available to obtain porous metals with unidirectional porosity. Now that the initial issues associated to metals have been fixed, we can expect more systematic assessments of their properties and potential.

Ice-templated aerogels

Because the solubility limit of pretty much anything in ice is extremely low, it is possible to start with an extremely dilute suspension and yet concentrate matter between the growing crystals. This unique feature has been used to ice-template a variety of aerogels (Fig. 2): carbon nanotubes [17][18][19], nanocellulose [20], carbon nanotubes/chitosan composites [21], or manganese dioxide [22]. Most of the work was nevertheless concentrated on graphene [23] and graphene oxide [24,25], when it was realized that ice-templating could be used to obtain bulk macroporous graphene with dimensions (centimeter) of practical sizes. Aerogels with a density as low as 0.14mg/cm 3 [26] and 0.16 mg/cm 3 [27] have been reported, and there is no reason why we could not obtain even lower densities, starting with even more dilute suspensions. These aerogels have been considered for a variety of applications such as sensors, catalysis, anodes for fuel cells, battery, or super capacitors, adsorption, or even tissue engineering. Centimeter-size, macroporous graphene with a wall thickness as small as 1.2 nm can be obtained [25].

Self-organisation, self-assembly

The systems investigated for a long time were powders with a broad particle size distribution and no specific morphology. It was recently realized that the concentration increase induced by the freeze-front could be used to trigger self-organization or self-assembly. The objective is thus not to just template the porosity, but also to create particular organizations of objects (particles, micelles) in the frozen structure. Two main ideas have been developed. The first idea is to use anisotropic objects such as platelets or fibers. If properly controlled, the freeze front can align and orient the anisotropic objects, and thus induce preferential orientations in the structure [28,29], but also create lightweight, percolating fiber scaffolds (SiC, cellulose) [30,31]. The alignment of objects can be used later to obtain specific morphologies and thus microstructures [32] and improves structural or functional properties. The increase of concentration between the ice crystals can also trigger selfassembly. This was reported with block copolymers, which can self-assemble into a variety of organized structures such as micelles when their concentration increases [33,34]. These micelles can then self-assemble into more complex configurations. Such self-assembly has been heavily investigated with evaporation, and its application to freezing is recent. This self-assembly has been used to elaborate porous materials with a hierarchical porosity: an organized mesoporosity templated by the micelles, combined with the usual ice-templated macroporosity.

The lure of ice-templating: what makes it unique ?

The first and maybe most unique feature of icetemplating is the extreme increase of concentration induced by the growth of ice crystals, which means that extremely high porosity content can be achieved [25]. If this is not of particular interest for metals and ceramicsmaterials cannot be handled anymore when the porosity is too high-, this is extremely valuable for the development of aerogels, as discussed above. The second asset of ice-templating is that self-organization takes place locally, but everywhere in the bulk at the same time. Compared to most of the self-assembly processes, the average processing time for large samples (centimeters) is much lower. In addition, because the growing crystals segregate the particles locally, large defects are unlikely to develop. A direct consequence is the possibility to obtain a high reliability of the materials processed [35]. The combination of directionality and range of macropore size put ice-templated materials in a sweet spot with little competition. If many alternatives exist to make directional macropores, few of them present the same versatility and even fewer can hit a similar pore size. Another possibly unique asset is the availability of many processing parameters to adjust the architecture and microstructure of ice-templated materials and thus decouple, to some extent, structure and properties. However, because many processing parameters are interdependent, such adjustments require careful, exhaustive studies and a good understanding and control of the process [36,37]. Ice-templating is also mostly material-agnostic. The main requirement is a low solubility limit of the material to be templated in the crystal, to make sure that the growing crystals expel the material and that the particles are small enough to be repelled and concentrated. The entry barrier is low, little equipment is required. In the most extreme case of a solvent that sublimates at room temperature and atmospheric pressure-such as camphene-and a material that does not need a high temperature treatment-a polymer-no equipment at all is required. It is possible to quickly master the basics of ice-templating and rapidly get started. Finally, we can combine ice-templating with other materials processing and shaping routes such as foaming [38], tape-casting [39], spraying [40], extrusion [41], co-extrusion [42], or additive manufacturing [43,44]. These combinations make it possible to achieve more complex, hierarchical architectures.

Roadmap

Ice-templating has reached today the status of a wellestablished processing route. However, despite such a status and countless papers, only two commercial products can be found (a cellulose foam and an alumina foam, both for thermal insulation). I outline below three main domains that seems promising, either in terms of new science to unveil, or opportunities in terms of development and applications (Fig. 3).

I want to emphasize first that progress in the domains discussed below will be facilitated by a multidisciplinary approach. The core of ice-templating is the interaction of objects with a freeze front, an ubiquitous phenomenon in nature and technology [START_REF] Deville | Freezing Colloids: Observations, Principles, Control, and Use, Engineering Materials and Processes[END_REF]. Linking icetemplating to these other occurrences will be a major facilitator both for the fundamental understanding and for the technological developments.

Fundamental understanding of the process

Our understanding of the phenomena today is still only partial and very experimental [36]. Ice-templating is based on three core features: crystal growth, particle redistribution, and the interactions between the solidification fronts and objects. We now need to focus in particular on what occurs at the particle scale, and in particular the role of multiple particle interactions. Studies and analytical models developed so far focused almost exclusively on single-particle models [45,46]. We have now many evidences that we should question the validity of these models in concentrated systems. Understanding and predicting the behavior of ice crystals and of the particles in concentrated systems is essential to understand and control the crystals morphology, which eventually dictates the pores morphology and thus some of the properties. It is also important to understand the organization of particles between the crystals. The particle organization controls the homogeneity of the microstructure (will defects develop? can we sinter these materials properly?) but can also be used to develop particular textures [32] or architectures [30]. We badly need experimental observation of crystal growth and particle redistribution at particle scale. Although a lot of attention was dedicated to X-rays imaging [47], we

are not yet able to use it to image such conditions (submicronic particle size, freeze front velocity in the 1-30 µm/s range). The recent developments of confocal microscopy [48], however, are an encouraging move in this direction. Optical microscopy, in particular in Hele-Shaw cells [49], have also certainly an important role to play in a near future. Such experimental observations should help us for further developments of modelling approaches of freezing colloids. Phase field, in particular, has been developed for many years in materials science (essentially in metallurgy) [50]. A working phase field model of freezing suspensions would certainly be a major advance.

Understanding ice-templated materials and their

properties We are still far from a complete understanding of the process/structure relations. Most of the attention so far focused on the strength/porosity relations and even in this case, the studies are complex. One of the difficulty is that we need multiple descriptors of the structure to completely characterize the materials: we need to measure the pore size and morphology (which are anisotropic), but also the morphology of the walls comprising the pores (thickness, roughness, density, etc.). We should do this characterization in 3D as the structure of ice-templated materials may evolve along the freezing direction. We then need a set of well-defined architectures to assess the relative influence of each of these structural parameters. This is particularly tricky, as many of these parameters are interdependent. A good understanding of the process and its control-as well as a lot of patience-are required to perform such investigations. Knowing such relations, we should get a better idea of the assets and limitations of ice-templated materials. We can then compare them to alternatives processes and materials, a critical step before any industrial application can be developed.

On a side note, the development of multifunctional ice-templated materials also raises questions regarding the coupling of properties in such materials and eventual size effects. Because we have many levers to control the size and characteristics of the porosity and architecture, ice-templating could also be an interesting playground to investigate such couplings.

Process and process technology: towards commercial applications?

The third key of the future of ice-templating is the development of process technologies. Just two commercial ice-templated materials can be found today. If we want ice-templating to become a successful industrial process like tape-casting or extrusion, more technological work on the process will be required. Fortunately, we do not have to start from scratch. We can take inspiration and knowledge first from the other materials processing routes, but also from low temperature processes and technologies in other domains. Costeffective techniques are already available at large scale in food industry [51] in particular and the process technology is already available to ensure a constant quality of the materials (ice cream) produced. There is thus no fundamental reason we could not ice-template in a cost-effective manner at large scale. The combination with other shaping and processing routes such as foaming or tape-casting not only provides additional levers to control the structure and properties of icetemplated materials, but may also facilitate the adoption of ice-templating. A key aspect not investigated so far is the reproducibility of the process (with the exception of [52]). Ensuring predictable and reproducible structures and properties is essential for any industrial development. Recent approaches (such as the wedge technique [53], which ensures a predictable control of the orientation of the ice-crystals perpendicular to the temperature gradient) should help, but more efforts are needed here. One aspect was nearly not addressed so far: near-net shape forming. Almost all studies reported simple shapes (brick, cylinder). Such shapes could be suitable for a number of applications (such as catalysts supports or filtration), but for many applications, more complex shapes will be required. A few steps have been made in this direction, for example with the development of technologies to make ice-templated tubes with a radial morphology [54,55] for oxygen transport membranes applications. More work towards applications is thus needed. As with any new process, we need to focus on niche applications first, where the added value is clear and the cost less of an issue. A close collaboration with the industry will be essential for this transition.

Conclusions

The segregation of particles by a freeze front is a ubiquitous phenomenon in nature and technology. Its use in ice-templating processes to obtain porous materials is one of the latest occurrences investigated and is thus relatively new. Despite being now a wellestablished topic, many opportunities await. We have now reached a stage where many proof of principle (in terms of materials, structure, properties) have been reported, but further work is required on the process and process technology developments. Despite being a simple process with a low entry barrier, ice-templating is an incredibly rich process. This is both an asset (as we have many levers to control the process, and thus the structure and properties of ice-templated materials) and an issue. Understanding the interdependent relations between all these parameters should still keep us busy for many years. A particular attention should be paid to the reproducibility and reliability of the associated processes and materials, which will determine if ice-templating may find practical applications or remain an academic topic of research, albeit a cool one.

Figure 1 :

 1 Figure 1: Ice-templated metals. (A) to (G): Ni, Ti, Cu, Mo, Ag, Fe, W. Credits: (A) Reprinted from Materials Letters, 153, M. Fukushima, Fabrication of highly porous nickel with oriented micrometer-sized cylindrical pores by gelation freezing method, p.99-101, c Copyright (2015), with permission from Elsevier (B) Reprinted from Materials Letters, 62(30), S. Yook, Porous titanium (Ti) scaffolds by freezing TiH 2 /camphene slurries, p.4506-4508, c Copyright (2008), with permission from Elsevier (C) Reprinted from Transactions of Nonferrous Metals Society of China, 22, S. Oh, Microstructure of porous Cu fabricated by freeze-drying process of CuO/camphene slurry, p.688-691, c Copyright (2012), The Nonferrous Metals Society of China. Published by Elsevier Ltd. All rights reserved. (D) Reprinted from Materials Letters, 139, S. Oh, Freeze drying for porous Mo with different sublimable vehicle compositions in the camphor-naphthalene system, p.268-270, c Copyright (2015), with permission from Elsevier (E) Reprinted from H. Gao et al., Macroscopic free-standing hierarchical 3D architectures assembled from silver nanowires by ice templating, Angewandte Chemie c Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (F) Metallurgical and Materials Transactions A, Processing, Microstructure, and Oxidation Behavior of Iron Foams, 47(9), 2016, p.4760-4766, H. Park et al., c Copyright 2016, The Minerals, Metals & Materials Society and ASM International, With permission of Springer (G) Reprinted from A. Röthlisberger et al., Synthesis, structure and mechanical properties of ice-templated tungsten foams, Journal of Materials Research, 31, 6, p.753-764, reproduced with permission.

Figure 2 :

 2 Figure 2: Ice-templated aerogels: (A) Carbon nanotubes (B) Nanocellulose/carbon nanotubes (C) Manganese dioxide (D) Graphene (E) Biomassderived carbonaceous nanofibres. Credits: (A) Reprinted from Polymer, 50(13), S. Kwon, Multiwalled carbon nanotube cryogels with aligned and non-aligned porous structures, p.2786-2792, c Copyright (2009), with permission from Elsevier. (B) Reprinted with permission from M. M. Hamedi, et al., Highly Conducting, Strong Nanocomposites Based on Nanocellulose-Assisted Aqueous Dispersions of Single-Wall Carbon Nanotubes, ACS Nano, 8, 2467-2476 (2014). c Copyright (2014) American Chemical Society. (C) Reproduced (in part) from [22] with permission of The Royal Society of Chemistry (D) Reprinted from J. Sun et al., Controllable Fabrication of Transparent Macroporous Graphene Thin Films and Versatile Applications as a Conducting Platform. Adv. Funct. Mater. 25, 4334-4343 (2015). c Copyright (2015) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (E) Reprinted from Y. Si et al., Ultralight Biomass-Derived Carbonaceous Nanofibrous Aerogels with Superelasticity and High Pressure-Sensitivity. Adv. Mater., 28(43), p.9512-9518 (2016). c Copyright (2016) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 3 :

 3 Figure 3: Overview of the proposed roadmap for the future of icetemplating.

Table 1 :

 1 . Many Main properties and applications targeted for ice-templated materials.

	Properties	Materials considered	Applications
	Porosity	Ceramics, polymers	Tissue engineering, filtration, membranes, thermal insu-
			lation, acoustic adsorption
	Mechanical strength	Ceramics, metals	Filtration, tissue engineering
	Toughness	Ceramics, metal/ceramic composites	Armour, impact resistance, high performance structural
			materials
	Specific surface area	Carbon, silica, alumina, titania, graphene, chitosan,	Catalysis, adsorption
		nanocellulose
	Adsorption	Carbon, cellulose, chitosan, graphene, graphene oxide,	Adsorption of gas and heavy ions
		copper, clay, silica, collagen
	Permeability	Alumina, silicon carbide, silicon nitride, collagen	Filtration, catalysis
	Conductivity	Graphene, carbon nanotubes	Batteries, supercapacitors, sensors
	Dielectric properties	Silicon nitride, SiAlON, lead zirconate, rare-earth silicate Microelectronics
	Piezoelectric properties Barium titanate, PZT, PZN
	Capacitance	Graphene, graphene oxide, carbon nanotubes, barium ti-	Batteries, supercapacitors
		tanate
	Permittivity	PZT, barium titanate	Dielectrics, microelectronics
	Thermal conductivity	Silica, rare-earth silicate, alumina, zirconia, silicon ni-	Thermal insulation, flame-retardancy, thermal energy
		tride, mullite, clay, carbon nanotubes	storage, thermoresponsive materials

Acknowledgements

The research leading to this paper has received funding from the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013) Grant Agreement 278004, FreeCo.