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Aligned Porous Structures by Directional Freezing
Abstract
Materials with aligned porous structures have broad potential in applications such as organic electronics, microfluidics, and tissue engineering. Materials of this type can be fabricated using techniques such as microfabrication, soft lithography, and photolithography. Directional freezing is a cheap, simple, and novel route to prepare aligned porous materials in the form of 2D surface patterns or 3D monolithic structures. A solvent - typically water but also organic solvents or carbon dioxide - is frozen unidirectionally and the pore structure is templated from the aligned solvent crystals that are formed. These methods can produce complex composite materials with a range of aligned pore architectures.
... In connection with this, several approaches for the generation of the required macroporous morphology in these scaffolds have been developed [10][11][12][13]. Among these approaches are the so-called cryostructuring techniques [14][15][16][17][18][19][20][21][22][23] that facilitate diverse possibilities for the preparation of wide-pore scaffolds that are well-suited to serve as sponge-like carriers for cell culturing [24][25][26][27][28][29][30][31][32][33][34][35]. ...
... With that, the junction knots of the spatial network in various cryogels can be covalent cross-links, low-dissociating ionic/coordination bonds, non-covalent (physical) cross-links such as H-bonds and hydrophobic interactions, or a combinations of the knots of different nature [14,18,19,23,27,[36][37][38]. The second type of cryogenically produced materials are the cryostructurates-also macroporous polymeric matrices formed as a result of cryogenic processing (the freezing of the initial precursor solutions (variant 2a) or gels (variant 2b) followed by the removal of the crystallized solvent via sublimation or cryoextraction) when no gelation occurs in a frozen system [16,17,21,23]. Very often, certain post-tanning (crosslinking) of the thus formed matter is required in order to make it insoluble [22,23]. ...
... A characteristic morphological feature inherent in all types of cryogenically structured polymer matrices is a system of interconnected large pores generated by the polycrystals of the frozen solvent that act as porogens [14][15][16][17][18][19][20][21]. The size and geometry of such pores depend on many factors, mainly the solvent and solutes' nature, the concentration of the precursors, and the temperature/time regimes of the cryogenic processing [14,17,19,21,23,36,43]. ...
Various gelatin-containing gel materials are used as scaffolds for animal and human cell culturing within the fields of cell technologies and tissue engineering. Cryostructuring is a promising technique for the preparation of efficient macroporous scaffolds in biomedical applications. In the current study, two new gelatin-based cryostructurates were synthesized, their physicochemical properties and microstructure were evaluated, and their ability to serve as biocompatible scaffolds for mammalian cells culturing was tested. The preparation procedure included the dissolution of Type A gelatin in water, the addition of urea to inhibit self-gelation, the freezing of such a solution, ice sublimation in vacuo, and urea extraction with ethanol from the freeze-dried matter followed by its cross-linking in an ethanol medium with either carbodiimide or glyoxal. It was shown that in the former case, a denser cross-linked polymer phase was formed, while in the latter case, the macropores in the resultant biopolymer material were wider. The subsequent biotesting of these scaffolds demonstrated their biocompatibility for human mesenchymal stromal cells and HepG2 cells during subcutaneous implantation in rats. Albumin secretion and urea synthesis by HepG2 cells confirmed the possibility of using gelatin cryostructurates for liver tissue engineering.
... The porous structure with controllable size and regular distribution can be obtained by directional freezing technology by controlling the time and location of ice crystal growth (Figure 3a) [63]. Zhang et al. obtained a wellarranged porous PVA hydrogel combination (Figure 3b) by limiting the growth direction of ice crystals by directional freezing and demonstrated the relationship between the freezing rate of hydrogel and channel spacing [64]. It can be seen in Figure 3c, that the pore size of hydrogel decreases with the increase in freezing rate. ...
... (c) Relationship between freezing rate and pore size. Reprinted with permission from Reference[64]. ...
... (c) Relationship between freezing rate and pore size. Reprinted with permission from Ref.[64]. ...
Hydrogel is a type of crosslinked three-dimensional polymer network structure gel. It can swell and hold a large amount of water but does not dissolve. It is an excellent membrane material for ion transportation. As transport channels, the chemical structure of hydrogel can be regulated by molecular design, and its three-dimensional structure can be controlled according to the degree of crosslinking. In this review, our prime focus has been on ion transport-related applications based on hydrogel materials. We have briefly elaborated the origin and source of hydrogel materials and summarized the crosslinking mechanisms involved in matrix network construction and the different spatial network structures. Hydrogel structure and the remarkable performance features such as microporosity, ion carrying capability, water holding capacity, and responsiveness to stimuli such as pH, light, temperature, electricity, and magnetic field are discussed. Moreover, emphasis has been made on the application of hydrogels in water purification, energy storage, sensing, and salinity gradient energy conversion. Finally, the prospects and challenges related to hydrogel fabrication and applications are summarized.
... The third strategy is construction of aligned porous structures by directional freezing [37][38][39][40][41][42][43][44]. A main feature of directional freezing is that the growth direction of solvent crystals is along a unidirectional temperature gradient during freezing [45,46]. Such unidirectional temperature gradient can be achieved by two ways: one is immersing a vessel containing the liquid sample into a cold bath at a controlled rate [37,38,41,42,44,45]; the other is frozen from the bottom of a container in contact with a cold bath [39,40,43,46,47]. ...
... A main feature of directional freezing is that the growth direction of solvent crystals is along a unidirectional temperature gradient during freezing [45,46]. Such unidirectional temperature gradient can be achieved by two ways: one is immersing a vessel containing the liquid sample into a cold bath at a controlled rate [37,38,41,42,44,45]; the other is frozen from the bottom of a container in contact with a cold bath [39,40,43,46,47]. Okaji et al reported a preparation of aligned porous structures based on urethane diacrylate in 1,4-dioxane using unidirectional freezing and UV-induced cryopolymerization [42]. ...
... It must be mentioned that the conventional cryopolymerization is demonstrated to be unable to obtain the aligned porous structure under the same recipes in comparison with directional freezing method [39,43,44]. It is widely accepted that randomly porous structures are the characteristic feature of conventional cryogels, and aligned porous cryogels, as a rule, should be constructed by directional freezing [7,[45][46][47][48][49]. To date, only a very few exceptions of cryogels having oriented porous structures could be observed by conventional freezing [50,51]. ...
To adapt to nature, wood forms an oriented tubular porous structure that is feasible for transporting water and nutrition. Inspired from this adaptability, we here demonstrate a universal approach to obtain rigid cryogels of aligned micro-sized tubular structures by a redox-induced cryopolymerization conducted in a freezer. In contrast to the classical directional freezing method, our method free of any special instrument leads to the aligned porous structures that are dependent on gravity due to the tubular porous structures parallel to gravity direction regardless of the sample placement direction. The versatility of our method is proven in the fabrications of a series of cryogels varying different hydrophilicity. Among the resultant samples, the three-function crosslinked cryogenic sample containing carboxyl group (polyMAA-TRIM, 40/60) was tested to have an unprecedented property integrated by high mechanical strength (9.0 MPa of compressive moduli), good hydrophilicity, and efficient purification of lysozyme (purity >90 %) from egg white solutions only driven by gravity.
... By controlling the direction of coagulation and the growth direction of ice crystals (a process called "directional freezing"), scaffolds with aligned porous or channeled structures can be obtained [26,39,41,43]. It has been widely reported that directional freezing could be achieved by choosing the appropriate cooling rate or by producing a temperature gradient between the two sides of freezing samples [39,41,44]. The principle is that the solvent coagulates from one side to the other and the ice crystals are formed in one direction. ...
... The solution or particles used are concentrated and excluded between the growing ice crystals. Finally, the aligned structures are obtained following the removal of the orientated ice by freeze-drying [44]. ...
Tissue engineering to develop alternatives for the maintenance, restoration, or enhancement of injured tissues and organs is gaining more and more attention. In tissue engineering, the scaffold used is one of the most critical elements. Its characteristics are expected to mimic the native extracellular matrix and its unique topographical structures. Recently, the topographies of scaffolds have received increasing attention, not least because different topographies, such as aligned and random, have different repair effects on various tissues. In this review, we have focused on various technologies (electrospinning, directional freeze-drying, magnetic freeze-casting, etching, and 3-D printing) to fabricate scaffolds with different topographic orientations, as well as discussed the physicochemical (mechanical properties, porosity, hydrophilicity, and degradation) and biological properties (morphology, distribution, adhesion, proliferation, and migration) of different topographies. Subsequently, we have compiled the effect of scaffold orientation on the regeneration of vessels, skin, neural tissue, bone, articular cartilage, ligaments, tendons, cardiac tissue, corneas, skeletal muscle, and smooth muscle. The compiled information in this review will facilitate the future development of optimal topographical scaffolds for the regeneration of certain tissues. In the majority of tissues, aligned scaffolds are more suitable than random scaffolds for tissue repair and regeneration. The underlying mechanism explaining the various effects of aligned and random orientation might be the differences in “contact guidance”, which stimulate certain biological responses in cells.
... The pores had a width of 20-40 μm and were alternately separated by much thinner walls; this structure can be considered as collagen fibers with wide gaps. Some researchers have thus used unidirectional solidification to fabricate fibers from suspensions of collagen molecules or fibrils [47][48][49]. Additionally, a modified technique has been developed to concentrate collagen axially and form a fiber-like construct [50]. ...
... As a result, those macromolecules are likely to be partially aligned because of the high aspect ratios. However, there is little evidence of the unidirectional alignment of collagen fibrils in the walls after freeze drying, whereas aligned thin walls or fiber-like structures were observed microscopically [45][46][47][48][49]. The authors believe that collagen in the micro structures is almost amorphous on a fibrillar scale, affecting cellular responses and morphologies. ...
Collagen has been used in various therapeutic medical devices, such as artificial dermis, bone, and cartilage, wherein the effectiveness of collagen mainly depends on its biological features of biocompatibility, biodegradability, bioresorbability, cell affinity, and weak antigenicity. Collagen is the main structural protein in the human body and is responsible for the mechanical properties of tissues and organs. The fundamental structural component of tendon tissue is uniaxially aligned collagen fibrils that run parallel to the geometrical axis. Thus, the fabrication of artificial tendons is an excellent example of developing biomaterials using collagen as a structural backbone. Previous attempts to construct aligned fibril-based biomaterials involved electrospinning, freeze drying, using a strong magnetic field, and mechanical methods, including shearing and tension during wet extrusion. Among these, mechanical methods have been extensively studied owing to their simplicity and effectiveness suitable for mass production. However, few review articles have focused on these mechanical methods. Thus, this article reviews the mechanical methods for creating biomaterials from aligned collagen fibril while discussing the other fabrication methods in brief.
... The most common hard templating technique is colloidal crystal templating, which gives rise to the negative replica of opal crystal structure (so-called inverse opal) [67,68]. As the diameter of pores Ice-templating strategy, also known as freeze casting, is useful for obtaining anisotropic porous morphology [69][70][71]. One-dimensionally (1D) aligned microchannels are produced with the aid of ice pillars grown upon the directional freezing of wet gels. The pore size is tunable typically in the range of tens of micrometers by varying the freezing rate, which eventually changes the size of ice crystals. ...
... Notably, since frozen solids are directly subjected to the freeze drying (sublimation by vacuum) resulting in cryogels, this technique is not necessarily combined with the sol-gel reactions to form a 3D network. On this account, porous carbon monoliths based on carbon nanotubes and graphenes (or graphene oxides) have been produced from their suspensions [70,71]. ...
To realize a prosperous and sustainable society in the future, continuous progress in electrical energy storage technology is essential. Carbon materials have played a pivotal role in the global economy since electrical power was harnessed in the 19th century. To improve electrode performance, it is vital to develop carbon electrodes with designed porous structures over multiple length scales. In this regard, a self-standing, binder-free carbon electrode has many advantages that are not available in conventional composite electrodes involving binders and other additives. This review highlights porous monolithic carbon electrodes derived from crosslinked organic gels in terms of their synthesis and the control of their pore structure by sol–gel techniques. Special focus is given to porous carbon monoliths with three-dimensionally interconnected macropores prepared by a phase separation strategy. Electrochemical studies on energy storage devices using the monolithic carbon electrodes are also overviewed.
... After removal of the template, aligned structures are obtained [166]. Figure 36. ...
... Schematic representation of the directional freeze-drying process. The particles are excluded from the ice crystals, which are formed from de the bottom to the top [166]. ...
La catalyse est l'un des piliers pour le développement de procédés durables, car elle permet d'utiliser moins de ressources en accélérant les réactions chimiques. Afin de fournir des catalyseurs plus performants, cette étude propose une nouvelle méthode de préparation de catalyseurs pour contrôler la distribution de nanoparticules (NPs) métalliques au sein des catalyseurs hiérarchiquement poreux (méso et macro) en combinant la synthèse de latex, la réduction sonochimique et le procédé sol-gel. La première étape est la synthèse d'une empreinte porogène de billes de polystyrène (latex) obtenues par polymérisation en émulsion aqueuse. La deuxième étape est la synthèse et le dépôt de NPs de métaux nobles sur la surface des billes de polymère par voie sonochimique dans l’eau. La troisième étape est la synthèse du support catalytique par un procédé sol-gel en milieu aqueux en utilisant le latex décoré et l’orthosilicate de tétraéthyle (TEOS) dans des conditions contrôlées pour moduler la porosité finale de la matrice de silice (mésoporeuse). Toutes les étapes de cette approche sont effectuées dans l'eau, ce qui limite les impacts environnementaux de la préparation du catalyseur. L'élimination du porogène (latex) par calcination génère les macropores. Le matériau résultant possède alors une morphologie inédite pour un catalyseur, avec des macropores fonctionnalisés par des NPs métalliques, dans une matrice de silice mésoporeuse. Ainsi, il a été possible de synthétiser un latex monodisperse de polystyrène (~130 nm), lequel a été décoré avec des NPs de Pt (~2.3 nm) par réduction sonochimique. Le matériau final de silice a présenté des mésopores (2-15 nm) reliant les macropores (110-400 nm) contenant les NPs de Pt. Il a été possible d'obtenir des surfaces spécifiques et des volumes poreux totaux de 615 m2/g et 0,74 cm3/g, respectivement. Dans un premier cas d'étude, des catalyseurs de Pt/SiO2 à porosité hiérarchique ont été évalués dans l'hydrogénation sélective du p-chloronitrobenzene (p-CNB) pour produire la p-chloroaniline. Ils ont présenté des activités catalytiques allant jusqu'à 91,7 ± 2,9 molCNB/(min molPt) et des sélectivités jusqu'à 100 ± 2% à 80% de conversion, par rapport à 47,7 ± 2,9 molCNB/(min molPt) et 91 ± 2%, respectivement, obtenus dans les mêmes conditions avec un catalyseur commercial. Dans un deuxième cas d'étude, des catalyseurs à base de Pd, Pd-Pt et Pd-CeO2 supportés sur de la silice à porosité hiérarchique ont été préparés et testés dans la synthèse directe du peroxyde d'hydrogène. La meilleure productivité a été obtenue avec le catalyseur bimétallique Pd-Pt avec 32500 molH2O2/(h molmétal) en batch, et la meilleure sélectivité a été obtenue avec le catalyseur Pd-CeO2/SiO2 (63 ± 2%) en semi-continu. En résumé, cette thèse propose une nouvelle méthode de préparation dans l’eau de matériaux fonctionnels à porosité hiérarchique en combinant la synthèse de latex, la réduction sonochimique et le procédé sol-gel. Il a été démontré que cette technique de préparation fournit une boîte à outils très puissante et polyvalente pour la préparation et l'optimisation des catalyseurs. Des perspectives pour améliorer davantage les morphologies et la distribution contrôlée des sites actifs sont également proposées.
... It is formed by controlling the directional freezing of water or organic solvent in a certain direction to form neatly arranged solution crystals, and then the template is dried under low pressure. Finally, porous ceramics are obtained by high-temperature sintering process [43,44]. Figure 13(b) is the SEM image of porous ceramics prepared by a freeze-drying method. ...
Porous silicon nitride ceramics is a promising functional ceramic material. In recent years, the research on the preparation of porous silicon nitride ceramics within different methods has been widely investigated. First, this work reviews the main synthesis methods of Si3N4 porous ceramics in detail, and compares the differences between strength and porosity caused by each method. The characteristics and advantages of different technologies under the current conditions were evaluated. Second, the dielectric properties, sound absorption properties and permeability properties of silicon nitride ceramics were compared and summarized based on the experimental results. Third, the applications fields of porous silicon nitride ceramics, such as smelting industry, catalyst carrier, sound absorption, wave-transparent, and biomedical fields were explored. Finally, the assessment of different silicon nitride ceramics preparation technologies was elaborated. This review gives an outlook on the porous silicon nitride ceramics, which shows great potential for further research in this field.
... There are numerous strategies for controlling the porosity and microarchitecture of TE hydrogels [33]. In particular, freeze casting can be utilized to regulate the microscopic molecular arrangement of such gels and control pore morphology by controlling the directional ice formation in the solute solution [34]. Ice crystals form and propagate through predetermined directions during freezing, and melted ice crystals form interconnected microchannels within the scaffold [35]. ...
The effectiveness of cell culture and tissue regeneration largely depends on the structural and physiochemical characteristics of tissue-engineering scaffolds. Hydrogels are frequently employed in tissue engineering because of their high-water content and strong biocompatibility, making them the ideal scaffold materials for simulating tissue structures and properties. However, hydrogels created using traditional methods have low mechanical strength and a non-porous structure, which severely restrict their application. Herein, we successfully developed silk fibroin glycidyl methacrylate (SF-GMA) hydrogels with oriented porous structures and substantial toughness through directional freezing (DF) and in situ photo-crosslinking (DF-SF-GMA). The oriented porous structures in the DF-SF-GMA hydrogels were induced by directional ice templates and maintained after photo-crosslinking. The mechanical properties, particularly the toughness, of these scaffolds were enhanced compared to the traditional bulk hydrogels. Interestingly, the DF-SF-GMA hydrogels exhibit fast stress relaxation and variable viscoelasticity. The remarkable biocompatibility of the DF-SF-GMA hydrogels was further demonstrated in cell culture. Accordingly, this work reports a method to prepare tough SF hydrogels with aligned porous structures, which can be extensively applied to cell culture and tissue engineering.
... Ice-templating, also known as freeze-casting, is a facile and diverse technology that has been widely used to create hierarchically well-aligned porous materials. [115][116][117] Encouragingly, icetemplating-driven self-assembly delivers great potential to fabricate superstructural carbon materials. As the rst demonstration, Yamauchi and coworkers synthesized nanoparticleconstituted 2D layered MOF superstructures through an icetemplating self-assembly approach without any surfactants or external elds (Fig. 6j). ...
Three-dimensional carbon superstructures with ingenious topographies and favorable functionalities present attractive prospects in energy fields. Compared to the simple low-dimensional segments (e.g., nanosheets, nanoparticles), carbon superstructures deliver excellent skeleton robustness, more uncovered electroactive motifs, and superior reaction kinetics, which are particularly useful for electrochemical energy storage. Therefore, there is valuable work ongoing to make the ordered arrangement of single-level building blocks into “one-piece” superstructure networks with tunable geometries and functional compositions. In this review, we first discuss the general strategies and underlying mechanisms for the fabrication of versatile carbon superstructures, such as flowers, urchins, and nanoarrays. The current design strategies are summarized and categorized into (i) the hydrothermal approach, (ii) the templating method, (iii) nanoemulsion assembly, (iv) spatially confined assembly, (v) modular self-assembly, and (vi) direct ink writing. Furthermore, we highlight the implementation performances of carbon superstructures as electrode materials for energy-storage devices, giving insights into the structure-property relationship in the family of nanomaterials. Ultimately, the challenges and outlooks of carbon superstructures in terms of design and uses are outlined to guide the future development of energy-related communities.
... The mechanical stability of PVA hydrogel materials has also been improved by freezing the container starting from the bottom up during multiple cycles of freeze-thawing, which promotes the production of ordered microcrystals [31]. All these methods allow for the preparation of a complex series of composites with a neatly arranged microporous structure [32,33]. This directional freeze-thawing results in anisotropic porous PVA hydrogels with high tensile strengths (0.3-1.2 MPa) and moderate compressive moduli (0.03-0.10 MPa). ...
As a biocompatible, degradable polymer material, polyvinyl alcohol (PVA) can have a wide range of applications in the biomedical field. PVA aqueous solutions at room temperature can be cast into very thin films with poor mechanical strength via water evaporation. Here, we describe a novel dehydration method, unidirectional nanopore dehydration (UND). The UND method was used to directly dehydrate a PVA aqueous solution to form a water-stable, anisotropic, and mechanically robust PVA hydrogel membrane (PVAHM), whose tensile strength, elongation at break, and swelling ratio reached values of up to ~2.95 MPa, ~350%, and ~350%, respectively. The film itself exhibited an oriented arrangement of porous network structures with an average pore size of ~1.0 μm. At 70 °C, the PVAHMs formed were even more mechanically robust, with a tensile strength and elongation at break of 10.5 MPa and 891%, almost 3.5 times and 2 times greater than the PVAHM prepared at 25 °C, respectively. The processing temperature affects the velocity at which the water molecules flow unidirectionally through the nanopores, and could, thus, alter the overall transformation of the PVA chains into a physically crosslinked 3D network. Therefore, the temperature setting during UND can control the mechanical properties of the hydrogel membrane to meet the requirements of various biomaterial applications. These results show that the UND can induce the ordered rearrangement of PVA molecular chains, forming a PVAHM with superior mechanical properties and exhibiting a greater number of stronger hydrogen bonds. Therefore, the novel dehydration mode not only induces the formation of a mechanically robust and anisotropic PVA hydrogel membrane with a porous network structure and an average pore size of ~1.0 μm, but also greatly enhances the mechanical properties by increasing the temperature. It may be applied for the processing of water-soluble polymers, including proteins, as novel functional materials.
... 79 Briey, freeze-drying involves the controlled solidication of a solution, suspension, sol or gel, followed by the sublimation of the solvent under reduced pressure. 80,81 Moreover, one outstanding advantage of freeze-drying is that the micro/ macro-structures of the prepared freeze-drying scaffolds can be easily tailored by adjusting the process parameters. 82 Therefore, biomaterials manufactured by freeze-drying have been developed into the most common forms for rapid hemostasis and wound healing benetting from their porous structure and absorption capacity. ...
The challenge for the treatment of severe traumas poses an urgent clinical need for the development of biomaterials to achieve rapid hemostasis and wound healing. In the past few decades, active inorganic components and their derived composites have become potential clinical products owing to their excellent performances in the process of hemorrhage control and tissue repair. In this review, we provide a current overview of the development of inorganic-based biomaterials used for hemostasis and wound healing. We highlight the methods and strategies for the design of inorganic-based biomaterials, including 3D printing, freeze-drying, electrospinning and vacuum filtration. Importantly, inorganic-based biomaterials for rapid hemostasis and wound healing are presented, and we divide them into several categories according to different chemistry and forms and further discuss their properties, therapeutic mechanisms and applications. Finally, the conclusions and future prospects are suggested for the development of novel inorganic-based biomaterials in the field of rapid hemostasis and wound healing.
... Directional freezing, also termed freeze casting (FC), has been well developed to fabricate hierarchical porous materials [97][98][99]. By confining the heat reduction along the axial direction, e.g., from bottom to top, ice crystals nucleate on the cooled bottom surface and propagate along the temperature gradient, offering an aligned ice-template to concentrate and squeeze the building blocks into the gaps between the crystal boundaries, yielding highly ordered constructs. ...
In the past decade, a thriving family of 2D nanomaterials, transition-metal carbides/nitrides (MXenes), have garnered tremendous interest due to its intriguing physical/chemical properties, structural features, and versatile functionality. Integrating these 2D nanosheets into 3D monoliths offers an exciting and powerful platform for translating their fundamental advantages into practical applications. Introducing internal pores, such as isotropic pores and aligned channels, within the monoliths can not only address the restacking of MXenes, but also afford a series of novel and, in some cases, unique structural merits to advance the utility of the MXene-based materials. Here, a brief overview of the development of MXene-based porous monoliths, in terms of the types of microstructures, is provided, focusing on the pore design and how the porous microstructure affects the application performance.
... In the demonstration, an intuitive approach using polydimethylsiloxane (PDMS) moulding and forming arrayed ice templates with dimensions of 300 μm  300 μm x 300 μm was used for the vapour sublimation and deposition process to fabricate the proposed structural MVP coatings. Theoretically, other methods of creating iced templates can also be prepared by softlithography [58], sculpturing [59], directional freezing and crystallinity [60], and direct assembly and modulation [61]. ...
A multicomponent vapour-deposited porous (MVP) coating with combined physical and biochemical properties was fabricated based on a chemical vapour sublimation and deposition process. Multiple components are used based on their natural thermodynamic properties, being volatile and/or nonvolatile, resulting in the sublimation of water vapour (from an iced template), and a simultaneous deposition process of poly-p-xylylene occurs upon radical polymerization into a disordered structure, forming porous coatings of MVP on various substrates. In terms of physical properties, the coating technology exhibits adjustable hydrophobicity by tuning the surface morphology by timed control of the sublimation of the iced template layer from a substrate. However, by using a nonvolatile solution during fabrication, an impregnation process of the deposited poly-p-xylylene on such a solution with tuning contact angles produces an MVP coating with a customizable elastic modulus based on deformation-elasticity theory. Moreover, patterning physical structures with adjustable pore size and/or porosity of the coatings, as well as modulation and compartmentalization to introduce necessary boundaries of microstructures within one MVP coating layer, can be achieved during the proposed fabrication process. Finally, with a combination of defined solutions comprised of both volatile and nonvolatile multicomponents, including functional biomolecules, growth factor proteins, and living cells, the fabrication of the resultant MVP coating serves devised purposes exhibiting a variety of biological functions demonstrated with versatility for cell proliferation, osteogenesis, adipogenesis, odontogenesis, spheroid growth of stem cells, and a complex coculture system towards angiogenesis. Multicomponent porous coating technology is produced based on vapour sublimation and deposition upon radical polymerization that overturns conventional vapour-deposited coatings, resulting in only dense thin films, and in addition, the versatility of adjusting coating physical and chemical properties by exploiting the volatility mechanism of iced solution templates and accommodation of solute substances during the fabrication process. The MVP coating and the proposed fabrication technique represent a simple approach to provide a prospective interface coating layer for materials science and are attractive for unlimited applications.
... Because freeze-drying and sublimation constitute a single step, it is a highly scalable method for the fabrication of centimeter-scale structures within 24 hours. By controlling the ice crystal nucleation and growth kinetics during cooling [22,23] and by controlling the cooling direction, the morphology and orientation of the ice crystal formation can be controlled, which essentially allows control of the direction of the pores in the fabricated 3D structure [24]. Our previous study of AgNW and cellulose nano bers demonstrated that one can design and control the mechanical properties of 3D structures by changing the pore direction [25]. ...
A 3D co-continuous polymer nanocomposite with high strength and high recoverability is demonstrated. This nanocomposite used hard-core-soft-matrix design which is suitable for obtaining the optimal strength. Polyvinyl alcohol (PVA) was freeze-dried together with silver nanowires (AgNW) to fabricate a 3D porous structure as hard-core phase, which was then filled with polydimethylsiloxane (PDMS) as soft-matrix phase via vacuum infiltration. The PVA + AgNW nanocomposite served as the hard core, with PDMS as the soft matrix, with this hard core-soft matrix design allowing for a combination of the excellent strength of the nanocomposite and the resilience of the PDMS. The addition of AgNWs strengthened the modulus of the freeze-dried structure over the 3 times and the comparison with the Halpin-Tsai model is indicated of AgNWs were well dispersed into the wall of the 3D structure. The vertical pore alignment of the freeze-dried structure resulted in an increased the strength. In addition, incorporation of hard core-soft matrix significantly increased the strength of the 3D nanocomposite up to 3.5 times that of the PDMS as a result of the co-continuous incorporation of hard and soft phases with well distributed 3D interfaces that also hindered crack propagation. Therefore, the PVA + AgNW 3D porous structure contributed by strengthening and toughening of the entire composite, resulting in increasing energy loss coefficients of nanocomposites, which showed good shock absorbance.
... Herein, the chitosan aerogel with ordered porous structure and wood-like structure was prepared by the method of directional freeze casting [40]. On the top of the aerogel, a MXene layer wrapped with polyethyleneimine (Ti 3 C 2 T x -PEI) was used as a solar absorbent to drive water evaporation under solar radiation. ...
Solar energy, as an endless clean energy, has great prospects for the use of water vapor from sewage purification and seawater desalination. Local heating of surface water has shown to maximize the energy efficiency of steam generation. Therefore, at first, the material is required to have a good ability of photo-thermal conversion. Among the photo-thermal converting materials, MXenes are arising candidates for effective photo-thermal conversion, which have shown excellent photo-thermal conversion efficiency and have been attempted for photo-thermal distillation. Although a high photo-thermal conversion efficiency has been realized, however, it is more challenging to prepare photo-thermal evaporating materials with rapid water transport capacity and generating solar steam. In this work, a photo-thermal distillation device was reported by combining a chitosan aerogel with a bio-inspired wood-like oriented porous structure prepared by directional freezing method, and Ti3C2Tx two-dimensional nanosheets coated with polyethyleneimine sprayed on the surface. The ordered microporous structure can quickly transport moisture from the bottom up to the evaporation surface by capillary force. The coating of Ti3C2Tx coated with polyethyleneimine on the evaporation surface has a strong ability to capture sunlight. The evaporation rate reaches 3.99 kg m⁻² h⁻¹ under the optical power density of 3000 W m⁻² and the efficiency reaches 90%, indicating the broad application prospects as a photo-thermal conversion material for efficiently producing clean fresh water.
Graphical abstract
... Bidirectional freezing technology is an environmentally friendly strategy that enables produce complex multi-scale architectures with long-range order porous structures controllably [20][21][22]. During freezing, ice crystals J Mater Sci directionally grow along the two temperature gradients [23]. The dispersed particles are expelled between the ice layers and the ice crystal gaps by adjusting the suspension viscosity. ...
Cellulose-based aerogels have become a great potential for oil absorption applications due to their wide range of sources, low density, and good adsorption properties. However, many reported cellulose-based aerogels to have the drawbacks of low oil recovery rate and poor mechanical, which limit their application. In this study, high reusable and excellent mechanical properties silylated TEMPO-oxidized nanofibers/polyvinyl alcohol aerogels (STPA) were fabricated through a three-step process of aqueous silylation, bidirectional freezing, and freeze-drying. A bidirectional freezing technique was employed to obtain two unique structural aerogels by adjusting the methyltrimethoxysilane concentration. The STPA possessed many excellent physical properties, such as ultralight (9.79 mg/cm3), hydrophobicity (136°), and compressibility; as an adsorbent, STPA showed high adsorption capacity (51–111 g/g) for various types of organic solvents. Furthermore, the excellent compressibility enables STPA fast and highly efficient recovery of the absorbed oil by simple mechanical squeeze. It possesses an excellent oil recovery rate of 80% and maintains around 90% of its initial saturated absorption capacity after 50 cycles of absorption–squeezing. Therefore, the successful microstructure design of the biomass-aerogel through bidirectional freezing could provide new thoughts for the design of multifunctional oil absorbents and STPA with good mechanical properties and reusability showing promising prospects in the field of water treatment.
Graphical abstract
... Directional freeze-casting is another simple yet effective method for fabricating graphene networks with orientated porous structures, which enables precise structural controllability, easy scalability, and versatility [187,188]. This technique is typically applied on wet graphene systems such as GHs and GO dispersions [189,190]. ...
Electronic devices generate heat during operation and require efficient thermal management to extend the lifetime and prevent performance degradation. Featured by its exceptional thermal conductivity, graphene is an ideal functional filler for fabricating thermally conductive polymer composites to provide efficient thermal management. Extensive studies have been focusing on constructing graphene networks in polymer composites to achieve high thermal conductivities. Compared with conventional composite fabrications by directly mixing graphene with polymers, preconstruction of three-dimensional graphene networks followed by backfilling polymers represents a promising way to produce composites with higher performances, enabling high manufacturing flexibility and controllability. In this review, we first summarize the factors that affect thermal conductivity of graphene composites and strategies for fabricating highly thermally conductive graphene/polymer composites. Subsequently, we give the reasoning behind using preconstructed three-dimensional graphene networks for fabricating thermally conductive polymer composites and highlight their potential applications. Finally, our insight into the existing bottlenecks and opportunities is provided for developing preconstructed porous architectures of graphene and their thermally conductive composites.
... Although traditional CNs hydrogels can achieve a high degree of biomimetic composition, they lack the ability to mimic the complex structure of biological tissues, for example, well-ordered structures [13] . Recently, several well-ordered hydrogels have been synthesized through directional freezing [14,15] , electrostatic repulsion [16] , magnetic methods [17] , stain-or compression-induced reorientation [18] , and self-assembly [19] . It has been reported in some literatures, epichlorohydrin was applied to form the loosely cross-linked points to mediate the cellulose chains' self-assembly and external stretching was used to promote fiber orientation [20][21][22] . ...
In nature, many biological tissues are composed of oriented structures, which endow tissues with special properties and functions. Although traditional hydrogels can achieve a high level of biomimetic composition, the orderly arrangement of internal structures remains a challenge. Therefore, it is of great significance to synthesize hydrogels with oriented structures easily and quickly. In this study, we first proposed and demonstrated a fabrication process for producing a well-ordered and dual-responsive cellulose nanofibers + hyaluronic acid methacrylate (CN+HAMA) hydrogels through an extrusion-based three-dimensional (3D) printing process. CN in the CN+HAMA hydrogels are directionally aligned after extrusion due to shear stress. In addition, the synthesized hydrogels exhibited responsive behaviors to both temperature and ultraviolet light. Since the temperature-responsiveness is reversible, the hydrogels can transit between the gelation and solution states while retaining their original qualities. Furthermore, the developed well-oriented CN+HAMA hydrogels induced directional cell growth, paving the way for potential applications in ordered biological soft-tissue repair.
... Resulting pores are aligned with the temperature gradient and the direction of solidification. The greater the gradient, the faster the solidification, the smaller the ice grains, and the narrower the spacing between the pore channels [11,14,[19][20][21][22]. ...
Freeze-drying is a process of choice to texture hydrogel scaffolds with pores formed by an ice-templating mechanism. Using state-of-the-art microscopies (cryo-EBSD, μCT, CLSM), this work evidences and quantifies the effect of crosslinking and ice nucleation temperature on the porous structure of thin hydrogel scaffolds freeze-dried at a low cooling rate. We focused on a polysaccharide-based hydrogel and developed specific protocols to monitor or trigger ice nucleation for this study. At a fixed number of intermolecular crosslinks per primary molecule (p = 5), the mean pore size in the dry state decreases linearly from 240 to 170 μm, when ice nucleation temperature decreases from −6 °C to −18 °C. When ice nucleation temperature is fixed at −10 °C, the mean pore size decreases from 250 to 150 μm, as the crosslinking degree increases from p = 3 to p = 7. Scaffold infiltration ability was quantified with synthetic microspheres. The seeding efficiency was assessed with MC3T3-E1 individual cells and HepaRG™ spheroids. These data collapse into a single master curve that exhibits a sharp transition from 100 % to 0 %-efficiency as the entity diameter approaches the mean pore size in the dry state. Altogether, we can thus precisely tune the porosity of these 3D materials of interest for 3D cell culture and cGMP production for tissue engineering.
... However, articular cartilage is characterized by a unique structure and performance that confers it with anisotropic microstructures, mechanical behaviors, and functionalities [9,10] . Most synthetic hydrogels that have been used for biomaterials engineering are isotropic, and thus, it is desirable to create novel biomaterials with tunable anisotropic structures, properties, and functionalities [11,12] . ...
The fabrication of bio tissues using anisotropic constructs capable of repairing and/or replacing diseased bio tissues remains a significant challenge in tissue engineering. Therefore, this study investigated the fabrication of an anisotropic PLGA-microspheres/PVA-hydrogel composite using a novel directional freezing-thawing (DFT) technique. The DFT technique altered the properties of the anisotropic PLGA-microspheres/PVA-hydrogel composite, such that its compressive strength improved from 14.64 ± 1.09 MPa after three DFT cycles to 45.77 ± 6.73 MPa after nine cycles. The utilization of the DFT technique was also shown to enhance the proliferation and adhesion of cultured chondrocyte cells. The obtained results demonstrated that DFT constituted a facile method to fabricate anisotropic microsphere/hydrogel composites with improved and directed cell adhesion characteristics.
... Several excellent reviews have been published on anisotropic hydrogel fabrication [31,32]; however, these do not focus on the strategies that are compatible with cell culture and do not discuss the effect of anisotropy on the behavior of cells. However, the reviews describing anisotropic scaffolds for cell culture either cover a particular fabrication method [33][34][35], discuss a specific class of hydrogels [36], describe a specific biological structure [37,38], or discuss nonhydrogel-based anisotropic systems [39]. ...
Many tissues in vivo exhibit structural anisotropy, which endows them with orientation-specific properties and functions. Structurally anisotropic hydrogels have emerged as promising scaffolds for tissue engineering applications, owing to their ability to recapitulate the structure and physical properties of anisotropic tissues in vivo and provide cells with the appropriate cues. This review highlights key progress in the development of biocompatible structurally anisotropic hydrogels for tissue engineering. We provide an overview of tissues with structural anisotropy and different fabrication methods of anisotropic hydrogels, summarize different characterization methods of hydrogel structural anisotropy, and provide insights on the physical properties of anisotropic hydrogels and their impact on cell behavior. We conclude with a critical assessment of future directions of structurally anisotropic hydrogels for tissue engineering applications.
Ice-templating technology holds great potential to construct industrial porous materials from nanometers to the macroscopic scale for tailoring thermal, electronic, or acoustic transport. Herein, we describe a general ice-templating technology through freezing the material on a rotating cryogenic drum surface, crushing it, and then re-casting the nanofiber slurry. Through decoupling the ice nucleation and growth processes, we achieved the columnar-equiaxed crystal transition in the freezing procedure. The highly random stacking and integrating of equiaxed ice crystals can organize nanofibers into thousands of repeating microscale units with a tortuous channel topology. Owing to the spatially well-defined isotropic structure, the obtained Al2O3·SiO2 nanofiber aerogels exhibit ultralow thermal conductivity, superelasticity, good damage tolerance, and fatigue resistance. These features, together with their natural stability up to 1200 °C, make them highly robust for thermal insulation under extreme thermomechanical environments. Cascading thermal runaway propagation in a high-capacity lithium-ion battery module consisting of LiNi0.8Co0.1Mn0.1O2 cathode, with ultrahigh thermal shock power of 215 kW, can be completely prevented by a thin nanofiber aerogel layer. These findings not only establish a general production route for nanomaterial assemblies that is conventionally challenging, but also demonstrate a high-energy-density battery module configuration with a high safety standard that is critical for practical applications.
2D metal-organic frameworks-based (2D MOF-related) materials benefit from variable topological structures, plentiful open active sites, and high specific surface areas, demonstrating promising applications in gas storage, adsorption and separation, energy conversion, and other domains. In recent years, researchers have innovatively designed multiple strategies to avoid the adverse effects of conventional methods on the synthesis of high-quality 2D MOFs. This review focuses on the latest advances in creative synthesis techniques for 2D MOF-related materials from both the top-down and bottom-up perspectives. Subsequently, the strategies are categorized and summarized for synthesizing 2D MOF-related composites and their derivatives. Finally, the current challenges are highlighted faced by 2D MOF-related materials and some targeted recommendations are put forward to inspire researchers to investigate more effective synthesis methods.
To remedy the drawbacks of weak solar‐thermal conversion capability, low thermal conductivity, and poor structural stability of phase change materials, pyramidal graphitized chitosan/graphene aerogels (G‐CGAs) with numerous radially oriented layers are constructed, in which the long‐range radial alignment of graphene sheets is achieved by a novel directional‐freezing strategy. A G‐CGA/polyethylene glycol phase change composite exhibits a thermal conductivity of 2.90 W m⁻¹ K⁻¹ with a latent heat of 178.8 J g⁻¹, and achieves a superior solar‐thermal energy conversion and storage efficiency of 90.4% and an attractive maximum temperature of 99.7 °C under a light intensity of 200 mW cm⁻². Inspired by waterlilies, solar‐responsive phase change composites (SPCCs) are designed for the first time by assembling the G‐CGA/polyethylene glycol phase change composites with solar‐driven bilayer films, which bloom by day and close by night. The heat preservation effect of the solar‐driven films leads to a higher temperature of SPCC for a longer period at night. The SPCC‐based solar–thermal–electric generator achieves output voltages of 499.2 and 1034.9 mV under light intensities of 200 and 500 mW cm⁻², respectively. Even after stopping the solar irradiation, the voltage output still occurs because of the latent heat release and the heat preservation of the films.
As the world’s demand for green energy storage systems continues to increase, electrochemical double-layer supercapacitors based on porous carbon electrodes are receiving more attention than ever. Porous carbon materials can be designed and synthesized from biomass residues/extracts through various technologies, which realizes the value-added utilization of biomass and meets the targets of circular economy. Pore characteristics are particularly important for the capacitive performance of carbon-based electrodes. This review outlines the biomass-derived porous carbon materials and their corresponding capacitive properties, which are divided into biomass-derived ultra/super-microporous carbon, mesoporous carbon, macroporous carbon and hierarchical porous carbon according to the category of pore size. The synthesis methods, pore characteristic testing probes/calculation models and capacitance performance are discussed in detail, and the corresponding advantages and bottlenecks are pointed out critically. Emphasis is laid on the research status and development trend of biomass-derived hierarchical porous carbon materials, including the effects of the structure and composition of biomass precursors, advanced nanotechnologies and the proportion of various pores in carbonaceous products on the capacitance performance. Finally, the coping strategies are proposed for the further development of high capacitance biomass-derived porous carbon materials.
In this study, we applied oblique angle deposition to a modified initiated chemical vapor deposition (iCVD) process to synthesize porous poly(methacrylic acid) (PMAA) films. During the modified iCVD process, frozen monomer molecules are first captured on a cooled substrate, then polymerization occurs via a free radical polymerization mechanism, and finally, the excess monomer is sublimated, resulting in a porous polymer film. We found that delivering the monomer through an extension at an oblique angle resulted in porous films with three morphological regions. Region 1 is located nearest to the monomer extension outlet and consists of porous polymer pillars; region 2 consists of densified pillars, which occur due to the recapturing and polymerization of the sublimated monomer; and region 3 is located furthest from the monomer extension outlet and consists of dendritic structures, which occur due to low monomer concentration. We investigated the role of substrate temperature and monomer deposition time on the growth process. We found that changing the extension angle influenced the location of the regions and the film coverage across the substrate. Our results provide useful guidelines for tuning the structures within porous polymer films by varying the angle of monomer delivery.
Lignocellulosic biomass is a potential sustainable feedstock to replace fossil fuels. Utilization of lignocellulosic biomass in a green and effective way is of great significance for sustainable development. The direct,...
The characteristics and properties of fumed oxides depend strongly on various external actions that is of importance from a practical point of view. Therefore, gelation or high-pressure cryogelation (HPC) of aqueous media pure or with 0.1 M NaCl, and mechanochemical activation (MCA) of dry or wetted powders of individual (silica, alumina, their mechanical blends) and complex (silica/titania, alumina/silica/titania, AST1, AST1/A–300) nanooxides were studied to analyze the influence of the nanooxide composition, particulate morphology, and preparation conditions on changes in the morphological and textural characteristics of treated samples. The temperature-pressure behavior of different phases (silica, alumina, and titania) under HPC can result in destroy of complex core-shell nanoparticles (100–200 nm in size) in contrast to small nonporous nanoparticles, NPNP (5–20 nm). The textural characteristics of nanooxides are sensitive to any external actions due to compaction of such supra-NPNP structures as aggregates of nanoparticles, agglomerates of aggregates, and visible structures in powders. The compaction of supra-NPNP enhances the pore volume but much weakly affects the specific surface area (with one exception of AST1) because small NPNP are relatively stable during any external actions (HPC, MCA). The compacted materials are characterized by enhanced mesoporosity shifted to macroporosity with decreasing specific surface area and increasing sizes of nanoparticles or to mesopores with increasing MCA time or amounts of water in wetted powders. At low hydration of the A–300 powder (h = 0.5 g/g), the value of SBET slightly increases if MCA is provided by stirring or ball-milling. Diminution of the freezing temperature from 208 to 77.4 K during HPC results in enhanced compaction of aggregates and agglomerates but this does not practically affect the primary nanoparticles. The degree of decomposition of core-shell nanoparticles of AST1 does not practically increase with decreasing freezing temperature from 208 to 77.4 K. Decomposition of core-shell AST1 particles is inhibited under HPC by added A–300 (1 : 1) working as a damper.
Biomass-based aerogel fibers have attracted increasing attention due to their renewable nature. However, their disadvantages, such as low mechanical strength, poor long-range order, and easy combustion, are still significant challenges. Herein, a directed freezing-assisted forced stretching strategy is developed to fabricate sheath-core structured Ca-alginate/polyvinyl alcohol (Ca-A/PVA) aerogel fibers with Long-range-ordered pores. The Ca-A/PVA aerogel fibers (3:2 m/m) exhibit the best comprehensive mechanical properties in terms of low thermal conductivity of 0.0524 W·m-1·K-1, a density of 0.1614 g·cm-3, a porosity of 89.9 %, a tensile strength of 8.72 MPa, a tensile modulus of 249.7 MPa, a toughness of 1.98 MJ∙m-3, and a self-extinguishing time from the fire of <1.2 s. The Ca-A/PVA fabrics showed a maximum absolute temperature difference of 11.4 °C at -20 °C and 14.0 °C at 60 °C compared to the plate temperature. The presented strategy is generalizable to other alginate-based aerogel fibers (e.g., alginate/guar gum).
Biomass-derived porous carbons have received an extensive importance as effective electrode materials owing to their abundance and low cost. The unique porous architectures and large specific surface areas of biomass are beneficial for manipulating the charge storage performance of assembled supercapacitor devices. Here, we demonstrate nitrogen and phosphorus self-doped hierarchical porous carbon (N/P-HPC) derived from yeast (Y) and phytic acid (PA) precursors via freeze-drying-assisted esterification reaction and pyrolysis treatment. The supercapacitive performance and charge storage capability of N/P-HPC were regulated by optimizing the Y/PA composition and controlling the carbonization temperature. Accordingly, the resultant N/P-HPC-Y:PA(2:1)-800 (fabricated with an optimized Y:PA ratio of 2:1 and carbonized at 800 °C) reveals a high specific surface area of 978 m² g⁻¹ and a large pore volume of 0.592 cm³ g⁻¹. As an electrode material, N/P-HPC-Y:PA(2:1)-800 delivers a high specific capacitance of 432 F g⁻¹ at a current density of 1 A g⁻¹ and can sufficiently retain about 250 F g⁻¹ at 20 A g⁻¹ under a three-electrode cell configuration in 1.0 M H2SO4 electrolyte. Moreover, the as assembled symmetric supercapacitor device operated with the N/P-HPC-Y:PA(2:1)-800 as both positive and negative electrode material exhibits an energy density of 13.6 Wh kg⁻¹ at a power density of 500 W kg⁻¹. Even at a larger current density of 20 A g⁻¹, the device maintains an energy density of 10.4 Wh kg⁻¹ and a maximum power density of 10 kW kg⁻¹. The constructed device displays a large capacitance retention of 93.3% after 10 000 charge/discharge times at a higher current density of 10 A g⁻¹, manifesting the enhanced cycling stability.
To improve the thermal conductivity of thermal interface materials (TIM), we constructed a dual-aligned scaffold, i.e., both aligned carbon nanofiber (CNFs) in the microscopic structures and directional microporous channels, serving as the heat conduction paths of TIMs. The dual-aligned scaffold was firstly fabricated by combining magnetic alignment and conventional freeze-casting, and the composites of the dual-aligned scaffold and silicone rubber (D-AS/SR) were then obtained by immersing the dual-aligned scaffold into the silicone rubber with vacuum assistance. Through this method, not only the high axial thermal conductivity of CNFs can be fully utilized, but also a long-range continuous and parallel structure can be fabricated as the heat conduction paths of TIMs. The results show that the thermal conductivity of the D-AS/SR composites reached 4.66 W/(m·K) at 7.73 vol% CNFs, which is 1.5 times higher than that of the composites constructed by conventional freezing casting and 25 times higher than that of pure silicone rubber. Additionally, the compressive strength of the D-AS/SR composites was greatly improved, whereas the electrical insulation of the composites was significantly reduced, which limited the use of D-AS/SR composites in conditions requiring good electrical insulation.
Here, we report the synthesis of two-dimensional (2D) layered metal-organic framework (MOF) nanoparticle (NP) superstructures via an ice-templating strategy. MOF NP monolayers and bilayers can be obtained by regulating the concentration of colloidal MOF NPs without any external fields during self-assembly. Adjacent polyhedral MOF NPs are packed and aligned through crystalline facets, resulting in the formation of a quasi-ordered array superstructure. The morphology of the MOF layers is well preserved when subjected to pyrolysis, and the obtained carbon NPs have hollow interiors driven by the outward contraction of MOF precursors during pyrolysis. With the advantages of large surface areas, hierarchical porosity, high exposure of active sites, and fast electron transport of the 2D layered structure, the mono- and bilayered carbon NP superstructures show better oxygen reduction activity than isolated carbon particles in alkaline media. Our work demonstrates that ice-templating is a powerful strategy to fabricate superstructures of various MOFs and their derivatives.
Enabling the direct infiltration of freeze-cast graphene structures with water-based ceramic suspensions, otherwise prevented by graphene’s intrinsic hydrophobic behaviour, can lead to the production of hierarchical graphene/ceramic composites in a cost-effective and replicable manner. In this study, the addition of a triblock copolymer (PF127) in the formulation of water-based alumina slurries was used to allow the integration with reduced graphene oxide (rGO) scaffolds combining freeze-casting, wet chemistry processing and Spark Plasma Sintering. Wettability and infiltration tests were performed to optimise the composition of the ceramic suspension, leading to the preservation of alignment in embedded rGO scaffolds and maintaining channel widths of 5–15▒μm upon sintering at 1500∘ C.
Polymer composites with high enough thermal conductivity, known as thermal interface materials (TIMs), are continuously pursued in the thermal management of high‐power density electronic and energy devices. In this article, to improve the heat transfer performance of TIMs, we constructed an aligned carbon nanofiber (CNFs) scaffold via the freeze‐casting method to serve as heat conduction paths in the TIMs. The composites of aligned CNF scaffold and silicone rubber were obtained by immersing the aligned scaffold into the silicone rubber with vacuum assist. We studied the influences of different CNFs diameters on the thermal conductivity of the composites, the pore sizes of the aligned CNF scaffold and the thermal conductivity of the composites fabricated under different freezing temperatures, and also the mechanical properties of the composites. Owing to the aligned scaffold, providing directional and continuous heat conduction paths, the thermal conductivity of the composites reached 3.17 W/(m·K) at 7.73 vol% CNFs. Additionally, the compressive strength and fracture strain of the composites were also greatly improved. Owing to the aligned scaffold, providing directional and continuous heat conduction paths, the thermal conductivity of the composites is significantly improved.
Beta''-Al2O3 powder was prepared through freeze-drying with liquid nitrogen, then pressed into bulk samples and sintered at 1570°C. The effects of Al2O3 morphology (rod shaped or flake shaped) on the properties of sintered samples were well discussed. The rod-shaped Al2O3 sample shows a Lotgering factor (LF) of 0.38, an ionic conductivity of 1.852 × 10–1 S cm–1 for the direction parallel to the pressure at 350°C and a bending strength of 192 MPa, all of which are much higher than those of the flake shaped sample (0.08, 178 MPa, and 3.08 × 10–3 S cm–1, respectively). For the rod-shaped sample, the freeze-drying process has evidently increased the degree of crystal orientation and therefore shortened the ion migration path, thereby enhancing its ionic conductivity.
Flexible lithium‐ion batteries (LIBs) have been in the spotlight with the booming development of flexible/wearable electronics. However, the dilemma of simultaneously balancing excellent energy density with mechanical compliance in flexible electrodes impedes their practical applications. Here, for the first time, a directional freezing assisted 3D printing strategy is proposed to construct flexible, compressible, and ultrahigh energy/power density LIBs. Cellulose nanofibers (CNFs) and carbon nanotubes (CNTs) are entangled with each other to form an interwoven network and uniformly wrap the active materials, ensuring fast electron transfer and stress release through the entire printed electrode. Furthermore, vertical channels induced by directional freezing can act as high‐speed ion diffusion paths, which effectively solve the sluggish ion transport limitation of flexible 3D printed electrodes as the mass loading increases. As expected, the 3D printed LIB delivers a record‐high energy density (15.2 mWh cm–2) and power density (75.9 mW cm–2), outperforming all previously reported flexible LIBs. Meanwhile, the printed flexible full LIBs maintains favorable electrochemical stability in both bending and compression states. This work suggests a feasible avenue for the design of LIBs that resolves the long‐standing issue of high electrochemical performance and mechanical deformation in wearable and smart electronics. A directional freezing assisted 3D printing technology is proposed to construct flexible, compressible, and ultrahigh energy/power density lithium‐ion batteries. The printed flexible full cell can maintain favorable electrochemical stability under different mechanical deformations and deliver a record‐high energy density. This work suggests a feasible avenue for the design of durable flexible lithium‐ion batteries.
The current demand for patients’ organ and tissue repair and regeneration is continually increasing, where autologous or allograft is the golden standard treatment in the clinic. However, due to the shortage of donors, mismatched size and modality, functional loss of the donor region, possible immune rejection, and so forth, the application of auto‐/allo‐grafts is frequently hindered in many cases. In order to solve these problems, artificial constructs structurally and functionally imitating the extracellular matrix have been developed as substitutes to promoting cell attachment, proliferation, and differentiation, and ultimately forming functional tissues or organs for better tissue regeneration. Particularly, polymeric materials have been widely utilized in regenerative medicine because of their ease of manufacturing, flexibility, biocompatibility, as well as good mechanical, chemical, and thermal properties. This review presents a comprehensive overview of a variety of polymeric materials, their fabrication methods as well applications in regenerative medicine. Finally, we discussed the future challenges and perspectives in the development and clinical transformation of polymeric biomaterials. With the evolution of the times, the definition of the essential nature of biological materials has undergone a fundamental change from biocompatibility to biomimicry, a process, which is still ongoing. Due to ease of manufacturing, flexibility, biocompatibility, as well as good mechanical, chemical, and thermal properties, polymeric biomaterials are widely applied in regenerative medicine, including bone, cardiovascular, skin, verve, biomedical devices, and so forth.
The ice templating assembly has been investigated to construct macroporous channels of functional nanomaterials with well‐defined homogeneous morphology. Recently, this templating method has been revisited integrating with other materials’ synthesis and processing methodologies (such as, spinning, spraying, filtration, hydrothermal, oxygenation, gelation, and 3D printing) for electrochemical energy conversion and storage applications. Herein, the recent progress on “integrative ice frozen assembly” focusing on the hierarchical structures and chemistries of functional nanomaterials such as, organic, inorganic, carbon, and composite materials for a rational design of energy application‐oriented materials is comprehensively reviewed. This integrative process allows functional nanomaterials to be assembled into various dimensions, such as, 0D, 1D, 2D, and 3D macrostructures, as well as, into larger bulk objects such as, fibers, films, monoliths, and powders. The fundamental understanding of the integrative ice frozen assembly is thermodynamically and kinetically discussed with the help of primitive freeze casting domain knowledge and the energy conversion and storage performances of the as‐designed electrodes with their hierarchical structures and chemistries are further correlated. The applications of the as‐assembled electrodes into batteries, supercapacitors, fuel cells, and electrocatalysis are also addressed. Finally, the perspective on the current impediments and future directions in this field is discussed. The recent progress on “integrative ice frozen assembly” is comprehensively reviewed focusing on the hierarchical structures and chemistries of functional nanomaterials and their correlations with energy conversion/storage performances.
Next-generation applications, such as flexible electronic devices, sensors, actuators, and soft robotics, require anisotropic functional soft materials with controlled, directional electrical and heat conductivities, mechanical properties, and responsiveness, as well as shape-morphing capability, complex designability, and wide operational temperature ranges. However, a combination of these functions in any single class of materials has been very rarely seen to date. In this study, a novel class of multi-anisotropic gels is developed to realize all these functions through a new fabrication route. The gels are synthesized by integrating cellulose with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in tripropylene glycol. The prepared gels exhibit high electrical and thermal conductivities of ≈200 S m⁻¹ and ≈1.49 W m⁻¹ K⁻¹, respectively, with exceptional Young's modulus (≈500 MPa) and tensile strength (≈55 MPa), which are much better than the previously reported mechanical properties of PEDOT-based gels (modulus/strength ≤ 10 MPa). Moreover, the gels exhibit self-welding ability and maintain their properties for 14 d over a wide temperature range (from −50 to 35 °C), covering almost the entire atmospheric temperature range on Earth surface. It is believed that the developed gels are promising candidates for application in many next-generation flexible devices, some of which are experimentally demonstrated in this study.
This book was compiled from joint scientific articles written by Professor Kulkov and Professor Gömze and their students in memory of the outstanding Russian scientist, physicist, friend and main organizer of the cmtp international conferences Prof. Dr. Segei N. Kulkov, who died in COVID in July, 2021.
Thanking to the fruit fill collaboration between Tomsk State University, Institute of Strength Physics of SB RAS and University of Miskolc several years a large number of scientific papers were published by groups under the direction of Prof. Sergei N. Kulkov and Prof. László A. Gömze. This book including only selected papers in which both professors are co-authors.
A variety of soft, hydrogel-like materials with sophisticated micro- and nanoengineered ordered structures that offer excellent functionalities exist in nature. Conventional synthetic hydrogels have an isotropic structure; therefore, they lack many of the functions of the aforementioned natural materials. Anisotropic hydrogels, which possess characteristics similar to that of natural analogs, have been receiving considerable attention in recent years because they display superior and new functionalities that their isotropic counterparts fail to exhibit. Although complex ordered structures are readily generated in nature, engineering such structures in bulk hydrogels has proven to be challenging. These materials contain considerable amounts of water, which makes their polymer interactions and microscale structures difficult to control. In this review, recently developed biomimetic anisotropic hydrogels are discussed systematically, along with state-of-the-art fabrication techniques, critical design principles, and superior functionalities. The extraordinary functionalities of anisotropic hydrogels make them promising candidates for applications in various fields. We reviewed their potential use in several areas, such as those involving tissue engineering, cell control, artificial implants, drug delivery, microfluidics, smart actuators and sensors, soft robotics, flexible electronics, and gel electrolytes for supercapacitors. Finally, the existing challenges and future implications in terms of fabrication, functionalities, and applications of these materials are outlined.
Lithium (Li) metal is regarded as an ultimate anode material for use in Li batteries due to its high theoretical capacity (3860 mA h g‐1). However, the Li dendrites that are generated during iterative Li plating/stripping cycles cause poor cycling stability and even create vital safety risks, and thus severely handicap the commercial utility of Li metal anodes. Herein, we describe a graphene‐ and carbon nanotube (CNT)‐based Li host material that features vertically aligned channels with attached ZnO particles (designated as ZnO@G‐CNT‐C) and show that the material effectively regulates Li plating and stripping. The ZnO@G‐CNT‐C is prepared from an aqueous suspension of Zn(OAc)2, CNTs, and graphene oxide using ice to template channel growth. ZnO@G‐CNT‐C was found to be mechanically robust and capable of guiding Li deposition on the inner walls of the channels without the formation of Li dendrites. When used as an electrode, the material exhibits relatively low polarization for Li plating, fast Li‐ion diffusion, and high Coulombic efficiency, even over hundreds of Li plating/stripping cycles. Moreover, full cells prepared using ZnO@G‐CNT‐C as Li host and LiFePO4 as cathode exhibit outstanding performance in terms of specific capacity (155.9 mA h g‐1 at 0.5 C), rate performance (91.8 mA h g‐1 at 4 C), cycling stability (109.4 mA h g‐1 at 0.5 C after 800 cycles). The methodology described can be readily adapted to enable the use of carbon‐based electrodes with well‐defined channels in a wide range of contemporary applications that pertain to energy storage and delivery.
The present work focuses on the utilization of biowaste material for a sustainable environment. In order to support the concept of ‘value-added product from waste’, Eggshell waste, which is a rich source of calcium is considered as a sustainable and renewable precursor for the synthesis of porous hydroxyapatite. This study introduces a novel, green and facile method for the synthesis of highly porous nano hydroxyapatite (nHA) and its magnetic composite aerogels ([email protected]) by freeze-drying technology. The synthesized materials have undergone through characterization. Perchlorate removal efficiency of the materials was investigated for the first time and the factors that influence the perchlorate adsorption capacity were explored and optimized. The magnetic composite of nHA showed an excellent separation in the presence of an external magnetic field from contaminated water after adsorption. XPS analysis and zeta potential measurements showed that the main driving force behind the adsorption of perchlorate is ion exchange and ion-pair formation/electrostatic interaction. Very fast removal kinetics was observed and the experimental maximum adsorption capacity for nHA and [email protected] were 148.4 and 305.8 mg g⁻¹ respectively. It is commendable that other commonly existing ions exhibited no remarkable effect on the adsorption performance, indicating [email protected] has an immense application potential. The adsorption process was found to follow by pseudo-second-order and intra-particle diffusion kinetic models. A novel method for the regeneration of perchlorate loaded adsorbent using phosphate buffer was also studied in this paper. The regeneration performance of spent magnetic adsorbent showed greater than 90% recovery on the adsorption capacity even after fifth cycle, revealing the stability, reusability and economic efficiency of the adsorbent. Thus, magnetic nanocomposite of the biogenic hydroxyapaptite can be exploited as a sustainable and efficacious material for the abolishment of perchlorate from aqueous media at near neutral pH.
Developing renewable porous materials with low thermal conductivity and high flame retardancy is of great significance in energy saving and fire safety. Herein, a series of bio-based anisotropic composite foams were fabricated from oil-in-water (o/w) high internal phase Pickering emulsions (Pickering HIPEs) stabilized by both bio-based poly (urethane-acrylate) (PUA) and poly (cyclotriphosphazene-co-4,4ʹ-sulfonyldiphenol) (PZS) particles. All Pickering HIPEs have good stability originated from the two kinds of particles irreversible adsorption at the oil-water interface and the formation of a “3D networks” of PUA particles aggregates in the aqueous phase. The microstructures and rheology of the double-particles stabilized HIPEs were adjusted by changing the contents of particles and the internal phase volume fractions. Then the anisotropic PZS/PUA composite foams with high porosity (up to 93.9%) and low thermal conductivity (down to 48.4 mW m⁻¹∙K⁻¹) were prepared by unidirectional freeze-drying these Pickering HIPEs. The anisotropic structure enhanced the mechanical strength in axial direction and thermal insulation property in radial direction. Compared with the PUA foam, loading only PZS of 10.7 wt%, the values of average heat release rate, total heat release and average effective heat of combustion of the composite foams were reduced by 19.3%, 12.8%, and 40.2%, respectively. Meanwhile, the value of limiting oxygen index was also increased from 16.3% to 18.2%, which was resulted from PZS functioned in the condensed phases during the combustion process. This work provides a strategy to prepare a kind of bio-based anisotropic foams which would find uses in the fields of fire safety and thermal insulation.
Cells are known to be surrounded by nanoscale topography in their natural extracellular environment. The cell behavior, including morphology, proliferation, and motility of bovine pulmonary artery smooth muscle cells (SMC) were studied on poly(methyl methacrylate) (PMMA) and poly(dimethylsiloxane) (PDMS) surfaces comprising nanopatterned gratings with 350 nm linewidth, 700 nm pitch, and 350 nm depth. More than 90% of the cells aligned to the gratings, and were significantly elongated compared to the SMC cultured on non-patterned surfaces. The nuclei were also elongated and aligned. Proliferation of the cells was significantly reduced on the nanopatterned surfaces. The polarization of microtubule organizing centers (MTOC), which are associated with cell migration, of SMC cultured on nanopatterned surfaces showed a preference towards the axis of cell alignment in an in vitro wound healing assay. In contrast, the MTOC of SMC on non-patterned surfaces preferentially polarized towards the wound edge. It is proposed that this nanoimprinting technology will provide a valuable platform for studies in cell-substrate interactions and for development of medical devices with nanoscale features.
The preparation of materials with aligned porosity in the micrometre range is of technological importance for a wide range of applications in organic electronics, microfluidics, molecular filtration and biomaterials. Here, we demonstrate a generic method for the preparation of aligned materials using polymers, nanoparticles or mixtures of these components as building blocks. Directional freezing is used to align the structural elements, either in the form of three-dimensional porous structures or as two-dimensional oriented surface patterns. This simple technique can be used to generate a diverse array of complex structures such as polymer-inorganic nanocomposites, aligned gold microwires and microwire networks, porous composite microfibres and biaxially aligned composite networks. The process does not involve any chemical reaction, thus avoiding potential complications associated with by-products or purification procedures.
A porous NiO–YSZ tubular support with radially aligned pore channels was prepared by freeze-drying of a water-based slurry. The slurry was frozen, controlling the growth direction of the ice, and it was sublimated at a reduced pressure. After sintering, the support tube showed radially aligned large pore channels parallel to the ice growth direction, and fine pores are formed around the outer surface of the tube. Bilayer consisting of dense thin electrolyte film of YSZ onto the support has been successfully fabricated using a slurry-coating process followed by co-firing. It is regarded that such a unique bilayer structure is suitable for constructing electrode-supported type electrochemical cells.
Zirconia hydrogel was prepared by dialyzing 1∼3M ZrOCl2 solution partially neutralized with an alkline solution for 96 ∼192 h. The gel was placed in a plastic cylinder and unidirectionally frozen by lowering the cylinder into a −78°C cold bath at 4 or 6 cm/h. A bundle of parallel fibers longer than 8 cm and 10∼60 μm in diameter was obtained after thawing. The fibers were amorphous and porous as-dried at 25°C. Their pores were greatly eliminated by a heat treatment in a vacuum at 290°C for 2 days. The chemical composition of the heat-treated fibers was approximately ZrO2·H2O.
Phase separation during directional freezing of liquid systems leads to a variety of microstructures of the components due to the interplay of heat transfer, diffusion kinetics, and interfacial surface energy1–6. One of the more dramatic effects is the in situ formation of parallel fibres in directionally solidified eutectic superalloy. We report here similar morphological consequences of freezing aqueous polysilicic acid. This system is unique not only because it has an amorphous component, polysilicic acid, but this substance undergoes accelerated, concentration-dependent polymerization immediately after phase separation. As a result, it becomes insoluble, and after thawing, preserves the structure that had been conferred on it by the surrounding ice. We have explored the relationships of polymerization, composition, and freezing conditions with the morphology of the products, in particular with regard to fibre formation.
We present a mathematical model of the unidirectional solidification of a suspension of hard-sphere colloids. Similarity solutions are obtained for the volume fraction and temperature profiles ahead of a planar solidification front. The highly nonlinear functional dependence of the diffusion coefficient on the volume fraction gives rise to a range of behaviours. For small particles, Brownian diffusion dominates and the system behaviour is reminiscent of binary-alloy solidification. Constitutional supercooling occurs at the interface under certain conditions, leading potentially to an instability in the shape of the interface. For larger particles, Brownian diffusion is weak and the particles form a porous layer above the interface. In this case constitutional supercooling reaches a maximum near the surface of the layer, and the porous medium itself is potentially unstable. In stable systems there exists the possibility of secondary nucleation of ice.
A new method for synthesizing well-defined porous materials by templating supercritical fluid emulsions was developed. The method, which did not involve use of volatile organic solvents and fine control over the porous structure, was achieved by tuning the CO2 density. The method finds application in producing crosslinked polyvinylchloride (PVA) materials by gelation of high internal phase emulsions (HIPE).
Ice-segregation-induced self-assembly is a versatile and biocompatible process that facilitates the preparation of hierarchical biohybrid materials exhibiting a very sophisticated structure with up to six levels of space organization: the ternary structure of esterase, the PVA domains surrounding esterase, the silica cages entrapping the PVA domains, and the macroporous structure resulting from ice segregation (see figure). The resulting materials are attractive for biomedical and sensor applications.
Porous silicon nitride with macroscopically aligned channels was synthesized using a freeze-drying process. Freezing of a water-based slurry of silicon nitride was done while unidirectionally controlling the growth direction of the ice. Pores were generated subsequently by sublimation of the columnar ice during freeze-drying. By sintering this green body, a porous silicon nitride with high porosity (over 50%) was obtained and its porosity was controllable by the slurry concentration. The porous Si3N4 had a unique microstructure, where macroscopically aligned open pores contained fibrous grains protruding from the internal walls of the Si3N4 matrix. It is hypothesized that vapor/solid phase reactions were important to the formation mechanism of the fibrous grains.
Highly oriented and crystalline regioregular poly(3-hexylthiophene) (P3HT) films have been obtained by directional solidification. Oriented P3HT crystallizes from a solution in 1,3,5-trichlorobenzene, which plays the dual role of solvent and orienting substrate. Periodic alternation of crystalline lamellae separated by amorphous zones, the typical fingerprint of the semicrystalline structure of P3HT, are observed (see figure).
Porous materials are used in a wide variety of applications, including catalysis, chemical separation, and tissue engineering. The synthesis and processing of these materials is frequently solvent intensive. In addition to reducing organic solvent emissions, supercritical fluids offer a number of specific physical, chemical, and toxicological advantages as alternative solvents for the production of functional porous materials. The figure shows an electron image of a porous polyacrylate produced by the templating of a concentrated CO2-in-water emulsion—a process that would otherwise be highly solvent intensive.
The significant control of thin-films microdomain patterns of lamellar- and cylinder-forming ultrahigh molecular weight block copolymers (BCP) over a via directional solidification (DS) of a solvent is presented. The directional solidification of BCPs with benzoic acid (BA) resulted in an excellent orientational control of the self-assembled microdomains over a large area. The difference in the microdomain orientation obtained from ultrahigh molecular weight BCPs and low molecular weight BCPs can be understood by diffusion- and solubility-controlled microphase separation of BCPs in solution during the directional extraction of the crystallizable solvent. These ultrahigh molecular weight BCP patterns have a domain size comparable to visible wavelengths and can be potentially used for fabricating useful thin-film photonic structures.
In this work, the unidirectional freezing method was applied to a wide variety of silica gel precursors; from silica sols which do not readily gel, to thoroughly aged silica hydrogels. It was found that in addition to the well-known structure of fibers with polygonal cross-sections which are commonly obtained in the unidirectional freezing method, porous silica gels having unique morphologies such as honeycomb, lamellar and flat fiber structures could also be obtained by changing the state of the precursor sol or gel. The obtained silica hydrogels were freeze-dried after exchanging the water included in its structure to t-butanol, and finally dry samples maintaining their wet state structures were obtained. The morphology and the porous properties of the obtained silica gels were systematically analyzed and the influences of preparation conditions, including pH, aging time before freezing, SiO2 concentration, freezing temperature, and storage time at the frozen state on both factors were examined in detail. It was found that the simultaneous controlling of both factors could be easily conducted by simply adjusting preparation conditions.
Macroporous monoliths of SiO2–Al2O3 cryogels were prepared. Macropores were generated by using ice crystals as the template, while the walls which surround the macropores were tailored as porous cryogels by freeze drying. Macropores and walls formed honeycomb-like structures, which were confirmed from scanning electron microscopy images of cross-sections of the samples. It was confirmed that the sizes of the macropores and the wall thicknesses were respectively in the ranges of 10–20 µm and 200–500 nm. Al mapping analysis by energy dispersive X-ray diffractometry showed that Al atoms were homogeneously dispersed throughout the samples without local aggregation. Moreover, Raman spectroscopy and 27Al NMR spectroscopy indicated that Al atoms were incorporated into the silica framework by forming an Al–O–Si polymeric network. Nitrogen adsorption–desorption measurements indicated that the walls were micro/mesoporous with high BET surface areas (>700 m2 g−1) and large pore volumes (>0.45 cm3 g−1). Moreover, NH3-TPD measurements revealed that the samples had acid sites, which allowed this material to be used as a solid acid catalyst.
Ordered macroporous silica, a silica gel microhoneycomb (SMH), has been prepared through a method which uses micrometer-sized ice crystals as a template. Template ice crystals, which have a continuous rod shape, a polygonal cross section, and ordered diameters, were grown inside precursor silica hydrogels under a condition where the pseudo-steady-state growth of them continues. Besides their ordered macroporosity, micro-/mesopores develop inside the honeycomb walls through the freeze-drying of SMHs soaked in tert-butyl alcohol. SMHs have straight and polygonal macroporous voids, which are created and retained through the formation and removal of the ice crystals. Micromorphology including macropore size and wall thickness, micro-/mesoporosity inside the honeycomb walls, and thermal stability of SMHs were investigated in detail through scanning electron microscopy observation, nitrogen adsorption - desorption measurements, and thermogravimetric analysis. It was found that the macropore size of the SMHs can be controlled by changing the immersion rate into a cold bath and the freezing temperature without changing the micro-/mesoporosity of their honeycomb walls. It was also found that the thickness of the honeycomb walls was affected by the SiO2 concentration and the macropore size. On the other hand, the porosity of the honeycomb walls could be controlled to be microporous as well as mesoporous by hydrothermal treatment of as-prepared SMHs in basic aqueous solutions. Moreover, it was found that SMHs with developed mesopores showed a higher stability against heat treatment.
The preparation of resorcinol-formaldehyde (RF) hydrogels by the sol-gel polycondensation of resorcinol (R) with formaldehyde (F) using sodium carbonate (C) as a basic catalyst and distilled water (W) was discussed. It was found that the ratios of resorcinol to formaldehyde (mol/mol; R/F), resorcinol to water (g/cm3; R/W) and resorcinol to catalyst (mol/mol; R/C) were 0.5, 0.2 and 100 respectively. The resulting hydrogels were washed five times with distilled water and placed into polypropylene tubes, which were filled with distilled water. These tubes were frozen at a constant rate of 20 cm/h, by dipping the tubes perpendicularly into a cold bath.
We investigated the contact guidance phenomenon of rat dermal fibroblasts (RDF) on microgrooved polystyrene substrates. Grooves were 1 microm deep, and between 1 and 10 microm wide. Light microscopy and digital image analysis (DIA) showed that RDF were oriented on all microgrooved substrates. Scanning electron microscopy showed that RDF cultured on 1 or 2 microm wide grooves were positioned on top of the ridges. On the wider 5 and 10 microm grooves, the cells were able to descend into the grooves. In confocal laser scanning microscopy, focal adhesions were lying in the same direction as the actin filament where they attached to. DIA confirmed an orientational behavior of focal adhesions and actin filaments on microgrooves. There were no differences in the measured orientation between the different grooves. Besides, no obvious preference was found for focal adhesions to lie along edges of the surface ridges. Transmission electron microscopy showed that focal adhesions were able to bend along the edges of ridges. On the basis of our observations, we suggest that the breakdown and formation of fibrous cellular components, especially in the filopodium, is influenced by the microgrooves. The microgrooves create a pattern of mechanical stress, which influences cell spreading and cause the cell to be aligned with surface microgrooves.
The wide array of tissue engineering applications exacerbates the need for biodegradable materials with broad potential. Chitosan, the partially deacetylated derivative of chitin, may be one such material. In this study, we examined the use of chitosan for formation of porous scaffolds of controlled microstructure in several tissue-relevant geometries. Porous chitosan materials were prepared by controlled freezing and lyophilization of chitosan solutions and gels. The materials were characterized via light and scanning electron microscopy as well as tensile testing. The scaffolds formed included porous membranes, blocks, tubes and beads. Mean pore diameters could be controlled within the range 1-250 microm, by varying the freezing conditions. Freshly lyophilized chitosan scaffolds could be treated with glycosaminoglycans to form ionic complex materials which retained the original pore structure. Chitosan scaffolds could be rehydrated via an ethanol series to avoid the stiffening caused by rehydration in basic solutions. Hydrated porous chitosan membranes were at least twice as extensible as non-porous chitosan membranes, but their elastic moduli and tensile strengths were about tenfold lower than non-porous controls. The methods and structures described here provide a starting point for the design and fabrication of a family of polysaccharide based scaffold materials with potentially broad applicability.
Creating a regular surface pattern on the nanometre scale is important for many technological applications, such as the periodic arrays constructed by optical microlithography that are used as separation media in electrophoresis, and island structures used for high-density magnetic recording devices. Block copolymer patterns can also be used for lithography on length scales below 30 nanometres (refs 3-5). But for such polymers to prove useful for thin-film technologies, chemically patterned surfaces need to be made substantially defect-free over large areas, and with tailored domain orientation and periodicity. So far, control over domain orientation has been achieved by several routes, using electric fields, temperature gradients, patterned substrates and neutral confining surfaces. Here we describe an extremely fast process that leads the formation of two-dimensional periodic thin films having large area and uniform thickness, and which possess vertically aligned cylindrical domains each containing precisely one crystalline lamella. The process involves rapid solidification of a semicrystalline block copolymer from a crystallizable solvent between glass substrates using directional solidification and epitaxy. The film is both chemically and structurally periodic, thereby providing new opportunities for more selective and versatile nanopatterned surfaces.
Acetylation of carbohydrates is proposed as a method for the synthesis of inexpensive CO2-philes that can easily be modified for a wide variety of green chemistry applications in liquid and supercritical CO2. The deliquescence of peracetylated sugars in contact with CO2 is reported in this communication as well as the unparalleled solubility of these compounds in supercritical CO2.
Porous and bioactive gelatin-siloxane hybrids were successfully synthesized by using a combined sol-gel processing, post-gelation soaking, and freeze-drying process to provide a novel kind of materials in the developments and optimization of bone tissue engineering. The pore sizes of the hybrids can be well controlled by varying the freezing temperature. The scaffolds were soaked in a simulated body fluid (SBF) up to 14 days to evaluate the in vitro bioactivity. The Ca(2+)-containing scaffolds showed in vitro bioactivity as they biomimetically deposited apatite, but the Ca(2+)-free scaffolds failed. Cytotoxicity and cytocompatibility of those scaffolds and their extracts were monitored by the MC3T3-E1 cell responses, including the cell proliferation and the alkaline phosphatase activity. It was demonstrated that appropriate incorporation of Ca(2+) ions stimulated osteoblast proliferation and differentiation in vitro.
Highly porous emulsion-templated materials were synthesized by polymerization of concentrated CO(2)-in-water (C/W) emulsions. The method does not use any organic solvents, in either the synthesis or purification steps, and no solvent residues are left in the materials. It was found that the emulsion stability is strongly affected both by the nature of the surfactant and by the viscosity of the aqueous continuous phase. By optimizing these parameters, it was possible to generate a highly porous, low-density polyacrylamide material with a pore volume of 5.22 cm(3)/g, an average pore diameter of 9.72 microm, and a bulk density of 0.14 g/cm(3). We have broadened the scope of this approach significantly by identifying inexpensive hydrocarbon surfactants to stabilize the C/W emulsions (e.g., Tween 40) and by developing redox initiation routes that allow the synthesis to be carried out at modest temperatures and pressures (20 degrees C, 65 bar). We have also extended the method to the polymerization of monomers such as hydroxyethyl acrylate, which suggests that it is possible to prepare a range of solvent-free biomaterials by this route.
In an effort to develop a permissive environment for neural stem cell differentiation, directional growth of astrocytes has been achieved on polymer substrates in vitro. Manipulating a combination of physical and chemical cues, astrocyte adhesion and alignment in vitro were examined. To provide physical guidance, micropatterned polymer substrates of polystyrene (PS) were fabricated. Laminin was selectively adsorbed onto the grooves of the patterned surface. Rat type-1 astrocytes were seeded onto the micropatterned PS substrates, and the effects of substrate topography and the adsorption of laminin to the PS substrates on the behavior and morphology of the astrocytes were explored. The astrocytes were found to align parallel to the micropatterned grooves at initial seeding densities of approximately 7500, 13,000, and 20,000 cells/cm(2) due to the effects of the physical and chemical guidance mechanisms. Adsorbing laminin in the microgrooves of the micropatterned PS substrates improved cell adhesion and spreading of cytoskeletal filaments significantly. At these initial seeding densities, over 85% astrocyte alignment in the direction of the grooves was achieved on the micropatterned PS substrates with laminin adsorbed in the grooves. This combination of guidance cues has the potential to provide a permissive substrate for in vivo regeneration within the central nervous system.
Monolithic silica gel microhoneycombs, which have an array of straight macropores within their structure and developed micro/mesopores inside their walls, were prepared using pseudosteady state growth of ice crystals which occurs during the unidirectional freeze-gelation of freshly gelled aqueous silica hydrogels, followed by a pore-protecting drying method, freeze drying.
We report here a method for generating structures with aligned macropores by templating solidified carbon dioxide. The CO2 template phase can be removed by direct sublimation to yield a dry, solvent-free porous material that may be useful in applications such as tissue engineering and aligned cell growth.
Materials that are strong, ultralightweight, and tough are in demand for a range of applications, requiring architectures
and components carefully designed from the micrometer down to the nanometer scale. Nacre, a structure found in many molluscan
shells, and bone are frequently used as examples for how nature achieves this through hybrid organic-inorganic composites.
Unfortunately, it has proven extremely difficult to transcribe nacre-like clever designs into synthetic materials, partly
because their intricate structures need to be replicated at several length scales. We demonstrate how the physics of ice formation
can be used to develop sophisticated porous and layered-hybrid materials, including artificial bone, ceramic-metal composites,
and porous scaffolds for osseous tissue regeneration with strengths up to four times higher than those of materials currently
used for implantation.
Although extensive efforts have been put into the development of porous scaffolds for bone regeneration, with encouraging results, all porous materials have a common limitation: the inherent lack of strength associated with porosity. Hence, the development of porous hydroxyapatite scaffolds has been hindered to non-load bearing applications. We report here how freeze casting can be applied to synthesize porous scaffolds exhibiting unusually high compressive strength, e.g. up to 145 MPa for 47% porosity and 65 MPa for 56% porosity. The materials are characterized by well-defined pore connectivity along with directional and completely open porosity. Various parameters affecting the porosity and compressive strength have been investigated, including initial slurry concentration, freezing rate, and sintering conditions. The implications and potential application as bone substitute are discussed. These results might open the way for hydroxyapatite-based materials designed for load-bearing applications. The biological response of these materials is yet to be tested.