Hierarchical structure formation by crystal growth-front instabilities during ice templating.

Directional solidification of aqueous solutions and slurries in a temperature gradient is widely used to produce cellular materials through a phase separation of solutes or suspended particles between growing ice lamellae. While this process has analogies to the directional solidification of metallurgical alloys, it forms very different hierarchical structures. The resulting honeycomb-like porosity of freeze-cast materials consists of regularly spaced, lamellar cell walls which frequently exhibit unilateral surface features of morphological complexity reminiscent of living forms, all of which are unknown in metallurgical structures. While the strong anisotropy of ice-crystal growth has been hypothesized to play a role in shaping those structures, the mechanism by which they form has remained elusive. By directionally freezing binary water mixtures containing small solutes obeying Fickian diffusion, and phase-field modeling of those experiments, we reveal how those structures form. We show that the flat side of lamellae forms because of slow faceted ice-crystal growth along the c-axis, while weakly anisotropic fast growth in other directions, including the basal plane, is responsible for the unilateral features. Diffusion-controlled morphological primary instabilities on the solid-liquid interface form a cellular structure on the atomically rough side of the lamellae, which template regularly spaced "ridges" while secondary instabilities of this structure are responsible for the more complex features. Collating the results, we obtain a scaling law for the lamellar spacing,  [Formula: see text] , where [Formula: see text] and [Formula: see text] are the local growth rate and temperature gradient, respectively.

A Hierarchical Hydrogel Impregnation Strategy Enables Brittle-Failure-Free 3D-Printed Bioceramic Scaffolds.

3D-printed bioceramic scaffolds offer great potential for bone tissue engineering (BTE) but their inherent brittleness and reduced mechanical properties at high porosities can easily result in catastrophic fractures. Herein, this study presents a hierarchical hydrogel impregnation strategy, incorporating poly(vinyl alcohol) (PVA) hydrogel into the macro- and micropores of bioceramic scaffolds and synergistically reinforcing it via freeze-casting assisted solution substitution (FASS) in a tannic acid (TA)-glycerol solution. By effectively mitigating catastrophic brittle failures, the hydrogel-impregnated scaffolds showcase three- and 100-fold enhancement in mechanical energy absorption under compression (5.05 MJ m-3) and three-point bending (3.82 MJ m-3), respectively. The reinforcement mechanisms are further investigated by experimental and simulation analyses, revealing a multi-scale synergy of fracture and fragmentation resistance through macro and micro-scale fiber bridging, and nano and molecular-scale hydrogel reinforcement. Also, the scaffolds acquire additional antibacterial and drug-loading capabilities from the hydrogel phase while maintaining favorable cell biocompatibility. Therefore, this study demonstrates a facile yet effective approach for preparing brittle-failure-free bioceramic scaffolds with enhanced biological functionalities, showcasing immense potential for BTE applications.

Tungsten Strongly Inhibits Sintering of Porous Iron During High-Temperature Redox Cycling.

Freeze-cast Fe-25 W (at%) lamellar foams show excellent resistance to degradation at 800 °C during steam-hydrogen redox cycling between the metallic and oxide states, with fast reaction kinetics maintained up to at least 100 redox cycles with full Fe utilization. This very high stability stems from the sintering inhibition of W combined with the freeze-cast architecture and the chemical vapor transport (CVT) mechanism of reduction. These three factors create a hierarchical porosity in the foam, consisting of i) macroscopic elongated channels, ii) micro-scale sintering inhibition pores, and iii) submicron CVT pores. Microstructural characterization via SEM and EDS is combined with in situ XRD to fully explore the phase evolution and microstructural impact of W on Fe during redox cycling. Comparison with tapped Fe-25 W (at%) powder beds reveals that the freeze-cast channels and lamellae are not critical to the performance of the material.

Scalable Fast-Charging Aligned Battery Electrodes Enabled by Bidirectional Freeze-Casting.

Over the past few years, lithium-ion batteries have been extensively adopted in electric transportation. Meanwhile, the energy density of lithium-ion battery packs has been significantly improved, thanks to the development of materials science and packing technology. Despite recent progress in electric vehicle cruise ranges, the increase in battery charging rates remains a pivotal problem in electrodes with commercial-level mass loadings. Herein, we develop a scalable strategy that incorporates bidirectional freeze-casting into the conventional tape-casting method to fabricate energy-dense, fast-charging battery electrodes with aligned structures. The vertically lamellar architectures in bidirectional freeze-cast electrodes can be roll-to-roll calendered, forming the tilted yet aligned channels. These channels enable directional pathways for efficient lithium-ion transport in electrolyte-filled pores and thus realize fast-charging capabilities. In this work, we not only provide a simple yet controllable approach for building the aligned electrode architectures for fast charging but also highlight the significance of scalability in electrode fabrication considerations.

Nanoarchitectonics of RGO-Wrapped CNF/GO Aerogels with Controlled Pore Structures by PVA-Assisted Freeze-Casting Approach for Efficient Sound and Microwave Absorption.

Acoustic absorption materials play an important role in eliminating the negative effects of noise. Herein, a polyvinyl alcohol (PVA)-assisted freeze-casting was developed for controllably fabricating reduced graphene oxide wrapped carbon nanofiber (RGO@CNF)/graphene oxide composite aerogel. During the freeze-casting, PVA was used as an icing inhibitor to control the size of ice crystals. While the concentration of PVA increased from 0 to 15 mg ⋅ ml-1 , the average pore size of the aerogel was reduced from 154 to 45 µm. Due to the modulation of the pore size and composition, the propagation path and friction loss for sound were optimized, especially at low frequency. For instance, the normalized sound absorption coefficient of RGO@CNF/GO-10 achieves 0.79 (250-6300 Hz). The sample also exhibits a desirable microwave absorbing property whose maximum reflection loss is -47.3 dB (9.44 GHz, d=3.0 mm). Prospectively, this synthetic strategy can be extended to develop other forms of elastic aerogel with a controlled pore size.

Lightweight, Fire-Retardant, and Anti-Compressed Honeycombed-Like Carbon Aerogels for Thermal Management and High-Efficiency Electromagnetic Absorbing Properties.

Ordered porous carbon materials (PCMs) have potential applications in various fields due to their low mass densities and porous features. However, it yet remains extremely challenging to construct PCMs with multifunctionalization for electromagnetic wave absorption. Herein, the honeycombed-like carbon aerogels with embedded Co@C nanoparticles are fabricated by a directionally freeze-casting and carbonization method. The optimized aerogel possesses low density (0.017 g cm-3 ), fire-retardant, robust mechanical performance (compression moduli reach 1411 and 420 kPa in the longitudinal and transverse directions at 80% strain, respectively), and high thermal management (high thermal insulation capability and high-efficiency electrothermal conversion ability). Notably, the optimized aerogel exhibits the excellent electromagnetic wave absorption properties with broad effective absorption bandwidth (13.12-17.14 GHz) and strong absorption (-45.02 dB) at a thickness of only 1.5 mm. Density functional theory calculations and the experimental results demonstrate that the excellent electromagnetic wave absorption properties stem from the synergetic effects among high electrical conductivity, numerous interfaces and dipoles and unique ordered porous structure. Meanwhile, the computer simulation technology (CST) simulation confirms that the multifunctional aerogel can attenuate more electromagnetic energy in a practical environment. This work paves the way for rational design and fabrication of the next-generation electromagnetic wave absorbing materials.

Hierarchically Porous Silk/Activated-Carbon Composite Fibres for Adsorption and Repellence of Volatile Organic Compounds.

Fabrics comprised of porous fibres could provide effective passive protection against chemical and biological (CB) threats whilst maintaining high air permeability (breathability). Here, we fabricate hierarchically porous fibres consisting of regenerated silk fibroin (RSF) and activated-carbon (AC) prepared through two fibre spinning techniques in combination with ice-templating-namely cryogenic solution blow spinning (Cryo-SBS) and cryogenic wet-spinning (Cryo-WS). The Cryo-WS RSF fibres had exceptionally small macropores (as low as 0.1 µm) and high specific surface areas (SSAs) of up to 79 m2·g-1. The incorporation of AC could further increase the SSA to 210 m2·g-1 (25 wt.% loading) whilst also increasing adsorption capacity for volatile organic compounds (VOCs).

An All-Freeze-Casting Strategy to Design Typographical Supercapacitors with Integrated Architectures.

The emergence of flexible and wearable electronics has raised the demand for flexible supercapacitors with accurate sizes and aesthetic shapes. Here, a strategy is developed to prepare flexible all-in-one integrated supercapacitors by combining all-freeze-casting with typography technique. The continuous seamless connection of all-in-one supercapacitor devices enhances the load and/or electron transfer capacity and avoids displacing and detaching between their neighboring components at bending status. Therefore, such a unique structure of all-in-one integrated devices is beneficial for retaining stable electrochemical performance at different bending levels. More importantly, the sizes and aesthetic shapes of integrated supercapacitors could be controlled by the designed molds, like type matrices of typography. The molds could be assembled together and typeset randomly, achieving the controllable construction and series and/or parallel connection of several supercapacitor devices. The preparation of flexible integrated supercapacitors will pave the way for assembling programmable all-in-one energy storage devices into highly flexible electronics.

Enhanced Adhesion of Polydimethylsiloxane Using an Interlocked Finger Structure.

Silicone-based polymers have been widely used for many applications, but their extremely low surface energies and the resulting poor adhesion have been the cause for continuous problems. Herein, a novel adhesion improvement technique using an interlocked finger structure is demonstrated, which enables up to 24.8 and 7.3-fold increases in adhesion compared to the untreated and conventional plasma-treated cases, respectively. The interlocked finger structure is fabricated by surface-confined dissolution and subsequent directional melt crystallization of a solvent. After removing the solvent crystals, porous surfaces are prepared from polyurethane, polyvinyl alcohol, and polystyrene, and these are used to fabricate interfaces of interlocked finger structures with polydimethylsiloxane. The improvement in adhesion strength linearly depends on the pore depth of the prepared surfaces. This novel technique of surface adhesion could improve the performance of polymers with intrinsically poor adhesion in future applications.

3D Printing Hierarchical Silver Nanowire Aerogel with Highly Compressive Resilience and Tensile Elongation through Tunable Poisson's Ratio.

Metallic aerogels have attracted intense attention due to their superior properties, such as high electrical conductivity, ultralow densities, and large specific surface area. The preparation of metal aerogels with high efficiency and controllability remains challenge. A 3D freeze assembling printing technique integrated with drop-on-demand inkjet printing and freeze casting are proposed for metallic aerogels preparation. This technique enables tailoring both the macrostructure and microstructure of silver nanowire aerogels (SNWAs) by integrating programmable 3D printing and freeze casting, respectively. The density of the printed SNWAs is controllable, which can be down to 1.3 mg cm-3 . The ultralight SNWAs reach high electrical conductivity of 1.3 S cm-1 and exhibit excellent compressive resilience under 50% compressive strain. Remarkably, the printing methodology also enables tuning aerogel architectures with designed Poisson's ratio (from negative to positive). Moreover, these aerogel architechtures with tunable Poisson's ratio present highly electromechanical stability under high compressive and tensile strain (both strain up to 20% with fully recovery).

New Aspects in the Formulation of Drugs Based on Three Case Studies.

The improvement of pharmaceutical dosage forms, such as tablets, towards drug delivery control and cost efficiency is of great importance in formulation technologies. Here, three examples: in situ coating, freeze casting and protein-based biocomposites are presented that address the above mentioned issues and contribute to further developments in formulation technologies. The in situ coating increases the economic efficiency by saving process steps in comparison to a conventional tableting process and provides a crystalline coating for a tailorable drug delivery rate. The freeze casting allows the control over the surface area of a drug delivery system (DDS) by providing different numbers and sizes of pores, which in conjunction with adequate additives offer an efficient instrument for drug delivery control, especially by accelerating the dissolution effect. Protein-based biocomposites are attractive materials for biomedical and pharmaceutical applications that can be applied as a polymeric DDS. They inherently combine degradability in vivo and in vitro, show a good biocompatibility, offer sites of adhesion for cells and may additionally be used to release embedded bioactive molecules. Here, a new approach regarding the incorporation of crystalline active pharmaceutical ingredients (API) into a protein matrix in one process step is presented. All three presented techniques mark decisive progress towards tailor-made drug delivery systems with respect to function, economic efficiency and the generation of additional values.

Enhancing the Thermal Conductivity of Epoxy Composites via Constructing Oriented ZnO Nanowire-Decorated Carbon Fibers Networks.

With the miniaturization and high integration of electronic devices, high-performance thermally conductive composites have received increasing attention. The construction of hierarchical structures is an effective strategy to reduce interfacial thermal resistance and enhance composite thermal conductivity. In this study, by decorating carbon fibers (CF) with needle-like ZnO nanowires, hierarchical hybrid fillers (CF@ZnO) were rationally designed and synthesized using the hydrothermal method, which was further used to construct oriented aligned filler networks via the simple freeze-casting process. Subsequently, epoxy (EP)-based composites were prepared using the vacuum impregnation method. Compared with the pure CF, the CF@ZnO hybrid fillers led to a significant increase in thermal conductivity, which was mainly due to the fact that the ZnO nanowires could act as bridging links between CF to increase more thermally conductive pathways, which in turn reduced interfacial thermal resistance. In addition, the introduction of CF@ZnO fillers was also beneficial in improving the thermal stability of the EP-based composites, which was favorable for practical thermal management applications.

Multiscale engineered artificial compact bone via bidirectional freeze-driven lamellated organization of mineralized collagen microfibrils.

Bone, renowned for its elegant hierarchical structure and unique mechanical properties, serves as a constant source of inspiration for the development of synthetic materials. However, achieving accurate replication of bone features in artificial materials with remarkable structural and mechanical similarity remains a significant challenge. In this study, we employed a cascade of continuous fabrication processes, including biomimetic mineralization of collagen, bidirectional freeze-casting, and pressure-driven fusion, to successfully fabricate a macroscopic bulk material known as artificial compact bone (ACB). The ACB material closely replicates the composition, hierarchical structures, and mechanical properties of natural bone. It demonstrates a lamellated alignment of mineralized collagen (MC) microfibrils, similar to those found in natural bone. Moreover, the ACB exhibits a similar high mineral content (70.9 %) and density (2.2 g/cm3) as natural cortical bone, leading to exceptional mechanical properties such as high stiffness, hardness, and flexural strength that are comparable to those of natural bone. Importantly, the ACB also demonstrates excellent mechanical properties in wet, outstanding biocompatibility, and osteogenic properties in vivo, rendering it suitable for a broad spectrum of biomedical applications, including orthopedic, stomatological, and craniofacial surgeries.

Development of Light, Strong, and Water-Resistant PVA Composite Aerogels.

A significant weakness of many organic and inorganic aerogels is their poor mechanical behaviour, representing a great impediment to their application. For example, polymer aerogels generally have higher ductility than silica aerogels, but their elastic modulus is considered too low. Herein, we developed extremely low loading (<1 wt%) 2D graphene oxide (GO) nanosheets modified poly (vinyl alcohol) (PVA) aerogels via a facile and environmentally friendly method. The aerogel shows a 9-fold increase in compressional modulus compared to a pure polymer aerogel. With a low density of 0.04 mg/mm3 and a thermal conductivity of only 0.035 W/m·K, it outperforms many commercial insulators and foams. As compared to a pure PVA polymer aerogel, a 170% increase in storage modulus is obtained by adding only 0.6 wt% GO nanosheets. The nanocomposite aerogel demonstrates strong fire resistance, with a 50% increase in burning time and little smoke discharge. After surface modification with 1H,1H,2H,2H-Perfluorodecyltriethoxysilane, the aerogel demonstrates water resistance, which is suitable for outdoor applications in which it would be exposed to precipitation. Our research demonstrates a new pathway for considerable improvement in the performance and application of polymer aerogels.

Freeze-Cast Ni-MOF Nanobelts/Chitosan-Derived Magnetic Carbon Aerogels for Broadband Electromagnetic Wave Absorption.

The exceptional benefits of carbon aerogels, including their low density and tunable electrical characteristics, infuse new life into the realm of creating ultralight electromagnetic wave absorbers. The clever conceptualization and straightforward production of carbon-based aerogels, which marry aligned microporous architecture with nanoscale heterointerfaces and atomic-scale defects, are vital for effective multiscale microwave response. We present an uncomplicated synthesis method for crafting aligned porous Ni@C nanobelts anchored on N, S-doped carbon aerogels (Ni@C/NSCAs), featuring multiscale structural intricacies─achieved through the pyrolysis of freeze-cast Ni-MOF nanobelts and chitosan aerogel composites. The well-ordered porous configuration, combined with multiple heterointerfaces adopting a "nanoparticles-nanobelts-nanosheets" contact schema, along with a wealth of defects, adeptly modulates conductive, polarization, and magnetic losses to realize an equilibrium in impedance matching. This magnetically doped carbon aerogel showcases an impressive effective absorption bandwidth of 8.96 GHz and a minimum reflection loss of -68.82 dB, while maintaining an exceptionally low filler content of 1.75 wt %. Additionally, the applied coating exhibits an astonishing radar cross-section reduction of 51.7 dB m2, signifying its superior radar wave scattering capabilities. These results offer key insights into the attainment of broad-spectrum microwave absorption features by enhancing the multiscale structure of current aerogels.

Anisotropic Elastomer Ionomer Composite-Based Strain Sensors: Achieving High Sensitivity and Wide Detection for Human Motion Detection and Wireless Transmission.

Anisotropic strain sensors capable of multidirectional sensing are crucial for advanced sensor applications in human motion detection. However, current anisotropic sensors encounter challenges in achieving a balance among high sensitivity, substantial stretchability, and a wide linear detection range. To address these challenges, a facile freeze-casting strategy was employed to construct oriented filler networks composed of carbon nanotubes and conductive carbon black within a brominated butyl rubber ionomer (iBIIR) matrix. The resulting anisotropic sensor based on the iBIIR composites exhibited distinct gauge factors (GF) in the parallel and vertical directions (GF∥ = 4.91, while GF⊥ = 2.24) and a broad linear detection range over a strain range of 190%. This feature enables the sensor to detect various human activities, including uniaxial pulse, finder bending, elbow bending, and cervical spine movements. Moreover, the ion-cross-linking network within the iBIIR, coupled with strong π-cation interactions between the fillers and iBIIR macromolecules, imparted high strength (12.3 MPa, nearly twice that of pure iBIIR) and an ultrahigh elongation at break (>1800%) to the composites. Furthermore, the sensor exhibited exceptional antibacterial effectiveness, surpassing 99% against both Escherichia coli and Staphylococcus aureus. Notably, the sensor was capable of wireless sensing. It is anticipated that anisotropic sensors will have extensive application prospects in flexible wearable devices.

The study of double-network carboxymethyl chitosan/sodium alginate based cryogels for rapid hemostasis in noncompressible hemorrhage.

Developing an injectable hemostatic dressing with shape recovery and high blood absorption ratio for rapid hemostasis in noncompressible hemorrhage maintains a critical clinical challenge. Here, double-network cryogels based on carboxymethyl chitosan, sodium alginate, and methacrylated sodium alginate were prepared by covalent crosslinking and physical crosslinking, and named carboxymethyl chitosan/methacrylated sodium alginate (CM) cryogels. Covalent crosslinking was achieved by methacrylated sodium alginate in the freeze casting process, while physical crosslinking was realized by electrostatic interaction between the amino group of carboxymethyl chitosan and the carboxyl group of sodium alginate. CM cryogels exhibited large water swelling ratios (8167 ± 1062 %), fast blood absorption speed (2974 ± 669 % in 15 s), excellent compressive strength (over 160 kPa for CM100) and shape recovery performance. Compared with gauze and commercial gelatin sponge, better hemostatic capacities were demonstrated for CM cryogel with the minimum blood loss of 40.0 ± 8.9 mg and the lowest hemostasis time of 5.0 ± 2.0 s at hemostasis of rat liver. Made of natural polysaccharides with biocompatibility, hemocompatibility, and cytocompatibility, the CM cryogels exhibit shape recovery and high blood absorption rate, making them promising to be used as an injectable hemostatic dressing for rapid hemostasis in noncompressible hemorrhage.

Freeze-Casting of Alumina and Permeability Analysis Based on a 3D Microstructure Reconstructed Using Generative Adversarial Networks.

In this study, alumina ceramics with hierarchical pores were successfully fabricated using freeze casting. Experimental studies show that both the solid loading of the slurry and the thermal insulation layer at the interface of the slurry and cooling plate can influence the pore characteristics of cast samples. In order to examine the pore characteristics and evaluate the permeability of the freeze-cast samples fabricated under different conditions, a generative adversarial network (GAN) method was employed to reconstruct the three-dimensional (3D) microstructure from two-dimensional (2D) scanning electron microscopy (SEM) images of the samples. Furthermore, GAN 3D reconstruction was validated against X-ray tomography 3D reconstruction results. Based on the GAN reconstructed microstructures, the permeability and pore distribution of the various samples were analyzed. The sample cast with 35 wt.% solid loading shows an optimal permeability.

Freeze casting to engineer gradient porosity in hydroxyapatite-boron nitride nanotube composite scaffold for improved compressive strength and osteogenic potential.

Graded porosity plays a crucial role in scaffolds for bone tissue engineering as it facilitates vital processes such as nutrient diffusion, cellular infiltration, and tissue integration. This paper explores the utilization of freeze casting (FC) as a technique to generate composite scaffolds comprising hydroxyapatite (HA) reinforced with 1D-boron nitride nanotubes (BNNTs) featuring graded porosity and improved compressive strength. Comparative studies were conducted using FC at room and sub-zero temperatures to assess the influence of temperature gradient and heat transfer rate on the production of gradient and aligned porosity in HA-BNNT composites. The FC process with a prolonged thermal gradient facilitated the creation of aligned pores in the HA-BNNT, exhibiting a wide distribution of 60% porosity ranging from 1 to 30 µm. Adding high strength 1 vol% BNNT reinforcement resulted in a remarkable 50% enhancement in compressive strength compared to the control sample. Osteoblasts seeded on the HA-BNNT substrate exhibited significantly higher alkaline phosphate activity, indicating accelerated mineralization compared to the control sample. Gradient porosity and wide pore distribution in the HA-BNNT scaffolds promoted osteogenic activities. Overall, the demonstrated FC processing technique and BNNT addition hold great potential for developing functional and biomimetic scaffolds that can effectively promote tissue regeneration, leading to improved clinical outcomes in bone tissue engineering applications.

A novel macroporous carboxymethyl chitosan/sodium alginate sponge dressing capable of rapid hemostasis and drug delivery.

Carboxymethyl chitosan (CMCS) and sodium alginate (SA), which are excellent polysaccharide-based hemostatic agents, are capable of forming polyelectrolyte complexes (PEC) through electrostatic interactions. However, CMCS/SA PEC sponges prepared by the conventional sol-gel process exhibited slow liquid absorption rate and poor mechanical properties post-swelling. In this work, a novel strategy involving freeze casting followed by acetic acid vapor treatment to induce electrostatic interactions was developed to fabricate novel PEC sponges with varying CMCS/SA mass ratios. Compared to sol-gel process sponge, the novel sponge exhibited a higher density of electrostatic interactions, resulting in denser pore walls that resist re-gelation and swelling according to FTIR, XRD, and SEM analyses. Additionally, the liquid absorption kinetics, as well as compression and tension tests, demonstrated that the novel sponge had significantly improved rapid blood absorption capacity and mechanical properties. Furthermore, in vitro coagulation and drug release studies showed that the novel sponge had a lower blood clotting index and clotting time, along with a slower drug release rate after loading with berberine hydrochloride, showcasing its potential as a rapid hemostatic dressing with controlled drug release capabilities.