Freeze Casting: From Low-Dimensional Building Blocks to Aligned Porous Structures-A Review of Novel Materials, Methods, and Applications

Freeze casting, also known as ice templating, is a particularly versatile technique that has been applied extensively for the fabrication of well-controlled biomimetic porous materials based on ceramics, metals, polymers, biomacromolecules, and carbon nanomaterials, endowing them with novel properties and broadening their applicability. The principles of different directional freeze-casting processes are described and the relationships between processing and structure are examined. Recent progress in freeze-casting assisted assembly of low dimensional building blocks, including graphene and carbon nanotubes, into tailored micro- and macrostructures is then summarized. Emerging trends relating to novel materials as building blocks and novel freeze-cast geometries-beads, fibers, films, complex macrostructures, and nacre-mimetic composites-are presented. Thereafter, the means by which aligned porous structures and nacre mimetic materials obtainable through recently developed freeze-casting techniques and low-dimensional building blocks can facilitate material functionality across multiple fields of application, including energy storage and conversion, environmental remediation, thermal management, and smart materials, are discussed.

Ice Templating Soft Matter: Fundamental Principles and Fabrication Approaches to Tailor Pore Structure and Morphology and Their Biomedical Applications

Porous scaffolds are widely used in biomedical applications where pore size and morphology influence a range of biological processes, including mass transfer of solutes, cellular interactions and organization, immune responses, and tissue vascularization, as well as drug delivery from biomaterials. Ice templating, one of the most widely utilized techniques for the fabrication of porous materials, allows control over pore morphology by controlling ice formation in a suspension of solutes. By fine-tuning freezing and solute parameters, ice templating can be used to incorporate pores with tunable morphological features into a wide range of materials using a simple, accessible, and scalable process. While soft matter is widely ice templated for biomedical applications and includes commercial and clinical products, the principles underpinning its ice templating are not reviewed as well as their inorganic counterparts. This review describes and critically evaluates fundamental principles, fabrication and characterization approaches, and biomedical applications of ice templating in polymer-based biomaterials. It describes the utility of porous scaffolds in biomedical applications, highlighting biological mechanisms impacted by pore features, outlines the physical and thermodynamic mechanisms underpinning ice templating, describes common fabrication setups, critically evaluates complexities of ice templating specific to polymers, and discusses future directions in this field.

High Axial Ratio Nanochitins for Ultrastrong and Shape-Recoverable Hydrogels and Cryogels via Ice Templating

High yield (>85%) and low-energy deconstruction of never-dried residual marine biomass is proposed following partial deacetylation and microfluidization. This process results in chitin nanofibrils (nanochitin, NCh) of ultrahigh axial size (aspect ratios of up to 500), one of the largest for bioderived nanomaterials. The nanochitins are colloidally stable in water (ζ-potential = +95 mV) and produce highly entangled networks upon pH shift. Viscoelastic and strong hydrogels are formed by ice templating upon freezing and thawing with simultaneous cross-linking. Slow supercooling and ice nucleation at -20 °C make ice crystals grow slowly and exclude nanochitin and cross-linkers, becoming spatially confined at the interface. At a nanochitin concentration as low as 0.4 wt %, highly viscoelastic hydrogels are formed, with a storage modulus of ∼16 kPa, at least an order of magnitude larger compared to those measured for the strongest chitin-derived hydrogels reported so far. Moreover, the water absorption capacity of the hydrogels reaches a value of 466 g g^-1. Lyophilization is effective in producing cryogels with a density that can be tailored in a wide range of values, from 0.89 to 10.83 mg·cm^-3, and corresponding porosity, between 99.24 and 99.94%. Nitrogen adsorption results indicate reversible adsorption and desorption cycles of macroporous structures. A fast shape recovery is registered from compressive stress-strain hysteresis loops. After 80% compressive strain, the cryogels recovered fast and completely upon load release. The extreme values in these and other physical properties have not been achieved before for neither chitin nor nanocellulosic cryogels. They are explained to be the result of (a) the ultrahigh axial ratio of the fibrils and strong covalent interactions; (b) the avoidance of drying before and during processing, a subtle but critical aspect in nanomanufacturing with biobased materials; and (c) ice templating, which makes the hydrogels and cryogels suitable for advanced biobased materials.

Topotactic Fibrillogenesis of Freeze-Cast Microridged Collagen Scaffolds for 3D Cell Culture

Type I collagen is the main component of the extracellular matrix (ECM). In vitro, under a narrow window of physicochemical conditions, type I collagen self-assembles to form complex supramolecular architectures reminiscent of those found in native ECM. Presently, a major challenge in collagen-based biomaterials is to couple the delicate collagen fibrillogenesis events with a controlled shaping process in non-denaturating conditions. In this work, an ice-templating approach promoting the structuration of collagen into macroporous monoliths is used. Instead of common solvent removal procedures, a new topotactic conversion approach yielding self-assembled ordered fibrous materials is implemented. These collagen-only, non-cross-linked scaffolds exhibit uncommon mechanical properties in the wet state, with a Young's modulus of 33 ± 12 kPa, an ultimate tensile stress of 33 ± 6 kPa, and a strain at failure of 105 ± 28%. With the help of the ice-patterned microridge features, normal human dermal fibroblasts and C2C12 murine myoblasts successfully migrate and form highly aligned populations within the resulting three-dimensional (3D) collagen scaffolds. These results open a new pathway to the development of new tissue engineering scaffolds ordered across various organization levels from the molecule to the macropore and are of particular interest for biomedical applications where large-scale 3D cell alignment is needed such as for muscular or nerve reconstruction.

Pressure-Responsive Hierarchical Chiral Photonic Aerogels

Pressure-responsive chiral photonic aerogels are fabricated by combining liquid crystal self-assembly and ice-templating processes. The aerogels have a hierarchical structure in which the primary 2D chiral nematic structured walls of cellulose nanocrystals form ribbons that support a secondary 3D cellular network. Owing to the flexibility of the aerogels in solvent, the 3D structure of the aerogel can easily be transformed to a 2D structure by pressure-induced rearrangement. The aerogels vary from white in color, which arises from light scattering, to a reflective photonic crystal displaying bright iridescent colors that depend on the immersed solvent. A solvent-sensitive ink that shows quick color response to different solvents is designed using the pressure-responsive photonic aerogel. This material demonstrates a new response mechanism for the design of smart and mechanoresponsive photonic materials.

Hierarchically Porous Graphitic Carbon with Simultaneously High Surface Area and Colossal Pore Volume Engineered via Ice Templating

Developing hierarchical porous carbon (HPC) materials with competing textural characteristics such as surface area and pore volume in one material is difficult to accomplish, particularly for an atomically ordered graphitic carbon. Herein we describe a synthesis strategy to engineer tunable HPC materials across micro-, meso-, and macroporous length scales, allowing the fabrication of a graphitic HPC material (HPC-G) with both very high surface area (>2500 m²/g) and pore volume (>11 cm³/g), the combination of which has not been attained previously. The mesopore volume alone for these materials is up to 7.53 cm³/g, the highest ever reported, higher than even any porous carbon's total pore volume, which for our HPC-G material was >11 cm³/g. This HPC-G material was explored for use both as a supercapacitor electrode and for oil adsorption, two applications that require either high surface area or large pore volume, textural properties that are typically exclusive to one another. We accomplished these high textural characteristics by employing ice templating not only as a route for macroporous formation but as a synergistic vehicle that enabled the significant loading of the mesoporous hard template. This design scheme for HPC-G materials can be utilized in broad applications, including electrochemical systems such as batteries and supercapacitors, sorbents, and catalyst supports, particularly supports where a high degree of thermal stability is required.

Fire-Retardant, Self-Extinguishing Inorganic/Polymer Composite Memory Foams

Polymeric foams used in furniture and automotive and aircraft seating applications rely on the incorporation of environmentally hazardous fire-retardant additives to meet fire safety norms. This has occasioned significant interest in novel approaches to the elimination of fire-retardant additives. Foams based on polymer nanocomposites or based on fire-retardant coatings show compromised mechanical performance and require additional processing steps. Here, we demonstrate a one-step preparation of a fire-retardant ice-templated inorganic/polymer hybrid that does not incorporate fire-retardant additives. The hybrid foams exhibit excellent mechanical properties. They are elastic to large compressional strain, despite the high inorganic content. They also exhibit tunable mechanical recovery, including viscoelastic "memory". These hybrid foams are prepared using ice-templating that relies on a green solvent, water, as a porogen. Because these foams are predominantly comprised of inorganic components, they exhibit exceptional fire retardance in torch burn tests and are self-extinguishing. After being subjected to a flame, the foam retains its porous structure and does not drip or collapse. In micro-combustion calorimetry, the hybrid foams show a peak heat release rate that is only 25% that of a commercial fire-retardant polyurethanes. Finally, we demonstrate that we can use ice-templating to prepare hybrid foams with different inorganic colloids, including cheap commercial materials. We also demonstrate that ice-templating is amenable to scale up, without loss of mechanical performance or fire-retardant properties.

Hierarchical Mesoporous Lithium-Rich Li[Li0.2Ni0.2Mn0.6]O2 Cathode Material Synthesized via Ice Templating for Lithium-Ion Battery

Tuning hierarchical micro/nanostructure of electrode materials is a sought-after means to reinforce their electrochemical performance in the energy storage field. Herein, we introduce a type of hierarchical mesoporous Li[Li0.2Ni0.2Mn0.6]O2 microsphere composed of nanoparticles synthesized via an ice templating combined coprecipitation strategy. It is a low-cost, eco-friendly, and easily operated method using ice as a template to control material with homogeneous morphology and rich porous channels. The as-prepared material exhibits remarkably enhanced electrochemical performances with higher capacity, more excellent cycling stability and more superior rate property, compared with the sample prepared by conventional coprecipitation method. It has satisfactory initial discharge capacities of 280.1 mAh g(-1) at 0.1 C, 207.1 mAh g(-1) at 2 C, and 152.4 mAh g(-1) at 5 C, as well as good cycle performance. The enhanced electrochemical performance can be ascribed to the stable hierarchical microsized structure and the improved lithium-ion diffusion kinetics from the highly porous structure.

Ultrahigh Damping Capacities in Lightweight Structural Materials

The demand to outperform current technologies pushes scientists to develop novel strategies, which enable the fabrication of materials with exceptional properties. Along this line, lightweight structural materials are of great interest due to their versatile applicability as sensors, catalysts, battery electrodes, and acoustic or mechanical dampers. Here, we report a strategy to design ultralight (ρ = 3 mg/cm³) and hierarchically structured ceramic scaffolds of macroscopic size. Such scaffolds exhibit mechanical reversibility comparable to that of microscopic metamaterials, leading to a macroscopically remarkable dynamic mechanical performance. Upon mechanical loading, these scaffolds show a deformation mechanism similar to polyurethane foams, and this resilience yields ultrahigh damping capacities, tan δ, of up to 0.47.

Linking Catalyst-Coated Isotropic Colloids into "Active" Flexible Chains Enhances Their Diffusivity

Active colloids are not constrained by equilibrium: ballistic propulsion, superdiffusive behavior, or enhanced diffusivities have been reported for active Janus particles. At high concentrations, interactions between active colloids give rise to complex emergent behavior. Their collective dynamics result in the formation of several hundred particle-strong flocks or swarms. Here, we demonstrate significant diffusivity enhancement for colloidal objects that neither have a Janus architecture nor are at high concentrations. We employ uniformly catalyst-coated, viz. chemo-mechanically, isotropic colloids and link them into a chain to enforce proximity. Activity arises from hydrodynamic interactions between enchained colloidal beads due to reaction-induced phoretic flows catalyzed by platinum nanoparticles on the colloid surface. This results in diffusivity enhancements of up to 60% for individual chains in dilute solution. Chains with increasing flexibility exhibit higher diffusivities. Simulations accounting for hydrodynamic interactions between enchained colloids due to active phoretic flows accurately capture the experimental diffusivity. These simulations reveal that the enhancement in diffusivity can be attributed to the interplay between chain conformational fluctuations and activity. Our results show that activity can be used to systematically modulate the mobility of soft slender bodies.

Characterization of Anisotropic Human Hair Keratin Scaffolds Fabricated via Directed Ice Templating

Human hair keratin (HHK) is successfully exploited as raw materials for 3D scaffolds for soft tissue regeneration owing to its excellent biocompatibility and bioactivity. However, most HHK scaffolds are not able to achieve the anisotropic mechanical properties of soft tissues such as tendons and ligaments due to lack of tunable, well-defined microstructures. In this study, directed ice templating method is used to fabricate anisotropic HHK scaffolds that are characterized by aligned pores (channels) in between keratin layers in the longitudinal plane. In contrast, pores in the transverse plane maintain a homogenous rounded morphology. Channel widths throughout the scaffolds range from ≈5 to ≈15 µm and are tunable by varying the freezing temperature. In comparison with HHK scaffolds with random, isotropic pore structures, the tensile strength of anisotropic HHK scaffolds is enhanced significantly by up to fourfolds (≈200 to ≈800 kPa) when the tensile load is applied in the direction parallel to the aligned pores. In vitro results demonstrate that the anisotropic HHK scaffolds are able to support human dermal fibroblast adhesion, spreading, and proliferation. The findings suggest that HHK scaffolds with well-defined, aligned microstructure hold promise as templates for soft tissues regeneration by mimicking their anisotropic properties.

Radial Sponges Facilitate Wound Healing by Promoting Cell Migration and Angiogenesis

The topographic cues of wound dressings play important roles in regulating cellular behaviors, such as cellular migration and morphology, and are capable of providing a prolonged stimulus for promoting wound healing. However, 3D porous dressings that can guide wound healing from the periphery to the center are poorly studied. Herein, radial sponges with adjustable lamellar spacing and microridge spacing by ice templating are developed to facilitate wound healing. With denser lamellae and microridges, fibroblasts achieve a more orderly arrangement, a larger elongation, and a greater migration rate. Meanwhile, the elongated state enables human umbilical vein endothelial cells to vascularization. The faster healing rate and a higher degree of vascularization based on radial sponges are further demonstrated in full-thickness skin defects in rats. Taken together, radial sponges with the densest lamellae and microridges perform the best in guiding the wound from the periphery to the center of the repair environment. It is believed that the proposed structure here can be combined with various biochemical factors to provide dressings with functions.

Freeform 3D Ice Printing (3D-ICE) at the Micro Scale

Water is one of the most important elements for life on earth. Water's rapid phase-change ability along with its environmental and biological compatibility also makes it a unique structural material for 3D printing of ice structures reproducibly and accurately. This work introduces the freeform 3D ice printing (3D-ICE) process for high-speed and reproducible fabrication of ice structures with micro-scale resolution. Drop-on-demand deposition of water onto a -35 °C platform rapidly transforms water into ice. The dimension and geometry of the structures are critically controlled by droplet ejection frequency modulation and stage motions. The freeform approach obviates layer-by-layer construction and support structures, even for overhang geometries. Complex and overhang geometries, branched hierarchical structures with smooth transitions, circular cross-sections, smooth surfaces, and micro-scale features (as small as 50 µm) are demonstrated. As a sample application, the ice templates are used as sacrificial geometries to produce resin parts with well-defined internal features. This approach could bring exciting opportunities for microfluidics, biomedical devices, soft electronics, and art.

Hybrid Transition Metal Dichalcogenide/Graphene Microspheres for Hydrogen Evolution Reaction

A peculiar 3D graphene-based architecture, i.e., partial reduced-Graphene Oxide Aerogel Microspheres (prGOAM), having a dandelion-like morphology with divergent microchannels to implement innovative electrocatalysts for the hydrogen evolution reaction (HER) is investigated in this paper. prGOAM was used as a scaffold to incorporate exfoliated transition metals dichalcogenide (TMDC) nanosheets, and the final hybrid materials have been tested for HER and photo-enhanced HER. The aim was to create a hybrid material where electronic contacts among the two pristine materials are established in a 3D architecture, which might increase the final HER activity while maintaining accessible the TMDC catalytic sites. The adopted bottom-up approach, based on combining electrospraying with freeze-casting techniques, successfully provides a route to prepare TMDC/prGOAM hybrid systems where the dandelion-like morphology is retained. Interestingly, the microspherical morphology is also maintained in the tested electrode and after the electrocatalytic experiments, as demonstrated by scanning electron microscopy images. Comparing the HER activity of the TMDC/prGOAM hybrid systems with that of TMDC/partially reduced-Graphene Oxide (prGO) and TMDC/Vulcan was evidenced in the role of the divergent microchannels present in the 3D architecture. HER photoelectron catalytic (PEC) tests have been carried out and demonstrated an interesting increase in HER performance.

Immobilizing Bimetallic RuCo Nanoalloys on Few-Layered MXene as a Robust Bifunctional Electrocatalyst for Overall Water Splitting

Doping Co atoms into Ru lattices can tune the electronic structure of active sites, and the conductive MXene can adjust the electrical conductivity of catalysts, which are both favorable for improving the electrocatalytic activity of the catalyst for water splitting. Here, ruthenium-cobalt bimetallic nanoalloys coupled with exfoliated Ti3 C2 Tx MXene (RuCo-Ti3 C2 Tx ) have been constructed by ice-templated and thermal activation. Due to the strong interaction between the RuCo nanoalloys and conductive MXene, RuCo-Ti3 C2 Tx not only exhibits an excellent hydrogen evolution reaction (HER) performance with a low overpotential and Tafel slope (60 mV, 34.8 mV dec-1 in 0.5 M H2 SO4 and 52 mV, 38.7 mV dec-1 in 1 M KOH), but also good oxygen evolution reaction (OER) performance in an alkaline electrolyte (266 mV, 111.1 mV dec-1 in 1 M KOH). The assembled RuCo-Ti3 C2 Tx ||RuCo-Ti3 C2 Tx electrolyzer requires a lower potential (1.56 V) than does the Pt/C||RuO2 electrolyzer at 10 mA cm-2 . A boosted catalytic HER activity from immobilizing the RuCo nanoalloys on MXene was unveiled by density functional theory calculations. This study provides a feasible and efficient strategy for developing MXene-based catalysts for overall water splitting.

Shape-Recoverable Macroporous Nanocomposite Hydrogels Created via Ice Templating Polymerization for Noncompressible Wound Hemorrhage

Uncontrolled hemorrhage resulting from severe trauma or surgical operations remains a challenge. It is highly important to develop functional materials to treat noncompressible wound bleeding. In this work, a shape-recoverable macroporous nanocomposite hydrogel was facilely created through ice templating polymerization. The covalently cross-linked gelatin networks provide a robust framework, while the Laponite nanoclay disperses into the three-dimensional matrix, enabling mechanical reinforcement and hemostatic functions. The resultant macroporous nanocomposite hydrogel possesses an inherent interconnected macroporous structure and rapid deformation recovery. In vitro assessments indicate that the hydrogel displays good cytocompatibility and a low hemolysis ratio. The hydrogel shows a higher coagulation potential and more erythrocyte adhesion compared to the commercial gauze and gelatin sponge. The noncompressible liver hemorrhage models also confirm its promising hemostasis performance. This strategy of combining a nano-enabled solution with ice templating polymerization displays great potential to develop appealing absorbable macroporous biomaterials for rapid hemostasis.

Open-Cellular Alumina Foams with Hierarchical Strut Porosity by Ice Templating: A Thickening Agent Study

Alumina replica foams were manufactured by the Schwartzwalder sponge replication technique and were provided with an additional strut porosity by a freeze-drying/ice-templating step prior to thermal processing. A variety of thickeners in combination with different alumina solid loads in the dispersion used for polyurethane foam template coating were studied. An additional strut porosity as generated by freeze-drying was found to be in the order of ~20%, and the spacings between the strut pores generated by ice-templating were in the range between 20 µm and 32 µm. In spite of the lamellar strut pore structure and a total porosity exceeding 90%, the compressive strength was found to be up to 1.3 MPa. Combining the replica process with freeze-drying proves to be a suitable method to enhance foams with respect to their surface area accessible for active coatings while preserving the advantageous flow properties of the cellular structure. A two-to-threefold object surface-to-object volume ratio of 55 to 77 mm⁻¹ was achieved for samples with 30 vol% solid load compared to 26 mm⁻¹ for non-freeze-dried samples. The freeze-drying technique allows the control of the proportion and properties of the introduced pores in an uncomplicated and predictable way by adjusting the process parameters. Nevertheless, the present article demonstrates that a suitable thickener in the dispersion used for the Schwartzwalder process is inevitable to obtain ceramic foams with sufficient mechanical strength due to the necessarily increased water content of the ceramic dispersion used for foam manufacturing.

Ice-Templated and Cross-Linked Xanthan-Based Hydrogels: Towards Tailor-Made Properties

The use of polysaccharides with good film-forming properties in food packaging systems is a promising area of research. Xanthan gum (XG), an extracellular polysaccharide, has many industrial uses, including as a common food additive (E415). It is an effective thickening agent, emulsifier, and stabilizer that prevents ingredients from separating. Nevertheless, XG-based polymer films have some disadvantages, such as poor mechanical properties and high hydrophilic features, which reduce their stability when exposed to moisture and create difficulties in processing and handling. Thus, the objective of this work was to stabilize a XG matrix by cross-linking it with glycerol diglycidyl ether, 1,4-butanediol diglycidyl ether, or epichlorohydrin below the freezing point of the reaction mixture. Cryogelation is an ecological, friendly, and versatile method of preparing biomaterials with improved physicochemical properties. Using this technique, XG-based cryogels were successfully prepared in the form of microspheres, monoliths, and films. The XG-based cryogels were characterized by FTIR, SEM, AFM, swelling kinetics, and compressive tests. A heterogeneous morphology with interconnected pores, with an average pore size depending on both the nature of the cross-linker and the cross-linking ratio, was found. The use of a larger amount of cross-linker led to both a much more compact structure of the pore walls and to a significant decrease in the average pore size. The uniaxial compression tests indicated that the XG-based cryogels cross-linked with 1,4-butanediol diglycidyl ether exhibited the best elasticity, sustaining maximum deformations of 97.67%, 90.10%, and 81.80%, respectively.

In Situ Incorporation of Pores and Nanoparticles into Polymer Surfaces Using Melt Crystallization

Engineering the surface properties of a material without affecting its bulk properties is desirable for many applications, but it is often not readily achievable because it requires a complex series of processing steps. This study presents for the first time a simple and straightforward method that can convert regular flat polymer surfaces into various porous composite surfaces. The controlled dissolution of the polymer surface by a dispersion and subsequent melt crystallization allows for the successful embedding of dispersed inorganic or organic particles in the pore walls. The porous surface morphology is not significantly affected by the type of dispersed particle, but the contact and sliding angles and surface wettability are. Both superhydrophobic and oil/water separation surfaces can be conveniently fabricated from flat polymer surfaces. This novel and versatile technique could provide engineering freedom for the future development of various functional materials.

Wood-Inspired Cement with High Strength and Multifunctionality

Taking lessons from nature offers an increasing promise toward improved performance in man-made materials. Here new cement materials with unidirectionally porous architectures are developed by replicating the designs of natural wood using a simplified ice-templating technique in light of the retention of ice-templated architectures by utilizing the self-hardening nature of cement. The wood-like cement exhibits higher strengths at equal densities than other porous cement-based materials along with unique multifunctional properties, including effective thermal insulation at the transverse profile, controllable water permeability along the vertical direction, and the easy adjustment to be water repulsive by hydrophobic treatment. The strengths are quantitatively interpreted by discerning the effects of differing types of pores using an equivalent element approach. The simultaneous achievement of high strength and multifunctionality makes the wood-like cement promising for applications as new building materials, and verifies the effectiveness of wood-mimetic designs in creating new high-performance materials. The simple fabrication procedure by omitting the freeze-drying treatment can also promote a better efficiency of ice-templating technique for the mass production in engineering and may be extended to other material systems.

Enhancing the Conductivity of PEDOT:PSS Films by the Confinement of Ice Crystals

Rapid developments in organic electronics demand highly conductive and freestanding (PEDOT:PSS) films. However, the synthesis of highly conductive PEDOT:PSS films requires toxic reagents, such as high-concentration acids and bases. Herein, an eco-friendly and cost-effective strategy is reported for improving the conductivity of PEDOT:PSS films through the confinement of ice crystals. The crystallization of water swelled by the film facilitated the phase separation of PEDOT and PSS, and the excess PSS in the skin layer is effectively removed. Moreover, under the confinement effect, the carrier mobility of the film is enhanced through the formation of a well-crystallized PEDOT molecular morphology. A detailed elucidation of aggregate structure evolution in PEDOT:PSS films during annealing, solvent post-treatment, and subsequent confined crystallization is presented herein. After multiple water crystallization cycles, the conductivity of the PEDOT:PSS film increased by over 85%, achieving a maximum of 2564 ± 142 S cm-1. Finally, compared to post-treatment with dimethyl sulfoxide (DMSO), the current strategy can improve the Seebeck coefficient by 5.6% and the power factor by 139%.

Highly Permeable and Regenerative Microhoneycomb Filters

Allergic reactions can profoundly influence the quality of life. To address the health risks posed by allergens and overcome the permeability limitations of the current filter materials, this work introduces a novel microhoneycomb (MH) material for practical filter applications such as masks. Through a synthesis process integrating ice-templating and a gas-phase post-treatment with silane, MH achieves unprecedented levels of moisture resistance and mechanical stability while preserving the highly permeable microchannels. Notably, MH is extremely elastic, with a 92% recovery rate after being compressed to 80% deformation. The filtration efficiency surpasses 98.1% against pollutant particles that simulate airborne pollens, outperforming commercial counterparts with fifth-fold greater air permeability while ensuring unparalleled user comfort. Moreover, MH offers a sustainable solution, being easily regenerated through back-flow blowing, distinguishing it from conventional nonwoven fabrics. Finally, a prototype mask incorporating MH is presented, demonstrating its immense potential as a high-performance filtration material, effectively addressing health risks posed by allergens and other harmful particles.

Independent Configuration and Reprogramming of Porous Characters in Macroporous Hydrogel Enabled by the Orthogonal Dynamic Network

Macroporous hydrogels have attracted much attention in both industry and academia, where the morphological characteristics of pores are essential. Despite significant improvements on regulating porous structures, the independent configuration and reprogramming of porosity and pore size still remain challenging owing to the lack of a chemical design to decouple the mechanism for adjusting each characteristic. Here, we report a strategy to adaptively control porous features relying on an orthogonal dynamic network. Disulfide bonds are employed to relax polymer chains during freezing via UV irradiation, thus, generating pores in hydrogels. On such a basis, the porosity is continuously switched from 0 to 75% by controlling network relaxation ratios. Subsequently, the pore size is further reversibly manipulated through the association or dissociation of dynamic metallic coordination. As a result, the porosity and pore size achieved independent configurations. Meanwhile, the dynamic nature of the network makes it possible to reprogram the porous character of a prepared hydrogel. Beyond these, the photopatterning of pores represents the capability to regulate the third feature. Our strategy provides an effective way to arbitrarily manipulate porous morphologies, which can inspire the design of future functional porous materials.

Self-Supported Branched Poly(ethylenimine) Monoliths from Inverse Template 3D Printing for Direct Air Capture

3D-printed inverse templates are combined with ice templating to develop self-supported branched poly(ethylenimine) monoliths with regular channels of varying channel density and ordered macropores. A maximum uptake of 0.96 mmol of CO2/g of monolith from ambient air containing 45.5% RH is achieved from dynamic breakthrough experiments, which is a 31% increase compared to the CO2 uptake from adsorption under dry conditions for the same duration. The breakthrough experiments show characteristics of internal mass-transfer limitations. The cyclic dynamic breakthrough experiments indicate stable operation without significant loss in CO2 uptake across eight cycles. Moreover, the self-supported monolith shows minimal loss in adsorption capacity (7.7%) upon exposure to air containing 21% oxygen at 110 °C, in comparison to a conventional sorbent consisting of poly(ethylenimine) impregnated on Al2O3 (18.9%). The monoliths exhibit good mechanical stability, contributed by elastic deformation, corresponding to up to 74% strain and lower pressure drop compared to many existing monoliths in the literature.

Oriented Porous NASICON 3D Framework via Freeze-Casting for Sodium-Metal Batteries

Sodium-metal batteries are promising candidates for low-cost, large-format energy storage systems. However, sodium-metal batteries suffer from high interfacial resistance between the electrodes and the solid electrolyte, leading to poor electrochemical performance. We demonstrate a sodium superionic conductor (NASICON) with an oriented porous framework of sodium aluminum titanium phosphate (NATP) fabricated by the freeze-casting technique, which shows excellent properties as a solid electrolyte. Using X-ray computed tomography, we confirm the uniform low-tortuosity channels present along the thickness of the scaffold. We infiltrated the porous NATP scaffolds with sodium vanadium phosphate (NVP) cathode nanoparticles achieving mass loadings of ∼3-4 mg cm^-2, which enables short sodium ion diffusion path lengths. For the resulting hybrid cell, we achieved a capacity of ∼90 mAh g^-1 at a specific current of 50 mA g^-1 (∼300 Wh kg^-1) for over 100 cycles with ∼94% capacity retention. Our study offers valuable insights for the design of hybrid solid electrolyte-cathode active material structures to achieve improved electrochemical performance through low-tortuosity ion transport networks.