Diffusion-Freezing-Induced Microphase Separation for Constructing Large-Area Multiscale Structures on Hydrogel Surfaces

Semi-crystalline polymers with supramolecular synergistic interactions: from mechanical toughening to dynamic smart materials

Semi-crystalline polymers (SCPs) with anisotropic amorphous and crystalline domains as the basic skeleton are ubiquitous from natural products to synthetic polymers. The combination of chemically incompatible hard and soft phases contributes to unique thermal and mechanical properties. The further introduction of supramolecular interactions as noncovalently interacting crystal phases and soft dynamic crosslinking sites can synergize with covalent polymer chains, thereby enabling effective energy dissipation and dynamic rearrangement in hierarchical superstructures. Therefore, this review will focus on the design principles of SCPs by discussing supramolecular construction strategies and state-of-the-art functional applications from mechanical toughening to sophisticated functions such as dynamic adaptivity, shape memory, ion transport, etc. Current challenges and further opportunities are discussed to provide an overview of possible future directions and potential material applications.

Nature's coatings: Sodium alginate as a novel coating in safeguarding plants from frost damages

Frost damage remains a significant challenge for agricultural practices worldwide, leading to substantial economic losses and food insecurity. Practically, traditional methods for frost management have proven ineffective and come with several drawbacks, such as energy consumption and limited efficacy. Hence, proposing an anti-freezing coating can be an innovative idea. The potential of sodium alginate (SA) to construct anti-freezing hydrogels has been explored in several sciences. SA hydrogels can form protective films around plants as a barrier against freezing temperatures and ice crystals on the plant's surface. Sodium alginate exhibits excellent water retention, enhancing plant hydration during freezing conditions. This coating can provide insulation, effectively shielding the plant from frost damage. The advantages of SA as a coating material, such as its biocompatibility, biodegradability, and non-toxic nature, are highlighted. Therefore, the proposed use of SA as an innovative coating material holds promise for safeguarding plants from frost damage. Following SA potential and frost's huge damage, the present review provides a comprehensive overview of the recent developments in SA-based anti-freezing hydrogels, their applications, and their potential in agriculture as anti-freezing coatings. However, further research and field trials are necessary to optimize the application methods and understand the long-term effects on productivity.

Leveraging Microgels Prepared from Poly(ethylene glycol) Bisepoxide and Polyetheramine for Versatile Surface Structuring of Agarose Hydrogels

We demonstrate a macromer-type bisepoxide, poly(ethylene glycol) diglycidyl ether, polymerizing readily with a trifunctional polyetheramine Jeffamine T-403 in water to facilitate the development of a series of microgels abbreviated as PMG. Simply by varying the concentration of the as-prepared thermoresponsive intermediate prepolymer from 1 to 2 and 4%, hydrodynamic sizes of the resulting P1MG, P2MG, and P4MG are easily tuned in the submicrometer to micrometer range shown by the dynamic light scattering results. Besides size difference, these microgels also deform differently, where the drying-induced deformation effect is most severe for P1MG and least prominent for P4MG. Simple evaporative deposition of PMG into multilayer packing provides versatile and green options for microgel-mediated surface structuring of agarose hydrogels. Specifically, deformabile P1MG- and P2MG-derived coatings render agarose gel microwrinkle textures by buckling against swelling-induced surface instability. Conversely, stiffer P4MG microgels lead to a patchy patterned hierarchical coating on agarose, similar to the cracking effect in drying colloidal films. The straightforward microgel-on-macrogel strategy allows integration of both wrinkle and patchy patterns to generate Janus-type agarose gels, just by rationally arranging the coating sequence. Diversifying topographic features attainable through microgel-based coatings on hydrogels could potentially make the sustainable and biocompatible material of agarose a more compelling choice for bioapplications. Brief demonstrations of the broad applicability of P1MG toward wrinkling of proteinaceous and synthetic hydrogels further highlight promising prospects of the PMG microgel-on-macrogel functionalization strategy.

Regulable Mixed-Solvent-Induced Phase Separation in Hydrogels for Information Encryption

The rapid progress of information technology is accompanied by plenty of information embezzlement and forgery, but developing advanced encryption technologies to ensure information security remains challenging. Phase separation commonly leads to a dramatic change in the transmittance of hydrophilic polymer networks, which is a potential method for information security but is often neglected. Here, taking the polyacrylamide (PAAm) hydrogel system as a typical example, facilely adjustable information encryption and decryption via its regulable phase separation process in ethanol/water mixed solvent, are reported. By controlling the osmotic pressure of the external and internal environment, it is demonstrated that the diffusion coefficient during deswelling and reswelling, as well as the corresponding change of transmittance of the gel, can be well controlled. Relatively high osmotic pressure leads to rapid phase separation of the initial gel but slow phase remixing of the phase-separated gel, opening the opportunity of applying the gel as a reversible information encryption device. As proof-of-concept demonstrations, several stable and reversible information encryption and decryption systems by making use of the phase separation process of the gels are designed, which are expected to inspire the development of next-generation soft devices for information technology.

ZnO/ZnS-Polyvinyl Alcohol Hydrogel for Photocatalytic H2-Generation

The separation of nanoparticles from a solution-based photocatalytic reaction is a significant problem in practical applications. To address the issue, we developed a new photocatalyst composite based on ZnO-ZnS heterojunction (ZnOS) embedded in polyvinyl alcohol (PVA) hydrogel, which showed satisfactory results for photocatalyst recycling. PVA-ZnOS composite hydrogel was fabricated by freezing-induced gelation, which enabled the encapsulation of ZnOS nanoparticles into polymeric matrices. PVA hydrogel served as a promising candidate in photocatalytic applications due to its excellent properties such as high transparency, porosity, hydrophilicity, and stability under ultraviolet (UV) light. PVA-ZnOS hydrogel showed worthy activity in H2 generation from Na2S/Na2SO3 aqueous solution under UV radiation with a production rate of 18.8 µmol.h−1. PVA-ZnOS composite hydrogel is a separation-free photocatalyst, which is prospective in a solution-based photocatalytic reactor.

Load-bearing hydrogels ionically reinforced through competitive ligand exchanges

Fast advances in soft robotics and tissue engineering demand for new soft materials whose mechanical properties can be interchangeably and locally varied, thereby enabling, for example, the design of soft joints within an integral material. Inspired by nature, we introduce a competitive ligand-mediated approach to selectively and interchangeably reinforce metal-coordinated hydrogels. This is achieved by reinforcing carboxylate-containing hydrogels with Fe³⁺ ions. Key to achieving a homogeneous, predictable reinforcement of the hydrogels is the presence of weak complexation agents that delay the formation of metal-complexes within the hydrogels, thereby allowing a homogeneous distribution of the metal ions. The resulting metal-reinforced hydrogels show a compressive modulus of up to 2.5 MPa, while being able to withstand pressures as high as 0.6 MPa without appreciable damage. Competitive ligand exchanges offer an additional advantage: they enable non-linear compositional changes that, for example, allow the formation of joints within these hydrogels. These features open up new possibilities to extend the field of use of metal reinforced hydrogels to load-bearing applications that are omnipresent for example in soft robots and actuators.

Composite Slow-Release Fouling Release Coating Inspired by Synergistic Anti-Fouling Effect of Scaly Fish

Inspired by the antifouling properties of scaly fish, the conventional silicone coating with phenylmethylsilicone oil (PSO/PDMS) composite coating was fabricated and modified with single layer polystyrene (PS) microsphere (PSO/PDMS-PS) arrays. The fish scale like micro-nano structures were fabricated on the surface of bio-inspired coating, which can reduce the contact area with the secreted protein membrane of fouling organisms effectively and prevent further adhesion between fouling organisms and bio-inspired coating. Meanwhile, PSO exuded to the coating surface has the similar function with mucus secreted by fish epidermis, which make the coating surface slithery and will be polished with the fouling organisms in turbulent waters. Compared to PSO/PDMS coating without any structure and conventional silicone coating, PSO/PDMS-PS showed better antiadhesion activity against both marine bacteria and benthic diatom (Navicula sp.). Additionally, the existence of PS microspheres can reduce the release rate of PSO greatly, which will extend the service life of coating. Compared to PSO/PDMS coating, the sustained release efficiency of PSO/PDMS-PS coating can reach 23.2%. This facile method for fabricating the bio-inspired composite slow-release antifouling coating shows a widely fabricating path for the development of synergistic anti-fouling coating.

Aggregation-Induced Emission-Active Gels: Fabrications, Functions, and Applications

Chromophores that exhibit aggregation-induced emission (i.e., aggregation-induced emission luminogens [AIEgens]) emit intense fluorescence in their aggregated states, but show negligible emission as discrete molecular species in solution due to the changes in restriction and freedom of intramolecular motions. As solvent-swollen quasi-solids with both a compact phase and a free space, gels enable manipulation of intramolecular motions. Thus, AIE-active gels have attracted significant interest owing to their various distinctive properties and promising application potential. Herein, a comprehensive overview of AIE-active gels is provided. The fabrication strategies employed are detailed, and the applications of AIEgens are summarized. In addition, the gel functions arising from the AIE moieties are revealed, along with their structure-property relationships. Furthermore, the applications of AIE-active gels in diverse areas are illustrated. Finally, ongoing challenges and potential means to address them are discussed, along with future perspectives on AIE-active gels, with the overall aim of inspiring research on novel materials and ideas.

Biosafe, self-adhesive, recyclable, tough, and conductive hydrogels for multifunctional sensors

As a bioelectronic material used in personalized medicine, it is necessary to integrate excellent adhesion and stretchability in hydrogels for ensuring biosafety. Herein, a high-performance multifunctional hydrogel of polyvinyl alcohol-sodium alginate-g-dopamine-silver nanowire-borax (PSAB) is reported. It can not only easily adhere to the surface of various substrates, but also exhibit excellent mechanical properties. Its tensile strength, elongation at break and toughness are 0.286 MPa, 500% and 55.15 MJ m^-3, respectively. The excellent mechanical properties and high conductivity guarantee that the PSAB hydrogel can successfully serve as a multifunctional sensor for detecting small activities and large-scale movements of the human body through strain and pressure changes. Meanwhile, the long-lasting potent and broad-spectrum antibacterial activity, combined with good in vitro biocompatibility, guarantees the biological safety and non-toxicity of the PSAB hydrogel. These compelling features, such as high flexibility and elasticity, high adhesion, multi-functional sensing and recyclability, as well as biological safety, pave the way for the application of PSAB hydrogel e-skin in biomedicine.

Tough Adhesion of Freezing- and Drying-Tolerant Transparent Nanocomposite Organohydrogels

Tough hydrogels with strong wet adhesion have drawn extensive attention for various applications. However, it is still challenging to achieve both excellent wet adhesion and freezing- and drying-tolerance in hydrogels. In this study, we present tough transparent nanocomposite organohydrogels based on the glycerol-water binary solvent system in the presence of Al(OH)₃ nanoparticles as a cross-linker. The resultant organohydrogels exhibited excellent tensile strength (∼0.9 MPa), high transparency (97%), superior anti-drying and anti-freezing properties, and good ionic conductivity. In particular, polyacrylic acid (PAA) was chosen as the bridging polymer to endow the organohydrogels with strong wet adhesion. The interfacial adhesion energy exceeded 2200 J m^-2, which was ascribed to the synergy of ionic coordination and hydrogen bonds between the nanoparticles and carboxyl groups in PAA chains. Interestingly, based on the strong wet adhesion, the transparent organohydrogels can be assembled into hydraulically driven soft variable-focus lenses with long-term ambient stability. This work will provide a new insight into controlled wet adhesion ̵of hydrogel and have great potential for hydrogel-based functional devices with long-term ambient stability.

Improved Ion Transport and High Energy Conversion through Hydrogel Membrane with 3D Interconnected Nanopores

To mimic and use the functions of the ion transport system that are central to biological processes, bioinspired ion-selective membranes are developed and show great potential in a variety of fields. However, the practical applications of them are now limited due to low pore density, low conductivity, or scale-up difficulty. Herein, we demonstrate a 2-hydroxyethyl methacrylate phosphate (HEMAP) hydrogel membrane with 3D interconnected nanopores and space charged through simple photopolymerization. The HEMAP hydrogel membrane exhibits high conductance and outstanding ion selectivity, and the membrane-based osmotic power generator shows the excellent output power density up to 5.38 W/m². Both experimentally and theoretically, the 3D interconnected structure is revealed to play a key role in enhancing charge-governed ion transport and energy conversion. This work highlights the advantages of 3D interconnected nanopores in ion diffusion and shows the potential of our designed hydrogel membrane in osmotic energy conversion, water desalination, and sensors.