Shear‐thinning and self‐healing hydrogels are being investigated in various biomedical applications including drug delivery, tissue engineering, and 3D bioprinting. Such hydrogels are formed through dynamic and reversible interactions between polymers or polypeptides that allow these shear‐thinning and self‐healing properties, including physical associations (e.g., hydrogen bonds, guest–host interactions, biorecognition motifs, hydrophobicity, electrostatics, and metal–ligand coordination) and dynamic covalent chemistry (e.g., Schiff base, oxime chemistry, disulfide bonds, and reversible Diels–Alder). Their shear‐thinning properties allow for injectability, as the hydrogel exhibits viscous flow under shear, and their self‐healing nature allows for stabilization when shear is removed. Hydrogels can be formulated as uniform polymer and polypeptide assemblies, as hydrogel nanocomposites, or in granular hydrogel form. This review focuses on recent advances in shear‐thinning and self‐healing hydrogels that are promising for biomedical applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48668.

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Synthetic fibrous materials were engineered to contract under cellular forces similar to natural extracellular matrix.

The incorporation of a secondary network into traditional single‐network hydrogels can enhance mechanical properties, such as toughness and loading to failure. These features are important for many applications, including as biomedical materials; however, the processing of interpenetrating polymer network (IPN) hydrogels is often limited by their multistep fabrication procedures. Here, a one‐pot scheme for the synthesis of biopolymer IPN hydrogels mediated by the simultaneous crosslinking of two independent networks with light, namely: i) free‐radical crosslinking of methacrylate‐modified hyaluronic acid (HA) to form the primary network and ii) thiol–ene crosslinking of norbornene‐modified HA with thiolated guest–host assemblies of adamantane and β‐cyclodextrin to form the secondary network, is reported. The mechanical properties of the IPN hydrogels are tuned by changing the network composition, with high water content (≈94%) hydrogels exhibiting excellent work of fracture, tensile strength, and low hysteresis. As proof‐of‐concept, the IPN hydrogels are implemented as low‐viscosity Digital Light Processing resins to fabricate complex structures that recover shape upon loading, as well as in microfluidic devices to form deformable microparticles. Further, the IPNs are cytocompatible with cell adhesion dependent on the inclusion of adhesive peptides. Overall, the enhanced processing of these IPN hydrogels will expand their utility across applications.

Granular hydrogels are an emerging class of biomaterials formed by jamming hydrogel microparticles (i.e., microgels). These materials have many advantageous properties that can be tailored through microgel design and extent of packing. To enhance the range of properties, granular composites can be formed with a hydrogel interstitial matrix between the packed microgels, allowing for material flow and then stabilization after crosslinking. This approach allows for distinct compartments (i.e., microgels and interstitial space) with varied properties to engineer complex material behaviors. However, a thorough investigation of how the compositions and ratios of microgels and interstitial matrices influence material properties has not been performed. Herein, granular hydrogel composites are fabricated by combining fragmented hyaluronic acid (HA) microgels with interstitial matrices consisting of photocrosslinkable HA. Microgels of varying compressive moduli (10–70 kPa) are combined with interstitial matrices (0–30 vol.%) with compressive moduli varying from 2–120 kPa. Granular composite structure (confocal imaging), mechanics (local and bulk), flow behavior (rheology), and printability are thoroughly assessed. Lastly, variations in the interstitial matrix chemistry (covalent vs guest–host) and microgel degradability are investigated. Overall, this study describes the influence of granular composite composition on structure and mechanical properties of granular hydrogels towards informed designs for future applications.

Numerous chemical modifications of hyaluronic acid (HA) have been explored for the formation of degradable hydrogels that are suitable for a variety of biomedical applications, including biofabrication and drug delivery. Thiol-ene step-growth chemistry is of particular interest due to its lower oxygen sensitivity and ability to precisely tune mechanical properties. Here, we utilize an aqueous esterification route via reaction with carbic anhydride to synthesize norbornene-modified HA (NorHA CA ) that is amenable to thiol-ene crosslinking to form hydrolytically unstable networks. NorHA CA is first synthesized with varying degrees of modification (∼15−100%) by adjusting the ratio of reactive carbic anhydride to HA. Thereafter, NorHA CA is reacted with dithiol crosslinker in the presence of visible light and photoinitiator to form hydrogels within tens of seconds. Unlike conventional NorHA, NorHA CA hydrogels are highly susceptible to hydrolytic degradation through enhanced ester hydrolysis. Both the mechanical properties and the degradation timescales of NorHA CA hydrogels are tuned via macromer concentration and/or the degree of modification. Moreover, the degradation behavior of NorHA CA hydrogels is validated through a statistical-co-kinetic model of ester hydrolysis. The rapid degradation of NorHA CA hydrogels can be adjusted by incorporating small amounts of slowly degrading NorHA macromer into the network. Further, NorHA CA hydrogels are implemented as digital light processing (DLP) resins to fabricate hydrolytically degradable scaffolds with complex, macroporous structures that can incorporate cell-adhesive sites for cell attachment and proliferation after fabrication. Additionally, DLP bioprinting of NorHA CA hydrogels to form cell-laden constructs with high viability is demonstrated, making them useful for applications in tissue engineering and regenerative medicine.

Microcapsules are commonly used in applications ranging from therapeutics to personal care products due to their ability to deliver encapsulated species through their porous shells. Here, we demonstrate a simple...

The primary objective of this review paper is to highlight the advances in Fuel Cell with the newly developed fuel cell materials and membranes as well as their potential applications. The major disadvantage that limits the commercialisation of conventional Platinum (Pt)/Palladium (Pd) electrode based fuel cells is the expense and hence restricts the use of fuel cells, though being more an environment friendly energy production method. With recent development and the newly emerging materials, these conventional technologies can be replaced with advanced materials and membranes. The synthesis, process chemistry, physical, electrical and chemical properties, output characteristics, workability and performance of recently developed electrode materials: Nitrogen doped Graphene electrode, Carbon Nanotube based electrodes for applications in conventional proton exchange membrane and microbial fuel cells; newly emerging polymer electrolyte membranes; Synthesis of new materials i.e. quantum dots (graphene and polyvinyl alcohol) as catalysts or support for catalysts, electron mediators and advanced electrolyte solutions have been studied in the paper. Recent advancements in the synthesis of porous materials called Metal Organic Framework for fuel cell applications have also been reviewed.