Hydrogels are flexible materials that have high potential for use in various applications due to their unique properties. However, their applications are greatly restricted by the low mechanical performance caused by high water content and inhomogeneous networks. This paper reports a universal strategy for easily preparing hydrogels that are tough and stretchable without any special structures or complicated processes. Our strategy involves tuning the polymerization conditions to form networks with many polymer chain entanglements to achieve energy dissipation. Tough and stretchable hydrogels can be prepared by free radical polymerization with a high monomer concentration and low cross-linker content to optimize the balance between physical and chemical cross-links by entanglements and covalent bonds, respectively. The strategy of using polymer chain entanglements for energy dissipation allows us to overcome the limitation of low mechanical performance, which leads to the wide practical use of hydrogels.

Target molecule-responsive hydrogels with β-cyclodextrin (β-CD) were prepared via molecular imprinting using bisphenol A (BPA) as a template. BPA-imprinted hydrogels showed greater shrinkage than non-imprinted hydrogels because CD ligands arranged at suitable positions formed CD-BPA-CD complexes that acted as crosslinks.

Core-cross-linked polyion complex (PIC) micelles entrapping trypsin in the core were prepared by mixing trypsin and poly(ethylene glycol)-block-poly(alpha,beta-aspartic acid) in aqueous medium, followed by the introduction of glutaraldehyde cross-linkages. Trypsin incorporated into the core-cross-linked micelles showed high storage stabilities, and the initial enzymatic activity of trypsin was maintained even after standing for one week at ambient temperature. Further, stable compartmentalization of trypsin into the core-cross-linked micelles led to a unique modulation in the enzymatic functions including an improved thermal tolerability with an increased maximum reaction rate compared to native trypsin.

We designed biotin-conjugated four-armed poly(ethylene glycol) (biotinylated Tetra-PEG) as biomolecularly stimuli-responsive polymers that underwent the phase transition from a sol to a gel state in response to avidin as a target biomolecule.

Temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm) gels were prepared via atom transfer radical polymerization (ATRP), and their mechanical and responsive properties were investigated from the viewpoint of their network homogeneity.

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Poly(l-lysine) (PLL) undergoes structural change from α-helix to random coil in response to a change in pH in an aqueous solution. By chemical crosslinking of PLL with ethylene glycol diglycidyl ether (EGDE), we prepared pH-responsive hydrogels that undergo a change in volume and conformation between α-helix and random coil in response to a change in pH. The difference in transition pH between a PLL linear polymer and a PLL hydrogel revealed that chemical crosslinking stabilized the helical structure of PLL chains in hydrogel networks.

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