Photodegradable Polyacrylamide Gels for Dynamic Control of Cell Functions

Cross-linked polyacrylamide hydrogels are commonly used in biotechnology and cell culture applications due to advantageous properties, such as the precise control of material stiffness and the attachment of cell adhesive ligands. However, the chemical and physical properties of polyacrylamide gels cannot be altered once fabricated. Here, we develop a photodegradable polyacrylamide gel system that allows for a dynamic control of polyacrylamide gel stiffness with exposure to light. Photodegradable polyacrylamide hydrogel networks are produced by copolymerizing acrylamide and a photocleavable ortho-nitrobenzyl (o-NB) bis-acrylate cross-linker. When the hydrogels are exposed to light, the o-NB cross-links cleave and the stiffness of the photodegradable polyacrylamide gels decreases. Further examination of the effect of dynamic stiffness changes on cell behavior reveals that in situ softening of the culture substrate leads to changes in cell behavior that are not observed when cells are cultured on presoftened gels, indicating that both dynamic and static mechanical environments influence cell fate. Notably, we observe significant changes in nuclear localization of YAP and cytoskeletal organization after in situ softening; these changes further depend on the type and concentration of cell adhesive proteins attached to the gel surface. By incorporating the simplicity and well-established protocols of standard polyacrylamide gel fabrication with the dynamic control of photodegradable systems, we can enhance the capability of polyacrylamide gels, thereby enabling cell biologists and engineers to study more complex cellular behaviors that were previously inaccessible using regular polyacrylamide gels.

Effects of the bonding intensity between hyaluronan and gelatin on chondrogenic phenotypic maintenance

Although there have been many reports on the use of crosslinked hyaluronic acid and gelatin derivatives as injectable hydrogels in cartilage tissue engineering, however, almost no reports have analyzed the kinds of bonding intensity that were most conducive for the maintenance of cartilage phenotypes. Herein, the biomimetic composite hydrogels based on thiolated hyaluronic acid and modified gelatin derivatives with physical mixed, weak, and strong bonding intensity were fabricated, wherein the thiolated hyaluronic acid ensured the basic network structure of composite hydrogels, and gelatin derivatives endowed the bioactivity to hydrogels. These physicochemical properties of composite hydrogels implied that strong bonding intensity (HA-GSH) contributed to the maintenance of a more uniform pore structure, and increased the ability of water retention and resistance to degradation. Further immunohistochemical and RT-PCR analyses demonstrated that the HA-GSH hydrogel greatly improved the expression level of the associated cartilage matrix and the possibility of hyaline cartilage formation in comparison to the physically blended HA-Gel gel and weak bonding crosslinked HA-GMA gel. Overall, all results proved that strong bonding intensity of the disulfide bonds in the HA-GSH hydrogel was more beneficial for the proliferation of chondrocytes and the maintenance of the hyaline cartilage phenotype, which might provide valuable inspiration for designing cartilage repair scaffolds.

Polyacrylamide crosslinked by bis-vinylimidazolium bromide for high elastic and stable hydrogels

A series of ionic compounds 1,n-dialkyl-3,3′-bis-l-vinylimidazolium bromide (C_nVIM) are prepared and employed to crosslink acrylamide for polyacrylamide (PAAM) hydrogel preparation via in situ solution polymerization. The swelling behavior, mechanical properties and thermal stability of the prepared C_nVIM crosslinked PAAM hydrogels are investigated. C_nVIM effectively crosslink the PAAM networks to form porous structures in the hydrogel, which could stably absorb water as much as 75.9 fold in weight without structural degradation. The prepared hydrogels could endure compressive stress up to 1.95 MPa and compressive deformation more than 90%. Meanwhile, the C_nVIM crosslinked networks show superior thermal stability, and could retain the structural integrity under 150 °C for more than 240 h. The swelling degradation resistance, mechanical strength and thermal stability of C_nVIM crosslinked hydrogels are much better than those of a conventional N,N′-methylenebisacrylamide crosslinked PAAM hydrogel. Using bis-vinylimidazolium bromides as crosslinkers provides an optional strategy for constructing thermally and mechanically robust hydrogel networks.

Polyacrylamide hydrogels crosslinked by bis-vinylimidazolium bromides achieved robust mechanical stability under various conditions, such as equilibrium swelling, compression and high temperatures.