Design of Dual Stimuli-Responsive Polymers that Undergo Sol-Gel Phase Transition in Response to Light and Biomolecule and Their Application as Cell Scaffolds

Abstract

Stimuli-responsive polymers that exhibit sol-gel phase transition in response to environmental changes such as temperature and pH have attracted considerable attention as biomaterials such as injectable polymers and scaffolds for cell culture. Although numerous stimuli-responsive polymers that undergo a sol-gel transition have been reported, the literature contains few accounts of biomolecularly stimuli-responsive polymers that undergo a sol-gel transition in response to a specific biomolecule. In previous works, we designed biomolecule-responsive hydrogels that undergo changes in volume in response to a target biomolecule; the strategy involves using biomolecular complexes as dynamic crosslinks in the gel networks. In the present study, we designed biotin-conjugated four-armed poly(ethylene glycol) (biotinylated Tetra-PEG) as biomolecular stimuli-responsive sol-gel transition polymers that underwent the phase transition from a sol to a gel state in response to avidin as a target biomolecule. Photo-dimerization is the reaction that occurs by photo-irradiation. Photo-responsive polymers are synthesized by the introduction of photo-dimerizable moieties. Focusing on the photo-dimerization of maleimide, we designed maleimide-conjugated Tetra-PEG as photo-responsive sol-gel transition polymers that transform from a sol to a gel state in response to photo-irradiation. In this study, we synthesized novel dual stimuli-responsive polymers exhibiting sol-gel phase transition in response to light and biomolecule by introducing both a photo-dimerizable group and a biological ligand in a poly(ethylene glycol) (PEG) chain. In addition, the dual stimuli-responsive sol-gel phase transition polymers were used as scaffolds for cell culture. A PEG derivative with both a maleimide group as a photo-dimerizable group and a biotin as a biological ligand was synthesized as follows: N-(2-aminoethyl)maleimide was conjugated to the a-biotinyl-w-succinimidyl poly(ethylene glycol) by the coupling reaction to obtain a-biotinyl-w-maleimide-poly(ethylene glycol) (BMP). The resultant BMP was mixed with avidin to form a BMP-avidin complex in a phosphate buffer solution (pH 7.4). An aqueous solution containing the BMP-avidin complex was exposed to UV. The exposure of the aqueous BMP-avidin complex solution to UV induced the phase transition from a sol to a gel state within 10 min. The photo-responsive gelation is attributed to the dimerization of maleimide groups that acted as dynamic crosslinks. Furthermore, the resultant gel changed to a sol state by the addition of free biotin because it resulted in a dissociation of BMP-avidin complexes as crosslinks. Cells were successfully entrapped within the hydrogels formed by the sol-gel phase transition of the PEG derivative, and were recovered by gel dissociation without cytotoxity. Cell behavior within the BMP-avidin complex hydrogel was quite different from that after the dissociation of the hydrogel by the addition of free biotin. The smart functions of BMP provide useful tools for regulating cell behavior.