Toward Dynamic Liquid Cell Scaffold: Photoreversible Ion Gels Exhibiting Light-Induced Sol-Gel Transitions.

Reversible sol-gel transitions are difficult to achieve in conventional water-swollen hydrogels in open aqueous environments, because polymer chains dissolve or diffuse once the network disassembles. Here, we present a proof-of-concept to overcome this limitation by introducing a water-immiscible and non-cytotoxic ionic liquid (IL) phase that confines polymer networks and prevents dissolution during reversible phase transitions. We report a photoreversible ion gel that crosses the rheological boundary (tan δ ∼ 1) under light, enabling reversible sol-gel switching within this closed IL environment. The material integrates an ABC triblock copolymer, P(AzoAm-r-NIPAm)-b-PBuA-b-PSt, with a solvent-quality-tunable blend of non-cytotoxic ILs ([P4,4,4,1][TFSI]/[P8,8,8,8][TFSI]). The photoresponsive A-block, P(AzoAm-r-NIPAm), exhibits a polarity-dependent solubility change with the cis/trans isomerization of azobenzene, providing a reversible light-controlled self-assembly. Time-resolved rheology confirmed repeated crossings of tan δ = 1 under alternating UV-vis illumination at 52°C. The switching mechanism is governed by the lifetime of reversible junctions, consistent with transient network theory. In addition, hMSCs adhered to and spread on the ion gel at 37°C, indicating the cytocompatibility of the ion gel itself. This light-programmable, water-immiscible ion gel has the potential to provide a reversible liquid-solid mechanical cue for next-generation mechanobiology.