Rapid Photogelation of Low-Concentration Polymer Solutions via Photodimerization of o-Ethylnitrobenzene

Polymers that exhibit significant property changes upon light stimulation are promising for biomaterials due to their minimally invasive nature and spatiotemporal controllability. In this study, o-ethylnitrobenzene (ENB) was identified as an effective photocrosslinking motif and incorporated into poly(vinyl alcohol) (PVA), yielding a new class of photocrosslinkable, water-soluble polymers. These polymers undergo rapid gelation (∼0.5 s) from dilute aqueous solutions (∼0.5 wt.%) upon UV irradiation to form flexible and elastic hydrogels. The resulting hydrogels exhibit high compressive resilience; for example, a cylindrical hydrogel prepared from a 1.5 wt.% solution withstands a compressive stress exceeding 10 MPa without fracture. Hydrogels formed from 2 wt.% solutions swell isotropically up to 200 fold in water while retaining sufficient mechanical strength for manual handling, even at a water content of 99.99 wt.%. Furthermore, the addition of the polymer to cell-containing media, followed by photoinduced gelation in the presence of cells, preserves cell viability. Cells are instantaneously immobilized in 3D space within the hydrogel, forming a highly permeable culture system that enables long-term culture through medium exchange and promotes the formation of large cellular spheroids. These results highlight the potential of ENB-based hydrogels for spatiotemporally controlled 3D cell culture applications.