Hydrogels with Preprogrammable Lifetime via UV-Induced Polymerization and Degradation

Hydrogels are 3D networks infused with water. When formed via radical polymerization, inherently stable hydrogels are created due to stable covalent carbon-carbon bonds. As such, they remain static soft scaffolds, unlike the dynamic tissue they are often compared to. Herein, a hydrogel capable of autonomously converting from a liquid hydrogel precursor solution into a solid hydrogel for a defined period of time, followed by a further transformation back into a liquid polymer solution, is designed. These antagonistic processes are initiated by the same UV light, which is used as stimulus for both a photopolymerization and a photodegradation simultaneously. It is demonstrated how the lifetime of the hydrogel state can be controlled and visualized on the minute scale. Both the photopolymerization and the photodegradation reactions are studied with various methods such as NMR, IR, and confocal microscopy. The different stages of the transformation, e.g., the hydrogel precursor (liquid), hydrogel (solid), and degraded hydrogel (liquid) are investigated with rheometry, viscosimetry, dynamic light scattering, and gel permeation chromatography. Small changes in the molecular composition of the precursor solution result in macroscopically measurable differences. Such time-dependent twofold photoreactive systems can be of interest for designing dynamic materials, such as glues and photoresists or for biomedical applications.