One-Pot and Fast Synthesis of Hydrogels with Photoswitchable Dynamic Coordination Cross-Linking for Tunable Healing, Photopatterning, and Light Sensing

A photoresponsive Fe3+-cross-linked poly(acrylic acid) hydrogel showing tunable healing, photopatterning, and light sensing is reported. The hydrogel is fabricated by simply mixing acrylic acid, Ag+, ammonium persulfate, Fe3+, and citric acid in water at room temperature. A gelation kinetics investigation demonstrates a short network occurrence time of 86 ± 3.5 s. The citric acid in the hydrogel inhibits covalent cross-linking and enables the photocleavage of Fe3+-COOH cross-linking by reducing Fe3+ to Fe2+, resulting in a solid-to-liquid transition. When the oxidizer H2O2 or O2 is imposed on the photoliquefied hydrogel, the Fe3+-COOH cross-linking is restored and the liquid-to-solid transition can be achieved. The photoresponsive hydrogel is demonstrated to be self-healable due to the dynamic nature of Fe3+-COOH cross-linking. It also shows fast light/H2O2-assisted healing through time-controlled photoliquefaction. The hydrogel sheet is also used to print portraits of people and a photo of a dog by photopatterning. We finally demonstrate that the hydrogel shows reversible electric resistance change in response to photoirradiation. We expect that the COOH-containing monomer/APS/CA/metal ion gelation chemistry can not only serve as a general strategy to readily and rapidly produce healable hydrogels, but also enrich the toolbox of manufacturing photopatterning materials and light sensors.