Multi-responsive chromatic hydrogel exhibiting reversible shape deformations

In this work, deformable and chromatic hydrogel triggered by multi-stimuli was fabricated. This hydrogel was composed of N-isopropylacrylamide (NIPAAm), acryl-6-aminocaproic acid (A6ACA), and spiropyran (SP) derivatives, which possessed ferric ion (Fe3+) and temperature-triggered shape deformations properties as well as chromatic behavior. Under ultraviolet light irradiation, the SP isomer in the hydrogel transformed to the merocyanine (MC) isomer, and the hydrogel showed the characteristic UV–Vis absorption peak of MC at 540 nm. The hydrogel revealed the selective chromatic response to Fe3+ with the disappearance of the absorption peak of MC. Using ionoprinting technology, the hydrogel deformed programmatically in the presence of Fe3+. Moreover, owing to the anisotropic structure of the chromatic hydrogel, it realized reversible bending deformation triggered by temperature. The behaviors of hydrogels in response to dual stimuli (Fe3+ and temperature) were further investigated, and the hydrogel showed programmable, spatiotemporal, and complex deformation behaviors, accompanying by the color change. This hydrogel will have great application potential in soft robotics, soft actuators, and environmental sensors.