Bilayer Asymmetric Hydrogels with Simultaneous Changes in Fluorescence Brightness and Shape Enabled by AIEgens

Abstract

The advancement of functional stimulus-responsive materials is highly important for achieving bionic artificial intelligence. Nevertheless, it is difficult to fabricate hydrogels that exhibit both fluorescence brightness and shape variation simultaneously in response to various stimuli. This work presents the design of a fluorescent hydrogel that responds to stimuli in a layered and asymmetric manner. The pH response layer consists of poly(acrylamide-sodium methacrylate) [P(AAm-NaMA)], while the T response layer consists of poly(acrylamide-N-isopropylacrylamide) [P(AAm-NIPAM)]. Furthermore, the hydrogel matrix contains a water-soluble polymer, tetraphenylethylene-3-sulfopropyl methacrylate potassium salt (TPE-PSPMA) with aggregation-induced emission (AIE). At strong acidic pH, the protonation of PNaMA chains leads to dehydration and shrinkage of the hydrogel network, resulting in hydrogel deformation toward the side of P(AAm-NaMA). When T is higher than lower critical solution temperature (LCST), PNIPAM has intramolecular interaction, causing the network to lose water and shrink, and then the hydrogel bends backward. Furthermore, the hydrogel network contracts when exposed to T or pH, which restricts the internal rotation and vibration of the TPE-PSPMA molecules. As a result, the hydrogel exhibits an AIE effect, leading to a shift in the fluorescence intensity. This finding offers valuable insights for the development of intelligent systems and holds significant potential for applications in the domains of soft robotics and smart wearable devices.