Highly sensitive smart hydrogels with pH-tunable toughness via signaling cascade amplification

The signaling cascade amplification is a common biological phenomenon which can effectively amplify the signal intensity and trigger huge physiological feedback. Inspired by this mechanism, herein we design a simple strategy to develop highly sensitive smart hydrogels of which the mechanical performance can be finely tuned with pH. Simply composed with hydrophobic rigid monomer phenyl acrylate with pH-responsive monomer acrylic acid, the hydrogels demonstrate high mechanical performance in an acidic environment. Upon pH rising, the gels reveal remarkable softening performance by dropping 3 orders of decrease in rigidity just in a narrow pH range. Especially, the gels reveal high sensitivity to pH near the pKa of acrylic acid, with Young's modulus dropping 50 times by merely elevating pH of 0.06. After systematical mechanism investigation, it proves that the pH signal affects the toughness with a step-by-step mechanism by gradually amplifying the trigger, similar to the signaling cascade amplification in nature. The pH enhancement first brings about a growth in ionization degree, followed by the upturning of hydration level. Then, the absorbed water serves as plasticizer by mobilizing the frozen network and lowering the softening temperature. Eventually, the gels are shifted to a rubbery state and their toughness significantly decreases. Moreover, the criterion to develop such smart hydrogels is proposed, which demonstrates universality to different systems. This novel bio-mimetic mechanism, we hope, may substantially inspire the development and application of smart hydrogels.