Regulating Boronic Ester Bonds in Bilayer Hydrogels toward Fabricating Multistimuli-Triggered Actuators

As a member of the dynamic covalent bond family, the boronic ester bond has attracted increasing attention in the fabrication of hydrogels because it can occur in an aqueous system at room temperature. When the hydrogels are applied as a soft sensor, the role of boronic ester bonding is well defined, namely, as a functional unit endowing the system with multistimulus responsiveness. However, this also limits applications of boronic ester bonding in hydrogels. In this work, a heterogeneous hydrogel is developed in which the boronic ester bonding simultaneously acts as structural and responsive units, integrating the actuating and sensing abilities together. The bilayer strategy is applied to give the system network heterogeneity, namely, constructing the physical cross-links of poly(vinyl alcohol) on one side, whereas building dual cross-links, namely, the physical cross-links and the covalent cross-links between carboxymethyl cellulose and poly(vinyl alcohol) together on the other side. Therefore, the as-prepared heterogeneous hydrogels can be used as temperature-triggered and pH-triggered actuators. Moreover, it well retains the strain resistance and temperature resistance effects of the homogeneous hydrogel with boronic ester bonding and can also be used as a flexible sensor monitoring human motions. More application scenarios and working modes for the boronic ester-bonded hydrogels are unlocked.