Sweat-enhanced adhesive hydrogel enables interfacial exchange coupling for wearable strain sensor

PVA-based wearable hydrogels have emerged as promising candidate for motion sensors due to their skin-like softness and unique biocompatibility. However, an open question is whether and how the strain-sensitive hydrogel designing can prevent adhesive performance loss caused by sweat between devices and human-skin and reduce interfacial failure. Herein, a versatile sweat-enhanced adhesive hydrogel-based motion sensor (VDBA) paradigm is engineered consisting of sweat sensitivity and self-adhesive function components. Taking advantage of catechol chemistry inspired mussel, the VDBA hydrogel enhances substrate adhesion behavior. The borate ions (B(OH)₄⁻) are found to efficient condensation form borate ester groups with phenolic hydroxyl. Interestingly, human sweat pH serves as a trigger for borate ester bonds protection-deprotection, whereby partial catechol primarily functions in non-sweating states and significantly activates during sweating. The VDBA hydrogel achieves a maximum adhesive strength of 13.45 kPa on skin at non-sweat, however, the maximum bonding strength increases to 24.0 kPa when sweating. The resultant VDBA hydrogel exhibits integration of exceptional mechanical properties and anti-freezing properties. Also, desirable conductivity appears in the hydrogel, allowing outputting accurate and repeatable signals for body motion sensing in various scenarios. Furthermore, VDBA hydrogel is demonstrated for use as a sports training times counter, which can achieve sensitive statistics for stretching and bending movements. This work provides a great hydrogel candidate for various motion sensor applications, particularly beneficial for people with sweaty skin and athletes.