A new mussel-inspired highly self-adhesive & conductive poly (vinyl alcohol)-based hydrogel for wearable sensors

Poly (vinyl alcohol) (PVA) hydrogel with nontoxicity, good biocompatibility and high mechanical strength was promising for wearable sensors. However, the traditional freezing/thawing or borax crosslinking method consumed large number of hydroxyl groups in PVA chains, causing insufficient self-adhesiveness of hydrogel. Herein, inspired by mussel-adhesive mechanism, we innovatively introduced carboxyl groups into PVA chains and fabricated PVA-COOH /polydopamine (PDA) hydrogel by incorporation with PDA as crosslinking agent. The formation mechanism of the hydrogel was confirmed through FTIR and 13C NMR analysis. With increasing PDA content, the crosslinking density increased significantly and the dense networks formed by dual covalent/ hydrogen bonding. The hydrogel exhibited impressive stretchability and extremely high adhesiveness. It could adhere to both hydrophilic and hydrophobic surface, and the adhesion strength to metal/ rubber/pigskin reached as high as 158.26 kPa/149.86 kPa/56.67 kPa, respectively, attributing to the incorporated enough free catechol/ quinone group, carboxyl/hydroxyl group and newly formed amide group in hydrogel, while the adhesion failure mode consisted of both interface failure and matrix failure of hydrogel, which consumed high energy under stress. Furthermore, the hydrogel showed conductive with high strain sensitivity, which could steadily and repeatedly monitor large-scale and subtle limb motions, exhibiting significant and diverse potential in wearable strain sensors.