Hydrogel Patch with Biomimetic Tree Frog Micropillars for Enhanced Adhesion and Perspiration Wicking

Hydrogels with superior adhesion in both wet and dry interfaces, excellent biocompatibility, and electrical conductivity are highly essential for biomedical engineering and wearable electronics. Although a variety of artificial adhesives have been developed to enhance adhesion in a wet interface, a single adhesion strategy has limitations in complex adhesion environments. Inspired by the ortho-hexagonal toe pads of the tree frog, we present a microstructured hydrogel adhesive (MS-HAD) that combines chemical adhesive and perspiration wicking introduced by a biomimetic microstructure. This MS-HAD is made by a biocompatible PDA/PAM hydrogel featuring ortho-hexagonal micropillars on the surface. The PDA/PAM hydrogel composition was optimized, balancing the hydrogel adhesive and mechanical strength to match the micropillar fabrication process. By studying the adhesion behavior of micropillar structures of various sizes, it is found that the liquid self-splitting and liquid self-sucking introduced by the microstructure were the synergistic mechanism of adhesion strength enhancement in the wet interface. The adhesion of MS-HAD in the wet interface is enhanced by 47% in comparison to a flat hydrogel adhesive (F-HAD). Finally, the application of the MS-HAD patches for ECG signal monitoring under dry and wet skins is demonstrated. For both dry and wet skins, the ECG signal measured by the MS-HAD is more stable and accurate compared to those measured by commercial gel electrodes and F-HAD. The MS-HAD reported here provides a potential adhesive electrode that could perform well in both wet and dry interfaces for future portable health monitoring devices.