Research on advanced photoresponsive azobenzene hydrogels with push–pull electronic effects: a breakthrough in photoswitchable adhesive technologies

Abstract

Smart materials that adapt to various stimuli, such as light, hold immense potential across many fields. Photoresponsive molecules like azobenzenes, which undergo E–Z photoisomerization when exposed to light, are particularly valuable for applications in smart coatings, light-controlled adhesives, and photoresists in semiconductors and integrated circuits. Despite advances in using azobenzene moieties for stimuli-responsive adhesives, the role of push–pull electronic effects in regulating reversible adhesion remains largely unexplored. In this study, we investigate for the first time photo-controlled hydrogel adhesives of azobenzene with different push–pull electronic groups. We synthesized the monomers 4-methoxyazobenzene acrylate (ABOMe), azobenzene acrylate (ABH), and 4-nitroazobenzene acrylate (ABNO₂), and examined their effects on reversible adhesion properties. By incorporating these azobenzene monomers into acrylamide, dialdehyde-functionalized poly(ethylene glycol), and [3-(methacryloylamino)propyl]-trimethylammonium chloride, we prepared ABOMe, ABH, and ABNO₂ ionic hydrogels. Our research findings demonstrate that only the ABOMe ionic hydrogel exhibits reversible adhesion. This is due to its distinct transition state mechanism compared to ABH and ABNO₂, which enables efficient E–Z photoisomerization and drives its reversible adhesion properties. Notably, the ABOMe ionic hydrogel reveals an outstanding skin adhesion strength of 360.7 ± 10.1 kPa, surpassing values reported in current literature. This exceptional adhesion is attributed to Schiff base reactions, monopole–quadrupole interactions, π–π interactions, and hydrogen bonding with skin amino acids. Additionally, the ABOMe hydrogel exhibits excellent reversible self-healing capabilities, significantly enhancing its potential for injectable medical applications. This research underscores the importance of integrating multifunctional properties into a single system, opening new possibilities for innovative and durable adhesive materials.