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

Smart materials that adapt to various stimuli, such as light, hold immense potential across many fields. Photoresponsive molecules like azobenzenes, which undergo <i>E</i>-<i>Z</i> 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 <i>E</i>-<i>Z</i> 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.