Thiol-5-methylene pyrrolones hydrogels with pH-tunable stress-relaxation and self-healing properties

Hydrogels containing dynamically crosslinked networks through covalent bonds have garnered substantial attention in both academia and technology sectors. This interest stems from their impressive mechanical stability and their unique spatiotemporal dynamic characteristics. Among the various type of dynamic covalent bonds used for preparation hydrogel, the 5-methylene pyrrolones (5MP) and thiol reaction stands out as one of the most prevalent. Given its reliability, efficiency and selectivity, thiol-5MP reaction has long been recognized as some of the most efficient Micheal additions. In this work, by utilizing thiol-5MP Michael addition with improved stability and specificity, a new type of dynamic hydrogel is easily prepared. Notably, the mechanical attributes of the resultant thiol-5MP hydrogels can be finely tuned by modulating the pH during their preparation process. Furthermore, hydrogels formulated under neutral (pH 7.5) or alkaline (pH 8.5) conditions display enhanced stress-relaxation response and superior self-healing capabilities compared to those generated under acidic conditions (pH 6.5). As revealed by single-molecule force spectroscopy assays, the pH-tunable mechanical properties are attributed to the pH-dependent dynamics of thiol-5MP bonds. This work showcases an innovative avenue for crafting dynamic hydrogels featuring pH-adjustable bulk characteristics, highlighting the versatility of thiol-5MP bonds as fundamental building blocks for the design of functional hydrogel materials.