Figure 11 (a) an EDC-driven writable transient gel (b) Fuel-driven transient fluorescence emission of an AIEgen. Copyright (2017) Royal Society of Chemistry.
With this strategy, X. Wang and coworkers developed another two kinetically stable self-healing materials. In the same year, they reported a temporary self-healing hydrogel via the crosslinking of ketone-type acylhydrazone bonds. [74] (Figure 13b) Such bonds show a dynamic feature in an acidic environment but are kinetically locked under alkaline conditions. Thus, the temporary self-healing ability of the hydrogel was achieved by the urea/urease-containing reaction network which can result in a temporary pH increase. [24] Another example is an acylhydrazone-based hydrogel containing NaHCO3. [75] (Figure 13c) The alkalinity of NaHCO3 made the hydrogel kinetically stable and inert, but incapable of spontaneous self-healing. However, applying a small amount of acidic buffer on the fracture surfaces created a temporary pH decrease, allowing the fast exchange of the acylhydrazone and the temporary self-healing of the severed hydrogel. Additionally, it is noteworthy that compared with the two examples above, this temporary self-healing hydrogel was achieved without using enzyme, consequently, the material has a better tolerance to various harsh treatment conditions including organic solvents, high ionic strength, drying, and extreme temperatures.