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.