Figure 4 (a) An alkali-driven temporal organogel. Copyright (2019) John Wiley and Sons. (b) A KOH-driven temporal supramolecular chiral G-quadruplex hydrogels. Copyright (2021) John Wiley and Sons.
3.1.2. Temporary gels based on pH-responsive gelator.pH-responsive gels are a sort of typical stimuli-responsive material which has been widely reported in recent decades. [38-41] They are thought to be good candidates for controlled drug delivery [42,43] or gel actuators. [42, 44-46] The key to fabricating a pH-responsive gel is to introduce a group, which is a weak acid (or base) or can be broken in presence of H+ (or OH-), to the 3D molecular network of the gel. It is therefore expected that combining a pH-responsive gel with a fuel-driven reaction network in which the pH value can temporarily change is an applicable strategy for creating a non-equilibrium temporary gel material. According to this hypothesis, some temporary gels based on pH-responsive gelators have been reported. In 2019, D. J. Adams and coworkers presented a temporary supramolecular gel by applying an amino-containing gelator. [47] As shown in Figure 4a, the hydrochloride of the gelator was well-soluble in water. To initiate the gelation, urease, and urea were added. NH3 yielded by their autocatalytic ureolysis rapidly enhanced the pH to 9.1–9.2, resulting in the deprotonation of the gelator. A semitransparent hydrogel can be obtained within 5 min. However, the increase in pH triggered the base-catalyzed saponification reaction of methyl formate, which was added together with the urease and urea. The formic acid generated by the saponification led to a slow decrease of pH to 6 or even 4. It will redissolve the gelator and destroy the hydrogel. The stiffness, viscosity, and lifetime can be enhanced by both increasing the concentration of urea and decreasing the amount of methyl formate. This work provided a simple example of pH-regulated temporal gel. Similarly, J. Li and coworkers reported a temporary supramolecular chiral G-quadruplex hydrogel realized by another pH-feedback reaction network. (Figure 4b) [48] Herein the pH was enhanced by directly adding KOH. It combined the guanosine and 5-fluorobenzoxaborole, yielding a benzoxaborolate complex that can self-assemble into a helical G-quadruplex, ultimately leading to the formation of a highly transparent hydrogel. The recovery of pH in this work was achieved by the hydrolysis of 1,3-propanesultone. It decomposed the gelator and damaged the temporary hydrogel. In addition to the alkalis such as NH3 and KOH, acid-induced temporal pH decrease has also been used to promote the formation of temporary gels. For example, A. Walther and coworkers applied citric acid as a chemical fuel to initiate the DSA of a pH-responsive dipeptide precursor. [24] (Figure 5a) The precursor immediately self-assembled into twisted nanofibrils and thereafter gelation occurred. The automatic pH recovery is caused by urase and urea premixed with the precursor. The rapid pH drop activated the urease that can catalyze the hydrolysis of urea into CO2 and NH3, resulting in the disassembly of the nanofibrils and the collapse of the hydrogel. Furthermore, the acid-driven temporal deformation of hydrogel based on the same principle has also been reported recently. X. Wang et al. developed a bilayer hydrogel actuator consisting of a tertiary amine-containing, pH-responsive layer, and a urease-containing non-responsive layer. [49] (Figure 5b) Upon the addition of acidic urea solution, a significant bend can be observed due to the swelling of the pH-responsive layer. Meanwhile, urease in the other layer would catalyze the hydrolysis of the urea. The resulting NH3led to the deswelling of the automatic relaxation of the actuator. It can be found that the key to constructing the above temporary gel material is to find a deactivator that can slowly and spontaneously self-decompose to neutralize the pH jump caused by the addition of alkali or acid. However, in some special cases, the pH can also be recovered by direct decomposition of the acidic/alkaline chemical fuel itself. For example, A. Quintard and coworkers presented a temporary gel that formed under the drive of CCl3COOH. [50] CCl3COOH protonated the precursor and led to its self-assembly and gelation. While the basic moieties in the reaction mixture, as a deactivator, catalyzed the decomposition of CCl3COOH into volatile chloroform and carbon dioxide. Unsurprisingly, the temporary gel gradually dissolved as the CCl3COOH was consumed.