Figure 2 (a) Temporal hydrogel driven by CH3I. Copyright (2010) John Wiley and Sons. (b) Temporal hydrogel driven by (CH3)2SO4. Copyright (2015) American Association for the Advancement of Science.
3.1.1. Temporary gels based on carboxyl group. Early in 2010, J. van Esch and J. Boekhoven et al applied a carboxyl-containing low-molecular-weight precursor to successfully fabricate a temporary hydrogel. [21] As shown in Figure 2a, the precursor was formed by coupling two dibenzoyl-(l)-cystine with two carboxyl groups on both sides. Thus, it is well-soluble after being mixed into an alkaline buffer. Then the chemical fuel, iodomethane, was added, and it quickly methylated the carboxyl groups in the precursor, producing the less soluble gelator, which can self-assemble into fibrous nanoaggregates. Hydrogel formed subsequently once the gelator increased to a critical concentration. However, unlike the static hydrogels under thermodynamic equilibrium, it spontaneously dissolved in the following days since the methyl carboxylate groups were slowly hydrolyzed by the OH-encapsulated in the hydrogel. Five years later, J. Boekhoven and J. van Esch et al used dimethyl sulfate, another highly active methylation agent, to realize a fuel-driven temporary hydrogel using carboxyl-containing gelators with a similar structure (Figure 2b). [22] The experimental results displayed that the lifetime, stiffness, and self-regenerating capability can be controlled by the amount of the chemical fuel or deactivator (OH-). It can be found that iodomethane and dimethyl sulfate are quite effective chemical fuels for initiating the temporal gelation of wide-range carboxyl-containing precursors because they are highly active and the methylation of the carboxyl group can remarkably decrease the hydrophilicity, which is conducive to the formation of the hydrogel. Meanwhile, the resulting methyl ester is unstable in the presence of OH- and can spontaneously revert back to the carboxyl groups, leading to a non-equilibrium self-decayed hydrogel. However, such highly active methylating agents show significantly poisonousness. It severely restricts the further development of the temporal material driven by them. Thus, to avoid the high toxicity, carbodiimide, for example, 1-ethyl-3-(3-(dimethylamino)propyl)-carbodiimide (EDC), was used to replace the iodomethane or dimethyl sulfate to initiate the temporary gelation of precursors containing carboxyl groups. [23, 31-37] As shown in Figure 3a, D. Haldar and coworkers presented a temporary hydrogel of which the precursor was benzyloxycarbonyl-L-phenylalanine (ZF). [35] Upon the addition of EDC, two ZF molecules rapidly condensed to an anhydride gelator, yielding a self-supporting, translucent gel. Due to the instability of the anhydride in the presence of water, the gel slowly redissolved after a period of time. Similar to the temporary gel formed by the drive of dimethyl sulfate, the author found that increasing the concentration of chemical fuel remarkably delayed the spontaneous dissolution of the gel. It exhibited once more that the lifetime of such kind of temporary gel is well-tunable. Additionally, the author also illustrated that the temporary gel was regeneratable by the refueling of EDC. In the same year, C. S. Hartley and D. Konkolewicz et al reported a polymeric temporary gel fabricated formed by the drive of EDC. [36] The precursor was the random polymer (Am-r-AA) formed by acrylic acid and acrylamide. (Figure 3b) The EDC-catalyzed condensation of carboxyl groups on the Am-r-AA chains resulted in an anhydride crosslinked polymer network, producing a yellow polymer hydrogel, which can slowly self-degrade. Unsurprisingly, many parameters, such as the concentration of EDC, temperature, the molecular weight of Am-r-AA, and the ratio of Am/AA units, can significantly affect the lifetime and stiffness of the temporary hydrogel. Besides the above fuel-driven “sol-gel-sol” processes, it was also reported that the carboxyl-containing hydrogel can also temporally deform with the addition of chemical fuel. In 2022, F. Xuan, Y. Wang, and coworkers demonstrated an EDC-driven hydrogel actuator based on poly(acrylic acid) (PAA). [37] As shown in Figure 3c, the addition of the EDC led to the deswelling of the PAA hydrogel, which can self-relax with the slow hydrolysis of the anhydride linkage. The temporary deformation is repeatable by the refueling of the EDC. It can be found that the mechanism of the temporary deformation was quite close to natural muscles, of which temporal contraction was driven by the energy from ATP or its analogs. Therefore, such fuel-driven temporarily-deformed hydrogel exhibited a potential application in developing artificial muscles and the manufacture of the soft robot.