Figure 7 (a) H2O2-driven transient organogel. Copyright (2020) John Wiley and Sons. (b) H2O2-driven transient gel-sol-gel process. Copyright (2021) John Wiley and Sons.
3.1.5. Other fuel-driven temporary gels. Besides the above typical reaction networks, there are some unusual strategies for constructing a fuel-driven temporary gel. In 2015, A. K. Das prepared a temporary hydrogel that could be used for human umbilical cords via a lipase-catalyzed reaction network. [59] As shown in Figure 8a, a peptide bolaamphiphile (HO-WYSuc-YW−OH) reacted with p-hydroxy benzylalcohol in the presence of lipase forming an activated diester building block (BHO-WYSuc-YW−OHB). It self-assembled to produce nanofibrillar thixotropic hydrogel. The subsequent hydrolysis of BHO-WYSuc-YW−OHB resulted in the dissipation of energy and collapse of the hydrogel. The author found that the hydrogel displayed a thixotropic behavior due to the dynamicity of hydrogen bonding interaction and other noncovalent interactions between the gelators. Hence, it was facially used as a supreme scaffold for human umbilical cord stem-cell proliferation. MTT, XTT, and DNA leaching experiments revealed that the hydrogel promoted the proliferation and survival of the stem cells. Later on, D. Das and coworkers demonstrated a simple fatty acid-based system that could temporarily gelate upon the addition of dimethylaminomethyl ferrocene (Fc-NMe2).As shown in Figure 8b, Fc-NMe2 acted as a counteraction to access unique hexagonal compartments resulting in the formation of a self-supporting gel. [60] However, an oxidizing environment created by pre-adding Fe(ClO4)3 contributed to the dissipation of energy by converting Fc-NMe2 to its oxidation products and the gel autonomously undergoes a transition to a sol. Furthermore, the authors also found that the temporary hexagonal nanostructures were able to host hemin and temporarily tune its peroxidase-mimicking activity up to an order of magnitude. Temporary enzyme regulation through compartmentalization is a feature seen in extant biology, therefore, this work indicated that fuel-driven temporary materials have the potential in mimicking the special function in living bodies. In 2020, T. M. Hermans and coworkers reported a “gel-sol-gel” temporary cycle driven by a reaction network initiated with dithionite (DT), glucono-δ-lactone (GdL), and hexamethylenetetramine (HMTA). [61] As shown in Figure 8c, the aldehyde saccharide hydrogelator (SachCHO) was converted to a negative-charged α-hydroxy sulfonate (SachSO3) by DT. Thus, SachCHO gels rapidly disassemble due to the electrostatic repulsion of SachSO3 to give a clear solution. The gradual re-gelation was achieved by the formaldehyde produced in situ by the slow reaction between GdL and HMTA which were added together with DT. Hence, it is understandable that the authors can control the lifetime of the sol state by modulating the concentration of GdL. Similarly, the “gel-sol-gel” temporary process was also realized by a thiuram disulfides-driven reaction network. As reported by Y. J. Zheng and coworkers [62], the hydrogel was synthesized by crosslinking a 4-armed, star-like, branched PEG terminated with a thiol group. As shown in Figure 8d, the addition of thiuram disulfides (chemical fuel) can break the disulfide linkages via dynamic disulfide exchange. It will soften the hydrogel and finally lead to its dissolution. However, the thiuram terminal group was unstable. Its automatic hydrolysis-coupling reaction will restore the disulfide crosslinking and cause the regelation. Finally, the author gave an interesting example of this fuel-driven “gel-sol-gel” cycle. As shown in Figure 8d, they encrypted an invisible hydrophilic image of “nature” on a hydrophobic aluminum substrate, while an “SHTU” made from the hydrogel was put on the substrate. Upon the addition of chemical fuel, the SHTU gel self-turned into a sol state. The liquid would wet the hydrophilic area and the excessive aqueous solution was automatically removed due to the hydrophobic interaction. Thus, the left solution spontaneously reformed a stable gel showing “nature”, the encrypted information, indicating that this cycle might have potential in anticounterfeiting applications.
3.2. Self-erased inks
Self-erased ink can be used for transmitting confidential messages, as the information written by such kind of ink can spontaneously disappear within a controllable period of time due to the consumption of chemical fuel. The key to developing self-erased ink is to find a material of which the color or fluorescent emission can change significantly as the reaction network proceeds. In 2017, J. Boekhoven and coworkers immobilized an aspartate-based precursor (Figure 9a) in a polyacrylamide polymer hydrogel to produce a transparent gel substrate. [31] High concentration of EDC, the chemical fuel, was applied as an ink to spread on the gel surface with a spray coater through a three-dimensional (3D) printed mask. The formation of insoluble anhydride made the exposed portion of the gel substrate turbid, yielding a clear trace. (Figure 9b) However, the anhydride was unstable and spontaneously hydrolyzed, resulting in the slow disappearance of the trace. The visibility of the traces decreased over time and was completely erased after roughly 10 h. (Figure 9b) Notably, the lifetimes of the visible traces could be tuned from roughly 200 to 500 min by changing the concentration of EDC, indicating that the availability of the presented information can be controlled. Similarly, S. J. George et al. presented a hydrogel of which color could be temporarily changed from red to purple by adding chemical fuel. [63] It made the writing more observable compared with the turbidity change. They designed a charge transfer (CT) system containing coronene salt (CS) and dodecyl methyl viologen (DMV). The CT interaction between CS and DMV produced a supramolecular amphiphile with red color. Once the concentration of the amphiphile reached 8 mM, it would undergo hydrogenation to yield red-colored CT gels. As shown in Figure 10a, sodium dithionite (SDT) was applied as chemical fuel to reduce the DMV to its cor-