Figure 5 (a) Temperal hydrogel driven by acid. Copyright (2015) John Wiley and Sons. (b) Acid-driven bilayer hydrogel actuator. Copyright (2022) American Chemical Society.
3.1.3. ATP-driven temporal gels. As a medium for energy exchange, ATP and its analogs widely exist in living organisms. Plenty of microscopic vital movements are directly driven by the energy provided by ATP or its analogs. In recent years, artificial ATP-driven dissipative molecular self-assemblies have been widely reported. [26, 51-55] The reaction network for such a kind of DSA has been successfully built. [26,53] However, temporary gel formation (“sol-gel-sol” cycle) driven by ATP or its analogs seems to be seldom reported. The possible reason is that the electrostatic attraction is a weak and non-oriented interaction which is detrimental to a long-range ordered aggregation or robust crosslinked structure. However, contrary to the temporary formation of the gel, the temporary “gel-sol-gel” conversion driven by ATP has emerged. As shown in Figure 6a, Q. Yan and coworkers reported a polymer gel that was prepared by mixing two linear polymers that contained β-cyclodextrin (β-CD) and adamantine (ADA) as side groups, respectively. [56] The host-guest interaction between the two side groups crosslinked the polymer chains to yield a highly transparent gel. However, ATP, which was the chemical fuel as well as a competitive guest, can displace the ADA group in the β-CD ring because it has a higher binding affinity with β-CD. (Association constant: Kβ-CD/ATP ≈ 106 M-1, Kβ-CD/ADA ≈ 104 M-1) The competition interrupted the crosslinked structure and led to the dissolution of the gel. Meanwhile, the potato apyrase preloaded in the hydrogel slowly hydrolyzed ATP to AMP. Since the Kβ-CD/AMP (≈ 102M-1) is much lower than Kβ-CD/ADA, the ADA reoccupied the β-CD ring causing the regeneration of the hydrogel. Furthermore, Q. Yan et. al also achieved ATP-driven temporary swelling of a microgel by slightly modifying the molecular design. As shown in Figure 6b, the β-CD-ADA supramolecular interaction was partially replaced by spiropyran-based linkers to avoid total dissolution of the hydrogel. The addition of ATP destroyed the β-CD-ADA interaction. The electrostatic repulsion between the ATP molecules enlarged the distance of the polymer chains and pull the spiropyran ring open. It can be found that the microgel significantly swelled and the color of the colloidal dispersion visibly changed. Finally, the ATP was totally hydrolyzed by the potato apyase, leading to the recovery of the microgel.