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.