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-