Biomedical Engineering Reference
In-Depth Information
convert the ionic group on an amino acid derivative into a neutral
group, resulting in the formation of a small-molecular hydrogela-
tor and supramolecular hydrogels. Gelation was processed by the
dephosphorylation of the PO
3
4
of Fmoc tyrosine phosphate under
basic conditions, which are mediated by alkaline phosphatase, one
ofthecomponentsofthekinase/phosphataseswitchesthatregulate
the protein activity. They prepared amphiphilic Fmoc-amino acid
derivatives as hydrogelators to adjust the hydrophilic/hydrophobic
balance of amino acids for hydrogelation. In a recent paper, Ulijn
et al
. reported a novel approach of enzyme-triggered hydrogelation
using proteases, which are enzymes that normally hydrolyze pep-
tide bonds in aqueous media, to perform the reverse reaction to
produce an amphiphilic peptide hydrogelator that self-assembles
to form a nanofibrous structure.
63
The researchers employed ther-
molysin from
Thermoprotelolyticus rokko
as a suitable enzyme to
test this approach. Thermolysin has been used in reverse hydrol-
ysis reactions and has a well-known preference for the hydropho-
bic/aromatic residues on the amine side of the peptide bond. The
researchers prepared Fmoc-conjugated peptide amphiphiles that
self-assemble into nanofibrous structures derived from
π
-stacking
of the highly conjugated fluorenyl group, further stabilized by the
formation of helical structures. They applied this system to a novel
PEG-based hydrogel particle that was responsive to enzymes for
controlled release.
64
A copolymer of PEG and acrylamide form a
hydrogelparticleincorporatedwithFmoc-peptideswithanenzyme-
cleavable linker.
6.4 Conclusions and Outlook
The chapter provides information on enzymatically triggered
in
situ
gel-forming biomaterials, which have attracted considerable
attentionamongintelligentscaffoldsfortissueregeneration.Hydro-
gels formed
in situ
have been developed steadily and are still
under investigation because of their potential applications. How-
ever, many efforts to accomplish practical hydrogels as injectable
biomaterials have ended in failure due to insu
ciencies in safe cell
encapsulation, solidity after hydrogelation, and signaling molecules
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