Biomedical Engineering Reference
In-Depth Information
Figure 9.20.
Cross-linking of HA-Tyr- conjugates by an enzymatic cou-
pling reaction.
65
peroxide (H
2
O
2
) and horseradish peroxidase (HRP), yielding enzy-
matically cross-linked, highly elastic, and biodegradable HA hydro-
gels (Fig. 9.20).
65
The HRP-mediated coupling reaction of phenol
moieties in HA-Tyr conjugates occurred either via a carbon-carbon
bond at the
ortho
positions and a carbon-oxygen bond between
the carbon atom at the
ortho
position and the phenoxy oxygen.
While the gelation time was dependent on the precursor concentra-
tions of enzyme or H
2
O
2
/Tyr ratios, the gel was degraded over sev-
eral months, due to slow hydrolysis of the newly formed urethane
bonds. In related hydrogels where the Tyr units were linked to the
HA via an ester-containing diglycolic group, degradation was, how-
ever, controlled more rapidly within several days. A double syringe
equipped with a mixing chamber was normally utilized for the syn-
thesisofHAhydrogels:onecompartmentcontainedtheHA-Tyrsolu-
tion, whereas the other one contained a mixture solution of H
2
O
2
and HRP. This double syringe system facilitated the administration
of the hydrogel system, leading to its applications in protein deliv-
ery and scaffold for tissue engineering. Furtehr grafting of dextran-
tyramine to HA was turned into an injectable hydrogel for cartilage
tissue engineeringin the presence of HRP and H
2
O
2
.
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