Biomedical Engineering Reference
In-Depth Information
be a reverse gelation process. The critical extent of reaction for gelation,
p
c
, may
also be used to predict the time to dissolution for photopolymerized or end-
linked hydrogels that are dissolved by hydrolysis or proteases [35, 79].
B
A
Fig. 4. Degradation of PEG hydrogels. A. Hydrogel formed from reaction of four-arm PEG-
vinylsulfone (thin grey lines) with an enzymatically degradable peptide (thick black lines).
Even after degradation of one of the four peptide that attach the four-arm PEG to the network, the
four-arm PEG is still connected to the network but now has a functionality of three. B. Hydrogel
formed by free radical polymerization acrylate-lactide-PEG-actide-acrylate in the presence of PVP.
The PEG chains (thin grey lines) are connected through poly(acrylate-co-VP) segments (thick black
line). Degradable lactide groups are represented as filled black circles.
Much more challenging is the prediction of swelling during degradation of
the hydrogels. Inaccuracies in the Flory-Rehner model may result in poor
predictions of the initial swelling of the hydrogels [70]. Network imperfections
are also likely present that are not well described and not included in the model.
In these cases, a scaling law may be applied to describe swelling from the initial
state during the degradation process:
−
3
5
Ä Ô
3
3
Q
∝
Z
∝
M
∝
Å
Æ Ö
[8]
5
5
c
V
0
Thus, as the gel degrades, the average molecular weight between crosslinks
increases and the density of crosslinks decreases [49]. The scaling law approach
is successful in fitting the swelling of degrading acrylate-oligolactide-PEG-
oligolactide-acrylate hydrogels to the predicted crosslink density [80]. For end-
linked hydrogels, the Flory-Rehner equation may be used directly if it is assumed
that the gels only reach a fraction of complete end group reaction, which may
vary with gel formation conditions and becomes an adjustable parameter [70]. As
hydrogels formed from end-linked multiarm polymers degrade, the number of
elastically inactive dangling ends may become quite large, while the presence of
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