Biomedical Engineering Reference
In-Depth Information
Free radical crosslinking of multifunctional monomers such as PEG-
diacrylate produces network structures that generally resemble crosslinking by
vulcanization but with important differences. Polyacrylamide gels used for
electrophoresis are a familiar example of this type of network structure. Gels for
electrophoresis have low ratios of N,N'-methylenebisacrylamide crosslinker
(functionality = 4) compared to acrylamide monomer (functionality = 2). The
polyacrylamide backbones that form during crosslinking serve the same role as
the backbone chains of vulcanized networks. An N,N'-methylenebisacrylamide
crosslinker may join two backbone polyacrylamide chains consisting primarily of
acrylamide monomers. The dangling ends of the polyacrylamide chains may be
accounted for by using equation 5, with the average molecular weight of the
polyacrylamide chains as M
0
and the ratio of acrylamide to N,N'-
methylenebisacrylamide used to calculate M
c
. Thus, the polyacrylamide gels can
be analyzed as vulcanized networks, although crystallization of the crosslinker
complicates analysis of these familiar gels [58]. The structure of PEG-DA
hydrogels is more difficult to analyze. PEG-DA is frequently mixed with N-
vinylpyrrolidone (NVP) in an aqueous solution and photopolymerized [59].
Within these hydrogels, the crosslinker is PEG-DA, which has a high molecular
weight and is present in high molar amounts compared to NVP. Similar to
polyacrylamide hydrogels, the relevant backbone chain is not the PEG but rather
the poly(acrylate-VP) copolymer that forms during crosslinking. The M
c
may be
calculated using the molar ratio of PEG-DA to VP assuming equal reactivity of
the acrylate and vinyl groups, but the molecular weight of the PEG chain is
generally so large that treating the PEG-DA as a point crosslink is not
appropriate. A simplifying strategy has emerged to analyze PEG-DA hydrogels
by the Flory-Rehner equation, in which swelling data are used to calculate an M
c
assuming a hypothetical, all-PEG network [59, 60]. This approach assumes that
the structure of the hydrogel is identical to that expected for vulcanization of
PEG [5]. The calculated 'mesh size' may be compared with the rate of diffusion
of proteins of different molecular weights through the hydrogel, with the mesh
size calculated from M
c
[61]. Hydrogel swelling and thus the calculated mesh
sizes are well correlated with the rate of release of proteins of different molecular
weights [60].
PEG-DA is often synthesized with degradable polyester linkages between the
PEG and the acrylate group, producing hydrolytically degradable hydrogels [13].
The poly(α-hydroxy acid) subunits have a profound effect on the solubility of
the PEG, leading to the formation of micelles in aqueous solution [62, 63]. This
may lead to high molecular weight poly(acrylate) formed within a single micelle,
with crosslinking due to bridging of two micelles by a single PEG. Such a model
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