Biomedical Engineering Reference
In-Depth Information
minutes), polymerization times are well within acceptable clini-
cal time scales, allowing for network formation intraoperatively. 75
In general, densely cross-linked networks formed from multi-
functional anhydride monomers degrade via a surface erosion
mechanism, with mass loss only at the surface erosion zones on
the exposed areas of the polymer. These erosion zones allow only
minimal penetration of water into the polymer, so hydrolysis occurs
exclusively near the polymer surface and is dictated by the poly-
mer chemistry. For example, a disk (
1.7 mm thickness) com-
posed entirely of methacrylated sebacic acid degrades in approxi-
mately three days, whereas a disk composed entirely of the more
hydrophobic methacrylated carboxyphenoxy hexane takes more
than one year to completely degrade. 76 Because erosion occurs
only at the polymer surface, structural integrity is maintained for
longer degradation periods than in polymers that degrade through-
out their bulk. For example, more than 90% of the tensile modu-
lusofpoly(methacrylatedsebacicacid)andpoly(methacrylatedcar-
boxyphenoxy hexane) networks are maintained for up to 40% mass
loss. 76 On the other hand in bulk eroding systems, polymer chains
are cleaved relatively homogeneously throughout the network and
mechanical properties can plummet even when little mass loss has
occurred. 77 , 78
8.3.2.1 Photo-cross-linkable poly(ethylene glycol)
PEG, a water-soluble polymer, has a long history of use in biomate-
rials. Thisisprimarily becauseof the extreme hydrophilicity of PEG,
which decreases the adsorption of proteins and can be used to alter
the interaction between materials and tissues and cells. Addition-
ally, the end groups on PEG are easily modified through a variety of
synthetic reactions. For instance, the reaction of PEG with acryloyl
chloride or methacryloyl chloride in the presence of triethylamine
is a simple technique for adding photoreactive vinyl groups. 79 Pho-
topolymerizablePEGhydrogelshavebeenusedfornumerousappli-
cations, including as membranes for the encapsulation of islets of
Langerhans, 79 - 81 as barriers to reduce intimal thickening after bal-
loon angioplasty, 82 as matrices for chondrocyte encapsulation in
cartilage regeneration, 83 - 87 for osteoblast encapsulation in bone
 
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