Biomedical Engineering Reference
In-Depth Information
and resistant to protein and cell adhesion. These characteristics are
an important starting point for fundamental studies of cell matrix
interactions and cell behavior in modified materials because the
dataarenotconfoundedbyhighnonspecificproteinadhesionwhich
influences the cell behavior. Further, several different chemistries
can be used to incorporate polymerize and add functionalities into
the hydrogel.
One system that has been studied extensively uses PEG
end-functionalized with diacrylate (PEG-DA) and/or methacry-
late (PEG-DM), which, when combined with a photoinitiator
(1-[4-(2-Hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1
-one,alsoknownasIrgacure2959,CIBASpecialtyChemicals),canbe
photopolymerized.Thiscross-linkingmechanismisadvantageousin
that it could be used as an injectable,
in situ
cross-linked treatment
for cartilage defects.
75
Using this system, the mechanical properties can be tuned over
a large range; the compressive modulus was varied from 60 kPa to
500kPainphotopolymerizedgelswithPEG-DMandahydrolytically
degradable poly(lactic acid) (PLA)-PEG-PLA dimethacrylate precur-
sors by altering the precursor percentage from 10% to 20%.
76
The
degradation of these constructs by hydrolysis was found to best
match the extracellular matrix accumulation by encapsulated chon-
drocytes at an intermediate macromer content of 15%. These sys-
tems can incorporate biological molecules by first functionalizing
the biomolecule with (meth)acrylate and reacting with PEG pre-
cursors. This technique has been used to generate gels functional-
izedwithRGDpeptidesforcelladhesion,methacrylatedchondroitin
sulfate
77
covalently bonded with the PEG network, and physically
entrapped collagen type I andhyaluronic acid.
78
,
79
Interestingly, these functionalized gels showed differential
effects on zonal chondrocytes, with superficial chondrocytes
expressingPRG4atthehighestrateanddeepchondrocytesexpress-
ing aggrecan and collagen II at the highest rate in PEG-CS gels.
79
Similar hydrogels have been used to form zonal constructs by
sequentialphotopolymerizationofPEG-DAgelscontainingchondro-
cytes from different zones (Fig. 36.6).
80
The immature bovine chon-
drocytes remained viable, remained within their respective layers,
and retained biosynthetic differences over three weeks in culture.
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