Biomedical Engineering Reference
In-Depth Information
poly(2-hydroxyethyl methacrylate) (PHEMA), and poly(
N
-isopropylacrylamide)
(PNIPAAm), and poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) seg-
ments. These synthetic polymer networks can be synthesized using various poly-
merization techniques. The polymer chemist can design and synthesize polymer
networks with molecular-scale control over structure such as crosslinking density,
and with tailored properties such as biodegradation, mechanical strength, and
chemical and biological response to stimuli. Cells can be encapsulated homo-
geneously within these synthetic polymer hydrogels.
From the viewpoint of the actions of the hydrogels toward cells, they can
be classified into three groups: cell adhesion-inducible matrices, tissue response-
inducible matrices, and bioinert matrices. Collagen, gelatin, and poly(
L
-lysine)
contain cell-adhesive ligands in their structure and are classified as cell adhesion-
inducible matrices. Hydrogels composed of PEG and the MPC polymer are classi-
fied as bioinert matrices. Other hydrogels such as polysaccharide derivatives are
classified as tissue response-inducible matrices. PEG-based hydrogels are particu-
larly intriguing because of their bioinert property, hydrophilicity, and ability to
be customized by changing the chain length to tune transport properties or by
incorporating biologically relevant molecules [
15
]. They have been used to immo-
bilize various cell types including osteoblasts and fibroblasts that can attach, grow,
and produce matrix. PEG-based hydrogels can be customized by incorporation of
adhesion domains of ECM proteins to promote cell adhesion, growth factors to
modulate cell function, and degradable linkages [
16
]. Hydrogels for tissue engi-
neering is a rapidly growing field because of their chemical flexibility for customi-
zation and the resulting tissue-like physical properties.
Collagen is the most widely used tissue-derived natural polymer, and is a main
component of the ECM of tissues. However, these gels lack physical strength, are
potentially immunogenic, and can be expensive. Furthermore, there can be big
variations between produced collagen batches. However, collagen meets many
of the biological design parameters, as it is composed of specific combinations of
amino acid sequences that are recognized by cells and degraded by enzymes
(collagenase) secreted from the cells.
Agarose is another type of marine algal polysaccharide, but unlike alginate it
forms thermally reversible gels. The gel structure is thought to be bundles of
associated double helices, and the junction zones to consist of multiple chain aggre-
gation. The physical structure of the gels can be mainly controlled by using a range
of agarose concentrations, which results in various pore sizes. The large pores and
low mechanical stiffness of the gels at low concentrations of agarose may enable
the migration and proliferation of cells, and these factors have been found to affect
neurite growth
in vitro
.
Chitosan has found many biomedical applications, including tissue engineering
approaches. Enzymes such as chitosanase and lysozyme can degrade chitosan.
However, chitosan is easily soluble in the presence of acid, and generally insoluble
in neutral conditions as well as in most organic solvents due to the existence of
amino groups and the high crystallinity. Therefore, many derivatives have been
reported to enhance the solubility and processability of this polymer.
