Biodegradable Elastomeric Polymers and MEMS in Tissue Engineering 37
clusters disassociate, and the material can be made to flow. When subsequently
cooled, the clusters reform and the material will again exhibit elastomeric prop-
erties. Some ABA triblock co-polymers will also show elastomeric properties.
30
For example, the thermal liable crosslinks in an ABA triblock co-polymer can
aggregate to form physical crosslinking between polymer chains.
Chemical crosslinking joins the polymer chains together into a network linked
by covalent bonds. Unlike physical crosslinks, the chemical crosslinks are gen-
erally irreversible, and display greater mechanical strength and elasticity. It
is well known that natural ECM components such as collagen and elastin are
crosslinked polymers. The crosslinking provides these natural materials with their
elastic nature. Due to this phenomenon, researchers have utilized the concept
of crosslinking in the creation of elastomers to meet the versatile needs in tissue
engineering and other biomedical applications.
For polycondensation mechanism, in order to create a polymer with a 3D
elastomeric network structure, at least one of the monomers chosen should be
multifunctional. In addition to providing the needed functional groups for
chain extension, a multifunctional monomer provides valuable unused functional
groups, which can be used in later post-processing to create a 3D crosslinked
network. Thus, by creating crosslinked network structure, a material with elastic
properties can be obtained.
3.2.3 Design Concerns
The three major concerns when designing a biodegradable elastomer for biomed-
ical applications are the biocompatibility, mechanical properties, and degradation
rate of the material. In the following section, a brief introduction of these three
properties and how they affect each other are discussed.
3.2.3.1 Biocompatibility
Biocompatibility is a term used to describe the ability of a material to perform
with an appropriate host response in a specific application. For the materials
used in biomedical applications, the biocompatibility should always be put as
the first concern. There are several factors that can affect the biocompatibility
of a material. For example, the hydrophilicity or hydrophobicity of a material
can greatly influence its biocompatibility. It has been demonstrated that the
degree of hydrophilicity/hydrophobicity should be balanced to achieve optimal
cell affinity.
31,32
The acidity of a material can also influence its overall biocompatibility. Certain
functional groups located on the polymer chain have the ability to greatly change
the pH of the surrounding area. In addition to the chemistry of the bulk material
causing pH changes, certain materials will degrade into acidic products to alter
the pH of the immediate area. This deviation in pH from the body’s normal values
can create a cytotoxic effect, which can later lead to adverse reactions.
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