Biomaterials for MEMS

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.

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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.

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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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