36 R. Tran et al.
biodegradable scaffold will be required. Engineered scaffolds must be strong
enough to withstand the mechanical demands asserted upon them when im-
planted into the body, and must be able to retain their mechanical properties over
time.
23
The utilization of an elastomeric scaffold is advantageous in that it can
sustain and recover from multiple deformations without causing irritation to the
surrounding tissue in a mechanically demanding environment.
24,25
Another advantage of elastomeric scaffolds is their ability to be used with
mechanical conditioning regimens to promote improved tissue formation. By
gradually transferring stress from the degrading synthetic matrix to the newly
forming tissue, scaffolds with applied cyclic mechanical strains have been shown
to increase collagen and elastin production in vascular smooth muscle cells, and
enhance the mechanical properties of the tissue engineered constructs in cardiac
applications.
26,27
Research groups have also shown that mechanical signals aid in
the development of tissue engineered cartilage.
28
3.2 DESIGN CRITERIA FOR BIODEGRADABLE ELASTOMERIC
POLYMERS
In order to fabricate constructs with the appropriate mechanical properties, many
important design criteria must be met when creating the starting materials for
the intended target application. The following section will discuss the design
requirements and concerns that should be taken into consideration when creating
an elastic material for soft tissue engineering applications.
3.2.1 Polymerization Mechanisms
The two main forms of polymerization for elastic polymers are polycondensation
and polyaddition reactions. Polycondensation reactions have stepwise growth
kinetics, and are characterized by the formation of by-products during synthesis.
For example, a diol can be reacted with a diacid to produce a polyester with water
as a by-product.
Polyaddition reactions display chain-growth kinetics, and require the use of an
initiator. Chain initiation, propagation, and termination are steps that characterize
a polyaddition reaction. Through this general mechanism, the average molecular
weight of the polymer increases during the reaction. High molecular weight poly-
mers and/or crosslinked polymers can be produced in a polyaddition reaction.
3.2.2 Methods to Incorporate Elasticity
The two methods to incorporate elasticity are physical crosslinking and chemical
crosslinking. Certain segments of polymer chain will form a crystalline structure,
which will serve as a means for physical crosslinking. In the case of polyurethanes,
the clusters of hard segments act as “pseudo cross-links”, and allow the material
to behave as an elastomer.
29
When the temperature is raised, the hard segment
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