Biodegradable Elastomeric Polymers and MEMS in Tissue Engineering 39
) will also affect the degradation rate due to the increased water diffusion rate
into the material. In order to prevent any changes in the elasticity of a material,
it is important to maintain the T
below the normal body temperature. In the
case of an elastomer, both the T
and mechanical properties are affected by the
degree of crosslinking. A higher crosslinked elastomer will normally have a slower
degradation rate, stronger mechanical strength and smaller elongation rate.
3.3 BIODEGRADABLE ELASTOMERIC POLYMERS
The use of elastomers in medical applications originates back to the beginning of
the rubber industry. Since then, numerous materials have played a major role in
Polyesters are the most widespread category of polymers
used in biomedical applications. The ester bond is important because it allows
for degradation through hydrolytic cleavage in the presence of water. Unlike
enzymatic degradation, this form of degradation is advantageous because of the
minimal site–to–site and patient–to–patient variations.
A polymer used in tissue engineering applications should show good degrad-
ability and biocompatibility when presented in vivo. Due to these requirements,
glycolic and lactic acid based poly(α–hydroxy acids) such as poly-L-lactide acid
(PLLA) and poly(lactic-co-glycolic acid) (PLGA) have gained attention in the past
few decades as suitable polyesters for various medical applications. Their use
can be seen in drug delivery systems, scaffolds for tissue regeneration, resorbable
sutures, staples, and orthopedic ﬁxation devices.
However, these α–hydroxyl acid polymers are inappropriate for soft tissue ap-
plications because of their stiff nature. Due to this major drawback, researchers are
advancing towards a new category of polyesters whose mechanical properties can
be tuned for particular soft tissue engineering applications such as blood vessels,
heart valves, ligaments, and tendons. Polyesters that possess elastic properties
to meet the requirements for soft tissue engineering are shown in Table 3.2. The
following section will focus on the polyester elastomers that have been used in the
ﬁeld of soft tissue engineering.
184.108.40.206 Polyhydroxyalkanoates (PHAs)
In the early 1920’s, the bacteria bacillus megaterium was recognized for producing
poly(3–hydroxybutyrate) (PHB), which is the most common polymer among the
polyester class. Since then, more than 150 different monomer combinations have
been used in the formation of different polymers within the PHA family.
different pathways have been revealed for the synthesis of PHA through the
process of biosynthesis, which has been mentioned in detail elsewhere.
advancements in the ﬁeld of genetic engineering, researchers have also used plants
as the production house for PHB-related polymers.
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