Biomedical Engineering Reference
In-Depth Information
large scale under controlled conditions and with predictable and reproducible
mechanical properties, degradation rate, and microstructure, are the most viable
alternatives to the naturally occurring materials.
Both PGA, PLA, and their copolymer PLGA belong to a family of linear ali-
phatic polyesters, which are most frequently used in tissue engineering.
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These have been shown as biocompatible materials that degrade into nontoxic
components with a controllable degradation rate in vivo that have been FDA
(US Food and Drug Administration) approved as degradable surgical sutures for
clinical use.
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Degradation of these polymers through hydrolysis of the ester
bonds
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leads to degradation products that are eventually eliminated from the
body in the form of carbon dioxide and water. The rates of degradation can be
tailored from several weeks to several years by modifying the chemical com-
position, crystallinity, molecular weight value, and distribution.
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As a result
of the hydrophilic nature of PGA, rapid degradation in aqueous solution or in
vivo, and ability to lose mechanical integrity in 2-4weeks, it is widely used as
polymer for scaffold.
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PGA in the form of nonwoven fibrous fabrics is one
of the most widely used scaffolds in tissue engineering.
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Compared with PGA, the extra methyl group in the PLA repeating unit
makes it more hydrophobic with less affinity to water that causes a slower rate
of hydrolytic de-esterification into lactic acid. The morphology and crystallinity
of PLA strongly influence the rate of biodegradation and mechanical proper-
ties
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making the scaffolds degrade slowly both in vitro and in vivo. Such
stability leads to mechanical integrity over several months.
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Various lactic and
glycolic acid ratios are used to synthesize PLGA to achieve intermediate degra-
dation rates between PGA and PLA.
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Currently, PLGA copolymers with
different PGA/PLA ratio (50:50, 65:35,75:25, 85:15, 90:10) are applied in skin
tissue regeneration and for suture applications.
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Among the few synthetic polymers, PLA, PGA, and PLGA are approved by
the FDA for certain human clinical applications.
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Other linear aliphatic poly-
esters that are also used for tissue engineering research include PCL
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and
PHB.
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Because PCL degrades at a significantly slower rate than PLA, PGA,
and PLGA,
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it is less attractive for biomedical applications and more attractive
for long term as implants as well as controlled release applications.
One good candidate for bone tissue engineering is PCL which showed suffi-
cient mechanical properties to serve as scaffold in bone substitution that requires
physical properties to be maintained for at least 6months.
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Scaffolds involved
in a bone regeneration process can be enhanced by the addition of a carbonated
apatite that is the main constituent of the inorganic phase of bone.
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6.5.2 Examples of Bionanocomposites
Different nanocomposites have been studied for tissue engineering applications.
A few of these materials are discussed below.
