Biomedical Engineering Reference
In-Depth Information
10.4.2 Polylactic acid (PLA)
Polylactic acid is a commonly used carbon-based biodegradable polymer
consisting of lactic acid monomers. Lactic acid is optically active, because it
contains an asymmetrical carbon atom. The
L
-confirmation of lactic acid is a
naturally occurring metabolic by-product, but either
L
- or
D
-PLA can be used in
polymer synthesis to create isotactic
L
-PLA or
D
-PLA or syndiotactically
alternating
D,L
-copolymers. The stereochemistry of the polymer monomers
affects its ability to crystallize and self-associate, so just by changing the
D
/
L
composition of PLA, it is possible to change its mechanical properties. For
instance
L
-PLA is a crystalline polymer with a tensile modulus of 2.23±3.85 kN
m/g, but by creating a syndiotactic
DL
-PLA, the tensile modulus is reduced to
0.80±2.36 kNm/g (van de Velde and Kiekens, 2002). PLA has a typical in vivo
degradation time of 30 to 50 weeks (Barns et al., 2007).
10.4.3 Polyglycolic acid (PGA)
Polyglycolic acid (PGA) is commonly copolymerized with PLA to form PLGA.
Unlike PLA, PGA is not optically active, and when not copolymerized, it is
highly crystalline. PGA is more hydrophilic than PLA due to its lack of
additional asymmetrical methyl groups. Consequently, increasing the weight
percent of PGA in a PLGA copolymer is one method to increase the wettability
of a biomaterial made from PLGA. Another consequence of the hydrophilicity
of PGA is its comparatively rapid degradation time. In vivo, PGA degrades in 2
to 4 weeks, losing 60% of its mass during the first two weeks.
10.4.4 Poly(-caprolactone) (PCL)
Poly(-caprolactone) (PCL) is a semicrystalline polymer produced by a
catalyzed ring opening of -caprolactone. The resulting polymer consists of
five methylene groups separating a polar ester group. PCL is biocompatible and
is currently used as a material for degradable sutures. PCL has a tensile modulus
of 0.19±0.38 kNm/g (van de Velde and Kiekens, 2002). PCL has a degradation
time of approximately two years, but PCL can be copolymerized with PLGA for
a more rapidly degrading polymer (Yang et al., 2001).
10.5 Methods for polymer matrix fabrication
This section will examine some common tissue engineering matrix fabrication
methods. The methods included in this section are solvent casting and par-
ticulate leaching, melt molding, emulsion freeze-drying, gas foaming,
electrospinning and computational techniques. Introducing porosity into a solid
polymer is one of the central goals of tissue matrix engineering and this task can
