Biomedical Engineering Reference
In-Depth Information
H
O
(
O
C
CH
2
C O
O
HO C COOH
H
n
C
O
O
Glycolic acid (GA)
Glycolide
Poly(glycolic acid) (PGA)
(Polyglycolide)
CH
3
CH
3
O
CH
3
O
C
(
HO C COOH
H
CH C O
O
C
O
n
O
CH
3
Lactic acid (LA)
(L-, D-, DL-)
Lactide
(L-, D-, DL-, meso-)
Poly(lactic acid) (PLA)
(Polylactide)
(L-, D-, DL-)
(
(
CH
2
CH
2
O
CH
2
C O
O
CH
2
CH
2
CH
2
CH
2
CH
2
C O
O
n
n
Poly-
p
-dioxanone
Poly-
ε
-caprolactone (PCL)
(
CH
2
CH
2
CH
2
O C O
O
n
Poly(trimethylene carbonate) (PTMC)
Fig. 7.2-7 Chemical structure of
a
-hydroxyacid polymers, copolymers, and their monomers.
degradation kinetics is the simple addition of kinetics of
each component degradation.
It takes a longer time than a few years for homopol-
ymers of LLA and 3
-
CL to be completely absorbed,
whereas TMC homopolymer degrades too quickly in the
presence of water. Homopolymers of poly(
D
,
L
-lactide)
(PDLLA) are bioabsorbed at a little higher rate than
PLLA, because of the absence of crystalline regions. In
case there is no need to distinguish between PLLA and
PDLLA, the term PLA will be used below to include
PLLA and PDLLA.
Homopolymers
The most widely used absorbable sutures are made
from polyglycolide (PGA) or poly(glycolide-co-lactide)
(PGLA) with a glycolide (GA)/
L
-lactide (LLA) ratio of
90/10. This PGLA with the 90% content of GA is in-
cluded in PGA here, because PGLAwith a GA/LLA ratio
of 90/10, which is commercially available as a multifila-
ment suture and a Vicryl mesh (Vicryl, Ethicon, USA),
exhibits properties quite similar to PGA (100% GA
polymer). Poly(
L
-lactide) (PLLA) has been clinically
used after molding into pin, screw, and mini-plate for
fixation of fractured bones and maxillofacial defects of
patients. Both PGA and PLLA are crystalline polymers
which can provide medical devices with excellent me-
chanical properties, but PGA degrades mostly too
quickly while PLLA degrades too slowly for use as scaf-
fold. Nevertheless, both of them have primarily been
chosen as polymers for scaffold fabrication in numerous
studies worldwide.
Non-woven PGA fabrics have extensively been used as
a scaffold material for cell growth in the effort to engi-
neer many types of tissues. However, scaffolds fabricated
from PGA fibers lack sufficient dimensional stability to
allow molding into distinct shapes and degrade rapidly to
disturb processing of this material after exposure to
aqueous media. To overcome these problems, the PGA
fabrics are often dipped in solution of polylactide (PLA),
followed by evaporation of the solvent to deposit stiff
PLA coating on the fabrics. In general, cell adhesion onto
such blended materials is influenced by the polymer
component existing at the outermost surface, while the
Copolymers
The aliphatic copolyester that has the largest clinical
application is poly(LA-
co
-GA) (PLGA) mostly with an
LLA/GA ratio around 50/50. This copolymer has clini-
cally been used as a carrier of peptide drugs for their
sustained release. In many cases, copolymers are pre-
ferred for scaffold fabrication because of their more
versatile, physicochemical properties.
Figure 7.2-8
shows
the decrease in tensile strength in phosphate buffered
solution (PBS) versus the hydrolysis time for various al-
iphatic polyesters (
Table 7.2-3
) in the fiber form
[3]
.
Copolymerization of monomers A and B offers a great
potential for modifications of polymers A or B, by con-
trolling the physical and biological properties of bioab-
sorbable polymers, such as degradation rate, hydrophilicity,
mechanical properties, and
in vivo
shrinkage.
Assume that homopolymer A is absorbed while ho-
mopolymer B exhibits no or insignificant absorption in
the body. Blending of homopolymers A and B does not
