Biomedical Engineering Reference
In-Depth Information
5.2.2 Glycolide-Based Biodegradable Copolyesters Having Aliphatic
Polyester-Based Co-Monomers
Other commercially successful glycolide-based biodegradable polymeric biomaterials are the copo-
lymers of glycolide with other monomers within linear aliphatic polyesters such as lactides, carbon-
ates, and ε-caprolactone. The glycolide−lactide random copolymers are the most studied and have a wide
range of properties and applications, depending on the composition ratio of glycolide to lactide. Figure
5.2 illustrates the dependence of biodegradation rate on the composition of glycolide to lactide in the
copolymer. For wound closure purposes, a high concentration of glycolide monomer is required for
achieving proper mechanical and degradation properties. Vicryl sutures, sometime called polyglactin
910, contain a 90/10 molar ratio of glycolic to l-lactide and this molar ratio is important for the Vicryl
suture to retain crystalline characteristics. For biomedical use, Lewis acid catalysts are preferred for the
copolymers (Wise et al., 1979).
Ethicon introduced a similar glycolide-l-lactide copolymer suture in late 1990s, Panacryl
®
, but basi-
cally reversed the molar ratio of Vicryl
®
, 5/95 molar ratio of glycolide to l-lactide. As a result of pre-
dominant l-lactide component in Panacryl suture, its absorption behaves more like PLA than PGA,
that is, Panacryl requires at least 1.5 years or longer to be completely absorbed in living tissues. Such a
prolonged absorption profile
in vivo
may be associated with some reported postoperation complications
(Farnsworth, 2002; Vakili et al., 2004; Goldstein et al., 2007). It is well known that a prolonged
in vivo
degradation period of absorbable sutures could expose patients to the risk of late-stage tissue reactions
and hence may cancel out the advantage of slow degradation. For example, Eittenmuller et al. (1989a)
reported that 47.7% (9 of 19 patients) treated with PLA plates and screws for ankle fracture had an
inflammatory reaction nearly 3 years after operation. Bostman (1991) expressed a similar concern as he
reported that late inflammatory reaction encountered in 8% of the patients is a worrisome complication.
As a result, Panacryl was withdrawn from the market by Ethicon in mid-2000. Deknatel introduced a
100% PLA absorbable suture (Orthodek
®
) that is intended for wounds required a prolonged strength
support like in orthopedic field. Vicryl sutures are sterilized by ethylene oxide like other synthetic
absorbable sutures.
If d,l- instead of l-lactide is used as the co-monomer, the U-shaped relationship between the level of
crystallinity and glycolide composition disappears. This is because polylactide from 100% d,l-lactide
6
4
2
0
0
25
50
PLA (%)
75
100
FIGURE 5.2
The effect of poly(l-lactide) composition in polyglycolide on the time required for 50% mass loss
implanted under the dorsal skin of rat. (From Miller, R.A., Brady, J.M., and Cutright, D.E., 1977.
J. Biomed. Mater.
Res.
, 11: 711. With permission.)
