Biomedical Engineering Reference
In-Depth Information
6.4.5
Degradable Synthetic Polymeric Tubes
In studies with degradable tubes, the historical goal has been to improve on the
silicone tube by eliminating the second surgical procedure (to remove the tube)
normally required when these biodurable tubes have been implanted.
Degradable synthetic polymers intended for implantation have been commonly
based on monomers, such as α-hydroxy acids, HO-CHR-COOH, that bear a simi-
larity to amino acids. The most well-known among these are polymers derived from
lactic acid (R = CH
3
) and glycolic acid (R = H). Glycolic acid (GA) is not optically
active and exists in only one configurational structure, while LA has an optically
active carbon atom and can be found in two enantiomeric forms,
l
- and
d
-lactic
acid. Polymers of α-hydroxy acids are readily synthesized by ring-opening po-
lymerization of the cyclic dimers of the corresponding α-hydroxy acids, known
as lactide and glycolide cyclic diesters or simply lactide and glycolide; for this
reason, poly(lactic acid) and poly(glycolic acid) are often called polylactide and
polyglycolide, respectively (Stevens 1990). Poly(glycolic acid; PGA), poly(lactic
acid; PLA), as well as copolymers of GA and LA have been synthesized and their
degradation rate in various in vitro and in vivo systems have been determined. A
commonly used copolymer has the composition 90/10 GA/LA (PGL; polyglactin
910). Poly (ε-caprolactone; PCL), another polyester, is produced by ring-opening
polymerization of ε-caprolactone (Stevens 1990). Copolymers of poly(lactic acid)
and ε-caprolactone have been used extensively in nerve regeneration studies.
The degradation rate of these polymers depends on the molecular weight, con-
figurational structure, comonomer ratio, residual monomer, molding conditions,
annealing, sterilization procedures, and, especially, on the fraction of crystallinity
(Vert et al. 1984; Vert and Li 1992). Poly(lactic acid) is a stiff and brittle polymer;
with addition of a plasticizer such as triethyl citrate it becomes quite flexible and
somewhat tougher. The most well-known uses of these condensation polymers cur-
rently are as sutures, as orthopedic materials, and as delivery media for controlled
release of drugs; however, experimental investigations of their value as implants
have also been reported in almost every anatomical site.
Some information on in vivo degradation times of these polymers that have been
used to tubulate transected peripheral nerves has occasionally appeared. Most of
the data were obtained after long periods and represent upper bounds to the half-life
of the tubes studied. A PLA tube plasticized with 10 % triethyl citrate almost com-
pletely degraded by 13 weeks (Seckel et al. 1984). Rigid and meshed PGA tubes
were not present 1 year after implantation in the monkey (Dellon and Mckinnon
1988). A tube constructed from nonwoven PGA fabric, implanted as a nerve bridge
in the monkey, was reported degraded by about 26 weeks (Tountas et al. 1993). A
PGA mesh coated with collagen had degraded by 12 weeks in cats; however, small
residual fragments of the tube were identified at 17 weeks (Kiyotani et al. 1996). A
PGA tube was reported to have disappeared by about 26 weeks (Keeley et al. 1991;
Aldini et al. 1996). Tubes constructed from PGL were also reported to have been
completely degraded by about 26 weeks (Gibson et al. 1991; Aldini et al. 1996).
