Types of bioresorbable polymers for medical applications - Durability and Reliability of Medical Polymers - page 5

as examples of production processes. PHas homopolymers and copolymers
were first introduced in the 1980s by ICI under the name of Biopol (Williams
and Martin, 2005), and were used as an alternative to petroleum-based
plastic. However, the production cost was too high and interest in PHas
faded rapidly. recently, PHas polymers have gained an important role in
the field of biomedical devices and tissue engineering because of their low
toxicity and bioresorbability properties.
Condensation polymers are materials obtained through a polycondensation
reaction (which follows a step growth polymerization mechanism) involving
one or more different monomers. The most representative materials of this
category are the poly-alpha-hydroxy-acids such as the poly(lactic acid)
(PLA) and poly(glycolic acid) (PGA) which were first introduced during
the 1950s. PGa is the simplest linear aliphatic polyester, obtained through
polycondensation of glycolic acid; it has a high crystalline ratio, a high
melting point, and it is insoluble in water and in almost all common organic
solvents. PGa degrades mainly because of hydrolysis, producing glycolic
acid, which is a natural metabolite and thus recognized by human body (Figs
1.4 and 1.5).
PGA was used to produce the first example of bioabsorbable synthetic
suture threads under the commercial name of Dexon, although the mechanical
strength of such devices decreased in a short period of time (2-4 weeks)
(ratner, 2004), limiting the application of this polymer. in order to improve
PGa mechanical characteristics, PGa and Pla copolymers were realized; Pla
is hydrophobic and limits water uptake, slowing the hydrolysis mechanism

O

OH

OH

(a)

￿ ￿ ￿ ￿ ￿ ￿

O

*

O

n

(b)

1.4 (a) Glycolic acid and (b) polyglycolic acid.

O

*

O

*

y

x

O

CH 3

1.5 Poly(lactic-co-glycolic acid).