Types of bioresorbable polymers for medical applications - Durability and Reliability of Medical Polymers - page 6

which is responsible for degradation phenomenon. This blend allowed the
introduction of new suture threads with the commercial name of Vicryl.
lactic acid contains a carbon atom with four different substituting groups,
and thus has two enantiomeric forms: l - and d -lactic acid (Fig. 1.6). This
implies that four morphologically different poly(lactic acid) (Pla) polymers
can be obtained: poly- l -lactic acid (Plla), poly- d -lactic acid (PDla) and
the racemic form poly- d , l -lactic acid (PDlla). The fourth form, meso Pla,
synthesized from d - and l l-lactide, is rarely used (ratner, 2004). Plla and
PDla are stereoregular and have a high crystalline ratio, thus making them
suitable for applications where high mechanical strength is required. However,
Plla is employed more frequently because l l-lactic acid is the monomer
recognized by human body, being involved in the Krebs's cycle. PDlla
is an amorphous polymer, suitable for drug delivery where a homogeneous
drug dispersion into a monophasic matrix is needed.
a crucial aspect of the synthesis of these polymers is water removal; the
polycondensation reaction produces water which causes polymer hydrolysis
during the process, since the reaction is reversible, reducing the molecular
weight of the final product. Moreover, in order to obtain high molecular
weights, these reactions are carried out with a catalyst, such as snCl 2 or
Ti(BuO) 4 (Harshe et al. , 2007). another example of aliphatic polyesters is
a material called 'Bionolle' (ishioka et al. , 2005), a bioresorbable polymer
obtained from a polycondensation of glycols (such as ethylene glycol and
1,4-butanediol) with dicarboxylic acids (such as oxalic acid and succinic
acid) (Fig. 1.7). By varying the initial monomer composition it is possible
to obtain materials with different properties. Two examples of Bionolle
are poly(butylene succinate) (PBs) and poly(butylene succinate-adipate)
(PBsa).
The fourth category involves polymers which are synthesized following
the rOP mechanism, a particular step-growth polymerization, where cyclic
monomers are involved. The most representative material belonging to this

￿ ￿ ￿ ￿ ￿ ￿

O

OH

OH

(a)

O

O

* n

CH 3

(b)

1.6 (a) l l-Lactic acid and (b) poly(lactic acid).