Types of bioresorbable polymers for medical applications - Durability and Reliability of Medical Polymers - page 3

(Woo et al. , 2006, Kim et al. , 2005, loo et al. , 2009, Wang et al. , 2005,
Mochizuki, 2005). another interesting application of bioresorbable polymers
is the synthesis of hydrogels, i.e. heterogeneous, strongly hydrophylic and
crosslinked polymeric matrices (Peppas, 1986). Hydrogels can be loaded with
drugs or with drug-containing nanoparticles, in order to perform a release
at a certain rate.

1.2 Aliphatic polyesters
aliphatic polyesters have a chemical structure characterized by the
presence of ester groups along the chains. They can be classified according
to their preparation method (albertsson and Varma, 2005) in four main
categories:

∑ naturally-occurring polyesters;

∑ microbial polyesters;

∑ condensation polyesters; and

∑ polyesters from ring-opening polymerization (rOP).

The most representative naturally-occurring polyester is 'shellac', a resin
secreted by female lac bugs in india and Thailand. shellac is a complex
mixture of monoesters and polyesters, whose backbone is mainly formed
of terpenic acids, aleuritic acid and several minor fatty acids (Wang et al. ,
1999). This is therefore a non-homogeneous material from a chemical point
of view, because it contains isolated and conjugated double bonds, and
aldehydic and primary alcoholic groups, that make shellac a polyfunctional
resin (Fig. 1.1).
examples of microbial polyesters are the so-called polyhydroxyalkanoates
(PHa), which are produced by bacterial fermentation of sugars and lipid, in
order to store carbon and energy. There are about 100 different monomers
that had been included into PHa polymers (rehm and steinbuchel, 2005);
this list comprises hydroxyalkanoate units with a substituted group (alkyl,
aryl, alkenyl, halogen, cyano, epoxy, ether, acyl, ester, and acid groups)
(Fig. 1.2).
However, PHA polymers are also classified according to monomers carbon
units. short-chain-length (sCl) PHas consist of 3-5 carbon units monomers,
whilst medium-chain-length (MCl) PHas have 6-14 carbon units monomers
(lee and Park, 2005). The most representative polymer belonging to this
family is poly-3-hydroxybutyrate (P3HB), whose main component is R -3-
hydroxybutanoic acid, a metabolite found also in human blood (Fig. 1.3).
The PH3B homopolymer is a stiff, brittle and rigid material with a Young
modulus similar to that of polypropylene (abe and Doi, 2005) and with a high
melting temperature, around 170 °C; this implies a difficult processability
(satkowski et al. , 2005) because the polymer degrades during processing.

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