Integrated Biomaterials Science

Drug Delivery Systems 837
exploitation of polymers is often dictated not only by the superiority of
properties, but by marketing and patenting situations as well.
29.4. Polymers for Controlled Release Applications
Some of the most important biodegradable and nonbiodegradable
polymers used in drug release technology are reported in this section
(Chasin and Langer, 1990; Dumitru, 1994; Park et al., 1993). The former are
preferred for internal application although some nonbiodegradable ones,
such as polysiloxanes, are largely used both inside the body as well as in
delicate body cavities.
29.4.1. Polysiloxanes
Polysiloxanes may be considered the first class of polymers employed
in controlled drug release. They are also known as silicones, a historic name
that originated from the old idea that they were ketones in structure. Their
use in drug release came from the observation by Folkman and Lony while
studying polysiloxane heart valves. These authors found that a lipid soluble
dye like Sudan was adsorbed and could then diffuse in silicon elastomer
membranes, unlike water-soluble dyes like methylene blue. A study with
dyes and drugs of different structures demonstrated that this property is
common to many substances and depends upon their hydrophilic/hydrophobic
ratio; furthermore, penetration into the polymer matrix is due to
hydrophobic interaction.
From the structure reported below, which is at the basis of the most
common products, one may understand the reason for the chemical inertness
of polysiloxanes. They may be degraded, in fact, by a few chemicals
such as ozone or nitrogen oxides. For this reason, heat or chemical means
are much used methods of sterilization.
Their special oligomeric properties are explained by the chemical
structure (I). The Si–O length of 1.64 exhibits high rotation for lower
intramolecular steric hindrance as compared to all the polymers based on
carbon backbone in which the C–C distance is 1.53 Å. The very low torsion
barrier for the rotation along the skeleton bonds is the reason for the high
flexibility and low Tg.