Innovative systems and applications - Physical Gels from Biological and Synthetic Polymers - page 339

the previous section. They can be prepared either by autoclaving or by the freeze/thaw
method ( Chapter 8 ), although the resultant structures are different.
Complexes formed by autoclaving can be used as scaffolds in cell culture. In
Figure 11.8 , hydrogen bonds between the hydroxyl groups of PVA and the hydroxyl or
amino groups of chitosan are thought to be the main interactions inside the complex.
Chitosan PVA blended gel membranes have been prepared by this procedure

(Koyano et al., 2000 ), but observations of the membrane by microscopy suggest that
the chitosan components for certain compositions are localized in islands in the air-
surface side of the membrane, because chitosan is less hydrophilic than PVA. Complexes
formed by autoclaving are readily soluble under acidic conditions and therefore not
suitable as drug delivery systems, although they do favour cell attachment: the higher the
chitosan ratio, the higher the water retention, which enhances the cell growth rate. Such
complexes can be used, for example, in

-

fibroblast cultures, where they provide a better

scaffold than collagen.
The chitosan

PVA blends formed by freeze/thaw methods have been used as drug
delivery systems. In addition to the interactions between the PVA hydroxyl groups and
the hydroxyl or amino groups on chitosan, crystallite junction zones between PVA
polymeric chains are formed (see Chapter 8 ). This complex with a less regular structure
forms gels with a high capacity to swell, which is an important property for drug delivery
systems. Since there is no pH-sensitive swelling, drug release is diffusion-controlled and
modulated by the chitosan ratio. Overall, chitosan

-

PVA hydrogels represent an interest-
ing biocompatible alternative to other types of gels, but they are not as versatile. Because
of the intrinsic properties of chitosan, its inclusion favours the adhesion of the system at
the site of administration, and the bioavailability of poorly absorbable drugs is enhanced.
Such complexes can be used, for example, for the controlled release of growth hormones
and for oral administration of certain drugs.

-

11.3.2 Applications of microspheres

11.3.2.1 Inhomogeneous alginate beads

Alginate gels are particularly versatile, especially for immobilization of living cells. The
most exciting prospect for such immobilized cells is their potential use in cell trans-
plantation, where the main goal of the gel is to act as a barrier between the transplant and
the immune system of the host. We have already mentioned in Chapter 5 the work on
encapsulation, and the implantation of alginate beads containing pancreatic cells into
insulin-dependent diabetics, where the alginate gel surrounds serve to protect from
immune reactive responses and so preserve cell viability (King et al., 2003 ). However,
there are problems with this approach, including the low biocompatibility and low
stability of such systems. By forming capsules with a strong polyanion

-

polycation
membrane on their surface and a low-swelling gel network in the core, one can obtain
systems that are stable for extended periods under physiological conditions. Such con-
ditions are achieved by forming inhomogeneous gels, where the highest polymer con-
centration is close to the surface and the lowest is in the centre of the capsule. Such