Innovative systems and applications - Physical Gels from Biological and Synthetic Polymers - page 337

or secondary (physical) interactions as in grafted chitosan hydrogels and in chitosan -

poly(vinyl alcohol) (PVA) hydrogels.

11.3.1.1 Ionic interactions

Ionic interactions between the negative charges of the cross-linker and positively charged
groups of chitosan depend on the type of cross-linker. Metallic ions induce formation of
coordinate-covalent (dipolar) bonds between the positively charged ammonium
groups of chitosan. Among anionic molecules, phosphate-bearing groups such as
β -glycerophosphate and particularly tripolyphosphate can be employed.

PECs are generally biocompatible networks and exhibit interesting swelling character-
istics, and these can be reinforced by the addition of ions. Typically Ca 2+ can be added to
alginate or pectin and Al 3+ to sodium carboxymethylcellulose (NaCMC), distinct from
ionically cross-linked chitosan gels since then chitosan is not cross-linked but plays the
role of the additional polymer. For example, as well as forming a PEC with the anionic
mucopolysaccharide chondroitin sulphate, chitosan can also be ionically cross-linked,
although control of pH during the cross-linking reaction is critical to generating a
reproducible product.
During complexation, polyelectrolytes can either coacervate, i.e. form dispersed
emulsion-like droplets, or form a more or less compact gel, whereas if ionic interactions
are too strong, precipitation rather than gel formation occurs. Such precipitation can be
avoided if electrostatic attractions are weakened by the addition of salts such as NaCl.
The presence of salts reduces the attraction between oppositely charged polyelectrolytes
by contributing to the counterion environment. To overcome the dif

culties encountered
with large-scale processing during PEC preparation, and so obtain a homogeneous
mixture, mixing can be carried out at a pH value where complexation does not occur.
This pH can then be adjusted to a value where interactions are favoured.
PEC gels exhibit pH-sensitive, and to a minor extent ion-sensitive, swelling. As
pH changes, the swelling rate is controlled by the diffusion of mobile ions and changes
in the degree of ionization. As swelling of PECs is in

uenced by many factors, it can

be used for fine modulation of drug release. Chitosan gels formed by a PEC can be

used as scaffolds in cell culture and enzyme immobilization. As no potentially toxic
molecules or covalent cross-linkers are added to these gels, they represent a better
medium for cell culture than covalently cross-linked systems. The GAGs, chondroitin
sulphate and hyaluronic acid are of interest here, since they can be used in cartilage
reconstruction and wound-healing, as carrier materials and as scaffolds for tissue
engineering.

11.3.1.2 Grafted chitosan hydrogels

Grafting (functionalizing) chitosan is a common way to improve properties such as
chelating and complexation capability, solubility in water or organic solvents, bacterio-
static effects and absorption properties. Depending on the nature of the graft, the
secondary interactions between grafted groups can be hydrogen bonds or hydrophobic