Chitin is the second most abundant polymer in nature after cellulose. It is a sustainable natural resource that is ubiquitous but still relatively under-exploited commercially. Chitin is a structural polysaccharide found in the cell walls of fungi, the cuticle of insects, shells and the exoskeleton of crustaceans – its main commercial source. In crustaceans, single-strands of chitin polymer are arranged into protein-coated fibrils. The fibrils are arranged into sheets, which are stacked at slight angles to the sheets above, creating a structural composite with a strong, plywood-like structure.
The healing properties of chitin have been known for centuries. Healers used apparent the haemostatic properties of chitin materials to treat cuts and abrasions – this property in fact coming from a natural degradation product of chitin; namely chitosan. Chitosan is produced by the natural, chemical or physical hydrolysis of pendant acetyl groups on the polymer chain. The primary amines that are left are positively charged under mildly acidic conditions, rendering chitosan soluble – unlike chitin – and providing it with bacteriostatic properties. The potential commercial applications of chitosan have grown steadily since the late twentieth century, but numbers of patent applications have exploded in the last 10 Years.
Chitosan is now sold within products for wound healing, cosmetics and toiletries, nutraceuticals and dietary aids, water treatment and agricultural and horticultural applications – and with bio-medical and biotechnological applications for chitosan becoming one of the strongest, high-value growth markets. The broad utility of chitosan comes from its physical properties; it is relatively rare in being a cationic natural polymer, it binds metal ions and is film forming, also from its biological properties; it is antibacterial, adheres selectively to biological surfaces (e.g. to tumour cells) and is biodegradable. However, the great commercial potential of chitosan is due to its chemical properties in that it can be readily degraded to oligomers and monosaccharide building blocks, but can also be readily modified, chemically or enzymically to enhance its natural properties, generate entirely new functional properties or confer chitosan-like properties onto other chemicals.
Chitosan does have some functional flaws however. Commercial processing to consistently yield a high quality product can be difficult and expensive and it is relatively insoluble in water above pH 6.5. The use of functionalizing reagents to generate substituted amine groups either at, or appended to chitosan primary amines, will overcome the solubility problem and has been shown to enhance the antibacterial properties of chitosan. However, further research and investment in this area will in time generate functionalized chitosan products at the right cost and of sufficient quality and purity to be suitable for the broader market.