Chitosan is a linear aminopolysaccharide made of glucosamine and N-acetylglucosamine units obtained by the deacetylation of chitin under alkaline conditions. Chitin is a native polymer extracted from the exoskeleton of crustaceans, such as shell fish, shrimps and crabs, as well from the cell walls of some fungi; it was discovered 200 years ago [5–7]. Together with chitin, chitosan is considered to be the second most abundant polysaccharide after cellulose. Chitosan is composed mainly of (1,4)-linked 2-amino-2-deoxy-β-D-glucan [8], with the chemical name of poly[-(1,4)-2-amino-2-deoxy-D-glucopiranose]. Chitosan (Figure 10.1) differs from chitin in that a majority of the N-acetyl groups in chitosan have been deacetylated.

Chitosan comes in several grades depending on the molecular weight as well as the degree of deacetylation. The degree of deacetylation has significant effects on the solubility and rheological properties of the polymer. These determine its physical and mechanical characteristics, including solubility, gel strength, mucoadhesion and sites for surface modification via reaction with the amine functional group. For example, medium molecular weight chitosan with a molecular weight of 190–310 kDa is 75–85% deacetylated. The amine functional group on the polymer has a pK_a in the range 5.5–6.5, depending on the source of the polymer [9]. At low pH, the polymer is soluble, with the sol–gel transition occurring at approximately pH 7. The poor solubility of chitosan in neural or alkaline medium, however, limits its usage. The solubility in aqueous solvents of up to a pH of 6.5 is, however, attainable via the protonation of the NH₂ functional group on the C-2 position of the glucosamine unit to produce soluble R-NH₃⁺ polyelectrolyte [10].

10.2.2 Functional Characteristics of Chitosan

The pH sensitivity, coupled with the reactivity of the primary amine groups, make chitosan a unique polymer for various drug delivery applications. Furthermore, its gel and matrix-forming ability make it useful for solid dosage forms, such as granules, microparticles and xerogels. Chitosan has many applications because it is the only pseudo-natural cationic polymer that is easily and readily functionalised to yield various derivatives with different functional properties. The key functional characteristics of chitosan include bioadhesion (mucoadhesion), biophysical (biomaterial), smart, permeation enhancing, hydrogel forming (swelling) and controlled (targeted) drug delivery properties. These properties make chitosan a highly versatile polymer and form the basis of its diverse applications covering pharmaceutical, agricultural, biomedical and clinical uses; these are discussed briefly here. Badawi and Rabea [11] noted that chitosan has certain major and unique characteristics that make it advantageous for various applications: ‘(i) a well defined chemical structure; (ii) easily modified chemically and enzymatically; (iii) physically and biologically functional; (iv) biodegradable and biocompatible with many organs, tissues, and cells; (v) it can be processed into several products including flakes, fine powders, beads, membranes, sponges, cottons, fibers and gels’.

10.2.3 Factors Affecting Mucoadhesive Performance

Various factors that can alter the interaction between the polymer and the mucosal layer can affect the mucoadhesive property of a polymer. These include polymer molecular weight [20,21], degree of derivatisation, polymer concentration [22] and method of drying. Grabovac et al. [23] reported that the method of drying is vital as it influenced the mucoadhesive potential of polymers. They showed that lyophilised thiolated chitosan produced greater mucoadhesion than others dried by precipitation in organic solvents. The addition of plasticisers to polymeric dosage forms also imparts flexibility, reduces brittleness, increases toughness and improves flow. It achieves this by interposing itself between the polymer chains and interacting with polymer functional groups to reduce interaction and intermolecular cohesive forces between the polymer chains [24].

Recently, it has been shown that the thiolation of chitosan can lead to an improvement of several properties of unmodified chitosan [25]. These thiolated chitosans have numerous advantageous features in comparison to unmodified chitosan, such as significantly improved mucoadhesive and permeation enhancing properties [26–28]. For instance, the mucoadhesive properties of a chitosan–4-thio-butyl-amidine conjugate were improved 250-fold in comparison to unmodified chitosan. The strong cohesive properties of thiolated chitosans render them highly appropriate excipients in controlled drug release dosage forms. Roldo et al. [25] also reported the effect of molecular weight and percentage thiol immobilisation on the mucoadhesive properties of chitosan and its conjugates. They concluded that mucoadhesion was increased when the level of 4-thio-butyl-amidine substructures immobilised on the polymer was greater whilst thiolated chitosan of medium molecular mass was relatively more mucoadhesive [29].

10.2.4 Permeation Enhancing Effect

The permeation enhancing capabilities of chitosan were first shown by Illum and co-workers [30]. The use of chitosan in various studies carried out on Caco-2 cell monolayers demonstrated a significant decrease in the transepithelial electrical resistance, leading to enhanced membrane permeation [31–33]. Chitosan is able to improve the transport of hydrophilic compounds, such as therapeutic peptides and antisense oligonucleotides, across the membrane via the paracellular route. Structural reorganisation of tight junction-associated proteins, which stems from the interaction between the positive charges of the polymer and the cell membrane, results in the permeation enhancing effect [34]. However, in the presence of mucus layer, the permeation enhancing effect is comparatively low, as chitosan cannot reach the epithelium due to size exclusion and/or competitive charge interactions with mucins [35]. On the other hand, the results obtained on Caco-2 cell monolayers were confirmed by in vivo studies, showing an enhanced intestinal absorption of the peptide drug, buserelin, in rats owing to the co-administration of chitosan hydrochloride [36].

Chitosan shows strong mucoadhesive properties [37], which make it useful in drug delivery. Interaction between the protonated amino group at the C-2 position and the negatively charged sites on the cell surfaces and tight junctions allows paracellular transport of large hydrophilic compounds such as proteins by opening the tight junctions of mucosal membrane barriers [31,32,38]. This has been demonstrated by a decrease in ZO-1 proteins and the change in the cytoskeletal protein F-actin from a filamentous to a globular structure [39,40]. These observations show the potential of chitosan as a penetration enhancer of mucosal paracellular pathways, which makes it useful as a mucosal drug delivery system. It has also been found to enhance the nasal absorption of degracalcitonin and insulin in rats and sheep [41], morphine-6-glucuronide and goserelin in sheep [42].

10.2.5 Swelling and Hydrogel Behaviour

One of the key characteristics of chitosan is its in situ gellation property [43]. In addition, chemical or physical modification (and/or combinations) allows the formation of different hydrogels with varying physicochemical properties for a wide range of applications. The swelling and gel forming behaviour of chitosan is critical, as it affects functional properties such as mucoadhesion, drug release and controlled drug release characteristics. The swelling and gelling behaviour is also dependent on the degree of acetylation (or deacetylation), pH and degree of substitution as well as cross-linking. The latter, in particular, determines its hydrogel forming ability, which ultimately affects its water holding capacity and structural integrity [44].

Chitosan hydrogels are normally obtained by use of chemicals, physical interaction or by irradiation. Rohindra et al. [45] prepared hydrogels by cross-linking chitosan with varying concentrations of glutaraldehyde and showed the swelling behaviour to be dependent on pH, temperature and the degree of cross-linking. Similarly, Singh et al. synthesised chitosan hydrogels by cross-linking with varying concentrations of formaldehyde and characterised their swelling and water absorption capacity characteristics with varying formaldehyde concentration, ionic strength, pH and temperature. The hydrogels they produced generally showed a typical pH and temperature responsive behaviour, such as low pH and high temperature has maximum swelling while high pH and low temperature showed minimum swelling. Their results showed that the hydrogels exhibited a high swelling capacity and equilibrium water content at an optimum pH of 7 and temperature of 35 °C. In addition, higher values of ionic strength resulted in decreasing the swelling of hydrogels at both low and high pH [46].

10.2.6 Smart Properties

The presence of hydrophilicity, functional amino groups and a net cationic charge has made chitosan a suitable polymer for the ‘intelligent’ delivery of macromolecular compounds, such as proteins and genes. These have further been improved by derivatisation of the amine functionality, as highlighted above. The polymer is sensitive to various external stimuli, including pH, temperature, solvent, and ionic strength. The smart property of chitosan is normally achieved by various modifications to form hydrogels that respond to different external stimuli including pH, temperature and ionic strength as discussed above [47]. This is particularly significant, as it allows the versatility of chitosan and its derivatives in various biomedical, clinical and pharmaceutical applications, including tissue regeneration, controlled and targeted drug delivery, which are discussed in more detail here.

10.2.7 Controlled and Targeted Drug Delivery

Chitosan has been developed as a suitable matrix for the controlled release of small molecules and protein or peptide drugs over the last two decades [48]. Formulations with bioadhesive properties have been reported to offer certain advantages for mucosal drug delivery, including prolonged residence time, ease of application [49,50] and controlled release of loaded drug. Due to its favourable gelling properties, chitosan can deliver morphogenic factors and pharmaceutical agents in a controlled fashion. Its cationic nature allows it to complex DNA molecules, making it an ideal candidate for gene delivery strategies [51]. In most cases, controlled drug release is achieved by modifying the swelling and drug diffusion process by combining with a wide range of other synthetic, biological, ionic or nonionic polymers. For example, it is possible to combine positively charged chitosan with negatively charged equivalents, including gelatine, alginic acid and hyaluronic acid, to obtain delivery systems with properties tailored for controlled and targeted drug release [52].

The effect of chitosan properties on swelling behaviour of alginate–chitosan microcapsules was investigated by Lui and co-workers [53]. They showed that microcapsules prepared from low molecular weight and high concentration of chitosan had low swelling capacity. Microcrystalline chitosan as a gel-forming excipient for matrix-type drug granules has been studied by Sakkinen et al. [54], who observed that crystallinity, molecular weight and degree of deacetylation were the major factors that affected the release rates from the chitosan-based granules. The excipient also has promise for site-specific delivery. Tozaki et al. [55] used chitosan capsules into which was encapsulated a 5-amino salicylic acid for colon-specific delivery to treat ulcerative colitis. It was observed that chitosan capsules delivered in vivo to male Wistar rats after induction of colitis disintegrated specifically in the large intestine as compared to the control formulation (in absence of chitosan), which demonstrated absorption of the drug in the small intestines.