A non-covalently cross-linked chitosan based hydrogel

Hydrogels are normally formed by the covalent cross-linking of linear polym ers. In the case of chitosan based hydrogels this cross-linking is often ac hieved with glutaraldehyde, glyoxal or other reactive cross-linking agents. Such hydrogel materials have limited biocompatibility and biodegradability . However by the attachment of hydrophobic palmitoyl groups to glycol chito san, a water soluble chitosan derivative, we have produced a version of the amphiphilic vesicle forming polymer-palmitoyl glycol chitosan (Uchegbu et al., 1998, J Pharm Pharmacol 58, 453-458). The level of palmitoylation in t his variant of the polymer (GCP11), as determined by proton neutron magneti c resonance spectroscopy, is 19.62 +/- 2.42% (n = 4). GCP11 has been used t o prepare soft, slowly eroding hydrogels suitable for drug delivery by simp ly freeze-drying an aqueous dispersion of the polymer. Non-covalent cross-l inking to form the gel matrix is achieved by the hydrophobic interactions o f the palmitoyl groups. The resulting material, as examined by scanning ele ctron microscopy, is porous and may be hydrated to up to 20 x its weight in aqueous media without any appreciable change in volume-transforming from a n opaque to a translucent solid. The slow erosion of this material in aqueo us environments gives a biodegradable and ultimately more biocompatible mat erial than covalently cross-linked hydrogels. Unlike most chitosan-based ge ls, the gel is hydrated to 20 x its weight at alkaline pH but only 10 x its weight at neutral and acid pH. This is as a result of the gradual erosion of the gel at lower pH values. Hydration is also reduced from 20 x the dry gel weight in water to 10 x the dry gel weight in the presence of dissolved salts such as sodium chloride. GCP11 hydrogels have been loaded to 0.1% w/ w with a model fluorophore, rhodamine B, by simply freeze-drying an aqueous dispersion of CP11 in the presence of a solution of rhodamine B dissolved in either water or phosphate buffered saline (PBS, pH = 7.4). The release o f this model fluorophore was retarded by between 8 and 12% when PBS was con tained in the gel in accordance with the hydration profiles. Rhodamine B re lease was also reduced by between 13 and 25% in the presence of acid as a r esult of the reduced solubility of rhodamine B at acid pH. (C) 1999 Elsevie r Science B.V. All rights reserved.