An investigation into the use of alginates as bioadhesive delivery systems

Sodium alginates are naturally occurring polymers consisting of two monomers, mannuronate and guluronate. Their gel-forming properties and their potential use as bioadhesives have attracted particular attention within the pharmaceutical industry. The aim of this study is to characterise the gelling behaviour of several batches of sodium alginates of differing chemical characteristics and to relate this behaviour to their bioadhesive properties. Three methods based on the internal bulking gelation were used to assess factors that may affect the basic characteristics of alginate gels, particularly gel strength. Isothermal oscillatory rheology and thermorheology, as well as texture analysis were used to assess the behaviour of these gels. It was possible from these analyses to demonstrate that the level of calcium, the method of preparing these gels, the fraction of guluronate, and the amount of sodium alginate all affect the gel behaviour. In order to relate these findings to bioadhesion, two formulations, solid compacts and solutions and their interactions with various mucin types were examined. It was found that sodium alginate interaction with all types of mucin was more pronounced when the alginate consisted of predominantly mannuronate monomer. Further examinations revealed that there was also a temperature-dependent thermogelation process when alginate/mucin mixes were subjected to heat/cool cycles. Finally, the bioadhesive potential of solutions of sodium alginate to porcine oesophageal tissues was assessed using a purpose-designed bioadhesion test system within a humidity chamber, based on flow channel technique. Fluorescently labelled sodium alginate solutions were applied to oesophageal tissues, continuously perfused with de-ionised water, phosphate buffer pH 7.4, or artificial saliva over 30 minutes. The total amount left after 30 minutes was calculated and expressed as a bioadhesive index. Mannuronate-rich alginates were found to exhibit better bioadhesive properties than their guluronate-rich alginates.