VISCOELASTIC PROPERTIES OF BIOADHESIVE, CHLORHEXIDINE-CONTAINING SEMI-SOLIDS FOR TOPICAL APPLICATION TO THE OROPHARYNX

Purpose. This study examined the viscoelastic properties of bioadhesiv e, chlorhexidine-containing semi-solid formulations, designed for topi cal application to the oropharynx. Methods. Oscillatory rheometry was performed using a Carri-Med CSL2-100 rheometer at 20.0 +/- 0.1 degrees C in conjunction with parallel plate geometry (2 cm diameter, 0.5 mm sample thickness). Samples were subjected to a constant strain (6.5 x 10(-3) rad) and defined viscoelastic parameters, namely storage modulu s (G'), loss modulus (G ''), loss tangent (tan 6) and dynamic viscosit y (eta'), measured over a defined frequency range (0.01-1.0 Hz). Resul ts. As the oscillatory frequency was increased, G' G '' of all formula tions increased, whereas both eta' and tan 6 significantly decreased. The magnitude of increase of G' and G '' as a function of frequency wa s relatively small, indicating that, in general, the formulations were non-cross-linked elastic systems. Increasing concentrations of HEC, P VP and PC significantly increased G', G '', eta' yet decreased tan del ta, observations that may be attributed to the physical state of each polymer in the formulations. Formulation elasticity increased (i.e. ta n delta decreased) as a result of increased entanglement of polymeric chains of dissolved components (i.e. HEC and PVP) and the restrained e xtension of swollen, cross-linked chains of PC. Additionally, in formu lations where the saturation solubility of PVP was exceeded and/or ins ufficient ''free-water'' was available for maximal swelling of PC, for mulation elasticity increased as a result of the increasing mass of di spersed solid particles of PVP and/or PC. Formulation eta' increased d ue to the attendent effects of polymer chain entanglement and polymer state on overall formulation viscosity. Conclusions. Following applica tion to the oropharynx, the formulations will behave as elastic system s. Thus, these formulations would be expected to offer advantageous cl inical properties, e.g., prolonged drug release, increased bioadhesion . However, it is noteworthy that the final choice of formulation for c linical evaluation will involve a compromise between viscoelastic char acteristics and acceptable textural properties, e.g. ease of product a pplication. This study has shown the applicability of oscillatory rheo metry for both the characterisation and selection of candidate, topica l bioadhesive formulations for clinical evaluation.