Rheological and mechanical properties of pharmaceutical gels. Part I: Non-medicated systems

Non-medicated gel systems based on polymers of different chemical nature and source have been characterized for rheological as well as for gel strength properties: a synthetic product, polyacrylic acid and a natural product, scleroglucan, both known to produce true gels, and two semi-synthetic materials, sodium carboxymethylcellulose and hydroxypropylmethylcellulose, both known to originate polymeric colloidal solutions, have been chosen. In particular, the rheological analysis involved the comparison of the different polymers on the basis either of viscosity (constant rate test) or of viscoelastic measurements (oscillation test). Subsequently the possible relationships between either viscosity or viscoelastic parameters (G', G", tg delta) and the gel strength parameter have been investigated. The mechanical resistance of the pharmaceutical gels examined didn't depend on the viscosity but rather on the viscoelasticity of the systems. In particular, low tg delta values were always accompanied by high gel strength values. A rank order correlation was found between the tg delta values obtained for the various samples at different frequency values and the relevant gel strength parameter: this suggests that mechanical resistance measurements, similarly to viscoelastic ones, give information on the structural properties of the samples and therefore represent a suitable tool in the optimization of gel formulations.