Gravity-induced coating flows of vaginal gel formulations: in vitro experimental analysis

Efficacy of topical microbicidal drug delivery formulations against HIV depends in part on their coating distributions and retention on vaginal epithelium. This study focused on gravity-induced coating flows of vaginal gels, and effects of formulation composition and surface wettability on coating. We hypothesized that presence of a yield stress, and surface wettability, affect coating. Experiments imaged and analyzed coating flows of gels on inclined model hydrophilic or hydrophobic surfaces. The in vitro wettability conditions bracket those believed to exist on vaginal epithelium in vivo. Six commercial vaginal gels were studied: three polyacrylic acid-based (PAA) and three cellulose-based. Our research group uses these gels in complementary human in vivo studies and other in vitro experimental analyses; this study is a first step in linking the in vivo and in vitro measurements. Coating by PAA gels was different from cellulose-based gels: the former exhibited yield stresses, which prevented initial gel shape from deforming during sliding. Coating flows of cellulose gels depended upon surface wettability. The slipping rates of the PAA gels ranked inversely with fitted yield stress values. The coating flow rates of the cellulose gels (hydrophilic surface) did not correlate with consistency index, but ranked inversely with the shear-thinning index. This study introduces a simple methodology for comparing trial formulations and relating their flows to gel constituents and physical properties. It also suggests differences in coating by current commercial gels.