Spermicide permeation through biocompatible polymers

Although spermicides are safe and effective contraceptive/prophylactic agents, they are inconvenient to use. Formulations that provide a controlled release of spermicide may improve user acceptance, and therefore effectiveness. Using a two-chamber diffusion cell, we measured the rates of permeation of nonoxynol-9 (N9), benzalkonium chloride (BC), and chlorhexidine (CH) through films of ethylene-vinyl acetate copolymer (EVAc) and silicone elastomer (SILASTIC). In addition, we encapsulated N9, BC, and CH into solid polymer matrices and measured the rate of spermicide release following immersion in water. We also developed equations for predicting the release rate of spermicide from a vaginal ring containing encapsulated spermicide, and tested these equations using hollow SILASTIC rings containing pure N9 or BC. N9 diffuses through a thin film of SILASTIC several orders of magnitude slower than through water. The rates of permeation of N9 through EVAc, BC through SILASTIC, and CH through SILASTIC were too low to detect over a one-week experiment. Polymer matrices of EVAc or SILASTIC released N9 at a controlled rate for several days. Based on these measurements, we predict that a vaginal ring containing an inner core of EVAc/N9 surrounded by a thin, permeable layer of SILASTIC will provide a controlled, constant release of N9 for over 30 days. Because of its low permeability in SILASTIC, BC is probably not a good spermicide for a long-acting vaginal ring. Because of its low solubility in water, CH is also not a good candidate for controlled release into the vaginal mucus.