Transient analysis of drug delivery from a toroidal membrane: Applications for medicated vaginal rings.

A mathematical construct is proposed to analyze drug released from matrix-type vaginal rings. This work is intended to support experimental studies and promote the fabrication of these devices. The transport of a dissolved drug through a toroidal membrane was predicted using diffusion equations and their solutions. This dynamic framework led to the estimation of the time elapsed before releasing 98% of the ethynodiol diacetate from the polymer. Closed-form expressions, easily adaptable to spreadsheet implementation, were developed to simulate the controlled delivery of levonorgestrel initially dispersed in a silicone vaginal ring. As the loading increased, a greater amount of medication was delivered. However, the fractional release decreased from 32.6% to 23.1% when the dosage changed from 4.137 g/cm3 to 8.274 mg/cm3. The expressions were further simplified for thin rings.