Model for a controlled-release drug delivery safety system with permeable and erodible coatings

Models of drug delivery devices that employ erodible permeable coatings must take care to avoid the unacceptably high rate of release that arises as the erodible coating disappears and the barrier to drug release vanishes. One solution to this safety problem has been to exhaust the drug reservoir just before this condition occurs. This design has the disadvantage of placing demands of high accuracy on the quality control in the fabrication of the device. A drug delivery system of cylindrical symmetry is proposed that uses two permeable coatings on a drug-containing core. Only the outer of the two coatings is erodible; the inner safety coating and the core are inert. Calculations are performed to design the device that can yield constant drug delivery rates while avoiding the possibility of explosive late drug release.