Micromachined silicon structures for modelling polymer matrix controlled release systems

Physical models of polymer matrix controlled release systems were constructed by using silicon micromachining to fabricate miniature pore networks having features comparable in size to those found in the polymer systems. The networks were formed from pyramidal cavities, ranging in size from 100-200 mu m on a side and from 70-140 mu m deep, etched into a silicon wafer. Narrow channels were etched to connect adjacent cavities and a glass wafer was bonded onto the silicon wafer to form linear pore networks similar to 1 mm long. The pore networks were loaded with an aqueous solution of sodium fluorescein, and the solute concentration during diffusion from the network was imaged using video microscopy. The time-dependent concentrations within the pores were fit to a Fickian diffusion model to determine effective diffusivities for the release of fluorescein from the networks. In an initial experiment, the change in effective diffusivity as a function of connecting channel width was consistent with mathematical models of diffusion in constricted networks. Since these techniques permit the production of precisely defined pores of virtually any size and geometry, micromachined silicon networks provide useful physical models for microporous controlled release polymers. In addition, extensions of the present techniques may be useful for production of new microfabricated controlled release systems, where desired release kinetics could be programmed into millimeter-sized devices by fabrication of intelligently-designed pore structures.