RELEASE CHARACTERISTICS OF IMPLANTABLE CYLINDRICAL POLYETHYLENE MATRICES

The geometrical relationship between a hemisphere and a cylinder has b een investigated for controlled-release systems. The relationship was tested by comparing dissolution results with results from mathematical calculation based on the principles of diffusion for matrix systems. A procedure has been developed for producing implantable, cylindrical, low-density polyethylene matrices, uncoated or coated with a thin imp ermeable film and a thick paraffin layer except for a hole on the flat faces of the cylinder. Drug matrices were prepared from a blend of so dium salicylate and polymer compressed in an appropriately designed st ainless-steel mould at 150 degrees C. Differential scanning calorimetr y revealed that no decomposition product was formed in the matrix. Whe n the surface area and the number of holes is increased, drug release also increases. When density is increased, however, drug release decre ases significantly. Zero-order drug release was obtained from high-den sity covered one-hole and two-hole matrices. The diffusion coefficient was calculated as 0.067 day(-1). The study suggested that true zero-o rder drug release could be obtained by drug diffusion from a hole, rat her than from geometric shapes in the matrix systems. In addition, for constant release the diffusion area has to increase by approximately 25 mm(2) every day, compared to the area of the previous day, because the diffusion distance increases logarithmically.