Diffusion testing measures the rate that an active pharmaceutical ingredient is released from a semisolid preparation, providing the quality control analyst with critical performance data. Diffusion testing using diffusion cells has become the industry standard due to the pioneering work of Dr. T. J. Franz who developed the “Franz cell.” This device consists of a small-volume, water-jacketed cell that contains a chamber for drug application, a membrane through which the drug may permeate, and a receptor media chamber from which samples may be extracted and analyzed for drug release. Later developments include non-water-jacketed, dry-heat cells such as Teledyne Hanson’s Phoenix™ line of diffusion testers.