Theory of controlled release of biologically active substances

Abstract

The available concentration of a slowly released, biologically active substance is the result of the interplay of the processes of release and of decay. In this paper, the theory of the release mechanisms based on diffusion and, also, on water-degradable polymers is examined in detail, and, based on a first-order decay of the released substance, the “present concentration” based on area is calculated. The attainment and maintenance of a constant present concentration over a chosen time period is considered as the ideal. The actual, long-term efficiencies which can be expected vary from 22% for diffusion to 100% for a well-chosen combination of water-degradable polymers and conventional substances. The efficiency varies conversely with toxicity; so, safety and efficiency are simultaneously attainable. The major benefits of controlled release appear only for times encompassing several half-lives of the active substance. This finding encourages the consideration of non-persistent pesticides.