Controlled release technology can solve a variety of problems that have in common the application of an active toxic compound to a system in such a way to accomplish a specific purpose while avoiding certain other possible responses. In controlled release systems, a drug, a pesticide or other bioactive agent is incorporated into a carrier, generally a polymeric material. The rate of release of the substance is determined by the properties of the polymer itself as well as environmental factors (such as the pH of bodily fluids). Controlled release systems are capable of delivering substances slowly and continuously for up to several years.
Three mechanism of controlled release must be considered: (1) Diffusion controlled (membranes and matrices), (2) Chemically controlled (erosion and pendent chain), and (3) Solvent activated (osmotic pressure and swelling). Of particular interest in our work are controlled release systems based on pendently bound bioactive agents.
Chemical bonding of a bioactive agent to polymers can be accomplished in several general ways based on methods of synthesis: (a) reaction on preformed polymers, (b) reactions on naturally-occurring polymers, (c) polymerization of vinyl monomers containing the active ingredient, and (d) step growth polymerizations. Each mechanism contains certain advantages and must be selected accordingly depending upon the agent to be released, as well as the conditions to yield favorable release.
Controlled release systems are usually designed to obtain better control over concentrations of the active agent. When the bioactive agent is chemically bounded to a polymer, the bond has to be cleaved by a chemical reaction usually enzymatic, hydrolytic, thermal, or photochemically. A variety of chemical and physical variables can affect the rate of bond cleavage and subsequent release of chemically attached materials from polymers including the nature of the labile bone, length of the spacer group, molecular weight, hydrophilicity, neighboring group effects, environmental factors and physical form and dimensions. Once again, to obtain controlled release of a particular agent, one must consider each of the above factors in designing a system capable of controlled release.
Research within our laboratories have focused on determining fundamental structure-property relationships that dictate controlled release behavior. Systems studied have utilized alleopathic compounds pendently attached to a hydrophilic polymer backbone. The alleopathic compound may be hydrolyzed from the backbone and released into an aqueous media. Release rates have been determined as a function of environmental conditions to determine which aforementioned factors contribute most to the release of the bioactive agent.