Physico-chemical properties of a rifampicin-releasing polydimethylsiloxane shunt

Infection of implanted polymeric devices is a major problem in modern medicine. Silicone shunts were modified in order to prevent microbial colonization by incorporating rifampicin. The release mechanism and the altered properties of the silicone were studied. Release rates of rifampicin out of the polymeric shunt materia were measured in vitro for up to 60 d. For high velocity of rifampicin in the polymeric matrix and long-lasting controlled release rates, high compatibility of polymer and drug was required. Compatibility and therefore miscibility of drug and polymer were estimated by reduced solubility and cohesion energy densities (Hansen parameter, solubility parameter delta). Mechanical properties of the polymer were influenced by incorporation of small drug amounts, characterized by stress-strain curves. Differential scanning calorimetry (DSC) measurements suggested thermodynamically controlled interaction of the macromolecules with the incorporated substance. The physico-chemical state of the drug in the internal phase and the surface of the polymer was studied by scanning electron micrography (SEM), showing homogeneous molecular dispersion of the drug in the polymeric material as well as crystalline structures on the surface.