Development, characterization and modeling of mosquito repellent release from microporous devices

Abstract

Nanocomposite strands with mosquito repellent DEET or Icaridin incorporated in a poly(ethylene-co-vinyl acetate) (EVA) matrix, with either pyrogenic silica or an organoclay as a nanofiller, were prepared by a twin-screw extrusion compounding technique. The nature and levels of the repellent and nanofiller that was used affected the material phase morphology. The repellent release was followed as a function of aging time in convection ovens set at 30 and 50 C. The experimental release data of the mosquito repellent from the microporous polymer swellable matrix strands was mathematically modeled and fitted using a range of semi-empirical models. In the majority of case, the Korsmeyer-Peppas power law model provided the best data fit. As expected, the wide range of internal morphologies also resulted in quite different release profiles. These models were found to be valuable as they provided insights into the mechanism of repellent release from EVA swellable matrices. It was possible to differentiate between diffusion and relaxation mechanisms. Surprisingly, strands containing nominally more than 30 wt% Icaridin showed accelerating mass loss during the initial phase, consistent with Super Case II transport. Diffusional exponents as high as 1.81 were found. Furthermore, the internal microporous region of the extruded EVA strands was covered by a surface membrane that acted a diffusion barrier that, in effect, controlled the release rate of the mosquito repellents. Some of the investigated samples exhibited release profiles that suggest that longer lasting effective release of repellents is possible than currently achieved by available commercially products.