Towards controlled release of a natural mosquito repellent from polymer matrices

Abstract

Malaria is still the most important parasitic disease in humans with most cases occurring in Sub-Saharan Africa (90% cases). It is transmitted via anopheles mosquitoes. Several vector control methods are available, e.g. long lasting insecticidal mosquito nets (LLINs), insecticide-treated nets (ITNs) and indoor residual spraying (IRS). However, they are effective only when a person is in-doors. Outdoor protection can be obtained for short periods (48-72 hours) using topical repellents. This preliminary study investigated the possibility to develop longer acting delivery forms based on polymer technology. The viability of two different approaches were considered for the controlled release of the natural repellent 3,7-dimethyloct-6-en-1-al (citronellal). The first idea was to dissolve the repellent in the polymer while controlling the rate of release by clay nanoplatelets dispersed in the matrix. Towards this, ethylene vinyl acetate (EVA) copolymer (18% VA) was modified with organically modified nanoclay. Release tests showed that this approach was not viable as only a small amount of repellent could be incorporated and it was lost within a day or two from thin polymer strands. The second approach targeted the use of a polymer in which the repellent is not soluble at ordinary temperature but where solubility is achieved at high temperatures. In this case polyethylene was used as host polymer. It was shown that large quantities of repellent can be trapped inside the polymer matrix using the temperature induced phase separation method (TIPS). Scanning electron microscopy revealed that a microporous co-continuous phase structure was obtained by shock cooling homogeneous mixtures to temperatures well below the spinodal phase boundary curve. The phase behaviour of the LLDPE-citronellal system was studied using cloud point determinations in a microscope fitted with a hot stage and by differential scanning calorimetry. The experimental data points on the bimodal phase envelope were used to fix parameter values of the Flory-Huggins equation. The latter was then used to predict the location of the spinodal lines. At 40 wt.% polymer the spinodal boundary is located at 96 C. However, experiments showed that quenching temperature of 5°C (i.e. the temperature of typical cooling baths used during filament extrusion) is sufficient to generate the desired microporous structure.