Slow release of mosquito repellents from microporous polyolefin strands

Abstract

Malaria is a principal cause of illness and death in countries where the disease is endemic. The indoor residual sprays of insecticides and indoor use of long-lasting insecticide-treated nets (LLINs) are practical methods of the prevention of malaria recommended by WHO. However, the elimination of malaria is creating difficulties as the current methods do not protect against mosquitoes biting outdoors. The purpose of this study was to develop a new product by incorporating repellents into inexpensive thermoplastic polymer namely poly(ethylene-co-vinyl acetate) (EVA) and linear low-density polyethylene (LLDPE) to control the release rate of mosquito repellents from microporous polyolefin strands, e.g. repellent bracelets and anklets which can be used for longer periods of time, say for three to six months. Four approaches were considered in this research. In the first study, the evaporation rate of repellents was determined using thermogravimetric analysis (TGA). The duration of protection against mosquitoes by the repellent is partially affected by the rate of loss of repellent due abrasion, penetration and evaporation. Therefore, the repellent evaporation rate can be regarded as one of the physical properties of repellents which might affect repellent efficiency. The results showed that three repellents, namely Icaridin, IR3535 and DEET, had a low volatility compared to the other repellents investigated. These three repellents are indicators of long protection time against mosquitoes. The second approach was to use the open-cell microporous strands as reservoirs for relatively large quantities of mosquito repellent. Repellents of interest include DEET, Icaridin, ethyl anthranilate and IR3535. Microporous polymer strands containing mosquito repellent were prepared by twin-screw extrusion compounding. A co-continuous phase structure was achieved by rapid quenching in an ice-water bath of the homogeneous polymer-repellent melt exiting the extruder. Phase separation occurred through spinodal decomposition which trapped the liquid repellent in the microporous polymer matrix. The extraction and TGA results corresponded well to the amount of repellent added in the compounding step, showing that very little repellent was lost during processing. The third approach showed that control of the repellent-release rate was possible with a skin-like membrane at the surfaces of the open-cell polymer-repellent strands extruded. The presence of a skin-like membrane of the polymer strands was studied using scanning electron microscopy and estimated by a mathematical model. It was found that some of the microporous polymer strands released the repellents at an almost constant rate. The experimental and predicted data fitted very well, showing the accuracy of the mathematical model developed. The last study demonstrated that the polyolefin strands that contained up to 30 wt% of either DEET or Icaridin provided effective protection against bites from the Anopheles arabiensis mosquito even after 12 weeks of ageing at 50 oC. This means that the bracelets or anklets made with polyolefin impregnated with DEET or Icaridin may offer a new effective control strategy which is cost effective for outdoor mosquito bites.