Controlled-release of mosquito repellents from microporous polymer strands

Abstract

Malaria parasite infects more than 200 million people and about 435 000 succumb to the illness annually (WHO, 2019). Victims are mostly young children and pregnant women. It is transmitted by the bite of the infected female Anopheles mosquitoes. Indoor protection is provided by bed nets and residual spraying of insecticides. Mosquitoes typically bite ankles and feet most of the time (93%) whilst in outdoor settings. Long lasting insect-repellent anklets/bracelets/footlets may provide a strategy for reducing mosquito bites outdoors in the lower limb regions. This study considered long-lasting repellent anklets that may be used for outdoor protection against mosquito bites. Experiments were performed to investigate the incorporating of mosquito repellents into the thermoplastic polymers, poly(ethylene-co-vinyl acetate) (EVA) and linear low-density polyethylene (LLDPE). Two different mosquito repellents, namely DEET and Icaridin, were employed. The target was to develop cost-effective bracelets with long-lasting efficacy, i.e., slow release of the active ingredient over extended periods. In this way, it is expected to protect people from acquiring mosquito-borne diseases during the time they spend outdoors. The proposed concept utilises microporous polymer strands manufactured via conventional plastic extrusion processes. The internal open-cell polymer foam structure serves both as a reservoir and a protective environment for the active ingredient trapped inside. An outer dense skin layer covering the strands may provide the necessary diffusion barrier that controls the release of repellent at effective levels over a considerable period. The objective was achieved by phase separation via spinodal decomposition (SD), triggered by extruding the molten strands directly into ice-cold water. Thermogravimetric analysis (TGA) and solvent extraction confirmed that all of the repellents were embedded in the polymer matrices. Scanning Electronic Microscopy (SEM) confirmed the porous co-continuous repellent-polymer microstructure. The stability of the polymer matrix was studied by estimating the swelling and shrinkage of the polymer matrix. The release of the active ingredient in the polymer/repellent system was followed as a function of oven-ageing temperature and time. The kinetics of the release rate of the repellent from microporous polymer matrix strands was mathematically modelled using semi-empirical models. The performance of the repellent-based strands was evaluated using foot-in-cage repellence testing. Finally, an attempt was made to predict the phase diagrams of the LLDPE/repellent system on the basis of alkane/repellent systems data. The results confirmed that EVA and LLDPE are suitable scaffold matrices, acting as reservoirs, for liquid repellents that were released at a constant rate. As expected, the repellent swelled EVA more than LLDPE. As a result, it also shrank significantly more when the repellent was released, i.e. EVA showed poor dimensional stability compared to LLDPE. The semi-empirical repellent release models were found valuable as they provided insights into the way that the repellent was being released. They allowed differentiating between diffusion and relaxation mechanisms. It was found that repellence efficacy can be maintained for more than 90 days. Future developments of sandals and anklets based on this approach may assists in preventing outdoor mosquito bites, thereby decreasing malaria infection rates.