Blooming of insecticides from polyethylene mesh and film

Abstract

Malaria remains a public health concern with vector control still the vital component of disease prevention, control, and elimination strategies. Recent years has seen a "stalling" in the progress made towards the reduction in the global malaria burden, highlighting the need to develop new, innovative, and safe alternative tools and delivery systems to achieve global malaria elimination. Interventions based on the use of indoor residual spraying (IRS) and long-life insecticidal bed nets (LLINs), i.e. insecticide-containing wall linings (IWLs), can contribute towards the reduction of malaria. Both LLINs and IWLs rely on the presence of insecticides on the fibre or filament surfaces. However, materials directly incorporating the insecticides into the polymer melt during extrusion, allows for effective killing of the mosquitoes when they come into contact with the surface of the material, only if there is insecticide present there. This means that the insecticide must migrate to the surface and precipitate there (bloom). Over time the internal concentration of insecticide will decay. This investigation was done using Fourier transform infrared spectroscopy (FTIR) in both the transmission and attenuated total reflection (ATR) modes to better understand the blooming of three World Health Organization-approved contact insecticides, i.e. alphacypermethrin, fipronil and chlorfenapyr, from mesh or film to better understand the likeliness of insecticides within the materials to migrate to the surface. Film-based samples were prepared in addition to wall lining mesh, because of their easier characterisation than the irregular shaped mesh filaments. FTIR, in ATR and in transmission modes, enabled the tracking of the migration of the three insecticides, over time to the surface of polyethylene mesh or film. This made it possible to estimate the apparent solubility of the insecticides in the polymer matrix. However, scanning electron microscopy (SEM) revealed that a portion of the insecticide is trapped, in a crystalline state, inside the polymer matrix. These results suggest the possibility of developing products-based insecticides for protection against infective mosquito bites in malaria-endemic regions.

Chlorfenapyr Crystal Polymorphism and Insecticidal Activity

“…This experiment mimics chlorfenapyr blooming in poly-(ethylene) insecticidal bednets, the process that has been already simulated experimentally but for which no crystallographic information has been obtained. 34,35 TEM imaging of chlorfenapyr deposited from chloroform revealed formation of a mixture of forms II and III, as evidenced by fast Fourier transforms (FFTs) (Figure 7) that correspond well to the diffraction of forms II and III (Figure 6). This confirms a trend to crystallize in a mixture of these forms also from solution.…”

“…Thus, form II crystallized on poly­(ethylene) fiber surfaces because of the blooming process (Figure G) is largely disordered and probably contains small domains of form III. This experiment mimics chlorfenapyr blooming in poly­(ethylene) insecticidal bednets, the process that has been already simulated experimentally but for which no crystallographic information has been obtained. , …”

ROY Crystallization on Poly(ethylene) Fibers, a Model for Bed Net Crystallography

“…HDPE is typically around 90% crystalline, and LDPE is less crystalline, containing more amorphous regions. Molecules of active ingredients can easily dissolve in a molten PE; however, as temperature decreases below the PE melting point, their miscibility drops and PE becomes supersaturated with the additive . This supersaturation drives a phase separation that results in the migration of the additive to the fiber surface where it becomes bioavailable.…”

“…In a subsequent Convening, experts emphasized the need for physical and chemical characterization to identify the surface chemistry characteristics that would be informative and the methods that should be developed . Scanning electron microscopy has revealed minute crystals in and on PE, ,, but the crystalline forms (a.k.a. polymorphs) have not been identified.…”

“…Molecules of active ingredients can easily dissolve in a molten PE; however, as temperature decreases below the PE melting point, their miscibility drops and PE becomes supersaturated with the additive. 34 This supersaturation drives a phase separation that results in the migration of the additive to the fiber surface where it becomes bioavailable. Subsequent surface diffusion of the emerged insecticide results in the growth of crystals of the active ingredients on the bed net fibers.…”