Microencapsulation decreases the skin absorption of N,N-diethyl-m-toluamide (DEET)

The insect repellent N,N-diethyl-3-methylbenzamide (DEET) is widely used and is generally regarded as safe when used according to label instructions. Yet many studies have shown it to be absorbed through the skin. The objective of this study was to determine whether the skin absorption rate of DEET could be decreased while maintaining an evaporation rate consistent with effective repellency. To this end, an aqueous suspension containing 14C-DEET (15% w/w) entrapped in walled polysaccharide microcapsules was prepared and tested for skin absorption in vitro using modified Franz cells maintained in a fume hood. The control formulation was 15% w/w DEET in ethanol. Two doses (3 μL and 5 μL per 0.79 cm2 cell) of each formulation were applied to split-thickness human cadaver skin (n = 8/dose), and permeation was monitored for 24 h. The microencapsulated DEET formulation lead to a 25-35% reduction of radiolabel permeation compared to the ethanolic DEET formulation. Skin levels of radioactivity at 24 h were comparable, indicating that DEET evaporation from the microencapsulated formulation was comparable to or greater than that from ethanol. Hence microencapsulation increased the ratio of DEET evaporation rate to skin penetration rate relative to unencapsulated control in this in vitro study.