The helical structure of sweat ducts: Their influence on the electromagnetic reflection spectrum of the skin

The helical structure of human eccrine sweat ducts, together with the dielectric properties of the human skin, suggested that their electromagnetic (EM) properties would resemble those of an array of helical antennas. In order to examine the implications of this assumption, numerical simulations in the frequency range of 100-450 GHz, were conducted. In addition, an initial set of measurements was made, and the reflection spectrum measured from the skin of human subjects was compared to the simulation results. The simulation model consisted of a three layer skin model (dermis, epidermis, and stratum corneum) with rough boundaries between the layers and helical sweat ducts embedded into the epidermis. The spectral response obtained by our simulations coincides with the analytical prediction of antenna theory and supports the hypothesis that the sweat ducts can be regarded as helical antennas. The results of the spectrum measurements from the human skin are in good agreement with the simulation results in the vicinity of the axial mode. The magnitude of this response depends on the conductivity of sweat in these frequencies, but the analysis of the phenomena and the frequencies related to the antenna-like modes are independent of this parameter. Furthermore, circular dichroism of the reflected electromagnetic field is a characteristic property of such helical antennas. In this work we show that: 1) circular dichroism is indeed a characteristic of the simulation model and 2) the helical structure of the sweat ducts has the strongest effect on the reflected signal at frequencies above 200 GHz, where the wavelength and the dimensions of the ducts are comparable. In particular, the strongest spectral response (as calculated by the simulations and measured experimentally) was noted around the predicted frequency (380 GHz) for the axial mode of the helical structure.