IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 3, NO. 2, MARCH 2013 207 The Helical Structure of Sweat Ducts: Their Influence on the Electromagnetic Reflection Spectrum of the Skin Itai Hayut, Alexander Puzenko, Paul Ben Ishai, Alexander Polsman, Aharon J. Agranat, and Yuri Feldman Abstract—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 mea- surements 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 hy- pothesis 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 con- ductivity of sweat in these frequencies, but the analysis of the phe- nomena 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 ef- fect 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. Index Terms—Electromagnetic (EM) simulations, skin, sub-mm wave band, sweat ducts. I. INTRODUCTION W ITH the advent of modern imagery of living human skin, using methods such as optical coherence tomog- raphy (OCT), it was found that the human eccrine sweat duct has a well defined helical structure [1], [2]. This brought forward the hypothesis that the sweat ducts could exhibit electromag- netic (EM) behavior reminiscent of an array of helical antennas. This concept was presented and experimentally explored in two Manuscript received July 12, 2012; revised August 28, 2012; accepted Oc- tober 26, 2012. Date of publication December 28, 2012; date of current version February 27, 2013. This work was supported by the Israeli Ministry of Science under Grant 3/4602. The authors are with the Department of Applied Physics, The Hebrew Uni- versity of Jerusalem, 91904, Jerusalem, Israel (e-mail: yurif@vms.huji.ac.il). Digital Object Identifier 10.1109/TTHZ.2012.2227476 Fig. 1. Optical coherent tomography imaging of (a) the human skin (repro- duced with permission from ISIS GmbH) and (b) a sketch of a helical an- tenna (see Balanis [12], reproduced with permission from John Wiley & Sons Ltd.). The helical sweat ducts are embedded within the epidermis. The roughness between the epidermis and the dermis is of the same order of magni- tude as the sweat ducts length. previous works, where changes in the electromagnetic reflec- tion of the skin were observed as a result of elevated activity of the sweat glands, in a frequency range of 70–110 GHz [3], [4]. It is reasonable to assume that the morphology and electro- magnetic properties of the skin have an impact on the reflected signal in this frequency range, as well as in higher frequencies in the sub-millimeter regime. To explore this assumption one must consider the structure of the skin. We consider a skin model, which is composed of different layers: 1) outermost stratum corneum (SC); 2) intermediate epi- dermis; and 3) inner dermis. For the purposes of investigating its electromagnetic response it is necessary to take into account the conductivity and permittivity values of each layer. This can be achieved by evaluating the bulk and bound water content in each layer (previously detailed in [4]). One of the principal roles of the human skin is the thermoreg- ulation of the body by sweat evaporation. Sweat is produced in the glands, located at the bottom of the dermis layer. The ec- crine glands are the most common type of sweat glands, and are distributed through most of our body [5]. When activated by the nervous system the eccrine glands se- crete the sweat liquid into the ducts—a tube-like micro organ. The ducts deliver the sweat up to the skin surface, where it evap- orates through a pore in the SC [6]. The upmost section of the ducts associated with the eccrine glands have a well-defined helical structure [1], [2] as can be seen in Fig. 1. Histological studies have shown that about 90% of the sweat ducts are right-handed spirals [7]. In the sweat, the proton hopping can be considered as a mech- anism for ultra-fast electrical charge mobility. Past studies have 2156-342X/$31.00 © 2012 IEEE