Bioelectromagnetics 33:375 ^382 (2012) The Remote Sensing of Mental Stress From the Electromagnetic Reflection Coefficient of Human Skin in the Sub-THz Range Eli Safrai, 1 Paul Ben Ishai, 1 Andreas Caduff, 1,2 Alexander Puzenko, 1 Alexander Polsman, 1 Aharon J. Agranat, 1 and Yuri Feldman 1 * 1 Department of Applied Physics,The Hebrew University of Jerusalem, Givat Ram, Jerusalem, Israel 2 Biovotion AG, Zurich, Switzerland Recent work has demonstrated that the reflection coefficient of human skin in the frequency range from 95 to 110 GHz (W band) mirrors the temporal relaxation of stress induced by physical exercise. In this work, we extend these findings to show that in the event of a subtle trigger to stress, such as mental activity, a similar picture of response emerges. Furthermore, the findings are extended to cover not only the W band (75–110 GHz), but also the frequency band from 110 to 170 GHz (D band). We demonstrate that mental stress, induced by the Stroop effect and recorded by the galvanic skin response (GSR), can be correlated to the reflection coefficient in the aforementioned frequency bands. Intriguingly, a light physical stress caused by repeated hand gripping clearly showed an elevated stress level in the GSR signal, but was largely unnoted in the reflection coefficient in the D band. The implication of this observation requires further validation. Bioelectromagnetics 33:375–382, 2012. ß 2011 Wiley Periodicals, Inc. Key words: human skin; sweat ducts; epidermis; reflection coefficient; Stroop effect; mental stress; antennas INTRODUCTION For more than 50 years, a number of phenome- na of nonthermal effects (high Q-factor resonances) of extremely high-frequency microwaves (MMW) on biological systems, and in particular, human skin, have been reported [Pakhomov et al., 1998; Pooley et al., 2003]. Intriguingly, the reported resonant fre- quencies are virtually absent in natural biological surroundings, making these effects all the more un- usual [Belyaev, 1992; Pakhomov et al., 1998]. While some insight has been gained, their interpreta- tion has remained controversial. Furthermore, there is ample experimental evidence of the beneficiary effects of low power (<20 mW/cm 2 ) MMW electro- magnetic (EM) radiation in a number of therapeutic avenues such as coronary artery disease, various can- cers, pain relief, wound regeneration, and hyperten- sion [Gandhi, 1983; Korpan and Saradeth, 1995; Pakhomov et al., 1998; Rojavin and Ziskin, 1998]. However, none of these studies could recognize a specific skin receptor related to these frequencies. Recent advances in the imaging of living skin [Huang et al., 1991] have revealed that the human perspiration system contains elements such as a coiled sweat duct in the epidermis [Knuttel and Boehlau-Godau, 2000; Knuttel et al., 2004]. Their morphology (Fig. 1), coupled with the dielectric properties of the skin, led to the concept that these coiled sections could behave like a classical helical antenna and have an EM response in the extremely high-frequency range (70–600 GHz) [Feldman et al., 2009; Shafirstein and Moros, 2011]. This was conclu- sively shown up to 110 GHz in our previous work [Feldman et al., 2008, 2009]. There, it was reasoned that the EM response due to the sweat duct would reflect the physiological state of the subject, demon- strated experimentally by subjecting a group of 13 volunteers to intense physical activity. Afterward, the relaxation of their physiological stress over time, evidenced by changes in systolic blood Grant sponsor: Brojde Center for Innovative Engineering and Computer Science at the Hebrew University of Jerusalem. *Correspondence to: Yuri Feldman, The Hebrew University of Jerusalem, Bergman Building, The Edmond J. Safra Campus, Givat Ram, Jerusalem 91904, Israel. E-mail: yurif@vms.huji.ac.il Received for review 9 May 2011; Accepted 21 October 2011 DOI 10.1002/bem.21698 Published online 14 December 2011 in Wiley Online Library (wileyonlinelibrary.com). ß 2011Wiley Periodicals, Inc.