Permittivity Characterization of Human Skin at Sub-THz Using Free Space Reflection Coefficients

This article introduces a novel approach to experimentally characterize the effective human skin permittivity at sub-terahertz (sub-THz) frequencies, specifically from $140$ to $210$ GHz, utilizing a quasi-optical measurement system. To ensure accurate measurement of the reflection coefficients of human skin, a planar, rigid, and thick low-loss dielectric reference plate is utilized to flatten the human skin surface. Then, a permittivity characterization method is proposed that considers phase correction for displacements and inclination of the reference plate due to the nonrepeatable behavior of human subjects in placing their body parts on the reference plate to flatten skin surfaces. The phase correction ensures good measurement repeatability. We show that the measured complex relative permittivity of the finger, palm, or arm of seven volunteers shows small standard deviations for repeated measurements of the same skin area of the same person. Meanwhile, we see significant permittivity variations across different skin regions of a human body and for different persons most likely due to varying conditions of layered structures of the skin and their moist levels. The proposed measurement system holds significant potential for skin permittivity characterization in sub-THz bands, facilitating in-depth studies on human-electromagnetic wave interactions based on the measured permittivity values.