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30/07/2021

Reflection Properties of the Human Skin From 40 to 110 GHz: A Confirmation Study

Andreas Christ, Adrian Aeschbacher, Fereshteh Rouholahnejad, Theodoros Samaras, Bernadetta Tarigan, and Niels Kuster, Bioelectromagnetics, Volume 42, No. 7, pp. 562 – 574, October 2021, online 21 July 2021; doi: 10.1002/bem.22362

In several recent theoretical dosimetric studies conducted at frequencies above 6 GHz, generic layered skin models were applied. For this frequency range, new experimental phantoms with a low-loss coating layer for over-the-air performance of wireless devices that simulate the impedance matching effects of the stratum corneum layer (SCL) were proposed. The aim of this study was to verify the skin models by comparing their S11 reflection coefficients with measurements performed on 37 human volunteers (21 males, 16 females, 5 – 80 years old) at 21 body locations (10 at the palm and 11 at the arm or face) with different SCL thicknesses, with waveguides covering frequencies from 40 to 110 GHz. Such measurements were also carried out with the phantom material. The statistical analysis strongly indicates that S11 depends on the thickness of the SCL, and there is no evidence that S11 depends on sex. The measured S11 values for thin and thick skin can be represented by SCL layers of 15 and 140 μm, respectively. These values correspond well with the assumptions stated in previous studies. However, it should be noted that the cohort did not include volunteers who perform heavy manual labor. The phantom material mimics the matching effect of the SCL with deviations from the waveguide measurements of less than 0.85 dB (22%), which confirms the suitability of the layered phantoms to represent the electromagnetic reflection/absorption of human skin.

The scientific and technical impact of the study can be summarized as:

  • The theoretically predicted reflection characteristics of the skin at mm-wave frequencies were confirmed experimentally
  • Increased absorption of radiofrequency (RF) energy at mm-wave frequencies, due to matching effects, was observed for regions with thicker SCL, such as the palms
  • The thin SCL of other body regions (but not including the soles of the feet) do not exhibit increased absorption
  • The phantom materials proposed for the palms and the rest of the body reproduce the increase in RF energy absorption with good accuracy over the specified frequency range