RF-Induced Temperature Increase in a Stratified Model of the Skin for Plane-Wave Exposure at 6 – 100 GHz

Andreas Christ, Theodoros Samaras, Esra Neufeld, and Niels Kuster, Radiation Protection Dosimetry, ncz293, online 16 January 2020; doi: 10.1093/rpd/ncz293

In this study, we use a stratified model of the skin with 4 or 5 layers to assess the maximum temperature increase in the skin induced by exposure to electromagnetic (EM) fields at frequencies of 6 – 100 GHz under plane-wave incidence. The skin model allows descrimination of the stratum corneum (SC) and the viable epidermis as the outermost layers of the skin. The analysis enables identification of the tissue layer structures that minimize reflection and maximize the temperature increase induced by the EM field. The maximum observed temperature increase is 0.4°C for exposure at the present power density limit for the general population of 10 W m⁻². This result is more than twice as high as the findings reported in a previous study. The reasons for this difference are identified as impedance matching effects in the SC and less conservative thermal parameters. Modeling the skin as a homogeneous dermis tissue can underestimate the induced temperature increase by more than a factor of 3.

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

• Modeling the skin as homogeneous dermis tissue leads to underestimation of the  temperature increase induced exposure to EM fields; the heterogeneous composition of skin tissue and the frequency dependence of effects that contribute to energy absorption and temperature increase lead to unsatisfactory correlation of the 1 g psSAR or ∆T with the transmitted power density
• At frequencies >15 GHz, the SC acts to impede incident EM fields; at >60 GHz, the power transmission coefficient can reach values of almost 1 for SC thicknesses of 360 and 700 μm
• The maximum ∆T of 0.4°C for the exposure limit of S_inc = 10 W m⁻² for the general population is observed at 60 GHz for plane-wave incidence on thick SC without a terminating muscle layer; with a muscle layer, the ∆T is lower by ca. 15% at the upper end of the frequency range regarded in this study
• Impedance matching does not occur for thin SC; the power transmission coefficient reaches approximately 0.7 and  the maximum ∆T for S_inc = 10 W m⁻² is 0.29°C at 100 GHz
• Application of mixed boundary conditions with a heat transfer coefficient of 7 W per m² per °C can result in ∆Ts lower by ca. 25% relative to those found with adiabatic boundary conditions

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