New Method for Determining Dielectric Properties of Skin and Phantoms at Millimeter Waves Based on Heating Kinetics

Recent progress in millimeter-wave (MMW) wireless body-centric applications triggered an increasing interest to characterize the interactions between the millimeter waves and the human body. The determination of the dielectric properties of skin and phantoms (artificial models with tissue-equivalent dielectric properties) at MMW is crucial for the accurate evaluation of the power absorption and distribution in the skin. In this study, we show that the heating kinetics resulting from the MMW exposure can be used for the accurate determination of the penetration depth (δ) and power density (I) in different samples (1% and 4% agar phantoms, 20% and 25% polyethylene powder (PEP) phantoms, and human skin). The samples have been exposed at 60.4 GHz using an open-ended waveguide. The temperature distribution and dynamics are recorded using an infrared camera. The values of δ and I are defined by fitting the analytical solution of the bio-heat transfer equation to the experimental heating kinetics. The values of δ are further used to retrieve the permittivity spectra of materials described by Debye equation. Simultaneously, δ is calculated using the permittivity directly measured using a slim coaxial probe. Both results are in good agreement. Finally, our results demonstrate that the permittivity of a 20% PEP phantom is close to that of skin. Hence, this phantom can be used to model the MMW interactions with skin and to characterize on-body wearable MMW antennas.