Two-Coaxial-Probe Method for Dielectric Spectroscopy of Two-Layer Materials Towards Biological Application

This paper proposes a novel measurement method for admittance modelling of open-ended coaxial probes that measure two-layer material independently of the thickness of the first layer for dielectric spectroscopy of the inner layer. The penetration depth of a coaxial probe depends on the probe aperture, which results in differences in the measured effective dielectric constant for layered materials. Therefore, our analytical admittance model uses two coaxial probes with different penetration depths, and we calculate the admittance using the effective dielectric constant measured by both probes. We evaluated the accuracy of the model by in vitro measurement, assuming biomedical samples such as human skin. The results show good agreement with measured admittance in the frequency range from 0.5 to 10 GHz. The error with the measured admittance was within 10%, though the thickness of the first layer was not included in the calculation. Since biological materials are composed of a two-layer structure to retain moisture, the proposed method is expected to be used for novel biological sensing applications.