Penetration of electromagnetic fields of an open-ended coaxial probe between 1 MHz and 1 GHz in dielectric skin measurements

An open-ended coaxial probe is often used for investigating the dielectric properties of biological tissues. The present study indicates that in addit ion to the probe size, the penetration of the electromagnetic (EM) fields o f an open-ended coaxial probe in contact with the skin is dependent on the applied frequency between 1 MHz and 1 GHz. At high frequencies, above 100 M Hz, the measured dielectric parameters are functions of the dielectric prop erties of different cutaneous layers and subcutaneous fat. At lower frequen cies, less than 10 MHz, the measurement is mainly dependent on the dielectr ic properties of superficial structures of the skin. The reason for this is that the probe, the surface of the skin, mainly stratum corneum, and under lying skin form a capacitance where the stratum corneum with low water cont ent lies between the well-conducting dermis and the probe. The situation is equivalent to the frequency-dependent Maxwell-Wagner interfacial polarizat ion. This result is verified by experimental dielectric measurements and wi th human skin in vivo.