Multifaceted Free-Space Tissue Sensing up to 100 GHz

Non-invasive microwave technologies are emerging in biomedical applications. One of the promising applications is muscular activities detection, where the microwave spectrum of sub-5 GHz is primarily exploited due to large skin penetration depth. However, spatial resolution is limited, and high range resolution is required to analyze electromagnetic signals reflected from the layered structure of the skin and muscle. The resolution is proportional to bandwidth, and thus this study examines a spectrum from 6 GHz to 100 GHz. Although the dielectric properties of skin and muscle can be derived from state-of-the-art models and measurements, they are inadequate for estimating the penetration losses of inhomogeneous multi-layered tissues. Thus, this study emphasizes the precise estimation of penetration losses through skin tissue layers and reflectivity from the muscle tissue surface, using porcine skin as a substitute for human tissue, with a novel focus on the volume scattering behavior. Skin tissue layers are analyzed in two forms, with the first including the epidermis and dermis layers, and the second additionally including the hypodermis or subcutaneous fat layer. In a validating approach, this study aims to compare measurement results obtained from the free-space method using a vector network analyzer with known tissue model parameters derived from previous works. Further, a novel aspect of this study is the derivation of a link budget estimation for muscle sensing based on estimated losses from skin and the radar cross section (RCS) of muscle. This provides a replicable framework and foundational principles for skin and muscle sensing in biomedical engineering.