Millimetric Inclusion Detection Through a Contactless Microwave Spiral Sensor for Biomedical Applications

This article introduces the design and fabrication of a microwave contactless sensor used to detect the presence of millimetric inclusions in a biological medium for biomedical applications. The hardware system comprises a self-resonant planar spiral resonator (SR) (sensing element) inductively coupled to an external concentric single-loop probe (reading probe), working at 648 MHz. The microwave sensor configuration relies on the ${Q}$ -factor maximization of the spiral coil, that is, the sensing element, through an optimization process, to obtain a stronger sensitivity and, thus, a millimeter resolution for the inclusions’ detection. The detection is achieved by recording both the amplitude variation and the frequency shift of the input impedance of the reading probe. To validate the proposed solution, full-wave simulations have been performed to design and preliminary evaluate the radiating system performance in detecting millimetric biological inclusions. As an applicative example, we focus on air bubbles detection in hemodialysis procedures; in particular, we carried out the experimental verification by employing an agar phantom to replicate the dielectric characteristics of the blood tissue and polylactic acid (PLA) samples of various sizes to represent the air inclusions. We proved that the theoretical assumptions were in excellent agreement with both the numerical and experimental results, encouraging further analysis of the potential use of such sensors in biomedical applications. Indeed, the radiating device can be very helpful for all operations where it is necessary to detect the presence of undesired and dangerous contaminants in a contactless way, making the procedure safer for patients.