A Microwave Sensor Based on Concentric Split-Ring Resonators for Blood Urea Detection: A Novel Tool for Chronic Kidney Disease Diagnosis

Chronic kidney disease (CKD) affects millions of people worldwide, making early diagnosis critical for effective management and healthcare cost reduction. Monitoring blood urea levels is essential for this purpose. In this paper, we present a microwave sensor capable of detecting urea concentrations within the clinical range required for CKD diagnosis, a sensitivity not achieved by previous microwave (MW) based approaches. The proposed device operates without reagents or enzymes, requires no specialized personnel and offers an extended lifespan, ease of use, and sensitivity comparable to conventional optical sensors. Our work consists of two main steps. First, we conducted a dielectric characterization of artificial blood plasma (ABPS) and urea-enriched ABPS (ABPSU, 320 mg/dL) over a frequency range of 500 MHz to 10 GHz. The results revealed that the most significant differentiation between samples occurs at low frequencies, particularly in the loss tangent rather than in the real part of the dielectric constant, thereby identifying the optimal frequency range for sensor design. In the second step, we designed a double concentric circular split-ring resonator (DCCSRR) on a 49 mm × 45 mm Rogers 5880™ substrate. A dedicated optimization protocol enhanced both the resonance peak amplitude and the quality factor of the sensor’s fundamental mode. After fabrication, the DCCSRR was tested for urea concentrations ranging from 10 to 320 mg/dL, exhibiting measurable variations in S21 (dB) and frequency shifts around 575 MHz. This study establishes the first microwave sensor operating within the urea concentration range necessary for CKD diagnosis, paving the way for practical microwave-based urea monitoring.