Fabrication and Testing of an Optically Controlled Microwave Sensor for Urea Level Detection in Urine

Monitoring urea levels in urine is crucial for assessing renal function and hydration status. Current methods often rely on intrusive, costly, or time-consuming biochemical assays, which are not ideal for point-of-care or continuous monitoring. This protocol describes the fabrication and testing of a novel, low-cost, and highly sensitive microwave sensor designed for urea level detection. The sensor integrates a circular spiral inductor (CSI), an interdigital capacitor (IDC), and a light-dependent resistor (LDR) on an FR4 substrate, operating at a resonance frequency of 1.22 GHz. The key innovation is the optical control via the LDR, which, when exposed to a fixed light source through a urine sample, modulates the sensor's insertion loss (S₂₁) in a linear and quantifiable manner relative to urea concentration. The design incorporates a back-loop trace and Hilbert fractal stubs to minimize diffraction effects and enhance impedance matching, thereby improving measurement accuracy. We detail the sensor's numerical simulation using CST Microwave Studio, its fabrication via chemical etching, and its experimental validation using human urine samples. The results demonstrate a consistent and repeatable shift in the S₂₁ parameter with varying urea levels, confirmed by a neural network model for data classification. This sensor presents a promising tool for non-invasive, real-time biomedical diagnostics.