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

Summary

This protocol outlines a low-cost, optically controlled microwave sensor for detecting urea. Using a light-dependent resistor and fractal design, it translates urine's optical properties into a linear RF impedance response. While the FR4-based design shows promise for point-of-care use, future work requires low-loss materials and clinical validation of a portable system.

Abstract

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.

Introduction

Microwave sensing technology has emerged as a powerful tool for non-invasive characterization of biological materials due to its minimal contact requirements, nondestructive penetration, and the safe nature of non-ionizing electromagnetic waves¹. The dielectric properties of bodily fluids like urine are influenced by their chemical composition, including urea concentration, making them detectable through changes in microwave propagation². While conventional microwave sensors exist, they often face challenges in sensitivity, miniaturization, and optimization of the quality factor (Q-factor) for detecting minute changes in....

Protocol

Ethical approval was obtained from the IATRC Review Board. All procedures involving human urine samples complied with the principles of the Declaration of Helsinki. Written informed consent was obtained from all participants, and all samples were anonymized to protect confidentiality.

Sensor design and numerical simulation

The first step in designing a sensor is to run an electromagnetic simulation. CST Microwave Studio is used to make the layout, which includes a resonator made up of a circular spiral inductor (CSI) in series with an interdigital capacitor (IDC). There is a connection point for a ....

Results

Upon fabrication that was designed and presented in Figure 1, the sensor's performance was first fabricated, see Figure 2, and validated without any sample. The measured S-parameters showed excellent agreement with the simulated results, with a primary resonance at 1.22 GHz and S₂₁ reaching -27 dB, as shown in Figure 2. The minor discrepancies (<5%) are attributed to fabrication tolerances and soldering effects.

Discussion

The protocol outlines the successful development of an innovative optically controlled microwave sensor specifically designed for urea detection in human urine. Key aspects of the protocol involve the meticulous design of Hilbert fractal stubs to optimize electromagnetic field distribution and ensure consistent placement of urine samples on a glass slide over LDR to facilitate reproducible optical coupling. The results highlight the effectiveness of the LDR in translating the optical properties of urine, which vary with .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
CST Microwave Studio	Dassault Systèmes	2023	Full-wave electromagnetic simulation
Digital multimeter	Fluke	87V	For resistance verification
Distilled water	Local supplier	N/A	For sample dilution and cleaning
Etching bath	Generic	N/A	Temperature-controlled, 50°C
Ethanol (70%)	Merck	100983	For cleaning glass slides
Ferric chloride (FeCl3)	Sigma-Aldrich	157740	0.5 M solution, etching agent
FR4 copper-clad laminate	Generic	N/A	Single-sided, 1.6 mm thickness, εr 4.3
Glass slides	Corning	2947-75X25	75 mm × 25 mm × 1 mm, for sample placement
Hot plate	Generic	N/A	For development and curing
Light source	Generic LED	N/A	White LED, 6000 K color temperature, 1000 lux fixed intensity
Light-dependent resistor (LDR)	Generic (GL5528)	GL5528	Resistance: 10 kΩ (light), 1 MΩ (dark); 5 mm diameter
MATLAB	MathWorks	R2023b	Data analysis and KNN classification; Statistics and Machine Learning Toolbox
Mechanical calibration kit	Agilent	85052D	Open, short, load, through (OSLT) for two-port calibration
Micropipette	Eppendorf	3120000062	0.5–10 µL volume range
Microsoft Excel	Microsoft	Office 365	Data recording and analysis
Oscilloscope	Tektronix	TBS1052B	For rectifier output voltage measurement
Photomask	Custom	N/A	High-resolution chrome mask with sensor pattern
Photoresist	MicroChem	SU-8 2000	Negative photoresist for photolithography
Pipette tips	Eppendorf	30015.119	0.01–10 µL, sterile
Polypropylene sample containers	Thermo Fisher	342020-0030	30 mL sterile containers
RF rectifier circuit	Custom	N/A	Schottky diode-based; converts RF to DC voltage
SMA female edge launch connector	Amphenol RF	132169	50 Ω, for microstrip connection
Urea (for spiking)	Sigma-Aldrich	U5128	Pharmaceutical grade, ≥99.5% purity
UV exposure system	Karl Suss	MA6	Exposure time: 120 s
Vector network analyzer (VNA)	Agilent	PNA Series	Frequency range: 0.1–4 GHz, IF bandwidth: 100 Hz, sweep points: 1001