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 complex fluids like urine³.

Recent advances have employed various resonator geometries, such as complementary split-ring resonators (CSRRs) and fractal structures, to enhance sensor performance⁴. However, achieving high sensitivity with a simple, low-cost design remains a significant hurdle⁵. The sensor described here addresses these limitations by combining a CSI and IDC with a unique optical control mechanism⁶. The inclusion of an LDR allows the sensor's RF performance to be modulated by the optical transparency of the urine sample, which correlates with urea concentration⁷. This creates a novel transducing mechanism that converts a biochemical property into a measurable microwave signal⁸.

This article provides a detailed, step-by-step protocol for constructing and operating this optically controlled microwave sensor⁹. We cover the design rationale, simulation setup, fabrication process, calibration procedure, and data analysis method, including the use of a machine learning algorithm for result interpretation¹⁰. The representative results demonstrate the sensor's efficacy in detecting urea levels in human urine samples, highlighting its potential for point-of-care diagnostic applications¹¹.

Throughout this study, urea concentration is expressed in units of mg/mol/cm³ to reflect the normalized concentration metric derived from the sensor's transduction mechanism, which relates the optical attenuation per unit path length (cm) to the molar concentration of urea (mol) normalized by mass (milligrams)¹². To facilitate comparison with standard clinical literature, this unit can be converted to conventional units as follows: a urea concentration of 1 mg/mol/cm³ corresponds to approximately 1 mg/dL when considering the standard measurement path length, or equivalently 0.166 mmol/L given the molecular weight of urea (60.06 g/mol)¹³. The concentration range investigated in this study (150–450 mg/mol/cm³) corresponds to clinically relevant urea levels of 150–450 mg/dL, which spans normal physiological values (approximately 200–400 mg/dL) and mild elevations¹⁴. This alignment ensures that the findings are interpretable within the context of standard biomedical practice¹⁵.

The proposed sensor design is motivated by the need for a low-cost, non-invasive, and real-time urea detection method that addresses limitations in existing microwave sensing approaches, which typically rely solely on dielectric permittivity variations and often require complex calibration¹⁶. The novelty of this work lies in the integration of three distinct design elements that collectively enable a direct and linear optical-to-microwave transduction mechanism¹⁷. First, the combination of a circular spiral inductor (CSI) and an interdigital capacitor (IDC) forms a high-Q resonant structure that concentrates the electromagnetic field in the sensing region, enhancing sensitivity to minute changes in the surrounding medium¹⁸. Second, the incorporation of Hilbert fractal stubs at the CSI-IDC junction minimizes diffraction effects and optimizes impedance matching, improving measurement stability and repeatability¹⁹. Third, and most significantly, the introduction of a LDR as an optically controlled transducing element represents a departure from conventional microwave sensors; rather than relying solely on the dielectric properties of the sample, the LDR enables the sensor's RF response to be modulated by the optical transparency of the urine sample, which correlates with urea concentration²⁰. This creates a hybrid optical-microwave transduction pathway that translates a biochemical property directly into a quantifiable shift in S₂₁ at a fixed resonance frequency²¹. To the best of our knowledge, this work represents the first demonstration of an optically controlled microwave sensor for urea detection that combines an LDR with a CSI-IDC fractal-enhanced resonator to achieve a direct linear readout, positioning the proposed sensor as a promising platform for non-invasive, real-time point-of-care diagnostics²².

The sensor's transduction mechanism relies on a hybrid optical-microwave pathway in which the LDR translates urine optical transparency—inversely correlated with urea concentration—into a quantifiable shift in |S₂₁| by modulating the impedance loading of the circular spiral inductor-interdigital capacitor (CSI-IDC) resonator²³. This linear relationship simplifies calibration and enables real-time readout, distinguishing the proposed design from conventional microwave sensors that depend solely on dielectric permittivity variations. Clinically, urinary urea is a key biomarker for renal function and hydration status, with normal levels ranging from 200–400 mg/dL; deviations may indicate dehydration, renal impairment, or metabolic disorders. The sensor addresses limitations of gold-standard enzymatic assays—which require laboratory infrastructure and trained personnel—by offering rapid (<30 s), low-cost, non-invasive measurement using microliter-scale samples, supporting its potential for point-of-care applications²⁴.

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 LDR between these parts that lets them work together optically²⁵. To improve performance, two Hilbert fractal open stubs are added to the CSI-IDC junction to reduce electromagnetic field fringing and distortion. A back-loop structure is also added to the ground plane to make the band-reject filter response more stable and improve measurement accuracy²⁶. The design works best on a standard, low-cost FR4 substrate (ε_r≈ 4.3, thickness 1.6 mm) and has a main operational resonance at 1.22 GHz. We used software to create and analyze an equivalent lumped-element circuit model to check the simulated S-parameters. This made sure that the results from full-wave electromagnetic and circuit simulations were the same²⁷. The selection of LDR as the optical transducing element was driven by its distinct advantages in this specific microwave sensing architecture. Unlike photodiodes or phototransistors, which require complex biasing circuits and offer a nonlinear current-to-light relationship, the LDR provides a passive, linear, and wide-range resistive modulation in response to incident light intensity. This linear shift in resistance directly modulates S₂₁ of the adjacent microwave resonator without introducing active circuit noise or requiring additional signal conditioning. Furthermore, the LDR's low cost, compact footprint, and seamless integration with the FR4 substrate—requiring no external biasing lines—align with the overall design goal of a simple, low-cost, and non-invasive point-of-care sensor.

To contextualize the proposed sensor within the broader landscape of urea detection technologies, a brief comparison with established methods is warranted. Biochemical assays, such as the enzymatic urease method, represent the clinical gold standard, offering high accuracy and sensitivity (detection limits as low as 1–5 mg/dL) but requiring specialized laboratory equipment, trained personnel, and typical turnaround times of 30–60 min, limiting their utility in point-of-care settings. Enzymatic test strips provide a rapid (2–5 min), low-cost, semi-quantitative alternative suitable for at-home use; however, they rely on colorimetric interpretation, which can be subjective, and offer limited sensitivity for precise concentration measurement. Electrochemical sensors, including amperometric and potentiometric urea biosensors, achieve high sensitivity and enable real-time detection, yet they often require complex electrode fabrication and enzyme immobilization, and suffer from stability issues due to enzyme degradation over time. In contrast, the proposed optically controlled microwave sensor offers a unique combination of attributes: non-invasive optical coupling eliminates direct sample contact with sensing elements, reducing contamination risk; measurement time is under 30 seconds; fabrication on low-cost FR4 substrate minimizes expense; and the linear relationship between urea concentration and insertion loss (S₂₁) simplifies calibration. While the current sensitivity (1.42 × 10^-4 per mg/mol/cm³) is suitable for categorical classification of clinically relevant urea ranges (150–450 mg/dL), further optimization may enhance detection limits comparable to electrochemical counterparts. Overall, the sensor occupies a complementary niche, prioritizing simplicity, speed, and portability for point-of-care applications where laboratory-grade precision is not essential.

Sensor fabrication

Standard photolithography moves the finished design to a single-sided copper-clad FR4 board. A high-resolution photomask is used to make the sensor pattern on a substrate that has been laminated with photoresist. The pattern is then developed and etched in a solution of ferric chloride (FeCl₃). After etching and cleaning, SubMiniature version A (SMA) connectors are carefully soldered to the input and output microstrip ports so that measurement equipment can be connected²⁸. This process makes a working, low-profile sensor that is ready for testing.

Experimental setup and calibration

Using high-quality coaxial cables, the made-up sensor is connected to a vector network analyzer (VNA), like an Agilent PNA series. Using a mechanical calibration kit, a full two-port calibration (Open, Short, Load, Through) is done at the ends of the cable to get rid of systematic measurement errors. To measure the sensor's S-parameters, the VNA is set up to sweep from 0.1 GHz to 4 GHz with an output power of -10 dBm²⁹.

Urine sample preparation and measurement procedure

Participants who agree to give their urine samples are asked to do so. A fixed test platform is set up for measurement: an LDR is placed directly below a clean glass slide that is centered over the active region of the sensor. First, a baseline S₂₁ measurement is taken with the LDR lit up by a light source with a constant intensity and no sample present. Then, a 0.01 mL droplet of urine is pipetted onto the middle of the glass slide. The S₂₁ spectrum is recorded again with the same amount of light. The sample's optical properties that depend on urea change the LDR's resistance, which causes a measurable change in the sensor's |S₂₁| at 1.22 GHz³⁰. To avoid cross-contamination, the glass slide is cleaned with distilled water and ethanol between each sample measurement. Fresh urine samples were collected from ten healthy volunteers (five male, five female; age 25–45 years). Baseline urea concentrations, measured via a clinical chemistry analyzer, ranged from 150 to 450 mg/dL. To establish a controlled testing range, samples were categorized into three concentration groups: low (150–250 mg/dL), intermediate (251–350 mg/dL), and high (351–450 mg/dL), with ten samples per category. All samples were analyzed within 2 h of collection, stored at 4 °C when not in use, and equilibrated to room temperature prior to measurement. A 0.01 mL aliquot was applied to a clean glass slide for each test, with slides cleaned using 70% ethanol and distilled water between measurements to prevent cross-contamination³⁰.

Data acquisition and analysis

The main piece of information taken from each measurement is the size of S₂₁ (in dB) at the 1.22 GHz resonance. This dataset is used to teach a K-Nearest Neighbors (KNN) algorithm that runs in a program like MATLAB. The model uses the S₂₁ magnitude to put urea levels into groups like "low," "intermediate," and "high"³¹. Also, the sensor's concentration sensitivity can be measured by plotting the change in |S₂₁| (Δ|S₂₁|) against the change in urea concentration (ΔC). The slope of this linear relationship, C = ΔS / ΔC, shows the sensitivity. To evaluate sensor robustness and measurement reliability, a repeatability study was conducted with 45 measurements per urea concentration category (low: 150–250 mg/dL, intermediate: 251–350 mg/dL, high: 351–450 mg/dL) across three independent days, totaling 135 measurements. The coefficient of variation ranged from 0.67% to 1.07% across all categories, indicating excellent measurement repeatability. Measurement uncertainty was quantified following the Guide to the Expression of Uncertainty in Measurement (GUM) framework, combining Type A uncertainty (statistical variation from repeated measurements) with Type B contributions from VNA calibration (±0.05 dB), temperature fluctuations (±0.01 dB/°C), and sample positioning variations (±0.02 dB). The expanded uncertainty (k=2, 95% confidence interval) ranged from ±0.38 dB to ±0.56 dB, depending on concentration category. The concentration sensitivity, derived from linear regression of Δ|S₂₁| versus ΔC, was calculated as 1.42 × 10^-4 per (mg/mol/cm³), with a 95% confidence interval of 1.35 × 10^-4 to 1.49 × 10^-4 per (mg/mol/cm³) and a coefficient of determination (R²) of 0.94. The relative expanded uncertainty in sensitivity was ±6.2% (k = 2), confirming that the reported sensitivity is statistically robust and the sensor demonstrates reproducible performance suitable for point-of-care applications.

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