On the Selectivity of Planar Microwave Glucose Sensors with Multicomponent Solutions

The development of glucose concentration sensors by means of microwave planar resonant technology is an active field attracting considerable attention from the scientific community. Although showing promising results, the current experimental sensors are facing some fundamental challenges. Among them, the most critical one seems to be the selectivity of glucose concentration against the variations of the concentrations of other components or parameters. In this article, we investigate the selectivity of microwave planar resonant sensors when measuring multicomponent solutions. Three sensors are involved, two of them having been designed looking for a more simplified system with a reduced size, and the third one has been specially developed to improve the sensitivity. The performance of these sensors is thoroughly assessed with a large set of measurements involving multicomponent solutions composed of pure water, NaCl, albumin at different concentrations and glucose at different concentrations. The impact of the simultaneous variations of the concentrations of glucose and albumin on the final measurements is analyzed, and the effective selectivity of the sensors is discussed. The results show a clear influence of the albumin concentration on the measurements of the glucose concentration, thereby pointing to a lack of selectivity for all sensors. This influence has been modeled, and strategies to manage this selectivity challenge are inferredThis research was partially funded by AEI (Spanish Research State Agency) through the Race project (reference PID2019-111023RB-C32). The work of C.G.J. was funded by the Ministry of Universities in the Government of Spain, the European Union–NextGenerationEU and the Miguel Hernández University of Elche through the Margarita Salas postdoctoral program, and also by Conselleria d’Innovació, Universitats, Ciència i Societat Digital in Generalitat Valenciana (Government of Valencia Region) and European Social Fund through the APOSTD postdoctoral program, grant number CIAPOS/2021/267. Partial funding for open access charge: Universidad de Málaga

Permittivity Extraction of Glucose Solutions Through Artificial Neural Networks and Non-invasive Microwave Glucose Sensing

An accurate low-cost method is presented for measuring the complex permittivity of glucose/water solutions. Moreover, a compact non-invasive RF/microwave sensor is presented for glucose sensing with the reasoning behind design parameters as well as simulation and measurement results. The complex permittivity values of aqueous solutions of glucose were measured with an in-house manufactured open-ended coaxial probe and the values were extracted from the measured complex reflection coefficients (S11) utilizing artificial neural networks. The obtained results were validated against a commercial probe. The values were fitted to the Debye relaxation model for ease of evaluation for a desired glucose concentration at a desired frequency. The proposed permittivity model in this paper is valid for glucose concentrations of up to 16 g/dl in the 0.3–15 GHz range. The model is useful for simulating and validating non-invasive RF glucose sensors

Dual Frequency Microwave Resonator for Non-invasive detection of Aqueous Glucose

A novel dual-band microwave sensor for noninvasive detection of glucose concentration is presented. The proposed sensor consists of an open-loop resonator coupled to the input and the output of the structure. The resonator is loaded with modified split-ring resonators (SRRs) for dual-band operation as a sensing area. The open-loop resonator with electric coupling operating at low band functions as a host. The SRRs embedded into the open-loop resonator operate at a high band. In the proposed sensor, the overall size is miniaturized using the embedded resonator structure. This configuration has two transmission poles (TPs) and one transmission zero (TZ) in transmission coefficients, which are all sensitive to glucose-level (GL) variation. A dielectric container made with 3-D printer is used for dropping the aqueous glucose samples on the sensing section of the sensor. The experimental results obtained from the prototype having a dielectric container shows two resonance frequencies at 1.8 and 2.67 GHz as well as a TZ at 2.32 GHz. A glucose solution with deionized water in the range from 89 to 456 mg/dL is used in the measurements. For this range of glucose concentrations, the experimental frequency resolutions are 0.78 and 0.95 MHz/(mg/dL) based on the TP and the TZ, respectively.</p

Simulating the Effects of Skin Thickness and Fingerprints to Highlight Problems with Non-invasive RF Blood Glucose Sensing from Fingertips

The non-invasive measurement of blood glucose is a popular research topic where RF/microwave sensing of glucose is one of the promising methods in this area. From the many available measurement sites in the human body, fingertips appear to be a good choice due to a good amount of fresh blood supply and homogeneity in terms of biological layers present. The non-invasive RF measurement of blood glucose relies on the detection of the change in the permittivity of the blood using a resonator as a sensor. However, the change in the permittivity of blood due to the variation in glucose content has a limited range resulting in a very small shift in the sensor’s frequency response. Any inconsistency between measurements may hinder the measurement results. These inconsistencies mostly arise from the varied thickness of the biological layers and variation of fingerprints that are unique to every human. Therefore, the effects of biological layers and fingerprints in fingertips were studied in detail and are reported in this paper

Towards Accurate Dielectric Property Retrieval of Biological Tissues for Blood Glucose Monitoring

(c) 2014 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works.This post-acceptance version of the paper is essentially complete, but may differ from the official copy of record, which can be found at the following web location (subscription required to access full paper): http://dx.doi.org/10/1109/TMTT.2014.2365019

A Novel Pressure Sensing Circuit for Non-invasive RF/Microwave Blood Glucose Sensors

A novel pressure sensing circuit for non-invasive RF/microwave blood glucose sensors is presented in this paper. RF sensors are of interest to researchers for measuring blood glucose levels non-invasively. For the measurements, the finger is a popular site that has a good amount of blood supply. When a finger is placed on top of the RF sensor, the electromagnetic fields radiating from the sensor interact with the blood in the finger and the resulting sensor response depends on the permittivity of the blood. The varying glucose level in the blood results in a permittivity change causing a shift in the sensor’s response. Therefore, by observing the sensor’s frequency response it may be possible to predict the blood glucose level. However, there are two crucial points in taking and subsequently predicting the blood glucose level. These points are; the position of the finger on the sensor and the pressure applied onto the sensor. A variation in the glucose level causes a very small frequency shift. However, finger positioning and applying inconsistent pressure have more pronounced effect on the sensor response. For this reason, it may not be possible to take a correct reading if these effects are not considered carefully. Two novel pressure sensing circuits are proposed and presented in this paper to accurately monitor the pressure applied

A Glucose Sensing System Based on Transmission Measurements at Millimetre Waves using Micro strip Patch Antennas

AbstractWe present a sensing system operating at millimetre (mm) waves in transmission mode that can measure glucose level changes based on the complex permittivity changes across the signal path. The permittivity of a sample can change significantly as the concentration of one of its substances varies: for example, blood permittivity depends on the blood glucose levels. The proposed sensing system uses two facing microstrip patch antennas operating at 60 GHz, which are placed across interrogated samples. The measured transmission coefficient depends on the permittivity change along the signal path, which can be correlated to the change in concentration of a substance. Along with theoretical estimations, we experimentally demonstrate the sensing performance of the system using controlled laboratory samples, such as water-based glucose-loaded liquid samples. We also present results of successful glucose spike detection in humans during an in-vivo Intravenous Glucose Tolerance Test (IVGTT). The system could eventually be developed into a non-invasive glucose monitor for continuous monitoring of glucose levels for people living with diabetes, as it can detect as small as 1.33 mmol/l (0.025 wt%) glucose concentrations in the controlled water-based samples satisfactorily, which is well below the typical human glucose levels of 4 mmol/l.</jats:p