Feasibility Evaluation of Metamaterial Microwave Sensors for Non-Invasive Blood Glucose Monitoring

Abstract

The use of microwave technology is currently under investigation for non-invasive estimation of glycemia in patients with diabetes. Due to their construction, metamaterial (MTM)-based sensors have the potential to provide higher sensitivity of the phase shift of the S₂₁ parameter (∠S21) to changes in glucose concentration compared to standard microstrip transmission line (MSTL)-based sensors. In this study, a MSTL sensor and three MTM sensors with 5, 7, and 9 MTM unit cells are exposed to liquid phantoms with different dielectric properties mimicking a change in blood glucose concentration from 0 to 14 mmol/L. Numerical models were created for the individual experiments, and the calculated S-parameters show good agreement with experimental results, expressed by the maximum relative error of 8.89% and 0.96% at a frequency of 1.99 GHz for MSTL and MTM sensor with nine unit cells, respectively. MTM sensors with an increasing number of cells show higher sensitivity of 0.62° per mmol/L and unit cell to blood glucose concentration as measured by changes in ∠S21. In accordance with the numerical simulations, the MTM sensor with nine unit cells showed the highest sensitivity of the sensors proposed by us, with an average of 3.66° per mmol/L at a frequency of 1.99 GHz, compared to only 0.48° per mmol/L for the MSTL sensor. The multi-cell MTM sensor has the potential to proceed with evaluation of human blood samples.

Keywords: dielectric properties; glucose monitoring; microwave sensor.

Figure 1

Geometries and photographs of sensors. Geometries of (A) MTM9 and (B) MSTL sensors. Photographs of (C) MTM9 and (D) MSTL sensor.

Figure 2

Tailor-made holder with MTM9 sensor in PLA box ready for measurement.

Figure 3

Computational domain geometry; (A) and (B) are perspective and top view of the computational domain geometry, respectively. (C) and (D) are detailed views of the cross-sections in xz and yz planes with dimensions in mm.

Figure 4

Measured values of relative permittivity.

Figure 5

Measured values of electrical conductivity.

Figure 6

The course of ∠S₂₁ for all tested sensors.

Figure 7

The course of $$\angle$$S₂₁ for MSTL and 4 mL.

Figure 8

Detail of MSTL at 1.99 GHz and 4 mL.

Figure 9

The course of $$\angle$$S₂₁ for MTM9 and 4 mL.

Figure 10

Detail of MTM9 at 1.99 GHz and 4 mL.