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By G. Guarin, M. Hofmann, G. Fischer, R. Weigel, and D. Kissinger.

The key factor to reduce costs and time of hospitalization and to increase the independency of patients with chronic disorders or elderly people is the development of noninvasive and continuous biosensors for local and remote monitoring. Lately, new biomedical sensors based on radiofrequencies offer the possibility to measure in a noninvasive way without the disadvantage of the ionizing radiations This article shows two new methods to measure blood glucose using microwave sensors and is based on two papers that were presented at the IMS 2013 in Seattle.

The key factor to reduce costs and time of hospitalization and to increment the independency of patients with chronic disorders or elderly people is the development of noninvasive and continuous biosensors for local and remote monitoring. This article is based on two IMS-2013 papers related to the theme.

The modern society is affronting big challenges, especially due to the demographic change and his implications. A great portion of the population is getting old which directly relates to an increase of the cost of health care systems. These situations added to the prevalence of chronic diseases on the people is resulting in a loss of independency and quality of life of the people affected with chronic diseases impacting in a direct form the global economy.

In order to reduce hospitalization costs and to increment the independence of the patients, new health monitoring systems have to be developed to supervise in a noninvasive way the health status of the persons. With this type of sensors, the independency of the patients will be increased and the hospitalization and monitoring costs will be reduced.

Diabetes exemplifies one of the most widespread chronic diseases at the moment. The loss of the self-regulation of the blood glucose demands the continuous monitoring of the patient’s blood glucose level and the medication to counteract the change in the blood glucose concentration. At the moment, the sensors available to measure the blood glucose in a continuous way and that make possible the feedback to an insulin pump are invasive or minimal invasive. Noninvasive sensors are still in development; however, sensors based on microwave impedance spectroscopy have made important advances in the last years. This type of sensors, that are based on the measurement in the change of the dielectric permittivity of a material, are suitable for noninvasive measurement of different biological variables since they use nonionizing radiations and it is possible to penetrate deeper on the tissue in comparison with optical sensors. The changes in concentrations of some substances in the body are related to some specific diseases. A change in the concentration of a substance can be traced to a change in the dielectric permittivity of the substance. Therefore, with the microwave impedance spectroscopy sensor it is possible to detect changes in the concentration of body substances in order to support the monitoring and diagnosis of health disorders.

The primary goal of our research is the development of noninvasive radio frequency biosensors based on microwave impedance spectroscopy to support the monitoring of biosignals and the diagnosis of chronic diseases like diabetes.

In our investigation the changes in the propagation constant of planar waveguides like microstrip or coplanar waveguides (CPW) are used to measure the change in blood glucose concentration as a function of the frequency and as a function of the dielectric permittivity. In such planar structures, the fringe field is not confined inside of the waveguide, and a portion of the electromagnetic field is guided outside the structure. If the waveguide is brought into contact with a tissue, the fringe field interacts with it and in the case of superficial blood vessels; it interacts with the blood in the vessel. Since the permittivity of the blood is a function of the glucose concentration, a change in the glucose concentration results in a change in the propagation constant of the structure.

Our group works in two different approaches to characterize the change of the propagation constant of planar structures to measure the blood glucose level.

The first approach is based on a coplanar waveguide sensor element and a six port structure to characterize the propagation constant of the sensor [1]. Fig. 1a, shows the change in phase and in magnitude measured with this approach as a function of the voltage in the diode detectors at 6.6 GHz. With a reduction on the glucose concentration, the voltage in the detector diode increases.

Figure 1: Measurements with the CPW sensor and with the microstrip sensor.

The second approach is based on a microstrip sensor and the generation of a pseudo random noise sequence [1]. By generating the sequence an ultra-wideband portion of the spectrum can be excited to characterize the impulse response and the propagation constant of the sensor. The advantages of this method are the reduction of measurement times in comparison with frequency sweep methods and the possibility to implement low-speed and low-cost analog-to-digital converters to sample high frequency signals. Fig. 1b shows the change in the relative phase of the sensor as a function of the concentration for different frequencies. There is an increment of the relative phase with a reduction of the glucose concentration.

In the future both sensor approaches can be further optimized regarding sensitivity using digital signal processing and miniaturized through monolithic integration, making possible the integration in devices used by the patient in the daily live like a wrist watch or a smart band aid as shown in Fig 2.

Figure 2: Miniaturization of the two systems through monolithic integration.

For Further Reading

1. M. Hofmann, S. Linz, R. Weigel, G. Fischer, and D. Kissinger, “A multiband 2-port VNA for biomedical applications based on two six-port junctions”, Microwave Symposium Digest (MTT), 2013 IEEE MTT-S International, 2-7 June 2013.

2. G. Guarin, M. Hofmann, R. Weigel, G. Fischer, d. Kissinger, “Determination of sugar concentration in aqueous solutions using ultra-wideband microwave impedance spectroscopy”, Microwave Symposium Digest (MTT), 2013 IEEE MTT-S International, 2-7 June 2013.

Contributors

Gustavo Guarin is currently pursuing his PhD in Electric Engineering at the University of Erlangen-Nuremberg. He works at the Institute for Electronic Engineering with Prof. Dr.-Ing. Dr.-Ing. habil. Robert Weigel and with Prof. Dr.-Ing. Georg Fischer. The focus of his research is on Ultra-Wideband Microwave Sensors for spectroscopy applications in biomedicine. Read More

Maximilian Hofmann received the Dipl.-Ing. degree in electrical, electronic and communications engineering from the University of Erlangen-Nuremberg, Nuremberg, Erlangen, Germany. He joined the Institute for Electronics Engineering, Erlangen, Germany, as a Research Assistant. Read More

Dietmar Kissinger received the Dr.-Ing. degree in electrical engineering from the University of Erlangen-Nuremberg, Erlangen, Germany. He holds a position as Lecturer and Head of the RF Integrated Sensors Group at the Institute for Electronics Engineering. His research interests include silicon-based microwave and millimeter-wave integrated circuits as well as wireless sensors and communication systems for ultra-low power, automotive, industrial, security, and medical applications. He is a member of IEEE MTT-S and SSCS, the European Microwave Association (EuMA) and the German Information Technology Society (ITG) and Society of Microelectronics, Microsystems and Precision Engineering (VDE/VDI GMM). He was a Guest Editor for the IEEE Microwave Magazine (2012) and currently serves as an Associate Editor for the IEEE Transactions on Microwave Theory and Techniques. Read More