Abstract—Self-measuring the blood glucose level (BGL) is part and parcel of diabetes management. People with diabetes need to self-monitor their BGL several times per day, so that they can perform the right actuations (either with glucose intake or with insulin injections) according to each measurement. Currently, the measuring methods are invasive and uncomfortable, often leading to a reduced, intermittent number of measurements. The development of a reliable non-invasive method able to provide the user with their BGL in a comfortable way, with capabilities of continuous BGL measurement, seems therefore highly desirable.

 

This thesis discusses the application of microwave sensors for the glucose concentration detection in aqueous and biological solutions. During the last years, some attempts have been made in the pursue of a non-invasive BGL measuring device. Among the studied principles, microwave technology has been shown to present convenient penetration depths for non-invasive measurements in the human skin and other biological tissues. With a proper configuration, these devices can achieve electrical responses sensitive to the glucose content of the medium being measured. This dissertation offers a comprehensive assessment of the feasibility of such approach, deepening on the sensor design guidelines, as well as on the detection of the glucose concentration in watery and biological solutions. Special emphasis is placed on its use in real application contexts, and on the open lines to be faced in the near future. The results achieved throughout this thesis are collected in four publications included in the Journal Citation Reports (JCR).

 

The working principle of these sensors relies on the idea that the changes in the glucose concentration provoke variations in the dielectric permittivity of the medium. Thus, sensors whose electrical response is sensitive to the dielectric permittivity of the surrounding media should be able to perform as glucose concentration trackers. To provide for an adequate design, these changes in the permittivity must be characterized. At the beginning of the thesis, a transmission/reflection line is used to measure and characterize the dielectric permittivity of water–glucose solutions at concentrations relevant for diabetes purposes.

 

With this information, sensor design guidelines are deeply discussed in the central part of the thesis. The sensors are focused on the microwave resonator as the fundamental component. Specifically, microstrip open-loop resonators are proposed with different configurations. The gap between the open ends is a highly capacitive area, and the response of the sensor is remarkably sensitive to the permittivity of the media in that area. These sensors are widely studied in this document, and the results of implementing and assessing them are shown. The sensors are tested with water–glucose solutions and multi-component human blood plasma solutions, and thorough discussion on their sensitivities is provided. Among different sensing parameters, the unloaded quality factor is highlighted.

 

Taking one step ahead, a portable version of one of the sensors is developed in the last part of the thesis. This device is used for non-invasive BGL measuring of a large number of individuals in a real multi-center clinical scenario. A comprehensive experimental study aimed to real application is carried out to evaluate the feasibility of the proposal and to identify possible improvement aspects. Finally, as a result of the conclusions reached throughout the work collected in this document, the open lines to be addressed in the pursue of reliable non-invasive BGL measurement are discussed, presenting novel sensing approaches to deal with some of them. The global conclusions are briefly described at the end of the document.