# Microwave Planar Resonant Solutions for Glucose Concentration Sensing: A Systematic Review
###### Abstract  
The measurement of glucose concentration finds interesting potential applications in both industry and biomedical contexts. Among the proposed solutions, the use of microwave planar resonant sensors has led to remarkable scientific activity during the last years. These sensors rely on the changes in the dielectric properties of the medium due to variations in the glucose concentration. These devices show electrical responses dependent on the surrounding dielectric properties, and therefore the changes in their response can be related to variations in the glucose content. This work shows an up-to-date review of this sensing approach after more than one decade of research and development. The attempts involved are sorted by the sensing parameter, and the computation of a common relative sensitivity to glucose is proposed as general comparison tool. The manuscript also discusses the key points of each sensor category and the possible future lines and challenges of the sensing approach. glucose sensor; insertion/return loss; microwave; phase measurement; planar; relative sensitivity; resonant frequency; resonator; unloaded quality factor +
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characterization of lossy organic liquids [13]. It is in this context where the attention of a part of the microwave community was attracted towards glucose sensing. Glucose concentration sensing finds potential application in several contexts. The benefits for the diabetes community would be evident. People with diabetes need to self-monitor their blood glucose level several times every day (desirably with continuous monitoring) so that corrective actions can be accordingly applied. This technology could potentially lead to more comfortable and convenient measurement systems, notably helping to alleviate this burden [14]. Outside of the diabetes community, but still within a health context, glucose tracking is also of interest as a key indicator for other disorders [15; 16], and a large enough surveillance could even be interesting for early detection and stopping of pandemic outbreaks [17]. In addition, glucose gauging also finds application in some industry processes [18], such as those dealing with the production of glucose-containing drinks (juices, sodas, beers, wines, spirits and so on). The current methods in these processes usually involve chemical probes [19; 20], and therefore inline sensors able to provide a continuous measurement of the glucose content of the product would be of interest. Several measuring technologies have been proposed for different contexts, such as electrochemical [21; 22; 23; 24] or optoelectronic devices [25; 26], among others. However, microwave devices, and especially planar resonant ones, show some beneficial features such as ease of production and integration, reduced size, competitive cost and interesting penetration depths, especially for non-invasive measurements, generally with low tissue scattering [27]. Under these circumstances, the study of microwave planar resonant techniques for glucose concentration sensing has become an intense research field during the last years. As a matter of fact, searching the keywords \"microwave\", \"resonator\" and \"glucose\" in the Scopus database yields 127 results at the moment of writing this article, 70.16% of them being from the last 5 years. For such a highly specific technical topic, these numbers give a clear idea of the current interest raised within the scientific community. Indeed, the recent trends point to already attained interesting sensitivities (still to be enhanced, though), and pose the selectivity, i.e., the ability of the device to respond to changes only in the glucose concentration without interference from other elements, as one of the main current challenges [28; 29]. Effectively, these devices are usually affected by undesired sensitivities to other physiological parameters [30; 31; 32].