# Microwave Planar Resonant Solutions for Glucose Concentration Sensing: A Systematic Review
## 7 Discussion
The use of microstrip line-based designs has been involved in some attempts, although closed-loop resonators and SRR-based designs have also proved good comparative sensitivities. These sensors can operate at a single frequency, thereby easing the development of the required driving system and thus reducing the associated expense, although sophisticated phase-detection stages are required (or other hybrid solutions [152]). Recently, the properties of meandered lines have been proved to enhance the sensitivity of this kind of sensor for dielectric characteristics retrieval [156], as well as capacitively loaded lines [157], concepts that have contrarily not been hitherto applied to glucose sensing. This sensing approach remains therefore open for further study and exploitation, and its development could lead to interesting complementary measurements in the future. As a general consideration, it is important to note that both real and imaginary parts of the permittivity change depending on the frequency, and therefore the operating frequency must be chosen carefully, also considering the rest of practical constraints. The references considered in this review have been ordered by the operating frequency, but this parameter is not considered in the \(RS(G)\) calculation (only for \(f_{\rm r}\)-based sensors). Therefore, conclusions drawn from \(RS(G)\) values for each specific sensor should be considered in a moderate way, since the frequency could have an impact on the sensitivity. That said, at the moment, considering the general reported \(RS(G)\) values, it seems that insertion/return loss-based sensors are more sensitive than \(f_{\rm r}\)-based ones. Additionally, \(Q_{\rm u}\)-based sensors report generally higher \(SR(G)\) values than \(f_{\rm r}\)-based ones, often similar to those from insertion/return loss-based approaches. These data prove how important it is to study, analyze and properly select the sensing parameter from the design stages. Additionally, it is interesting to analyze the information provided by each sensing parameter. In this sense, according to (5), \(f_{\rm r}\) is of interest for tracking considerable changes in \(\epsilon_{\rm rs}\)\({}^{\prime}\). The insertion/return losses, however, seem more interesting for detecting the variations in \(\epsilon_{\rm rs}\)\({}^{\prime\prime}\), as shown in (7). Finally, according to (11) and (14), \(Q_{\rm u}\)- and phase-based approaches seem interesting for measuring combinations of both variations. These conclusions, in fact, come from the theoretical analysis of the generic model, and they are not strictly linked to the nature of the sample. This discussion is therefore applicable to any sensing scenario considering planar resonant elements, whatever the MUT. Therefore, the sensing parameter should be accordingly chosen depending on the expected variations of the dielectric characteristics of the samples. In general, it can be seen how microwave planar glucose sensors show the general trend of considering SRR in the designs, chiefly due to its high capacitive, permittivity-dependent characteristics. Other elements, such as interdigital capacitors or electric \(LC\) resonators, have also been proved to yield good results. During recent years the convenience of considering mutually coupled resonators has also raised some attention. Regardless of the design, these sensors are convenient for their simplicity, ease of integration and even often for their non-invasive capabilities. However, they are also notably affected by undesirable cross-sensitivities to certain environmental factors, such as temperature, pressure and humidity, as well as intrinsic factors of the measurement context such as variations in other components different to glucose. Due to these reasons, their real use would require accurate calibration and, most likely, redundant information acquisition. Therefore, the analysis of the possibilities to extract information from different sensing parameters and its implications seem of interest.