# Microwave Planar Resonant Solutions for Glucose Concentration Sensing: A Systematic Review
## 7 Discussion  
The use of planar microwave resonant sensors for dielectric properties characterization has been widely studied during the last decades. Their unique features in terms of ease of integration and cost-effectiveness, as well as frequency range of operation, have drawn the attention of a considerable part of the scientific community willing to develop glucose concentration sensors. The development of the topic has led to numerous publications during the last years, showing a wide variety of designs, contexts of use and results. At this point, a review of the most relevant ones seems convenient. While relying on the same technological principles, different ways of extracting the information from the measurement have been proposed, basing on different sensing parameters. This is why, once the technology and working principle have been narrowed down, this review proposes a classification according to the sensing parameter. This, in fact, can be understood as a subcategorization within more general, larger classification schemes based on technological working principles [48], focused on the specific context of glucose sensors. However, whereas sorting and displaying the approaches according to the sensing parameter can be useful for comparison and discussion within each category, the general, cross-type study becomes intricate in the absence of common performance indicators. To overcome this issue, a general relative sensitivity to glucose \(RS(G)\) has been defined and applied to all the analyzed references. In a general glucose measurement context, where not only physiological measurements are involved, the \(RS(G)\) proposes a standard tool that normalizes the obtained sensitivity regardless of the sensing parameter relative to an also normalized glucose content (1 wt%). It is worthwhile to note that the \(RS(G)\) implicitly implies a penalty for sensors operating at a relatively high value of their sensing parameter, as is usual for any normalizing tool. This can be interesting to account for the extra difficulties in real devices to achieve high resonant frequencies or quality factors, or measuring too low values for the transmission coefficient, for example. In this sense, although convenient for general analysis and comparison, careful study of each attempt in its specific context should always be considered. The sensors measuring a shift in resonant frequency find a wide variety of applications, and their use in the glucose context has been considered in many cases. Their working principle is mainly based on the changes in the real part of the permittivity of the sample. The use of SRR-based configurations has been reported in different cases with good results, especially when two or three of them are considered in the design. Additionally, complimentary electric \(LC\) resonators have shown promising results both in aqueous and biological solutions. These sensors often require high-quality factors for proper functioning, especially for low concentration changes detection, since a sharp resonant peak (or notch) is needed to allow distinction of slight frequency shifts. Active methods can be applied to enhance this aspect and raise the final practical sensitivity. While simple and easy to design, these sensors might require extremely high frequency resolution for low concentrations and wide band interrogation signals for high concentrations detection, which can lead to complex and costly final systems, depending on the application. Insertion/return loss-based sensors rely on similar principles but extract the information from amplitude changes, which are more related to the losses in the sample. Some dual approaches have been reported as able to relate the glucose content changes to both the measured resonant frequency and insertion/return loss, which are of interest from the point of view of robustness. Within this approach, in addition to SRR-based designs, interdigital capacitor elements have been also considered in some cases with promising results.