# Microwave Planar Resonant Solutions for Glucose Concentration Sensing: A Systematic Review
## 7 Discussion
These sensors can achieve high \(RS(G)\) values, often greater than those from\(f_{\text{r}}\)-based sensors, both for aqueous and biological solutions. Additionally, this sensing approach may require narrow interrogation signals (which could be even reduced to a single tone, depending on the case), thus easing the final implementation. However, this sensing parameter is especially sensitive to noise, electronic calibration and load mismatch effects, which can hinder the detection process when high sensitivities and resolutions are targeted. The unloaded quality factor, which finds application in other measurement contexts, has also been proposed for glucose measurements, and some \(Q_{\rm u}\)-based approaches have been studied in recent years. Indeed, according to (8) and (9), this attempt combines the parameters considered in the previous two ones in addition to the measured bandwidth (which was also proved to be convenient for glucose measurements [90]), searching for a more robust estimation of the glucose level. It is therefore interesting to study the individual influence of \(f_{\rm r}\), \(\Delta f_{\rm 3 dB}\) and \(S_{21 max}\) in the finally obtained \(Q_{\rm u}\). Let us consider, for simplicity, the maximum amplitude of the transmission parameter in linear scale: \(S_{21 max,\rm lin}=10^{521 max/20}\). Then, applying (8) into (9) and solving for the partial derivatives yields  
\[\frac{\partial Q_{\rm u}}{\partial f_{\rm r}}=\frac{1}{\Delta f_{\rm 3 dB}(1 -S_{21 max,\rm lin})}, \tag{16}\]  
\[\frac{\partial Q_{\rm u}}{\partial\Delta f_{\rm 3 dB}}=\frac{-Q_{\rm u}}{\Delta f _{\rm 3 dB}}, \tag{17}\]  
\[\frac{\partial Q_{\rm u}}{\partial S_{21 max,\rm lin}}=\frac{Q_{\rm u}}{(1-S_{ 21 max,\rm lin})}. \tag{18}\]  
This means that, for a sensor with a given value of \(Q_{\rm u}\), the variations in this parameter will be more sensitive to changes in: \(\Delta f_{\rm 3 dB}\) for narrower \(\Delta f_{\rm 3 dB}\) (17); \(S_{21 max,\rm lin}\) for greater \(S_{21 max,\rm lin}\) (18), i.e., greater amplitude levels; and \(f_{\rm r}\) for a proper combination of the prior cases (16). In other words, the designs relying on this sensing approach can benefit from the information contained in \(f_{\rm r}\), \(\Delta f_{\rm 3 dB}\) and \(S_{21 max}\), and the influence of each parameter can be adjusted from the design, thus adapting to different sensing scenarios. This approach has also been applied with SRR-based designs for measurements with watery and biological solutions, although other proposals have been made considering open-loop line resonators and coplanar designs. The attained \(RS(G)\) values show good comparative results with the rest of approaches, and a considerable sensitivity raise was reported recently for coupled SRR techniques, as it has also been shown for some insertion loss-based attempts. Despite that, no measurements of physiological glucose concentrations have been hitherto reported with this approach. These sensors may require moderately narrow interrogation signals, especially for low \(f_{\rm r}\) variations, although it depends on each design and application. They are also expected to provide more robust measurements. The measurements with this approach are partially related to the dielectric losses in the sample. The main interest of this sensing parameter is that it allows us to take into account changes in both the real and imaginary parts of the permittivity of the sample. Conversely, this approach requires the use of proper RF detection techniques, which can become complex in some cases, and further postprocessing of the measured signal to obtain the desired information. Regarding the phase-based sensors, a few attempts have been reported in the literature. The \(RS(G)\) concept seems not to be directly applicable to this sensing approach with the available data in the literature, and therefore solely the raw sensitivities have been included in this review. Only water-glucose solutions have been considered with these sensors, most of them addressing physiological concentrations.