# Microwave Planar Resonant Solutions for Glucose Concentration Sensing: A Systematic Review
## 4 Insertion/Return Loss-Based Sensors  
The sensors tracking variations in the insertion or return losses are mainly (but not exclusively) based on the variations in the dielectric losses in the sample due to changes in the glucose concentration. Indeed, these losses, represented as \(G_{\rm s}\) in the model of Figure 1, have an impact on the conductance associated with the MUT, thus affecting the finally measured insertion/return losses. This effect is mostly associated with variations in \(\varepsilon_{\rm rs}\)\" linked to changes in the concentration of electrolytes in the sample [121]. These variations in the conductance of the MUT provoke variations in the amplitude of the measured response [133]. In terms of the scattering parameters, considering a simple two-port microstrip resonator, the magnitude return loss can be defined as [134]  
\[\mathrm{mag}(S_{11})=20\log\left|\frac{R_{\rm in}-Z_{0}}{R_{\rm in}+Z_{0}} \right|=20\log\left|\frac{1-\frac{G_{\rm in}}{Y_{0}}}{1+\frac{G_{\rm in}}{Y_{0} }}\right|, \tag{6}\]  
where \(Z_{0}\) and \(Y_{0}\) are the effective impedance and admittance of the microstrip line with no sample, and \(R_{\rm in}\) and \(G_{\rm in}\) are the input resistance and conductance seen from port 1. For the model in Figure 1, the input admittance is:  
\[Y_{\rm in}=G_{\rm in}+jB_{\rm in}=G_{\rm r}+G_{\rm s}+j\left(\omega(C_{\rm r} +C_{\rm s})-\frac{1}{\omega L_{\rm r}}\right), \tag{7}\]where \(\omega\) is the angular frequency and \(B_{\text{in}}\) is the input susceptance. The input conductance in (6) is therefore \(G_{\text{in}}=G_{\text{r}}+G_{\text{s}}\). A similar analysis can be applied for the insertion loss. The influence of the sample in the measured insertion or return loss is therefore evident, although it depends on the configuration. Some works have provided accurate descriptions of the return/insertion loss in terms of the lumped elements associated with the MUT for different configurations, such as a magnetically coupled SRR [135] or interdigital capacitor-based SRR [125]. The relationship between the targeted parameter and the sensing parameter is not so evident in this approach, and many attempts can be found in the literature trying to maximize the sensitivity by means of diverse configurations. The main drawback of these sensors is the sensitivity to noise effects and load mismatches, which can pose a considerable challenge when highly accurate insertion/return loss measurements are required, especially for commercial, portable applications. Notwithstanding, these sensors can require notably narrow band interrogation signals for practical measurements, thereby easing the associated driving and data acquisition electronics. This has been a common sensing approach leading to a sizable number of publications. Some of the \(f_{\text{r}}\)-based attempts have also been considered with these sensing parameters, providing in some cases for interesting dual measurement capabilities [121; 127; 129; 130; 132] (Figure 4a). Again, the SRR is present in many of the proposed designs. Single SRR were considered with different designs for water-glucose solutions [94; 136; 137], aqueous microfluidic [135; 138] and blood plasma [31] measurements. Additionally, the influence of ambient temperature on glucose concentration retrieval and a correction strategy were studied in [135], and additional environmental effect elimination was provided in [118] with a dual SRR approach. Two SRR for differential mode measurements, based on the splitter/combiner microstrip structure [104], were proposed in [92] for aqueous microfluidic measurement of physiological concentrations. An approach based on inter-resonator coupling of two mutually coupled SRR was studied for water-glucose solutions measurement [105]. Proposals combining larger amounts of SRR have shown promising results able to address physiological concentrations in aqueous solutions [120]. Considering other configurations, a coplanar electric _LC_ resonator was proposed for glucose concentration measurement in watery and PBS solutions [139].