# Flexible UHF RFID Tag for Blood Tubes Monitoring
## 5 Application of the Proposed Tag as Volume Sensor  
In this section, a study of the proposed tag as sensor to determine the blood levels in tubes is proposed. This study is a consequence of the variation of the tag power sensitivity previously measured for the proposed tag (Figure 12). In addition, how blood level affects to the reading range is also presented. The aim of this study is to test if the proposed tag exhibits good level sensing capabilities, and if the variation in the blood levels can have impact on the reading range. As for the previous section, the measurements were carried out by means of an anechoic chamber and the Tagformance Voyantic System, and for the smallest clinic blood tube. Figure 15 represents the minimum transmitted power (tag power sensitivity) required to activate the IC chip (NXP UCODE G2 XL) of the proposed tag attached to a clinic tube with different blood volumes. The results have been obtained when the volume of the blood inside the clinic tube varies from 4 mL (full tube) to 0 mL (empty tube). It can be observed that the minimum power required \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Tube/Bag/Liquid** & \
\begin{tabular}{c} **Flexible** \\ **(Yes/Not)** \\ \end{tabular} & **Antenna Type** &
\begin{tabular}{c} **Max. Reading** \\ **Range (m)** \\ \end{tabular} & **2-D Size (mm)** \\ \hline
[18] Figure 3 & Bag & Not &
\begin{tabular}{c} Inductive \\ dipole \\ \end{tabular} & – & \(50\times 50\) \\
[19] Figure 1 & Tube & Not &
\begin{tabular}{c} Capacitive \\ dipole \\ \end{tabular} & 0.04 & \(25\times 25\) \\
[20] Figure 3 & Bag & Not & Traveling wave & 0.015 & \(120\times 50\) \\
[21] Figure 1 & Bag & Yes &
\begin{tabular}{c} Inductive \\ dipole \\ \end{tabular} & 2 & \(90.37\times 61.09\) \\
[22] Figure 2 & Bag & Yes &
\begin{tabular}{c} Inductive \\ dipole \\ \end{tabular} & 1.25 & \(82.4\times 15\) \\
[23] Figure 4 & Liquid (water) & Yes &
\begin{tabular}{c} Inductive \\ dipole \\ \end{tabular} & 2 & \(87.8\times 57.9\) \\
[24] Table 1 & Liquid (water) & Yes &
\begin{tabular}{c} Inductive \\ dipole \\ \end{tabular} & 0.79 & \(86\times 22.5\) \\
[25] Figure 6 & Liquid (water) & Yes & \begin{tabular}{c} Capacitive \\ dipole \\ \end{tabular} & 3.2 & 29 \\ This work & \
\begin{tabular}{c} Tube \\ \end{tabular} & Yes &
\begin{tabular}{c} Capacitive \\ dipole \\ \end{tabular} & 2.2 & \(49.5\times 15\) \\ Figure 9 & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison between the proposed RFID tag and other designs for blood and liquid traceability. to activate the tag shifts to higher frequencies when the volume of the blood is decreased inside the tube. This is a consequence of the capacitive design proposed in this paper, which increases the sensitivity of the tag to the presence of blood. In any case, by comparing the different states shown in Figure 15, it is easy to establish what are the blood levels inside a particular tube. This can simply be done by monitoring the minimum power required to activate the IC chip at the center of frequency \(f_{c}=867\) MHz of the UHF RFID band. This study is completed with the reading range. Figure 16 shows the results of this critical parameter measured with the proposed tag and with the same previous conditions. The results confirm once more that at the center of frequency \(f_{c}=867\) MHz of the band of interest, a maximum reading range is achieved when the clinic tube is full of blood (4 mL). The reading range gradually decreases at the same frequency (\(f_{c}=867\) MHz) when the volume of blood inside the tube decreases. As it can be seen in Figure 16, this is due to the shift of the resonant frequency of the tag to higher values. In addition, the peak values of the reading ranges increase when the volume of blood decreases. This is because lower quantities of blood introduce smaller absorptions in the power radiated by the tag.