# Flexible UHF RFID Tag for Blood Tubes Monitoring
## 2 Electromagnetic Characterization of the Materials Involved in the Design of the Tag Antenna  
Following the strategy proposed in this work, the design of the UHF tag antenna will be carried out considering the presence of the flexible substrate, the clinic tube, and the blood. Therefore, it is important to extract the electrical properties (permittivity, loss tangent) of these elements, before starting with the design tasks of the tag. The main techniques to characterize dielectric materials are based on free space, transmission lines, coaxial probe, parallel plate, and resonators [27; 28; 29; 30]. One of the most popular technique to measure the complex dielectric permittivity of solid and liquid materials is the coaxial probe method [31; 32]. This electromagnetic characterization technique is non-destructive and allows to carry out fast broadband measurements of biological substances [33; 34]. The complex permittivity of the tag antenna substrate, clinic tube and blood were measured between 0.5 GHz and 2 GHz, by employing the coaxial probe method developed in [35] and a vector network analyzer (Agilent Technologies E5070B). The results were obtained at room temperature. For the tag antenna substrate, a flexible DuPont(tm) Kapton polyimide [26] was used, while for the blood storage container, a polyethylene terephthalate (PET) tube [12] was employed. The blood sample was prepared from animal blood powder to achieve a real relative permittivity (\(\varepsilon\) '\({}_{r}\)) similar to human blood, which ranges from 58 to 62 in the measured frequency band. Figure 1 presents the measured results of the materials involved in the tag design. The real relative permittivity obtained for the Kapton polyimide in the frequency range from 0.5 GHz to 2 GHz is presented in Figure 1a. Results show a constant real part (\(\varepsilon\) ') of the relative permittivity around the value \(\varepsilon\) '\(r\) = 3.2, which is in good agreement with the typical manufacturer data \(\varepsilon\) '\(r\) = 3.1 [26]. The imaginary part of the relative permittivity of this material is not given, because it has a very small magnitude (tan\(\delta\) = 0.0015) and the electromagnetic characterization technique using the coaxial probe  
does not allow to measure it accurately. This magnitude can be neglected during the design of the tag antenna, since dielectric loss and power dissipation in the polyimide substrate will be very small. Figure 1b,c represents, respectively, the real part (\(\varepsilon\) '\(r\)) and loss factor (tan\(\delta\)) of the dielectric complex permittivity for the PET material. This material is used to make blood sample collection tubes. As it can be seen in Figure 1b the real part of the complex permittivity is constant in the frequency range 0.5 GHz to 2 GHz. The measured value is \(\varepsilon\) '\(r\) = 3.1. Its value is very similar to that of Kapton polyimide. However, the dielectric loss tangent (tan\(\delta\) = 0.016) depicted in Figure 1c is an order of magnitude higher (tan\(\delta\) = 0.0015 for Kapton polyimide). Both values (\(\varepsilon\) '\(r\) = 3.1, tan\(\delta\) = 0.016) match  
Figure 1: Measured electrical parameters of the materials involved in the tag design: (**a**) real part of the relative permittivity for Kapton polyimide; (**b**) real part of the relative permittivity for polyethylene terephthalate (PET) tube; (**c**) dielectric loss tangent for PET tube; and (**d**) real part of the relative permittivity for blood; (**e**) conductivity for blood. with data (\(\varepsilon\) ', \(r\) = 3.2, tan\(\delta\) = 0.016 at 1 GHz) reported in [36]. Therefore, the measured values will be used during the design of the tag antenna. Figure 1d,e shows the real relative permittivity (\(\varepsilon\) ', \(r\)) and conductivity (\(\sigma\)) of the blood, respectively. In the frequency range 0.5 GHz to 2 GHz, the value of the real relative permittivity (\(\varepsilon\) ', \(r\)) decreases from 62 to 60, while the conductivity (\(\sigma\)) value increases from 1 S/m to 1.65 S/m.