# Flexible UHF RFID Tag for Blood Tubes Monitoring
## 1 Introduction
The main drawback of the general-purpose commercial RFID tags is that the antenna impedance is matched to the IC chip impedance without considering the properties of the tagged product. In the case of blood, the product to be tagged is a liquid, which exhibits high dielectric permittivity and loss. As a consequence, this implies a drastic reduction of typical commercial tag reading ranges below 0.5 m [16], due to high power loss caused by absorption, and mismatch between the antenna and the IC chip [8; 17]. Recently, several blood management RFID systems have been proposed [18; 19; 20; 21; 22]. All of them are used in the monitoring of blood bags, except one which is applied to blood tubes. The antennas are mostly of inductive dipole type, although there is an example of a capacitive dipole type in [19] and a traveling wave antenna type in [20]. Two of the antennas are flexible [21; 22], while the other three are not. Among the different antennas proposed, only two of them are for far-field applications [21; 22]. Both are fabricated on flexible substrates and are of inductive dipole type. The first has a square slot configuration and requires a ground plane, while the second one [22] is a meandered dipole with a reflector. Both tag antennas have been designed by considering high permittivity and dielectric loss of blood. Thus, the negative effects of both tag antennas attached to blood bags are minimized and reading ranges above 1 m have been reported. However, their sizes cannot be adapted to a blood sample collection tube. In addition, important research effort has also been conducted to develop tags with good performance when they operate close to different liquids, such as water or ethanol [23; 24; 25]. In [23; 24], inductive designs are proposed for water bottles. The large area offered by the bottle containers allows the design of electrically large antennas to increase radiation efficiency, but incompatible with the size of blood tubes. The tag proposed in [25] has a capacitive design that uses the concept of shunt scaling, to achieve good matching for a range of different liquids. The tag is intended for flat surfaces, especially petri dishes, and it remains unknown the effects that the strong bending required by blood tubes will have in the performance of the tag. In this paper, a novel miniature, low-profile and flexible UHF RFID tag antenna for blood tubes monitoring is presented. The key idea of the design strategy is to consider the presence of the blood, dielectric substrate and tubes during the design of the tag. In this way, the antenna will operate in optimum conditions when it is attached to a blood tube, which is the final intended operational environment. For this purpose, it is very important to characterize the electrical properties of the blood, together with a good understanding on the materials employed in the fabrication of clinic tubes and the tag antenna (substrate). Once these elements are properly characterized, their presence is taken into account during the design of the tag antenna, in a similar way as it was done in previous studies [23; 24; 25]. While most of commercially available tags are based on inductive designs using the T-match technique, the proposed tag antenna consists of two compact coupled capacitive meandered structures. Due to the high relative permittivity value of the blood, inductive designs tend to have smaller sizes, leading to extra drop in the efficiency of the antenna. On the contrary, the proposed capacitive design exhibits reasonable size for the tube, and it represents a good compromise between compactness and radiation efficiency. In addition, the capacitive tag also presents higher sensitivity to the presence of blood. Thanks to this behavior, the reading range is increased when the tag antenna is attached to a clinic tube full of blood, while the reading range drastically drops when the tube is empty. This property may also be an advantage for certain traceability systems, in order to reduce non-relevant information relative to empty tubes.