# Flexible UHF RFID Tag for Blood Tubes Monitoring
## 1 Introduction  
In recent years, radio frequency identification (RFID) technology is rapidly developing and is becoming very important for monitoring and tracking applications [1; 2; 3]. Usual applications with this technology include security identification, retail item management, inventory, access control, and tracking. Thus, RFID technology can be used in the healthcare sector and, more specially, within medical centers for the management of blood products [4; 5]. It may have some benefits with regards to optical technologies, such as quick response codes (QR) and barcodes [6], since it is possible to increase the automation level of stock, identification, tracking and monitoring tasks and, therefore, reduce possible human intervention errors. Passive tags working at the ultra-high frequency (UHF) European band (865 MHz-868 MHz) can be used for this type of applications. A passive RFID system is based on a reader or interrogator and a tag or transponder [7]. An RFID tag is a passive device, which can be attached to or introduced within a product. It uses inductive coupling (near-field) or electromagnetic radiation (far-field) to communicate with the reader. A passive RFID tag is based on an antenna connected to an integrated circuit (IC), which includes specificidentification data about the product. No internal power source is required by a passive tag, since it is powered by the rectified RF signal sent by the reader. The tag antenna is an important element in an RFID system. It represents the largest part of the tag. Depending on the application, the tag may be single purpose or multi-purpose, fragile or robust, flexible or rigid, and may have a small or large size. Its performance depends on its size and the composition of the tagged product, which can be a powdery, solid or liquid material. Large antennas, in general, provide better directivities and gains than small antennas [8], while the properties of the tagged product have a negative influence on the reading range when permittivity and dielectric loss are high [9; 10]. In medical centers, human blood is collected in bags or tubes [11; 12]. Bags are produced from flexible PVC plastics and are used to store a large volume of blood, while tubes are made from glass or polyethylene terephthalate (PET) rigid containers, which are filled with a small sample of blood. For a suitable blood monitoring, it is better to handle a flexible tag than a rigid tag. In this way, it can easily be attached and molded to the shapes of blood bags or tubes. The tag should be restricted to have small size to achieve a versatile label that can be used in real health applications. The location of the tag with respect to the reader should also be considered in practical designs. A far-field RFID solution can cover a wide area (0-5 m) [13], while a near-field RFID solution works within a very close distance (10-20 mm) [1]. In addition, there are other RFID alternatives like the vicinity cards [14], which can reach intermediate distances (above 1 m). The UHF-RFID system works in the far field region. Therefore, an UHF RFID with far-field tags and reader antennas, that provides long reading distances, is the preferred option to carry out stock, automatic identification, tracking and monitoring tasks of many blood containers within medical centers. However, vicinity cards, with their intermediate reading distances may also be attractive for these applications. Further research is needed for this system, especially to adapt vicinity tags to the space available in blood tubes, to assess the effects of tube curvature and presence of blood in the overall performance of the system, and issues related to blood absorption in the frequency of operation of this standard. In general, passive far-field UHF RFID tags have a capacitive IC chip impedance, while their dipole antennas, usually designed in printed technology, present an inductive impedance in order to facilitate the impedance matching and reduced sizes [15].