Karoliina Koski

Miniature implantable and wearable on-body antennas: towards the new era of wireless body-centric systems [antenna applications corner]

Wireless body-centric sensing systems have an important role in the fields of biomedicine, personal healthcare, safety, and security. Body-centric radio-frequency identification (RFID) technology provides a wireless and maintenance-free communication link between the human body and the surroundings through wearable and implanted antennas. This enables real-time monitoring of human vital signs everywhere. Seamlessly integrated wearable and implanted miniaturized antennas thus have the potential to revolutionize the everyday life of people, and to contribute to independent living. Low-cost and low-power system solutions will make widespread use of such technology become reality. The primary target applications for this research are body-centric sensing systems and the relatively new interdisciplinary field of wireless brain-machine interface (BMI) systems. Providing a direct wireless pathway between the brain and an external device, a wireless brain-machine interface holds an enormous potential for helping people suffering from severely disabling neurological conditions to communicate and manage their everyday life more independently. In this paper, we discuss RFID-inspired wireless brain-machine interface systems. We demonstrate that mm-size loop implanted antennas are capable of efficiently coupling to an external transmitting loop antenna through an inductive link. In addition, we focus on wearable antennas based on electrically conductive textiles and threads, and present design guidelines for their use as wearable-antenna conductive elements. Overall, our results constitute an important milestone in the development of wireless brain-machine interface systems, and a new era of wireless body-centric systems.

Design and Implementation of Electro-Textile Ground Planes for Wearable UHF RFID Patch Tag Antennas

Localization, wireless monitoring, and emerging body-centric wireless systems demand low-cost and low-power devices that are efficient, maintenance-free, and comfortable to wear. Patch-type UHF radio frequency identification (RFID) tag antennas realized using electro-textiles are promising candidates for this purpose. In this letter, we design a patch-type tag antenna operating at 900 MHz and investigate how different types of ground planes using electro-textiles affect the antenna performance. Various conductive fabrics and embroidery structures are considered. We demonstrate that depending on the ground plane structure and density, it is possible to influence the tag impedance behavior and radiation characteristics. Furthermore, wireless reflectometry measurements are conducted to characterize the sheet resistance for the investigated electro-textiles. We then use the sheet resistance in modeling the conductive fabric ground planes in a full-wave electromagnetic solver. Our results contribute to deeper understanding of the complex electro-textiles structures and guidelines for future practical wearable antenna designs.

Fundamental Characteristics of Electro-Textiles in Wearable UHF RFID Patch Antennas for Body-Centric Sensing Systems

Electro-textiles feature excellent radio frequency (RF) performance. They have an enormous potential to be widely accepted in body-centric sensing systems as wearable antenna materials. However, the RF community lacks comprehensive knowledge of electro-textile fundamental characteristics. This paper provides critical electro-textile design parameters for wearable radio frequency identification (RFID) ultra-high-frequency (UHF) patch antennas. For the first time, modeling parameters for embroidered antenna ground plane and patch structures are established and verified. The results highlight a new unprecedented fundamental characteristic of embroidered antennas: all embroidered structures, modeled as uniform and infinitely thin textile layer, have an existing imaginary part in their sheet impedance modeling parameter. It accounts for the antenna input inductance contributed from the electro-textile. Ideally, such as for highly conductive fabrics and pure conductors, the imaginary part is negligible. Utilizing the proposed design guidelines, a fully wearable and flexible embroidered RFID patch antenna is for the first time realized. Its performance may be considered as a benchmark for future designs. The design parameters and findings form an important milestone in the development towards wearable intelligence and foster industrial developments of garment-integrated RFID antennas.

Fabrication of embroidered UHF RFID tags

A laboratory-fabrication of embroidered antennas for ultra high frequency (UHF) radio frequency identification (RFID) tags is presented. The embroidered antenna patterns are formed from conductive thread using software controlled embroidery machine. Measurements of fully assembled embroidered UHF RFID tag verify that competitive tag performance can be achieved with the presented fabrication method.

WLAN and RFID Propagation channels for hybrid indoor positioning

Indoor localization based on Received Signal Strengths (RSS) or on some form of power measurements is a low-cost and low-complexity solution gaining more and more interest in the research and commercial worlds. Typically, Wireless Local Area Network (WLAN) signals are employed for such purpose, due to the fact that they are widely spread in indoor environments. Nevertheless, any wireless signal available in indoor scenarios can be used for positioning based on similar power measurement approaches. One example is the radio frequency identification (RFID), which enable portable localization systems in the form of wearable RFID tags. Such RFID-enabled systems are highly demanded for health-state monitoring, object tracking, and security. These applications are mostly indoor applications, and thus, we can envision a near future where multiple RFID and WLAN signals will co-exist on multiple frequency bands. The existing signal diversity can offer a benefit in indoor positioning, providing that the signal propagation effects for both WLAN and RFID are well understood and taken into account. However, measurement-based studies on indoor channel modeling, including path loss and shadowing effects of RFID signals are still missing. A comparison between RFID and WLAN channel models for positioning purpose has yet to be made. It is the purpose of our paper to address the path-loss channel models and shadowing effects for WLAN and RFID signals based on extensive measurement campaigns in an office environment.

Design and realization of stretchable sewn chipless RFID tags and sensors for wearable applications

This paper presents the design of a sewed chipless RFID tag and sensor, on a fabric for wearable applications. The proposed design is based on three sewn scatterers on cotton textile. The tag is realized using a computer-aided sewing machine and electro-thread plated with silver. The simulation and frequency-domain measurement results validate the design from 3 to 6 GHz. The tags static backscattered response can be identified in free space and on the human body. Some preliminary results from a sewn stretchable sensor are also given to demonstrate the potential for biomedical applications. Finally, we discuss the main challenges concerning the practical implementation of this technology.

Embroidered RFID tags in body-centric communication

Wearable Radio Frequency Identification (RFID) tags for body-centric communication can be used in security, healthcare and biomedical applications. Embroidered tags using conductive threads are strong candidates for the implementation of wearable antennas. They provide several important features, such as flexibility, integrability, and light-weight structures. In this paper, the on-body performance of embroidered dipole-type ultra-high frequency (UHF) RFID tags is studied. First, modeling techniques for the embroidered tag antennas and for the human body are presented. The simulation models are then used to design and optimize an embroidered dipole tag to provide a read range of 2.5 m when separated from the human arm with a 1-mm thick layer of cotton fabric.

An Embroidered Two-Dimensional Chipless Strain Sensor for Wireless Structural Deformation Monitoring

A wireless chipless sensor for structural deformation monitoring is proposed. The sensor is based on two resonant scatterers sewn on stretchable fabrics with conductive threads. The combination of the two scatterers allows for the detection of the strain in a 2-D plane. The backscattered response, as a function of frequency, is detected with the help of frequency-stepped continuous wave radar technique. The radar performs the detection by scanning the two orthogonal polarizations. It operates in the ISM band at 2.45 GHz. The proposed processing method achieves the 2-D extraction of the strain and the tensile force with the electromagnetic response of the sensor. Simulation and measurement results validate this new concept.

Practical read range evaluation of wearable embroidered UHF RFID tag

In this study, the on-body read range performance of a wearable ultra-high frequency (UHF) radio frequency identification (RFID) tag is evaluated. The tag is fabricated using conductive thread and an embroidery machine. A measurement scenario is described, in which such an embroidered tag is attached to human clothing. The measurement results demonstrate the practical read ranges of the tag utilizing the new concept of using conductive threads. A maximum read range of 1.30 meters is attained.

Electro-textile UHF RFID patch antennas for positioning and localization applications

Passive ultra-high frequency (UHF) radio frequency identification (RFID) provides an attractive technology for indoor positioning and localization. It offers contactless communication, non line-of-sight readability, compactness, and low cost. Localization techniques and algorithms have been actively researched recently from accuracy point of view. However, less attention has been paid to the antenna and its effects on the achieved localization accuracy. Particularly, wearable antennas in human localization applications are key components affecting the achieved accuracy and reliability. In this paper, we elaborate on the material choice for wearable UHF RFID patch antennas. Two highly attractive and flexible conductive electro-textiles are discussed and compared: anisotropic embroidered textile structure and uniform commercially available copper fabric. A patch tag antenna is designed for each electro-textile to operate at 866 MHz. The on-body readability and Received Signal Strength (RSS) in office environment for the antennas are recorded and analyzed. Based on the results we decide on the antenna feasibility in localization applications.