Toni Björninen

Inkjet-Printed Humidity Sensor for Passive UHF RFID Systems

This paper presents a novel inkjet-printed humidity sensor tag for passive radio-frequency identification (RFID) systems operating at ultrahigh frequencies (UHFs). During recent years, various humidity sensors have been developed by researchers around the world for HF and UHF RFID systems. However, to our best knowledge, the humidity sensor presented in this paper is one of the first passive UHF RFID humidity sensor tags fabricated using inkjet technology. This paper describes the structure and operation principle of the sensor tag as well as discusses the method of performing humidity measurements in practice. Furthermore, measurement results are presented, which include air humidity-sensitivity characterization and tag identification performance measurements.

A Minimally Invasive 64-Channel Wireless μECoG Implant

Emerging applications in brain-machine interface systems require high-resolution, chronic multisite cortical recordings, which cannot be obtained with existing technologies due to high power consumption, high invasiveness, or inability to transmit data wirelessly. In this paper, we describe a microsystem based on electrocorticography (ECoG) that overcomes these difficulties, enabling chronic recording and wireless transmission of neural signals from the surface of the cerebral cortex. The device is comprised of a highly flexible, high-density, polymer-based 64-channel electrode array and a flexible antenna, bonded to 2.4 mm × 2.4 mm CMOS integrated circuit (IC) that performs 64-channel acquisition, wireless power and data transmission. The IC digitizes the signal from each electrode at 1 kS/s with 1.2 μV input referred noise, and transmits the serialized data using a 1 Mb/s backscattering modulator. A dual-mode power-receiving rectifier reduces data-dependent supply ripple, enabling the integration of small decoupling capacitors on chip and eliminating the need for external components. Design techniques in the wireless and baseband circuits result in over 16× reduction in die area with a simultaneous 3× improvement in power efficiency over the state of the art. The IC consumes 225 μW and can be powered by an external reader transmitting 12 mW at 300 MHz, which is over 3× lower than IEEE and FCC regulations.

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.

Embedded wireless strain sensors based on printed RFID tag

Purpose – The purpose of this paper is to develop a wireless strain sensor for measuring large strains. The sensor is based on passive ultra high‐frequency radio frequency identification (RFID) technology and it can be embedded into a variety of structures.Design/methodology/approach – Silver ink conductors and RFID tags were printed by the screen printing method on stretchable polyvinyl chloride and fabric substrates. The development of the strain‐sensitive RFID tag was based on the behavior of the selected antenna and substrate materials. Performance of the tags and the effect of mechanical strain on tag functioning were examined.Findings – The results showed that large displacements can be successfully measured wirelessly using a stretchable RFID tag as a strain‐sensitive structure. The behavior of the tag can be modified by selection of the material.Research limitations/implications – New tag designs, which are more sensitive to small levels of strain and which have a linear response will be the subje...

Printed humidity sensor for UHF RFID systems

This paper presents a printable humidity sensor for RFID systems at UHF frequencies. The sensor is printed onto Kapton HN polyimide substrate using silver ink. The sensor measures humidity capacitively through the permittivity change in the polyimide. The actual measurement result is acquired from the tag using a threshold power sweep performed by the reader device. The operation principle is explained and simulation results are shown. Simulation results predict good sensor performance and thus the sensor is ready for the prototype stage.

Analysis of electrically conductive silver ink on stretchable substrates under tensile load

Electrical conductors were printed by the screen printing method on stretchable PVC substrates and on fabrics. Polymer thick film silver ink was used as the conductive medium. The electrical performance and the structure of the ink film were investigated in unloaded conditions and under strain. In addition, the ink film morphology was examined. The goal of this study was to provide information for developing a strain sensor for large strain levels using the materials under investigation. An additional aim was to assist the integration of electronics into other structures. The results showed that strain sensitive structures can be made using the materials selected for this study and these materials provide an opportunity to develop strain sensors. The structures also tolerated large strain levels and thus they can be integrated into other materials which are exposed to strain.

Small and Flexible Metal Mountable Passive UHF RFID Tag on High-Dielectric Polymer-Ceramic Composite Substrate

A small and flexible metal mountable UHF RFID tag antenna, utilizing a high-permittivity substrate material is presented. The tag is composed of a small single-layer T-matched dipole antenna, on a flexible ceramic (BaTiO3) polymer (polydimethylsiloxane) composite substrate, with a thickness of 1.5 mm. The flexibility of the substrate allows it to be mounted on flat and cylindrical metallic surfaces. The performance of the developed tag is evaluated through simulations and measurements on both flat and cylindrical metallic platforms of various sizes. The results show that the tag achieves state-of-the-art size-performance ratio while meeting the key requirements of an affordable RFID tag with a simple and flexible structure.

Effects of Sewing Pattern on the Performance of Embroidered Dipole-Type RFID Tag Antennas

Embroidered tag antennas can be used in on-body applications, such as access control, human monitoring, and sensor tag antennas. The objective of this letter is to investigate the performance of dipole-type tag antennas sewed with different thread densities and with two different sewing patterns. This letter shows that the performance of sewed dipole-type tag antennas improves if the sewing pattern consists of sewed lines along the direction of current flow in the antenna. The sewed simple dipole, which consists of sewed lines along the length of the dipole, achieved up to 7.5 m read range in free space, which is comparable to the performance of the corresponding copper dipole.

SAR reduction and link optimization for mm-size remotely powered wireless implants using segmented loop antennas

This paper describes an approach to reduce the average specific absorption rate (SAR) of a wireless power link for mm-size cortical implants, while decreasing the loss of the overall link by using a segmented loop transmit antenna. It further shows that, for a given receive antenna size and antenna separation, an optimum transmit antenna size-frequency pair that minimizes the link loss exists. A case study of a wireless link for a mm-size cortical implant optimized for minimum loss shows a reduction of the average SAR of more than 30 %, and a link loss improvement of approximately 10 %, leading to a 57 % increase in power available to the implant compared to a conventional loop antenna.

The effects of recurrent stretching on the performance of electro-textile and screen-printed ultra-high-frequency radio-frequency identification tags

Future welfare and healthcare applications require wearable radio-frequency identification (RFID) tags where the tag antenna is an integral part of clothing and endures repeated stretching. In this study, wearable passive ultra-high-frequency (UHF) RFID tag antennas were fabricated from silver-plated stretchable fabric and by screen printing them on non-conductive, stretchable fabric. The reliability of the tags was studied by stretching them repeatedly from the initial length of 10 cm to 13.5 cm, up to 200 stretching cycles. According to our results, the electro-textile tags achieved read ranges of 6.5 meters, also after the 200 harsh stretches. The screen-printed tags initially achieved read ranges of 9.5 meters and after the 200 stretches the read ranges were only 2.5 meters shorter, that is, still about 7 meters. These measurement results and the strengths and weaknesses of both types of wearable tags are discussed in this paper.