Yuehui Ouyang

High Frequency Properties of Electro-Textiles for Wearable Antenna Applications

A systematic study of the high frequency electrical properties of electro-textiles is presented in this paper. First, conductive thread characterization is completed with a waveguide cavity method. The effect of conductive thread density and comparison of several different types of conductive threads are included. Second, comparisons of knitted patterns and weave patterns are made in terms of effective electrical conductivity through a microstrip resonator method. The effect of various weave patterns on conductive and dielectric loss is detailed. Finally, the relevance of the high frequency characterization of the electro-textile materials is shown through electro-textile patch antenna fabrication and measurements. The efficiency of the fully fabric patch antenna is as high as 78% due to the use of low loss electrotextiles characterized in this paper.

Evaluation of Cardiovascular Stents as Antennas for Implantable Wireless Applications

In this study, we explore the use of stents as radiating structures to support transcutaneous wireless telemetry for data transfer of internal measurements from within the circulatory system. The implant location is chosen for the specific application of heart failure detection by monitoring internal pressure measurements of the pulmonary artery. The radiative properties of the single stent are quantified in free space within an anechoic chamber and compared with measurements taken while implanted in a live porcine subject. The in vivo studies of our 2.4-GHz stent-based transmitter, implanted at a depth of 3.5 cm within the chest, showed a 32-35-dB power reduction at a receive distance of 10 cm for both co- and cross-polarizations. The approximate far-field H-plane antenna pattern is quantified at a distance of 50 cm both in free space within an anechoic chamber and while implanted within a porcine chest. These results are used to explore the accuracy of a high-fidelity simulation model developed using Ansofts High Frequency Structural Simulator and components of their Human Body Model to provide a model that is validated with empirical data. This study provides insight into the effects of tissue on high-frequency electromagnetic transcutaneous transmission and develops a high-fidelity model that can be used for further design and optimization.

Effect of fabric patterns on electrotextile patch antennas

Two different electrotextile antennas made of the same e-textile material, but of different structure, are compared. Details of the fabric construction of the electrotextiles and their effect on the efficiency of the e-textile patch antennas are discussed. Finally, one fully-fabric patch antenna is demonstrated for potential WLAN applications. This fabric antenna with measured gain of 6.59 dB and impedance bandwidth of 3.4% is comparable to a standard rigid patch antenna of similar structure and dimension.

Body-Worn Distributed MIMO System

In this paper, we analyze the performance of novel wearable multiple-input-multiple-output (MIMO) systems, which consist of multiple electrotextile wearable antennas distributed at different locations on human clothing. For wearable applications, a semidirectional radiation pattern of the wearable patch antenna is preferred over an omnidirectional radiation of conventional dipole antennas to avoid unnecessary radiation exposure to the human body and radiation losses. Additionally, the spatial distribution of the antennas is not constrained as a typical handheld unit. Through theoretical modeling and simulation, the wearable MIMO system is shown to demonstrate a significantly higher channel capacity than a conventional system on a handheld platform (e.g., a compact dipole array or a single dipole), due to enhanced spatial diversity and antenna pattern diversity. The unique effects of antenna directivity and location on the MIMO system capacity are investigated in terms of antenna correlation and effective gain under different wireless channel models. The advantage of a wearable system over a conventional system was further confirmed by detailed physical modeling through the combination of full-wave electromagnetic and ray-tracing simulations. Finally, complex channel response matrices were measured to characterize the performance of a body-worn MIMO system in comparison with a reference full-size dipole antenna. The 319% improvement in 10% outage capacity for the body-worn system over the reference system made of a full-size dipole antenna is consistent with the 288% improvement projected by theoretical modeling and the average 300% improvement found in the physical simulation of two typical indoor scenarios.

Wireless Powering and the Study of RF Propagation Through Ocular Tissue for Development of Implantable Sensors

This paper evaluates RF powering techniques, and corresponding propagation through tissue, to supply wireless-energy for miniature implantable devices used to monitor physical-conditions in real-time. To improve efficiencies an impulsive powering technique is used with short duty-cycle high instantaneous-power-bursts, which biases the rectifier in its nonlinear regime while maintaining low average input-powers. The RF rectifier consists of a modified two-stage voltage multiplier which produces the necessary turn-on voltage for standard low-power CMOS systems while supplying the required current levels. The rectifier, fabricated on the TI 130 nm CMOS process, measures 215 μm × 265 μm, and is integrated with an antenna to quantify wireless performance of the power transfer. In-vivo studies performed on New Zealand white rabbits demonstrate the ability of implanted CMOS RF rectifiers to produce 1 V across a 27 kΩ load at a distance of 5 cm with a transmit-power of just over 1.5 W. Using a pulsed-powering technique, the circuit generates just under 0.9 V output with an average transmit-power of 300 mW. The effects of implantation on the propagation of RF powering waves are quantified and demonstrated to be surmountable, allowing for the ability to supply a low-power wireless sensor through a miniature rectifier IC.

High frequency transcutaneous transmission using stents configured as a dipole radiator for cardiovascular implantable devices

In this work we explore the use of stents as radiating structures to support transcutaneous wireless telemetry. Stents are well established Food and Drug Administration (FDA) approved structures with a matured surgical delivery technique. Incorporating stents with a miniature implantable sensory device allows for internal monitoring of nearly any location within the cardiovascular system. We assembled an implantable stent-based transmitter by integrating a 2.4 GHz wireless transmitter, battery, and two stents configured as a dipole radiator. The radiative properties of the dipole stents was quantified through free space, ex vivo experiments on excised tissue, and in vivo studies on porcine subjects. The in vivo results from various receive distances (10 cm to 1 m) showed a 33–35 dB power reduction while implanted at a 3.5 cm depth within the chest. This validates the ability of using stents to wirelessly transmit data from deep within a living body.

Diversity characterization of body-worn textile antenna system at 2.4 GHz

A body-worn textile antenna system which consists of four electrotextile patch antennas is demonstrated in this paper. A series of experiments to characterize the diversity performance of the antenna for indoor multipath environments have been conducted. From these experiments, we show that the proposed wearable antenna configuration delivers an 8-12 dB improvement in signal level in fading that occur 10% of the time, compared to a single antenna in the same environments

Measurement of electrotextiles for high frequency applications

Two methods to measure effective surface resistance and conductivity of different classifications of electrotextile are presented. A waveguide technique is applicable for materials that are metal plated after forming the textile. The measurement data of two different conductive fibers is provided. Alternately, a resonant transmission line is useful for weave patterns consist of conductive and insulating materials. Detailed discussion of woven fabric is presented, specifically the isolation of dielectric and conductive loss. Measured and simulated results for conductivity are presented, along with techniques to separate different aspects of textile material.

3D packaging technique on liquid crystal polymer (LCP) for miniature wireless biomedical sensor

This paper presents a novel 3D packaging technique for a miniature wireless biomedical sensor. The end goal of this sensor is for implantation into the eye of a mouse, therefore size is extremely limited. The application makes the packaging of the sensor and control of IC a difficult challenge. The thickness of the unit must be less than 300 microns total. It is demonstrated in this paper that the thickness requirement can be met using novel epoxy interconnects and that micro-vias can be implemented in the package to distribute signals vertically to limit the eventual area of the device. First, the layer-to-layer interconnection between silicon and liquid crystal polymer (LCP) layers is demonstrated using a magnetically aligned Z-axis anisotropic conductive adhesive (ACA). The total thickness of the IC and the packaging layer is less than 150 microns. The resistance through Z-axis ACA represented 1.15 Ω on average for 75 micron pads. Second, 3D transitions through LCP via holes of 20 µm are demonstrated, which are suitable to distribute signals through the small form factor unit. In this paper, we demonstrate transition from an antenna layer through the LCP to a layer above where a rectifier resides. RF power received by a loop antenna on the bottom LCP layer is transferred to the rectifier on the top layer and generates 5 volts of DC voltage. These miniature 3D packaging techniques could make it possible to integrate all components in the small area (500 µm × 500 µm) to implement an implanted wireless biomedical micro-sensor.

Distributed Body-worn Transceiver System with the Use of Electro-textile Antennas

In this paper, we propose a distributed body-worn transceiver system for mobile communication, which integrates highly integrated transceivers and electro-textile antennas. The system benefits such as channel capacity were evaluated through the use of wireless sensor network and electro-textile patch antennas. Significant increase in channel capacity was found for the body-worn diversity system as compared to the single traditional small form factor antenna on wireless handheld device, such as the whip antenna. There are two major features causing the capacity increase for the proposed system: 1) the high efficiency of the electro-textile antennas that can be seamlessly embedded into human clothing; 2) significant system diversity gain due to the wide spacing among antennas in the distributed body-worn system. We demonstrate a practical integrated design of a button-size LTCC transceiver package directly interfaced to a wearable wide band stacked patch antenna. The electro-textile antenna with LTCC feed circuit was fabricated and measured to prove the integration feasibility.