David Bull

Converting a Wireless Biotelemetry System to an Implantable System Through Antenna Redesign

The two major challenges associated with the conversion of a wireless system operating in air to an implantable version, antenna detuning and biocompatibility, are addressed in a coherent way. An RF identification (RFID)-based biomedical telemetry system designed for free-space operation was chosen as the starting reference. A new pin-compatible space-saving antenna with a ground plane was designed, fabricated, and tested to replace the original “free-space” antenna in the active RFID tag without making any other changes to the tag circuit, such that the tag would function well when it is placed under rat skin and fat. Biocompatibility and potential antenna detuning due to rat tissue variations were addressed in the design process, without significantly increasing the tag physical height, by applying a thin coating of biocompatible material directly over the antenna. The operation of the medical telemetry system was successfully demonstrated, with the tag placed under rat skin and fat, and its range of 60-72 cm was found to be sufficient to support medical research experiments conducted with rats in cages. Due to the biocompatible coating over the antenna, antenna matching is very insensitive to changes in tissue dielectric constants and thickness. The footprint of the new antenna is 33% less than that of the original antenna, its measured 10-dB return-loss bandwidth is 100 MHz or 11%, and overall efficiency is 0.82% at 920 MHz.

Design of an implantable antenna to acquire physiological signals in rats

A miniaturized partially-folded planar inverted F-antenna (PF-PIFA) is proposed for a wireless physiological data acquisition system, being developed for small rodents including rats. The system operates in the Australian 900MHz ISM band (915-928 MHz). The overall size of the antenna is 26×19×4.4mm3 and it has been designed to be implanted just under the skin of a rat. The 10dB return-loss bandwidth of the antenna is 2.8%, which is sufficient for this telemetry system. The RF link distance between the implanted and external antennas is assumed to be less than 2m.

A miniaturized implantable PIFA antenna for indoor wireless telemetry

An implantable, compact, planar inverted F-antenna (PIFA), designed for a wireless telemetry system operating in the Australian 900MHz ISM band (915-928 MHz), is described in this paper. The antenna has been designed to operate well when it is positioned inside the body of a rat. The overall dimensions of the antenna are 12×12×4mm3. The predicted bandwidth of the antenna is found to be 8.5% (from 880 MHz to 960 MHz) at 10dB return loss. The sensitivity of antenna impedance matching to variations in the rats body material is investigated. It is found that the antenna should operate well in the ISM band even when the body tissue parameters change within a significant range.

Bandwidth enhancement of an implantable RFID tag antenna at 900 MHz ISM band for RF telemetry

In this paper, we present a compact PIFA antenna (11 mm×13 mm×5.6 mm) operating at 900 MHz Australian ISM band, custom designed for implantation under the skin of a rat. The antenna bandwidth is enhanced by partially connecting the antenna ground plane with the RFID circuit ground plane, leaving an optimized slot between them. The antenna operating environment is modeled and included when the antenna is simulated and optimized. Our results show that directivity of the antenna also improves with the ground connection. The purpose of this research is to modify an RFID-based telemetry device, which operates well in free space, for implantation into the body of an experimental rat.

Implantable compact antennas for wireless bio-telemetry: A comparative study

This paper compares the performance of two compact implantable planar inverted-F antennas (I-PIFAs) designed to operate in the UHF ISM band around 900MHz. The total dimensions of Antenna 1 (Hilbert I-PIFA) and Antenna 2 (planar strip I-PIFA) are 25×33×7.6mm3 and 13.5× 11× 4.25mm3, respectively. A trade-off between the size, bandwidth and radiation efficiency is demonstrated. The larger antenna has a wider bandwidth as expected but both antennas operate well over the Australian ISM band. The radiation efficiencies of the two antennas, when implanted inside a rat, are 1.3% and 0.38%, respectively.

Making a telemetry system implantable: Challenges and opportunities in antenna design

Design of a proximity coupled implantable planar inverted F-antenna (I-PIFA) is presented. Its to be operated in the Australian 900MHz ISM band (915-928MHz). This antenna, integrated to an existing Radio Frequency Identification (RFID) tag, will be implanted under the skin of rats for medical research. The main purpose of this wireless telemetry system is to transmit the extracted physiological signals and indicators from the body of the rat to an external RFID reader connected to a computer. The total volume of the antenna is 9.25mm×8mm×3.2mm and the larger ground plane of the RFID circuit board is utilized to improve the performance of the antenna. A skin-fat-muscle planar tissue model has been developed to represent the immediate environment around the antenna, which has been coated with a bio-compatible material. The simulations have shown that the effect of rat skin and the bio-compatible coating on antenna matching is significant. Yet with the conservative antenna design proposed here that has a nominal 10dB return loss bandwidth of 15%, the antenna remain matched over the entire ISM band even if the dielectric constants of the rat skin and bio-compatible coating change over a wide range.

An implantable Hilbert PIFA antenna for RFID based telemetry

In this paper, a Hilbert planar inverted F-antenna (H-PIFA) is presented that has been designed to be functional in the Australian 900MHz ISM band (915-928 MHz). The antenna is intended to be attached to an RFID PCB board which is further implanted under the skin of laboratory rats. The overall dimensions of the antenna including the ground plane are 25×25×7.6 mm3. Although the bandwidth of the antenna is only ~3%, the total efficiency is around 60% which is significant for implantable antennas. Return loss, impedance bandwidth and radiation patterns have been investigated and found being useful for a short range (2~3m) RFID telemetry link.

Effects of rat skin on the resonance frequency: An experiment with a commercial antenna for an implanted telemetry system

An implantable wireless data transmission system, under development, requires the implanted antenna to be located under the skin of a rat. Here we investigate variations in the resonance frequency of a commercial antenna (ANT403 SP1, Antenna Factor ™) with and without covering by rat skin. Frequency shifting due to ageing effects of the rat skin is also measured in this experiment. It is found that 15%–20% decrease in the resonance frequency may occur when the antenna is covered by rat skin, which has an average thickness at around 2mm. However, only slight frequency variations were observed due to the ageing effect of rat skin. Fresh rat skin samples were collected and stored in salt solution before, and after, measurements. A comparative study of frequency shifting between micro-strip (MS) feed and co-planar waveguide (CPW) feed of the antenna is highlighted in this paper.

Antenna design and placement options for an implantable wireless medical telemetry system

The main aim of our research is to convert an existing ultra-high frequency (UHF) telemetry system, operating in free space, to an implantable telemetry system. This system, based on active radio-frequency identification (RFID) technology, will be initially implanted in rats for transmission of physiological signals to a monitoring station outside the cage. Designing an implantable compact antenna to suit the space available in the active RFID tag is the most important step in this conversion. In this paper, two placements options are considered - on the ground-plane side and on the component-side of the tag. Novel implantable planar inverted-F antennas (PIFAs) with biocompatible material coatings were designed for each case. The ground plane of the antenna, together with the tag circuit ground, helped to reduce radiation into the rats body. A rat tissue model is considered and the critical design parameters such as biocompatible coating permittivity, that have a profound effect on antenna performance, are identified. The input reflection coefficient, radiation pattern and efficiency are presented.