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Wearable RFID Tags

Antennas must be lightweight, low-cost and powerful enough to support sensors in health-care applications.
By Damith Ranasinghe and Thomas Kaufmann
Nov 26, 2012The miniaturization of sensors and wireless systems is generating explosive growth in applications based on wearable electronic devices. At the Auto-ID Lab at the University of Adelaide, in Australia, we are working with the Queen Elizabeth Hospital and the Hospital Research Foundation to develop wearable sensor-equipped passive RFID tags for use in health-care applications, such as monitoring the daily activities of the elderly in hospitals and at home. Part of our research is aimed at developing lightweight, low-profile, low-cost antennas.

It is a considerable challenge to develop antennas that can be integrated with passive tag circuitry capable of delivering the power required to run the tag logic and sensory circuits, such as accelerometers. In particular, the antennas need to be concealable, flexible and designed for comfort, especially during the wearer's sleep. It's also essential that the electrical connections remain intact when the tags are worn around the clock.

Damith Ranasinghe (left) and Thomas Kaufmann
Another challenge is that most antennas are easily detuned when located on or near a human body, resulting in reduced read range. Thus, the effects of human tissue, such as skin and fat, in close proximity to the antenna must be included in design considerations.

To address these problems, we are taking advantage of recent developments in conductive fabrics. Instead of using metal foil to construct an antenna, we use fabrics made from silver thread and conductive plastics. Generally, eliminating the effects of the human body on antenna performance requires the use of large metal sheets between the body and the antenna (ground plane). Such designs are not desirable for a wearable tag, so we use conductive fabrics to produce a barrier and achieve good performance. This produces a flexible antenna and a garment that can be easily laundered. In addition, we are building the antenna on low-cost, highly flexible substrates, such as foams, which ensures that the overall antenna structure does not cause discomfort to the wearer.

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