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Research Paper

Radio Frequency Identification as a Testbed for Integration of Low Frequency Radio Frequency Sensors Into Orthopedic Implants

[+] Author and Article Information
Mary Beth M. Grabowsky

Department of Biomedical Engineering,
Rensselaer Polytechnic Institute,
110 8th Street, Troy, NY 12180

Richard L. Uhl

Division of Orthopaedic Surgery,
Albany Medical College,
47 New Scotland Avenue,
Albany, NY 12208

Mona M. Hella

Department of Electrical, Computer,
and Systems Engineering,
Rensselaer Polytechnic Institute,
110 8th Street, Troy, NY 12180

Eric H. Ledet

Department of Biomedical Engineering,
Rensselaer Polytechnic Institute,
110 8th Street, Troy, NY 12180
e-mail: ledete@rpi.edu

In the United States, RF communications in medical devices are generally limited to frequencies in the Industrial Scientific and Medical (ISM) bandwidths, which are defined by the Federal Communications Commission (FCC). For communications with implantable sensors, the range is effectively limited to 130 kHz (±5 kHz), 6.78 MHz (±0.015 MHz), 13.56 MHz (±0.007 MHz), 40.68 MHz (±0.02 MHz), and 915 MHz (±13 MHz).

1Corresponding author.

Manuscript received March 12, 2012; final manuscript received January 8, 2013; published online February 13, 2013. Assoc. Editor: Ming-Yih Lee.

J. Med. Devices 7(1), 011008 (Feb 13, 2013) (8 pages) Paper No: MED-12-1036; doi: 10.1115/1.4023499 History: Received March 12, 2012; Revised January 08, 2013

Recent advances in radio frequency (RF) sensor systems provide new opportunities to wirelessly collect data from inside the body. “Smart implants” instrumented with sensors have been used as research tools for decades, but only recently have implantable sensors become small enough and robust enough to be used in daily clinical practice. In orthopedic surgery, implants provide a vehicle onto which small RF sensors can be mounted to gather data for diagnostics. However, the sensors must function in a challenging environment which requires long term functionality under demanding physical and mechanical conditions. The purpose of this study was to parametrically test low frequency RF systems under simulated in vivo conditions to determine feasibility of sensor integration into orthopedic applications. Three low frequency RF systems were tested in several clinically relevant scenarios in vitro to characterize (1) strategies for maximizing communication range, (2) physical robustness, and (3) mechanical performance. Systems were tested in air, saline, soft tissue, bone, and in proximity to metal. Hermeticity was assessed during a 208 week period. Effects of γ-irradiation and repeated steam sterilized were measured. Strain at failure was measured by mechanical testing of various packaging configurations. All systems were capable of greater than 20 cm read range under ideal conditions. Saline, soft tissue, and bone had minimal effect on signal transmission, but read range was sensitive to the proximity of stainless steel. The electronics were tolerant of steam sterilization but not of γ-irradiation. Polymer encapsulation is robust enough for many orthopedic applications, but ceramic encapsulated sensors need to be optimized for weight-bearing applications to avoid brittle failure. Although sensor packaging remains a challenge, the technology exists to incorporate passive wireless implantable sensors into orthopedic daily practice.

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References

Figures

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Fig. 1

The tags evaluated include (a) 128 kHz 2 × 12 mm cylindrical glass tag, (b) 125 kHz 2 × 12 mm cylindrical glass tag, and (c) 125 kHz 12 × 2 epoxy disk tag. The scale shown is mm.

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Fig. 2

The tag (square) was placed within the intramedullary canal of a porcine femur to assess signal transmission through bone and soft tissue. The fine scale shown is mm.

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Fig. 3

Glass tags were tested in three point bending to determine maximum strain at failure of the packaging. Contact spacing was 10 mm and the samples were tested at 0.1 mm per min.

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Fig. 4

Linear offset between the tag and reader affected read range for all three systems. Maximum read range (MRR) was over 20 cm for all tags when aligned in air. Bars are 95% confidence interval.

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Fig. 5

Angular misalignment between the tag and the reader affected read range for all three systems. Bars are 95% confidence interval. MRR, maximum read range.

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Fig. 6

Tag collision (proximity of a second tag) dramatically affected read range of the Tag 128C system which has no anticollision feature. The effects were much more limited with the Tag 125C system which incorporates anticollision. Bars are 95% confidence interval.

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Fig. 7

Relative to maximum read range, saline, soft tissue, and cortical bone had no effect. Recessing in stainless steel and γ-irradiation significantly reduced the read range of all systems. *p < 0.001 relative to maximum read range. Bars are standard deviation.

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Fig. 8

Failure of the glass tags was initiated by cracks which formed at the point of contact with the center loading plateau

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