Passive Wireless MEMS Microphones for Biomedical Applications

[+] Author and Article Information
A. S. Sezen, S. Sivaramakrishnan, S. Hur, R. Rajamani, W. Robbins, B. J. Nelson

 University of Minnesota, 1100 Mechanical Engineering, 111 Church Street SE, Minneapolis, MN 55455

J Biomech Eng 127(6), 1030-1034 (Jul 08, 2005) (5 pages) doi:10.1115/1.2049330 History: Received April 01, 2005; Revised July 08, 2005

This paper introduces passive wireless telemetry based operation for high frequency acoustic sensors. The focus is on the development, fabrication, and evaluation of wireless, batteryless SAW-IDT MEMS microphones for biomedical applications. Due to the absence of batteries, the developed sensors are small and as a result of the batch manufacturing strategy are inexpensive which enables their utilization as disposable sensors. A pulse modulated surface acoustic wave interdigital transducer (SAW-IDT) based sensing strategy has been formulated. The sensing strategy relies on detecting the ac component of the acoustic pressure signal only and does not require calibration. The proposed sensing strategy has been successfully implemented on an in-house fabricated SAW-IDT sensor and a variable capacitor which mimics the impedance change of a capacitive microphone. Wireless telemetry distances of up to 5 centimeters have been achieved. A silicon MEMS microphone which will be used with the SAW-IDT device is being microfabricated and tested. The complete passive wireless sensor package will include the MEMS microphone wire-bonded on the SAW substrate and interrogated through an on-board antenna. This work on acoustic sensors breaks new ground by introducing high frequency (i.e., audio frequencies) sensor measurement utilizing SAW-IDT sensors. The developed sensors can be used for wireless monitoring of body sounds in a number of different applications, including monitoring breathing sounds in apnea patients, monitoring chest sounds after cardiac surgery, and for feedback sensing in compression (HFCC) vests used for respiratory ventilation. Another promising application is monitoring chest sounds in neonatal care units where the miniature sensors will minimize discomfort for the newborns.

Copyright © 2005 by American Society of Mechanical Engineers
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Figure 1

A typical SAW device configuration for passive wireless impedance sensing

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Figure 2

(a) Experimental results showing acoustic reflectivity with changing capacitance. (b) Simulated relationship between the acoustic pressure and the corresponding capacitance.

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Figure 3

The microfabrication process for the silicon MEMS microphone

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Figure 4

(a), (b) The fabricated surface acoustic wave sensor with 2.5μm wide IDT fingers. (c) The microfabricated silicon MEMS microphone.

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Figure 5

Data showing the 400ns SAW-IDT interrogation pulse and its reflection

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Figure 6

Experimental setup for testing the varactor-controlled SAW-IDT sensor

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Figure 7

Experimental results (a) Comparison of the power spectral density of the input and measured signals. (b) Comparison of the original and passive wirelessly retrieved signals.



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