Passive Wireless MEMS Microphones for Biomedical Applications

Abstract

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.

Microchemomechanical Systems

“…To overcome this challenge, researchers have been developing schemes to couple wireless electromagnetic energy to antennas mounted on MEMS devices. For example, pulse modulated surface acoustic wave interdigital transducers (SAW-IDT) (Figure 1 d) have been coupled with a microphone that can be used to monitor sounds, from breathing to cardiac murmurs, with time delays as small as 1 μ s. [ 24 ] Radio Frequency (RF-MEMS) refers to a class of MEMS-based devices that use wireless actuation. Here, the challenge is miniaturization of the antenna, which ultimately limits the overall size of the device.…”

A radio frequency controlled microvalve for biomedical applications

“…Successful realization of a SAW-Interdigital Transducer (IDT) based passive (battery less) wireless sensor is found in the literature [3]. The developed sensor can be used in implants such as in the neonatal intensive care units to continuously monitor the neonatal newborns, causing minimal discomfort for the newborns.…”

“…One possible way to overcome this issue is to design the system to operate in the near field, to operate only when the external RF source is in near proximity. The strength of the electromagnetic coupling between the antennas affects the telemetry distance between the sensor antenna and the control unit antenna [3]. However the electromagnetic coupling greatly depends on the antenna geometry, size and directivity.…”

“…In order to interrogate the microvalve, it is important to design a near proximity operation of a wireless system. Passive and wireless operations are desirable in applications where an unrestricted operation is more critical than the long distance communication [3]. The ability to store energy in SAW and the possibility of handling high frequencies enables the construction of passive sensors with the capability to be interrogated by RF signals [6].…”

“…Various different actuation schemes are widely used for microvalves and micropumps [1,3,7,[8][9][10]. Different actuation schemes are discussed to identify which actuation mechanism is best suited for the application domain in concern.…”