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Second Biennial Hearing Aid Research and Development Conference

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September 22-24, 1997
National Institutes of Health
Bethesda, Maryland



Implantable Hearing Aid Issues: Optimizing the Position and Force of a Piezoelectric Malleus Sensor

Iain L. Grant, Department of Otolaryngology, Ohio State University, Columbus, Ohio, Kai Kroll, Manager Pre-Clinical Engineering, St. Croix Medical Incorporated, Minneapolis, Minnesota, D. Bradley Welling, Associate Professor, Department of Otolaryngology, Ohio State University, Columbus, Ohio, Samuel C. Levine, Associate Professor, Department of Otolaryngology, University of Minnesota, Minneapolis, Minnesota

Purpose:
A piezoelectric bimorph applied to the malleus head can function as a microphone for an implantable hearing aid or cochlear implant. This concept takes advantage of the natural amplification and filtering inherent in the pinna and external auditory canal. The purpose of this work was to ascertain the optimal position on the malleus head and the ideal applied force required to maximize the transducer signal.

Methods:
The epitympanic recess was exposed using conventional surgical approaches in freshly harvested human temporal bone specimens. Care was taken to preserve the ossicular suspensory ligaments. The incus was removed and a piezoelectric bimorph as positioned using a micromanipulator at defined positions on the malleus head. A swept sine stimulus was applied to the tympanic membrane and the sensitivity of the transducer output plotted as a ratio of the voltage generated to the sound pressure applied using a spectrum analyzer. A force sensor was then introduced and the bimorph sensitivity recorded as a function of force.

Results:
Typical sensitivity was 25 mV at 100 dB SPL in the auditory canal. This varied minimally among the different positions on the malleus head. No one position was consistently better across all the specimens, however, individual specimens gave the strongest signal at particular positions. Maximal signal strength was obtained with an applied force of 20 - 50 mN (milli-Newtons). Although signal strength dropped slightly above 50 mN, frequency response deteriorated markedly.

Conclusions:
This study proves the feasibility of using a piezoelectric sensor applied to the malleus head as an implantable hearing aid microphone. The position of the sensor has minimal effect on the sensitivity. Varying the force significantly alters sensor output and quality.

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