An Implantable Piezofilm Middle Ear Microphone: Performance in Human Cadaveric Temporal Bones

TL;DR

This study evaluates the DrumMic, a implantable piezofilm microphone that senses umbo motion via a PVDF membrane, as a candidate for a totally implantable cochlear implant microphone. In cadaveric temporal bones, the DrumMic shows robust, linear performance across a wide dynamic range (46–100 dB SPL) and bandwidth (approximately 100 Hz–8 kHz), with an EIN around 54 dB SPL (0.1–10 kHz) that improves to ~46 dB SPL when outer-ear pressure gain is included. Sensitivity to ear-canal pressure remains consistent across multiple devices and ears, aided by mechanical stabilization with shims; outer-ear filtering is essential to approach external mic performance. Key challenges include achieving reliable encapsulation and biocompatibility, EMI shielding, and avoidance of neural structures, but findings indicate that an umbo-based implantable microphone is a viable path toward a fully implantable CI microphone platform. Overall, the DrumMic demonstrates potential for enabling fully implantable cochlear implants through a middle-ear microphone that leverages natural outer-ear gain and robust umbo sensing.

Abstract

Purpose: One of the major reasons that totally implantable cochlear microphones are not readily available is the lack of good implantable microphones. An implantable microphone has the potential to provide a range of benefits over external microphones for cochlear implant users including the filtering ability of the outer ear, cosmetics, and usability in all situations. This paper presents results from experiments in human cadaveric ears of a piezofilm microphone concept under development as a possible component of a future implantable microphone system for use with cochlear implants. This microphone is referred to here as a drum microphone (DrumMic) that senses the robust and predictable motion of the umbo, the tip of the malleus. Methods: The performance was measured of five DrumMics inserted in four different human cadaveric temporal bones. Sensitivity, linearity, bandwidth, and equivalent input noise were measured during these experiments using a sound stimulus and measurement setup. Results: The sensitivity of the DrumMics was found to be tightly clustered across different microphones and ears despite differences in umbo and middle ear anatomy. The DrumMics were shown to behave linearly across a large dynamic range (46 dB SPL to 100 dB SPL) across a wide bandwidth (100 Hz to 8 kHz). The equivalent input noise (0.1-10 kHz) of the DrumMic and amplifier referenced to the ear canal was measured to be 54 dB SPL and estimated to be 46 dB SPL after accounting for the pressure gain of the outer ear. Conclusion: The results demonstrate that the DrumMic behaves robustly across ears and fabrication. The equivalent input noise performance was shown to approach that of commercial hearing aid microphones. To advance this demonstration of the DrumMic concept to a future prototype implantable in humans, work on encapsulation, biocompatibility, connectorization will be required.

Figures (7)

• Figure 1: Left: View of the DrumMic’s sensitive PVDF membrane with electrodes on both surfaces. A metal ring is glued to one surface of the PVDF. Right: Three views of the DrumMic with connectorizations.
• Figure 2: Diagram of the experimental setup. The cadaveric specimen was held using a NOGA positioner. A speaker was used to produce an ear canal pressure that was measured by a probe-tube microphone. A laser Doppler vibrometer (LDV) was used to measure the motion of the umbo (at the center of the tympanic membrane) through the ear canal.
• Figure 3: DrumMic implanted in the middle ear. Left: View of a left middle-ear cavity with the DrumMic's PVDF membrane interfacing the umbo. Two shims have been inserted between the metal ring base and cochlear promontory. Right: Histological cross section with sketch of DrumMic and shims demonstrating how the DrumMic's PVDF membrane interfaces and senses umbo motion. Photo credit: MengYu Zhu for providing the histology picture.
• Figure 4: A representative DrumMic sensitivity with respect to umbo displacement (a) magnitude and (b) phase. DrumMic sensitivity with respect to ear canal pressure (c) magnitude and (d) phase. The output of the DrumMic is electrical charge measured in units of femtocoulombs (fC).
• Figure 5: Umbo displacement (a) magnitude and (b) phase normalized to ear canal (EC) pressure. The blue bar in (a) indicates the frequencies where there is significant separation in umbo displacement between no insertion versus insertion. DrumMic sensitivity (c) magnitude and (d) phase normalized to ear canal pressure. The intrinsic noise of the amplifier and DrumMic is plotted with a solid gray line in (c) and the pickup due to electrical coupling is plotted in a dashed gray line. The shaded gray regions in (a) and (c) show the bootstrapped 95% confidence interval of the mean for the response magnitudes.