The UmboMic: A PVDF Cantilever Microphone

TL;DR

The study tackles the need for an internal microphone in totally-implantable cochlear implants by presenting the UmboMic, a PVDF-based differential cantilever microphone that senses umbo motion and is paired with a custom low-noise differential charge amplifier. The authors demonstrate a mid-band gain of $20$ V/pC using a $C_{ m f}=1$ pF feedback capacitor, and achieve an equivalent input noise of $32.3$ dB SPL when outer-ear pressure gain is considered, with a flat response from roughly $100$ Hz to $6$–$7$ kHz. Measurements in fresh cadaveric temporal bones show low harmonic distortion (<0.1% at 94.5 dB SPL) and good linearity across a wide dynamic range, with strong EMI rejection (~$30$–$40$ dB CMRR). Together these results establish the feasibility of PVDF as a biocompatible sensing material for implantable microphones and outline a path toward fully internalized cochlear implants with performance competitive to conventional hearing-aid microphones.

Abstract

Objective: We present the "UmboMic," a prototype piezoelectric cantilever microphone designed for future use with totally-implantable cochlear implants. Methods: The UmboMic sensor is made from polyvinylidene difluoride (PVDF) because of its low Young's modulus and biocompatibility. The sensor is designed to fit in the middle ear and measure the motion of the underside of the eardrum at the umbo. To maximize its performance, we developed a low noise charge amplifier in tandem with the UmboMic sensor. This paper presents the performance of the UmboMic sensor and amplifier in fresh cadaveric human temporal bones. Results: When tested in human temporal bones, the UmboMic apparatus achieves an equivalent input noise of 32.3 dB SPL over the frequency range 100 Hz to 7 kHz, good linearity, and a flat frequency response to within 10 dB from about 100 Hz to 6 kHz. Conclusion: These results demonstrate the feasibility of a PVDF-based microphone when paired with a low-noise amplifier. The reported UmboMic apparatus is comparable in performance to a conventional hearing aid microphone. Significance: The proof-of-concept UmboMic apparatus is a promising step towards creating a totally-implantable cochlear implant. A completely internal system would enhance the quality of life of cochlear implant users.

Figures (13)

• Figure 1: An image of a human cadaveric middle ear cavity with the UmboMic sensor inserted. The UmboMic sensor tip is touching the underside of the eardrum at the umbo. The umbo is the point where the end of the manubrium of the malleus attaches to the tympanic membrane. The malleus, incus, and stapes make up the ossicular chain.
• Figure 2: Differential PVDF cantilever diagram (not to scale). We construct the cantilever from two layers of PVDF sandwiching a flex PCB base. We sputter-coat the charge sense electrodes onto the flex PCB and capacitively couple the electrodes to the PVDF through the thin glue layer. Note that the border around the electrodes is not shown in this figure.
• Figure 3: The fabrication process for the UmboMic sensor.
• Figure 4: The assembled differential amplifier board.
• Figure 5: The differential sensor (outlined in red) and charge amplifier topology. We model the piezoelectric sensor as the voltage source $v_{\rm piezo}$ in series with the capacitor $C_{\rm piezo}$, together with a parasitic capacitor $C_{\rm par}$, leakage resistor $R_{\rm par}$, and capacitor to ground $C_{\rm gnd}$. Estimates of the piezoelecric and parasitic component values are given in Table \ref{['parasitics']}. Our implementation of the differential amplifier uses $R_{\rm f} = 10GΩ$, $C_{\rm f} = 1pF$, $R_{\rm a} = 90kΩ$, $R_{\rm b} = 10kΩ$, $C_{\rm b} = 100nF$, $R_{\rm o} = 100kΩ$, and $C_{\rm o} = 100nF$.