These authors contributed equally as first authors to this work.

These authors contributed equally as first authors to this work.

[2]Hideko Heidi Nakajima

1]Massachusetts Institute of Technology

2]Harvard University and Mass Eye and Ear

3]Columbia University

2023

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

John Z. Zhang  Lukas Graf  Annesya Banerjee  Aaron Yeiser  Christopher I.
McHugh  Ioannis Kymissis  Jeffrey H. Lang  Elizabeth S. Olson
<EMAIL_ADDRESS>[  [  [

###### 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 (over a bandwidth of 0.1 – 10 kHz) of the DrumMic and
amplifier referenced to the ear canal was measured to be about 54 dB SPL in
the temporal bone experiment 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 (related to the
lowest level of sound measurable) 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.

###### keywords:

Implantable, Cochlear Implant, Piezofilm, Microphone, Implantable Microphone

##  1 Introduction

More than one million cochlear implants (CIs) have been implanted worldwide
zeng2022celebrating , changing the lives of many by enabling them to hear and
understand speech. Younger patients, and those with less profound hearing
loss, are now benefiting from cochlear implants. More patients are also
receiving bilateral CIs, increasing their ability to understand speech in
noisy environments potts2019improving . When the sensory mechanism has been
greatly compromised (yet the auditory nerve is functioning), CIs can sometimes
allow better hearing than conventional hearing aids.

Despite advances in CI technology, totally implanting all CI-associated
hardware internally has not been generally realized. With a totally
implantable CI system, wearers could experience multiple advantages: various
activities would not be limited, hearing would be possible all the time (for
safety and to increase a child’s auditory development), noise such as that
from wind would be reduced, and stigmatization due to external hardware would
be diminished shinners2008implantable . If designed accordingly, the system
could use the natural advantages of the outer ear’s pressure gain and
filtering, improve speech comprehension in noisy environments, and may enable
binaural processing shaw1974transformation . Advances in technology—batteries,
non-contact charging, low-power electronics, miniaturization of
electronics—all work towards the potential of a totally implantable CI system.
However, the major barrier is the absence of an implantable microphone that is
sensitive, low noise, consistent, reliable, and uses the benefits of the outer
ear trudel2022remaining .

Prior work in this direction has been summarized in the related work section
of past theses and review articles zhang2008acoustic ; hake2021piezoelectric ;
calero2018technical . Commercial and experimental implantable microphone
devices have limitations. For example, Carina pulcherio2014carina  is
implanted under the skin, which limits the signal-to-noise ratio due to the
skin attenuating the external sound recorded, while picking up internal body
noise. The Envoy Esteem senses incus body motion with a needle tip. However,
because the incus body moves with varying modes with respect to frequency and
its surface is smooth, stability and consistency across varying middle ear
anatomy for surgical insertion are of concern chung2013optimal . Experimental
devices, such as the intracochlear hydrophone concept proposed by the
University of Zurich group pfiffner2016mems , have limited bandwidth (500 Hz
to 3.5 kHz) and an equivalent input noise of 65 dB SPL at 1 kHz as reported in
pfiffner2016mems .

To overcome many of the challenges of realizing an implantable microphone and
to incorporate the acoustic advantages of the outer ear, we have developed an
implantable microphone concept that senses the motion of the umbo (the tip of
the malleus) where it attaches to the center of the tympanic membrane
cary2022umbo . An advantage of sensing umbo motion is that the umbo has the
largest predominantly one-dimensional motion among other points along the
ossicular chain, and it is a good representation of the external sound
nakajima2005experimental . Our umbo microphone, which we refer to as the Drum
Microphone (DrumMic), is constructed of a piezoelectric polymer,
polyvinylidene fluoride (PVDF). The design has a sensitive PVDF membrane
“drum” fixed at the circular perimeter of a metal ring. Within the middle-ear
cavity, the PVDF drum membrane presses against the umbo to sense its motion,
and the opposite end of the ring sits on top of the cochlear promontory. This
design allows for simple surgical implantation. Our analyses of how the
DrumMic functions, studied with two types of computational models (analytical
and finite element models), agreed with each other. These two modeling results
agreed with bench testing results with an artificial umbo, as described in
Cary et al. 2022 cary2022umbo . Preliminary tests in a human cadaveric
temporal bone also provided results that were similar cary2022umbo . In this
study, we improve the fabrication of the DrumMic to produce more consistent
and reliable sensors through the use of a laser cutter and gluing procedure to
adhere the PVDF to the support structure as described in Methods Section 2. We
also report on the performance of various DrumMics in different cadaveric
specimens.

##  2 Materials and Methods

###  2.1 Microphone Design and Construction

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.

We investigate the performance (sensitivity, linearity, noise) of DrumMics
subject to variations in fabrication, ear anatomy, and the effect of drumhead
static displacement controlled through the number of shims placed between the
DrumMic base and the cochlear promontory. Five DrumMics were fabricated and
labeled as (1, 2, 3, 4, 5) and experiments were carried out across four
different ears labeled (A, B, C, D). We refer to an experiment conducted with
a particular ear and DrumMic using a designation such as C4, meaning the
combination of ear C and DrumMic 4. Figure 1 shows our improved design and
implementation of the DrumMic. In summary, a PVDF sheet (PolyK Technologies,
Philipsburg, PA, USA) with copper-nickel electrodes on both sides was laser
cut (Protolaser U4, LPFK, Hannover, Germany) into a semicircular shape at one
end for the drum, and the other straight end for connectorization. The laser
beam ablates a rim of the electrode around the PVDF edge, helping to prevent
shorting between the electrode layers. Note that the laser cutter used must
have a sufficiently quick pulse duration or the PVDF material will be burned
rather than cut. Two of the earliest DrumMics (in experiments A1, B2) were
vinyl cut, and were also functional, though vinyl cutting was generally less
reliable.

A brass ring of $3\text{\,}\mathrm{m}\mathrm{m}$ diameter (with height between
$1\text{\,}\mathrm{m}\mathrm{m}$ to $1.5\text{\,}\mathrm{m}\mathrm{m}$) was
initially held lightly on top of the semicircular-shaped end of the PVDF sheet
with a thin layer of cyanoacrylate glue (Gorilla Glue Company, Cincinnati, OH,
USA). Subsequently, the brass ring was firmly fixed in place to the PVDF
surface with UV-curing glue (NEA123 Optical Glue, Norland Products, East
Windsor, NJ, USA) at the outer edge of the brass ring. Care was taken to
prevent any adhesive material from seeping onto the PVDF surface inside the
brass ring. The PVDF film beyond the edge of the hardened UV-curing glue was
trimmed ($0.5\text{\,}\mathrm{m}\mathrm{m}$ beyond the outer edge of the brass
ring). The resulting dimension of the DrumMic prevented contact to surrounding
structures such as the stapes, stapedial tendon, and tensor tympani. On the
other end of the PVDF sheet, top and bottom electrodes of the PVDF were
connected to flex wire connectors (HST-9805-210, Elform Inc., Reno, NV, USA)
with a hot bar bonder (Thin-Line Series 180, HiyachiUnitek, Monrovia, CA,
USA). The other ends of the flex wires were bonded to a receptacle PCB
(OSHPark, Portland, OR, USA) that was connected to a custom low-noise charge
amplifier yeiser2022fully  with a U.FL cable (Hirose, Tokyo, Japan). The flex
wires allowed flexibility yet sufficient rigidity during DrumMic insertion to
press against the umbo.

To insulate the DrumMic from moisture of the middle-ear cavity, the DrumMic
and flex wires were lightly sprayed with silicone (422B Silicone Modified
Conformal Coating Spray, MG Chemicals, Burlington, Ontario, Canada).
Additionally, the edge of the PVDF with 50-micron separation between the upper
and lower electrode layers was brushed with silicone to prevent shorting.

For this experiment we abstained from shielding the DrumMic to electro-
magnetic interference (EMI) pickup. While PVDF is FDA approved
teo2016polymeric , we disregarded the biocompatibility of certain other
materials as the goal of this study was to prove feasibility.

###  2.2 Preparation of the Temporal Bone Specimen

Five fresh (unexposed to fixative) previously frozen human cadaveric temporal
bone specimens (2 right, 3 left) were harvested with mean postmortem time of
36.2 hours (standard deviation $\pm$12.2 hours) from 4 male donors (mean age
71.5 years standard deviation $\pm$13.7 years) at Massachusetts General
Hospital (Boston, MA, USA). The donors did not have known histories of
previous ear pathology, and the ears appeared normal upon inspection.

The middle-ear cavity was accessed through the facial recess in a similar
manner used for CI surgery. However, the facial recess was expanded to enable
clear views of the middle-ear structures and DrumMic insertion. The middle-ear
and inner-ear structures were kept intact.

The bony external auditory canal was shortened to about 1 cm from the
posterior tympanic annulus. Two holes were drilled through the bony anterior
ear canal (EC) wall: one for a metal tube that connected to the rubber tubing
from the speaker, and another for a metal tube sleeve for the microphone
probe-tube. Both tubes were glued in place with cyanoacrylate glue. The whole
specimen was glued with cyanoacrylate glue to a multi-directional articulated
holder (Noga Engineering & Technology, Shlomi, Israel).

###  2.3 Stimuli and Measurements

As shown in Figure 2, the experimental setup with the specimen was supported
by an air table (TMC, Peabody, MA, USA) inside a soundproof chamber. The
DrumMics were inserted by hand and an adjustable stand was used to provide
strain relief for the cables connecting the DrumMic to the amplifier. To
prevent drying, moistened Gelfoam pledgets (Pfizer, New York City, NY, USA)
were placed in the middle ear and mastoid cavity, but not touching middle ear
structures or the sensor.

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.

Analog output signals (logarithmic chirps of
0.1–$20\text{\,}\mathrm{k}\mathrm{H}\mathrm{z}$ or sine sequences of
0.95–$18.2\text{\,}\mathrm{k}\mathrm{H}\mathrm{z}$) were produced by a
National Instruments DAQ (National Instruments Austin, TX, USA). An audio
amplifier (Crest Audio, Meridian, Mississippi, USA) drove a vibration-isolated
and electrically-shielded Beyer DT48 speaker (Beyerdynamic, Heilbronn,
Germany) that presented sound to a sealed ear canal.

Ear canal sound near the tympanic membrane was measured with a calibrated
Knowles probe-tube microphone (Knowles EK-23103, Itasca, IL, USA), with probe-
tube tip approximately 1–3 mm from the umbo. The intensity of the sound
stimuli was confirmed to be in the linear range. To measure tympanic membrane
motion at the umbo, a laser Doppler vibrometry (LDV) (CLV 1000, Polytec,
Waldbronn, Germany) beam was aimed at a reflector on the umbo of the tympanic
membrane through a clear acrylic cover-slip window that was sealed at its
perimeter to the bony ear canal opening. Time and frequency domain measurement
signals were collected by a National Instruments DAQ system running custom
LabVIEW software (National Instruments Austin, TX, USA).

###  2.4 Experimental Protocol

Initially, to check the integrity of middle-ear-plus-inner-ear input
admittance, ear canal umbo velocity referenced to ear canal pressure was
confirmed to be normal nakajima2005experimental . In the middle-ear cavity, a
DrumMic was inserted between the umbo and cochlear promontory, with the center
of the sensitive PVDF DrumMic membrane contacting the umbo, and the open
circular bottom of the metal cylinder base sitting on the cochlear promontory
(as shown in Figure 3). Because the distance between the umbo and promontory
varied (1.7–$2.3\text{\,}\mathrm{m}\mathrm{m}$ stieger2006anatomical ), 1–2
plastic shims (each shim was 0.65 mm in thickness) were inserted between the
metal cylinder and the promontory. We investigated the effect of zero, one, or
two shims. The number of shims used was limited by the space between cochlear
promontory and umbo. The coupling force was not measured, but rather the
static displacement of the umbo and the DrumMic was visually observed. Once
the deflection was deemed sufficiently large to achieve good sensitivity then
no additional shims were needed. To insert the shims, first we visually
observed that the DrumMic membrane contacted the umbo, then we lifted up the
umbo/manubrium to fit the shim(s) under the DrumMic. The promontory shape and
its superior-inferior and lateral-medial position in relation to the umbo
varied across specimens. When the promontory was not near symmetric under the
umbo, the bottom ring of the DrumMic cylinder had complex motion (e.g.,
rocking motion). This motion could be reduced by gluing the shim to the
promontory with carboxylate cement (Durelon 3M, Saint Paul, MN, USA).

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.

The input voltage to the speaker and measurements of ear canal pressure, umbo
velocity, and DrumMic (after charge-amplifier) output were simultaneously
recorded and averaged 1000 times for chirp signals and 20 times for sine-
sequences. Noise floor measurements were conducted by recording 10 seconds of
voltage output from the DrumMic/charge-amplifier with no acoustic stimuli
before and after DrumMic implantation. Control measurements with the speaker
tubing decoupled, and sound from the speaker housing blocked with putty,
verified that electrical coupling was negligible. Linearity was checked with
varying levels of sound input from 35 dB SPL to nearly 117 dB SPL. We
demonstrated that the device behaves linearly down to the sensor-amplifier
system noise floor so the normalized responses are valid for any input level
that still exhibits linearity.

###  2.5 Post-Experiment Data Analysis

All data was processed using MATLAB (R2022b, MathWorks, Natick, MA, USA). The
charge amplifier transfer function was used to convert the measured output
voltage of the charge amplifier to charge from the PVDF. A standard
calibration method corrected for the effect of the probe-tube to estimate ear
canal pressure at the tympanic membrane. LDV velocity data were converted to
displacements in the frequency domain. DrumMic input-to-output measurements
from several sine sequences and DrumMic harmonics in the frequency spectrum
determined linearity. All other frequency response data used log chirp sound
stimuli.

Power spectral densities of the noise were computed from the measured time
series data. First, a time series of the amplifier output was recorded by a
National Instruments 6122 A/D converter with 16-bit resolution for a full
range of $\pm$10 V and sampling rate set to 198 kHz. Next, this was
transformed into a power spectral density and then segmented and averaged with
a rectangular window following the Bartlett method bartlett1948smoothing .
Using the sensitivity transfer function, we converted the output noise-voltage
power spectral density to an input-referred noise pressure power spectral
density at the tympanic membrane in sound pressure. Equivalent input noise
(EIN) spectral densities were integrated numerically to calculate the
equivalent input noise in dB SPL. Note that we calculated both the 1/3-octave
bandwidth EIN and the EIN across the entire bandwidth from 100 Hz to 10 kHz.
By taking into account the outer ear gain, we then computed the EIN referenced
to a free field sound illustrating the advantage of implantable microphones
that can benefit from the natural filtering of the outer ear. The pressure
gain of the outer ear (pinna plus ear canal) was implemented using a
polynomial fit to well-known measurements from Shaw shaw1974transformation .

###  2.6 Equivalent Input Noise Calculation

The equivalent input-referred noise, or simply equivalent input noise (EIN),
is commonly used to report noise performance of microphones used in commercial
hearing aids such as the Knowles EK-23103 and the Sonion 65GC31. EIN is a
useful indicator of the lowest sound pressure level that the microphone can
detect. EIN is derived from a measurement of the noise at the output of the
system. It is the acoustic noise presented to a noiseless microphone and
amplifier that produces same output as does a noisy microphone and amplifier
in the absence of acoustic stimulus. The output-referred noise is then divided
by the sensitivity transfer function of the microphone plus amplifier system
resulting in the EIN. Finally, the EIN is typically reported in two ways for
microphone specifications: (1) as EIN (1/3-octave bands) in a plot over
frequency and (2) as a single-number metric EIN (0.1–10 kHz) often A-weighted
calculated across the entire bandwidth (see the Brüel & Kjær handbook
handbook1995technical ). To compare the EIN of the DrumMic to a realistic
hearing aid microphone EIN we present data measured in bench experiments for
the Knowles EK-23103, which is a hearing aid microphone that is also the same
model of microphone we use as the probe-tube microphone in the temporal bone
experiments. In this work, we report the microphone and amplifier system
together, though during the design process it is possible to consider each
separately.

##  3 Results

Five different microphones (labeled 1,2,3,4,5) were tested in four different
ears (labeled A, B, C, D) following the procedure outlined in Methods Section
2. The results of these experiments are shown in Figure 4 through Figure 7.
The key metrics which are reported include sensitivity, linearity, bandwidth,
and noise.

We studied the effect of drumhead static offset through the use of shims.
Figure 4 shows the magnitude and phase of the DrumMic charge with respect to
umbo displacement (a & b) or with respect to ear canal pressure (c & d). In
Figure 4, we present the data from experiments D3 and D5. The output of the
DrumMic is electrical charge since it is a piezoelectric material connected to
a charge amplifier. Note that displacements are reported in nanometers (nm),
pressures are reported in pascals (Pa), and the charge is reported in units of
femtocoulombs (fC). The sensitivities are normalized quantities and the actual
acoustic stimuli used were on the order of $0.3\text{\,}\mathrm{P}\mathrm{a}$
(84 dB SPL) or lower, which were within the the linear range of the DrumMic.

Figure 4 shows a representative experiment. In this ear, having no shim (gray
curves) has the lowest sensitivity with large fluctuations with frequency.
Good contact and some tension between the DrumMic PVDF membrane and umbo was
achieved with one shim (green) and two shims (pink). It is evident that the
spread in sensitivity with respect to umbo displacement between one and two
shims is almost 20 dB (Figure 4a), however, the spread in sensitivity with
respect to ear canal pressure has a spread less than 3 dB (Figure 4c).

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 presents frequency response data for all experiments across five
different DrumMic devices and four different cadaveric ears. For achieving a
good response, we found that the minimum combined thickness of shims(s) that
allowed for contact between the DrumMic and umbo without looseness after
DrumMic insertion was sufficient. This resulted in a slight observable static
displacement of the umbo causing a static deflection of the DrumMic membrane.
Figure 5a and Figure 5b show the magnitude and phase of the umbo displacement
normalized to the input ear canal pressure. The dashed lines indicate the umbo
displacement before insertion of the DrumMic and the solid lines indicate
response after insertion. At low frequencies there is a 10-20 dB drop in umbo
displacement after insertion of the DrumMic. A two-sample t-test shows that
there is a significant separation between the umbo displacement before and
after insertion up to $2\text{\,}\mathrm{k}\mathrm{H}\mathrm{z}$. The ranges
(95% confidence intervals) around the mean of the umbo motion before and after
insertion were both less than 9 dB. Note in Figure 5b, the phase for B2
deviates at higher frequency (possibly due to a complex mode of ossicular
motion).

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.

Plotted in Figure 5c and Figure 5d, DrumMic sensitivity with respect to ear
canal pressure is about
$25\text{\,}\mathrm{f}\mathrm{C}\mathrm{/}\mathrm{P}\mathrm{a}$ across five
DrumMics and four ears. The frequency response magnitude (Figure 5c) is
tightly clustered. The entire range (95% confidence intervals) around the mean
is less than 3 dB. The response magnitude -3 dB point is approximately
$5\text{\,}\mathrm{k}\mathrm{H}\mathrm{z}$, but the response has good signal-
to-noise up to $8\text{\,}\mathrm{k}\mathrm{H}\mathrm{z}$. The phase drops by
half a cycle after the cut-off frequency.

Comparing Figure 5a and Figure 5c it is clear that there was no relationship
between umbo motion and DrumMic sensitivity with respect to ear canal pressure
for these experiments. The 95% confidence intervals are smaller for the
DrumMic sensitivity (Figure 5c) than for the umbo displacement (Figure 5a). In
general, the spread of DrumMic sensitivities is tight and no significant
correlation exists between the umbo motion (before or after insertion) and the
DrumMic sensitivity. The apparent nature and mechanics of this are explained
in Discussion Section 4.

Figure 5c also displays the intrinsic noise (solid gray line) and the
electrical pickup (dashed gray line) of the DrumMic and amplifier. The noise
and pickup presented here has units of charge. The noise is reported in
1/3-octave bands. The electrical pickup is the equivalent charge output of the
DrumMic in the absence of ear canal pressure (see Section 2.4). Noise and
electrical pickup are generally several orders of magnitudes lower than the
DrumMic charge output referenced to ear canal pressure (note that the sound
stimuli used were less than 84 dB SPL, within the linear range).

Figure 6 shows a typical example of DrumMic charge output versus ear canal
pressure input for several frequencies. Linearity of the DrumMic was
maintained throughout a large dynamic range (46 dB SPL to 100 dB SPL).

Figure 6: DrumMic (D5) input-output curves show linearity up to 100 dB SPL
input for different frequencies. Dashed line is a reference for linearity.

Figure 7 compares the EIN (1/3-octave bandwidth) (EIN see Section 2.6) of the
DrumMic implanted in the middle-ear cavity with the EIN (1/3-octave bandwidth)
of a Knowles hearing aid microphone. The EIN (0.1-10 kHz) reported is
integrated across a bandwidth of 100 Hz to 10 kHz. Without an outer ear, the
best performing implanted DrumMic had a EIN (0.1-10 kHz) of 54 dB SPL. The
measured EIN (1/3-octave bandwidth) (pink line in Figure 7) trends upwards at
the higher frequencies and is approximately 20 dB higher than the EIN
(1/3-octave bandwidth) of the hearing aid microphone (black line in Figure 7).

However, the outer ear with pinna and full length of the ear canal would
result in a pressure gain to the tympanic membrane from a free-field sound
stimulus. If the gain due to the outer ear were taken into consideration, we
would expect a DrumMic EIN (0.1-10 kHz) of 46 dB SPL (green in Figure 7). As a
comparison, the EIN (0.1-10 kHz) for a representative external hearing aid
microphone, such as the Knowles EK-23103, was 34 dB SPL. Therefore, the
DrumMic has the potential of providing performance closer to the current
state-of-the-art external microphones used in cochlear implants and hearing
aids, with the advantage of being fully implantable.

Figure 7: Typical EIN (1/3-octave bandwidth) of an implanted DrumMic (D5) with
and without the simulated pressure gain of the outer ear as compared to the an
external hearing aid microphone (Knowles EK-23103) that does not leverage the
filtering of the outer ear.

##  4 Discussion

Using a short-term experimental setup in cadaveric temporal bones we
demonstrate that the DrumMic’s ring base should be mechanically stable in
relation to the cochlear promontory and that the PVDF drum membrane should
have good contact to the umbo to achieve good microphone performance (see
Figure 4). Because the distance between the umbo and promontory varies across
ears, the height of the DrumMic system must vary accordingly. Additionally,
the curvature of the cochlear promontory with respect to the umbo varies
across ears. In some ears the promontory is nearly symmetric directly under
the umbo with only slight circumferential curvature, while in other ears the
promontory is uneven and is asymmetric with respect to the umbo.

When necessary, stabilizing the DrumMic base mechanically on the cochlear
promontory with shim(s) and sometimes glue allows the DrumMic to be stable and
for the PVDF membrane to be in good contact to the umbo. Mechanically
stabilizing the DrumMic results in smoother frequency response of the DrumMic
sensitivity. This is likely due to reduction in complex modes of motion, such
as rocking, of the DrumMic cary2022umbo .

DrumMic sensitivity can vary due to static drumhead offset. This variation can
affect DrumMic sensitivity in two ways. First, an increased offset distorts
the shape of the drumhead due to the umbo pushing against the drum membrane.
The steepness of the cone-shaped distortion increases microphone sensitivity
as can be seen in the sensitivity relative to umbo displacement plot in Figure
4a. With increased steepness, a subsequent normal displacement of the drumhead
by the umbo geometrically leads to increased stretching of the drumhead, and
hence increased charge. Second, an increased offset also results in an
increased tension applied to the PVDF membrane and umbo. This mechanical
loading leads to an increased ossicular stiffness that reduces drumhead motion
in response to an acoustic stimulus. The reduction in drumhead motion
decreases the apparent sensitivity of the DrumMic with respect to ear canal
pressure. In Figure 4 and Figure 5, the same DrumMic with a greater static
offset must undergo a smaller acoustically-driven displacement for the
sensitivity to ear canal pressure to remain the same. In summary, the effect
is a cancelling out of these two behaviors resulting in a DrumMic sensitivity
with respect to ear canal pressure that works similarly over a large range of
offsets. This variation across ears of decreasing umbo motion due to the
DrumMic (Figure 5a) has a minor influence on the DrumMic sensitivity with
respect to stimulus sound measured at the ear canal (Figure 5c).

For an implantable microphone, the critical metric is sensitivity to ear canal
pressure. As seen in Figure 5c, the DrumMic sensitivity to ear canal pressure
is consistent across multiple DrumMics and different ears for our current
fabrication method. Therefore, a major advantage of the DrumMic system is its
reliable and repeatable sensitivity with respect to ear canal pressure with
low inter-individual variation despite differences in static drumhead offset
and differences in umbo motion across ears and DrumMics.

Our experiments in human cadaveric temporal bones demonstrate the robustness
and repeatability of the DrumMic across ears and devices to sense ear canal
sound. A bandwidth up to $8\text{\,}\mathrm{k}\mathrm{H}\mathrm{z}$ with a
generally flat region below 3–$5\text{\,}\mathrm{k}\mathrm{H}\mathrm{z}$ can
be achieved. The DrumMics are linear across a large dynamic range from 20 dB
SPL to 90 dB SPL across a wide bandwidth.

Part of the success of signal-to-noise and consistency of the DrumMic design
is due to its placement at the umbo where the ossicular motion is largest,
whereas other middle ear ossicles can have complex modes of motion at
reasonably low frequencies. Previous implantable microphone designs that are
sensing motion at the head of the malleus or the body of the incus are limited
in performance (bandwidth) and are challenging to position consistently and
robustly chen2004phase ; jenkins2007us ; hake2021design . Complex modes of
motion are more problematic further down the ossicular chain and the
displacement magnitude is largest at the umbo because of the flexibility of
the joints ravicz2004mechanisms . Other middle ear devices face challenges due
to these properties of the middle ear, which are avoided by the DrumMic.

The noise level of the DrumMic system was higher than microphones used in
hearing aids (see Figure 7). The EIN (0.1-10 kHz) was found to be 54 dB SPL
for the best DrumMic, which is 20 dB higher than a typical external microphone
for hearing aids, such as the Knowles EK-23103. The EIN (1/3-octave bandwidth)
of the DrumMic trends upwards at the higher frequencies as the DrumMic
sensitivity drops off. From the noise analysis of the DrumMic and charge
amplifier, we believe the primary increase in intrinsic noise is due to the
effect of the moist environment on the sensor impedance values. If the design
of the DrumMic included shielding that would prevent the environment from
loading the charge amplifier, the EIN would be expected to be greatly reduced.
Furthermore, an implanted DrumMic would take advantage of the pinna and full
length of the ear canal with the advantageous increase of pressure near the
umbo as compared to near the pinna where hearing-aid microphones are situated.

A drawback of the DrumMic concept as presented here is the relative
uncertainty in mechanical fixation of the sensor due to changing middle ear
pressure. The wet environment of the middle ear cavity poses yet another
challenge to the DrumMic due to the change in electrical resistance and the
necessity of long-term hermetic sealing and biocompatibility. These issues
must be addressed for a fully implantable solution. A further limitation of
the prototype design of the DrumMic is that the base of the DrumMic sits on
top of the cochlear promontory where Jacobson’s nerve (tympanic nerve)
courses. This is problematic because it has been reported that this nerve
lacks a bony covering in about 80% of ears reddy2019endoscopic ;
porto1978anatomic ; kanzara2014clinical . The Jacobson’s nerve provides
sensory innervation to the lining of the mesotympanum and Eustachian tube and
gives parasympathetic supply to the parotid gland.

Future design of an implantable microphone that senses umbo motion would
require electrical shielding and be able to avoid trauma of the Jacobson’s
nerve. This can be accomplished through new encapsulation methods and
microphone designs. The EIN could be further improved by advances in amplifier
design, however, investigation of the sound processing capabilities of
cochlear implants may prove that the current suite of implantable microphones
we have demonstrated are sufficiently performant. Ongoing progress in
implantable microphone research provides hopeful signs that totally
implantable cochlear implants may be possible in the near future.

Declarations The authors declare that they have no conflicts of interest.

Acknowledgments This work was supported by the NIH through grant R01DC016874,
the NSF through GRFP Grant Number 1745302, the Foundation Prof. Dr. Max
Cloëtta, and Research Fund of the University of Basel. We thank Mike Ravicz,
Charles Hem, and Anbuselvan Dharmarajan for their help.

## References

  *   * (1) Zeng, F.-G.: Celebrating the one millionth cochlear implant. JASA Express Letters 2(7) (2022)
  * (2) Potts, W.B., Ramanna, L., Perry, T., Long, C.J.: Improving localization and speech reception in noise for bilateral cochlear implant recipients. Trends in Hearing 23, 2331216519831492 (2019)
  * (3) Shinners, M.J., Hilton, C.W., Levine, S.C.: Implantable hearing devices. Current Opinion in Otolaryngology & Head and Neck Surgery 16(5), 416–419 (2008)
  * (4) Shaw, E.A.: Transformation of sound pressure level from the free field to the eardrum in the horizontal plane. The Journal of the Acoustical Society of America 56(6), 1848–1861 (1974)
  * (5) Trudel, M., Morris, D.P.: The remaining obstacles for a totally implantable cochlear implant. Current Opinion in Otolaryngology & Head and Neck Surgery 30(5), 298–302 (2022)
  * (6) Zhang, L.A.: Acoustic sensors for totally implantable cochlear implant. PhD thesis, University of Melbourne, Department of Otolaryngology (2008)
  * (7) Hake, A.: Piezoelectric mems accelerometers for sensing ossicular vibration. PhD thesis, University of Michigan (2021)
  * (8) Calero, D., Paul, S., Gesing, A., Alves, F., Cordioli, J.A.: A technical review and evaluation of implantable sensors for hearing devices. Biomedical engineering online 17(1), 1–26 (2018)
  * (9) Pulcherio, J.O.B., Bittencourt, A.G., Burke, P.R., Monsanto, R.d.C., De Brito, R., Tsuji, R.K., Bento, R.F.: Carina® and esteem®: a systematic review of fully implantable hearing devices. PLoS One 9(10), 110636 (2014)
  * (10) Chung, J., Song, W.J., Sim, J.H., Kim, W., Oh, S.-H.: Optimal ossicular site for maximal vibration transmissions to coupled transducers. Hearing Research 301, 137–145 (2013)
  * (11) Pfiffner, F., Prochazka, L., Péus, D., Dobrev, I., Dalbert, A., Sim, J.H., Kesterke, R., Walraevens, J., Harris, F., Röösli, C., et al.: A mems condenser microphone-based intracochlear acoustic receiver. IEEE Transactions on Biomedical Engineering 64(10), 2431–2438 (2016)
  * (12) Cary, B.G., Zhang, J.Z., McHugh, C.I., Kymissis, I., Olson, E.S., Nakajima, H.H., Lang, J.H.: An umbo microphone for fully-implantable assistive hearing devices. IEEE Sensors Journal (2022)
  * (13) Nakajima, H.H., Ravicz, M.E., Merchant, S.N., Peake, W.T., Rosowski, J.J.: Experimental ossicular fixations and the middle ear’s response to sound: evidence for a flexible ossicular chain. Hearing research 204(1-2), 60–77 (2005)
  * (14) Yeiser, A.: A fully-implantable low-noise emi-resistant piezoelectric-polymer microphone and amplifier for the middle ear. PhD thesis, Massachusetts Institute of Technology (2022)
  * (15) Teo, A.J., Mishra, A., Park, I., Kim, Y.-J., Park, W.-T., Yoon, Y.-J.: Polymeric biomaterials for medical implants and devices. ACS Biomaterials Science & Engineering 2(4), 454–472 (2016)
  * (16) Stieger, C., Djeric, D., Kompis, M., Remonda, L., Häusler, R.: Anatomical study of the human middle ear for the design of implantable hearing aids. Auris Nasus Larynx 33(4), 375–380 (2006)
  * (17) Bartlett, M.S.: Smoothing periodograms from time-series with continuous spectra. Nature 161(4096), 686–687 (1948)
  * (18) Brüel&Kjaer Microphone Handbook Vol. 1. Technical report, Brüel&Kjaer (1995)
  * (19) Chen, D.A., Backous, D.D., Arriaga, M.A., Garvin, R., Kobylek, D., Littman, T., Walgren, S., Lura, D.: Phase 1 clinical trial results of the envoy system: a totally implantable middle ear device for sensorineural hearing loss. Otolaryngology—Head and Neck Surgery 131(6), 904–916 (2004)
  * (20) Jenkins, H.A., Atkins, J.S., Horlbeck, D., Hoffer, M.E., Balough, B., Arigo, J.V., Alexiades, G., Garvis, W.: Us phase i preliminary results of use of the otologics met fully-implantable ossicular stimulator. Otolaryngology–Head and Neck Surgery 137(2), 206–212 (2007)
  * (21) Hake, A.E., Zhao, C., Sung, W.-K., Grosh, K.: Design and experimental assessment of low-noise piezoelectric microelectromechanical systems vibration sensors. IEEE Sensors Journal 21(16), 17703–17711 (2021)
  * (22) Ravicz, M.E., Rosowski, J.J., Merchant, S.N.: Mechanisms of hearing loss resulting from middle-ear fluid. Hearing research 195(1-2), 103–130 (2004)
  * (23) Reddy, R., Jufas, N., Bance, M., Patel, N.: Endoscopic tympanic neurectomy for otic neuralgia—a case series. Australian Journal of Otolaryngology 2 (2019)
  * (24) Porto Jr, A.F., Whicker, J., Proud, G.: An anatomic study of the hypotympanic branch of jacobson’s nerve. The Laryngoscope 88(1), 56–60 (1978)
  * (25) Kanzara, T., Hall, A., Virk, J.S., Leung, B., Singh, A.: Clinical anatomy of the tympanic nerve: A review. World Journal of Otorhinolaryngology 4(4), 17–22 (2014)