International Conference on Natural Polymers (ICNP2010),
Kottayam, India, 24, 25 & 26, September 2010

High-sensitivity body-conducted sound sensor
Tatsuya Hirahara (Faculty of Engineering, Toyama Prefectural Univ., Japan),Tooru Ooya (Graduate School of Engineering, Kobe Univ., Japan)
1. Introduction
We have developed a compact high-sensitivity sensor for body-conductedsounds such as heartbeats, lung sounds, blood-flow and gait sounds. The
sounds are generated either inside or outside of the body, propagated throughthe body, and appear as weak vibrations of the body surface. The
body-conducted sounds have many low-frequency components because thehuman body exhibits a strong low-pass filtering property [1].
Acoustic stethoscopes are commonly used by physicians to listen toheartbeats. One problem with acoustic stethoscopes is that the sound level islow. An electronic stethoscope surmounts this problem; a microphone installed ina stethoscope head senses air-pressure changes in the head cavity, then thesensed signal is amplified electronically. However, amplification of the signal
sometimes generates artifacts and limits the bandwidth. A piezoelectric
acceleration sensor has also been used to sense body-conducted sounds; itssensitivity, though, is not sufficient.
2. Sensor architecture
Our newly developed body-conducted sound sensor has an electret condenser
microphone (ECM) with an exposed diaphragm that is covered with a softpolymer material so as to provide better acoustical impedance matching with thesoft tissue of the skin. The sensor architecture is similar to that of non-audiblemurmur (NAM) microphone, which can sense very weak whispers inaudible tonearby listeners and even the speaker when it is placed on the skin at the lower
part of the mastoid process, i.e. high on the neck behind the ear. [2].
Figure 1 depicts the architecture of the newly developed body-conductedsound sensor. The ECM mounted on a printed-circuit board is installed in acylinder- or cone-shaped concave metal housing. The concave is filled withurethane elastomer, which adheres to skin. The vibration of the skin istransmitted to the exposed diaphragm of the ECM via the urethane elastomer.
φLurethaneelastomerφL

L = 20, D = 4, Cone-typeDECM

ECM
PC boardcone-shaped
concave housing

Figure 1:cylinder-shaped
concave housing

Structures and photographs of body-conducted sound sensor.

IL141-1

International Conference on Natural Polymers (ICNP2010),
Kottayam, India, 24, 25 & 26, September 2010

Key design changes from the NAM microphones [2] are (1) fixing of the ECM
position, (2) use of a metal housing, (3) use of a cylinder- or cone-shaped
concave housing, and (4) installation of a pre-amplifier circuit. Fixing of the ECMand using a concave housing contributed to improving the sensor sensitivity. The
use of a metal housing contributed to reducing hum noise.
3. Sensor sensitivity
The sensitivities of body-conducted sound sensors were measured using anaudio analyzer (3560C/3100; Brüel & Kjær), an accelerometer (NR-3211; OnoSokki) that has a flat response of acceleration less than 10 kHz, a vibrator (4810;Brüel & Kjær) and a urethane elastomer cylinder of 75-mm diameter and 50-mm
height, which simulates human soft tissues. The vibrations transmitted throughthe cylinder were subsequently sensed by the sensor and the accelerometer,and then mutually compared.
Figure 2 shows the pressure and acceleration sensitivity of body-conductedsound sensors that have a cone- (blue line) or cylinder-shaped (red line)
concave housing of 20-mm diameter with 4-mm depth. The pressure sensitivityof the cone-housing sensor is above -55 dB (0 dB = 1 V/Pa) and that of thecylinder-housing sensor is above -70 dB. The acceleration sensitivity of thecone-housing sensor is above -15 dB (0 dB = 1 V/ms2) below 300 Hz and that of
the cylinder-housing sensor is above -20 dB. They are much higher than that ofa commercially available piezoelectric-accelerator-based sensor (S-17; Promo).
200L=20φ, D=4-10-20
cone-housing-30

cylinder-housing-40-50-60-70-80Figure 2:0.11Frequency in kHz

acceleration sensitivity
Sensitivity in dB [ ref 1 V/ms2 ]Sensitivity in dB [ ref 1 V/Pa ]pressure sensitivity

10

L=20φ, D=410
cone-housing

0cylinder-housing-10-20-30-40-50-60

S-17 (Primo)
0.1

1Frequency in kHz

Pressure and acceleration sensitivity of body-conducted sound sensors.

4. New soft polymer material for sensor
Urethane elastomer (EXSEAL Corp., Japan) is used as a soft polymer material
because it reversibly adheres to skin. We further investigated two polymer
materials that can possess reversible adhesive power to skin.
One material investigated was a nano-fiber of segmented polyurethane
fabricated by an electrospinning method. The nano-fabrication should promoteinteraction between the polyurethane and human skin because of the increased
surface area. The nano-fiber film could cover the whole ECM area; it was found,however, that the strength of adhesion on skin was insufficient compared withurethane elastomer.
IL141-2

International Conference on Natural Polymers (ICNP2010),
Kottayam, India, 24, 25 & 26, September 2010

Another material tested was a new class of adhesive gel based on N-[2-(3,4dihydroxyphenyl)-ethyl]-2-methyl-acrylamide (dopamine methacrylamide, DMA).
The use of DMA was inspired by adhesive proteins (APs) that the blue mussel,Mytilus edulis, biologically synthesizes. The APs contain significant amounts of
L-3,4-dihydroxy phenylalanine (DOPA). APs are believed to show adhesive andcohesive characteristics due to DOPA-mediated intermolecular interaction.
DMA was prepared according to the previously reported protocol by H. Lee etal. [3]. The chemical structure of DMA was confirmed by 1H- and 13C-nuclear
magnetic resonance (NMR) spectroscopy. The obtained DMA (0.68 g, 3.07mmol) and 2-hydroxyethylmethacrylate (HEMA) (0.4 g, 3.07 mmol) wereco-polymerized by a conventional radical polymerization method with
N,N’-azobisisobuthylonitrile (AIBN) (42 mg) in dimethylacetamide (DMF) (9.5 ml)
at 60°C overnight (Figure 3). The crude product was washed with ether threetimes to obtain the co-polymer [poly(DMA-co-HEMA)].
The poly(DMA-co-HEMA) (0.5 g) was mixed with dicyclohexylmethane-4,4'-diisocyanate (0.25 g) in an ECM, and then it was settled at room temperature
overnight. As a result, gel-like resin was obtained as shown in Figure 4. This wasO

AIBN+NHOOO

DMAc
60 C, overnightNHOOoHOHOnOHOHO

OH

2-hydroxyethyl methacrylate

DMAFigure 3:OHmRadial polymerization of dopamine methacrylamide and HEMA.

NHOOOOOHOOln-l
NHOHOHmOH
HONHOOHO

OOO

O

HNO

Figure 4: Proposed structure of gel obtained by crosslinking reaction andpoly(DMA-co-HEMA)-crosslinked gel-deposited body-conducted sound sensor.
IL141-3

International Conference on Natural Polymers (ICNP2010),
Kottayam, India, 24, 25 & 26, September 2010

due to a crosslinking reaction between hydroxyl groups of the poly(DMAco-HEMA) and diisocyanate group (Figure 4).
When the poly(DMA-co-HEMA) gel-deposited sensor was attached onto theback of a hand and the hand was turned upside down, the sensor remained
attached for at least for 30 min. In contrast, in the same situation, the urethane
elastomer-deposited sensor detached from the hand after 4 min. Thus, theadhesion strength of the poly(DMA-co-HEMA) gel was much greater than that of
the urethane elastomer. Although the poly(DMA-co-HEMA) gel-deposited sensor
revealed a strong adhesive property, the adhesive strength decreased with time.
Improving the durability is a future issue.
5. Recording body-conducted sounds
Using the sensor, we could record various kinds of body-conducted soundssuch as cardiac, blood-flow, vesicular, bowel peristalsis sounds and muscle
murmur. Figure 5 shows typical waveforms of cardiac sounds sensed at thechest (left) and finger pulsation sounds sensed at the index finger (right) with thesensor.
cardiac sounds

Normalized
amplitude

1finger pulsation sounds

0-1

0

1

2

3
Time [s]

45

6 02

4

6
Time [s]810

Figure 5: Examples of body-conducted sounds' waveforms recorded with sensor.

6. Conclusion
A compact high-sensitivity body-conducted sound sensor was developed.
Urethane elastomer was used as a soft polymer material for the sensor because
it reversibly adheres to the skin. A poly(DMA-co-HEMA) gel revealed a strong
adhesive property but its durability was poor. Various kinds of body-conductedsounds could be recorded with the sensor, suggesting that it can be used as an
input device for a vital-signs monitoring system as well as a life-log system.
Acknowledgements:
This work was supported by SCOPE of the Ministry of Internal Affairs andCommunications of Japan.
References[1] Makoto Otani et al., Numerical simulation of transfer and attenuation characteristics
of soft-tissue conducted sound originating from vocal tract, Applied Acoustics 70(3),469-472, 2009.
[2] Tatsuya Hirahara et al., Silent-speech enhancement using body-conducted vocaltract resonance signals, Speech Communication 52(4), 301-313, 2010.
[3] H. Lee, et al., A reversible wet/dry adhesive inspired by mussels and geckos, Nature448, 338-342, 2007.
IL141-4