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5 Hearing

Figure 5.10: A piezoelectric material on a flexible substrate is used as an energy source for implanted

devices [18]. (a) Schematic illustration of the fabrication process and biomedical application of flexible

PMN-PT piezoelectric energy harvester. (b) Cross-sectional SEM image of the PMN-PT thin film on a PET

substrate. The inset shows an XRD pattern of PMN-PT thin film. (c) Raman spectrum obtained from PMN-PT

thin film. The indexed sharp spectra agrees well with the typical features of perovskite PMN-PT.

makes it also effective as a vibration sensor. But the film can also be used as an energy

source for implanted devices (Figure 5.10) ([17] and references therein).

5.4 Summary and the Bigger Picture

Our acoustic sensor, the ear, is actually rather complicated: vibrations in air are con-

verted into mechanical vibrations in a membrane, into vibrations in water, back into

mechanical vibrations of a membrane that then are transduced into electricity via the

bending of fiber that mechanically opens an ion channel. Why is the ear so complicated,

when a microphone can record sound much more easily and still produce excellent

sound quality? Basically, the sensitivity of the ear is important because any sound is

a mixture, and the basilar membrane is able to separate out the wavelengths and am-

plify and detect each of them separately. The wavelengths are only mixed together again

in the brain to create the original sound.

There is research currently being carried out into biomimetic hair cells to try to

copy these additional features on several length scales, but no one has yet achieved a

microphone with the efficiency of the basilar membrane. Additionally, vibration sensors

can be used as flow sensors and energy harvesting devices.