The requirements of a piezoelectric device based inner ear cochlear implant are high sensitivity, millimeter dimensions, flexibility, acoustic impedance matching and biocompatibility. Two cantilever devices, based on cochlear structures, were designed for investigation as cochlear implants. The specific material chosen was poly(vinylidene fluoride)(PVF2), used in the bending/flexure mode of deformation. Fabrication of devices of approximately implantable size was demonstrated. The piezoelectric sensitivity of devices to both airborne and underwater acoustic waves was measured. Very high sensitivities have been obtained; the response in air is several Volts for conversational sounds at close range. Underwater sensitivity is ≈ -200 dB in the 2 - 10 kHz range. Dependence of sensitivity on device dimensions and dielectric coating was investigated. Also, 2-element devices were found to be approximately 1.5 times more sensitive than 1-element devices, proving that amplification can be obtained by increasing the number of piezoelectric elements. A sub-kHz experimental modal analysis study revealed that cantilevers in the length range ≈ 2 - 20 mm have three bending resonances < 1 kHz, all resonances are piezoelectrically active (keff = 0.2 - 0.35), and the resonance frequency depends upon material properties, film dimensions and electrode properties. The suitability of PVF2 material for use in the temperature and frequency conditions faced by an implant was also confirmed. Also investigated was the origin of bending piezoelectricity in single films of PVF2. Thermal current studies revealed the existence of single-polarity space charges, which can contribute to bending piezoelectricity. Small angle x-ray scattering studies revealed the lamellae are not perpendicular to the machine direction, but make an angle with all the film axes. Lamellae are stacked in the form of fibers oriented in the direction of film drawing. Thus the mesoscale structure of the polymer is heterogeneous, and bending piezoelectricity is believed to originate as a result of the heterogeneous structure and from space charge effects. Lastly, some first generation devices have been specified.
|Advisor:||Roseman, Rodney D.|
|School:||University of Cincinnati|
|School Location:||United States -- Ohio|
|Source:||DAI-B 79/10(E), Dissertation Abstracts International|
|Keywords:||Acoustic, Cochlear implant, Piezoelectric, Polymer, Polyvinylidene flouride|
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