Development of piezoelectric acoustic sensor with frequency selectivity for artificial cochlea

In this paper, we report a novel piezoelectric artificial cochlea which realizes both acoustic/electric conversion and frequency selectivity without an external energy supply. The device comprises an artificial basilar membrane (ABM) which is made of a 40 mu m thick polyvinylidine difluoride (PVDF) membrane fixed on a substrate with a trapezoidal slit. The ABM over the slit, which mimics the biological system, is vibrated by acoustic waves and generates electric output due to the piezoelectric effect of PVDF. The width of ABM is linearly varied from 2.0 to 4.0 mm along the longitudinal direction of 30 mm to change its local resonant frequency with respect to the position. A detecting electrode array with 24-elements of 0.50 x 1.0 mm rectangles is made of an aluminum thin film on ABM, where they are located in a center line of longitudinal direction with the gaps of 0.50 mm. Since the device will be implanted into a cochlea filled with lymph fluid in future, the basic characteristics in terms of vibration and acoustic/electric conversion are investigated both in the air and in the silicone oil which is a model of lymph fluid. The in vitro optical measurements show that the local resonant frequency of vibration is varied along the longitudinal direction from 6.6 to 19.8 kHz in the air and from 1.4 to 4.9 kHz in the silicone oil, respectively. Since a resonating place vibrates with relatively large amplitude, the electric output there becomes high and that at the other electrodes remains to be low. Thus, the electric voltages from each electrode realize the frequency selectivity. Furthermore, the effect of surrounding fluid on the vibration is discussed in detail by comparing the experimental results with the theoretical predictions obtained by the Wentzel-Kramers-Brillouin asymptotic method. The theoretical prediction indicates that the surrounding fluid of the higher density induces the larger effective mass for the vibration that results in lower resonant frequency. From these findings, the feasibility of artificial cochlea is confirmed both experimentally and theoretically. (C) 2009 Elsevier B.V. All rights reserved.