Ferroelectric Polymer for Bio-Sonar Replica

The sensorial knowledge paradigm has captured the interest of many eminent scholars in past centuries (the philosophical trend of “Sensism” was developed around the “Gnoseologic Paradigm”, which has found its highest expression in Etienne Bonnot de Condillac, 1930) as well as in the modern era, particularly in the attempt to interface the external environment to humans through artificial systems. Of the five human senses, which have been investigated by scientists involved in artificial perception studies, vision, touch and hearing have received the most attention, each one for different reasons. When referring to hearing as the sense which perceives sound (the mechanical perturbation induced in a medium by a travelling wave at suitable frequency), a distinction should be made. Indeed, sound between 100 Hz and 18 kHz refers mainly to the range of human perception, while infrasound (up to 20 or 30 Hz) and low frequency ultrasound (from 20 to 120 kHz) refer to animal (mammalian) perception. Low frequency ultrasounds have been amply investigated in the last century and the resulting applications have been made in both military and civil fields. In any case, it appears relevant and necessary to improve the performance of the ultrasonic system (more properly named sonar) for use in a variety of industrial, robotic, and medical applications where ranging plays a basilar role. Nevertheless, other important information can be extrapolated through proper use of the ultrasonic signal as is evident from the study of the biology and mammalian behaviour (Altringham, 1996). Up to now, attempts have been made to try to emulate animal auditory systems by using both commercial or custom piezoelectric transducers. In this context, the latest investigation in artificial perception was mainly inspired by bat bio-sonar, which has been extensively studied and described by biologists. As a result of the damping exerted by the propagation medium, which increases as the ultrasound frequency increases, conventional transducers normally function at relatively low frequencies (40 ÷ 50 kHz) in air. Sometimes this restricts choices of piezoelectric materials, besides transducer shape and dimensions. In order to increase the frequency and hence to improve the performances of ultrasonic transducers, flexible plastic materials, such as the ferroelectric polymer polyvinylidene fluoride (PVDF) were investigated and assembled in different geometries. It was discovered that, when properly shaped, PVDF films can resonate at frequencies superior to 100 kHz, covering the full range frequency of the majority of bat bio-sonars (20 ÷ 120 kHz). The first part of a work aimed at emulating the auditory system of Pteronotus Parnellii, (also known as the moustached bat) is described in this chapter. We have simulated some of this