Researchers have discovered that polypropylene foam (used widely in
packaging) acquires piezoelectric properties after it is zapped with
several thousand volts. Air in the pores break down into electrons and
positively charged ions that cling to opposite walls of the cavities.
It is already being used in keypads and musical instrument pickups.
Future applications include conformable ultrasound arrays for medical
instrumentation and even embedded ultrasonic "bar code" type configurations
built into products for security and identification. The only drawback
that needs to be overcome is the material loses its piezoelectric properties
above 55° C.