Piezoelectric foam? 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.