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.