Flexible Hybrid Audio Speaker and Circuit

Flexible hybrid electronics (FHE) are being developed for sensors and smart tags.  Flexible actuating devices are another interesting technology which add further possible applications.  PARC has developed a flexible audio speaker system based on piezoelectric PVDF films and a FHE circuit to drive the speaker.  The PVDF speakers are about 25 micron thick, 7-10 cm in diameter, deposited on 10 µm thick polyimide substrates and operate at ± 30V.  Single layers and bi-morphs comprising two speakers bonded back-to-back were compared. Modeling shows that the bimorph gives a larger displacement amplitude and this was confirmed by measurement of the sound level.  The speakers produce easily audible sound with low harmonic distortion but the spectral response shows a sound power level (SPL) that increases with frequency. Two different FHE operating circuits were designed and fabricated.  One comprised an operational amplifier to take the headphone output from a cell phone and the second used an audio IC to record and play back a message.  Both circuits are made on flexible substrates using jet-printed Ag for the interconnects and jet-printed carbon nanotube inks for printed resistors. Printed resistors in the range 1-100 kohm have stability within about 3% when laminated with a thin plastic foil.  Both circuits operate from a 3V dc supply with a conventional PCB to give ± 30V to drive the speakers.  The demonstration of an FHE audio system opens the possibility of a similar technology applied to other actuator applications.