An Exploration into the Structuring of Soft Piezoelectric Transducers for Wearable Applications

Wearable ultrasound applications require a new generation of ultrasound systems, which should be compact, lightweight and facilitate good skin contact while measuring. These characteristics are difficult to achieve using conventional probes. One possible alternative to conventional rigid transducers is the use of ‘soft’ polymeric piezoelectric materials. The fabrication of these transducers requires patterning of the piezoelectric layer to reduce acoustical crosstalk. Here we present on the inherent trade-offs present when structuring such a soft piezoelectric, PVDF-TrFE, using hot embossing with a PDMS stamp. We demonstrate that the stamping fidelity is governed by the geometrical properties of the stamp. An optimum geometry was found in hexagonal elements, to maximize the effective transducer surface area while ensuring the integrity of the PDMS stamp during the embossing step. In addition, we show that the thickness of the residual layer after embossing significantly influences the transducer performance, since the electric field above the air cavities in between the piezoelectric pillars affect the field lines in the pillar region. Finally, we have measured the efficiency of the transducer using a split transmit and receive phased array transducer in combination with a hydrophone setup. We measured a peak transmit transfer of 5.0 kPa/V after electrical tuning at 8.2 MHz with ∼80% -6 dB bandwidth, and a peak receive transfer at 7.3 MHz of ∼150 μV/Pa with a -6 dB bandwidth of 105%. The maximum electrical crosstalk was measured to be <-63 dB, while no acoustical crosstalk could be observed.