Multi-material freeform 3D printing of flexible piezoelectric composite sensors using a supporting fluid

Abstract

Three-dimensional (3D) printed flexible piezoelectric devices featuring conformal or freeform geometries show great potential for being fabricated into wireless sensors and energy harvesters with tailorable mechanical stiffness and piezoelectric response. In our work, supporting fluid-assisted multi-material extrusion-based 3D printing technology has been successfully utilized to fabricate flexible conformal or freeform piezoelectric composite sensors with integrated electrodes. The printing technique relies on the extrusion of two paste-like materials: piezoelectric and conductive composite inks. A comprehensive characterization of the inks (i.e., piezoelectric or conductive behaviors, rheological and mechanical properties) and the rheological behavior of the supporting fluid are performed. Polydimethylsiloxane (PDMS)/30 vol% lead zirconate titanate (PZT), PDMS/25 vol% silver (Ag) and mineral oil/6% (w/v) fumed silica are the formulations for piezoelectric ink, conductive ink and supporting fluid, respectively. Three types of piezoelectric composite demonstrators which are a multi-layer planar film, a conformal non-planar hemisphere and a 3D structure composed of six spirals printed vertically between two hexagon layers (freeform spiral-hexagon), are fabricated and tested under tension or compression tests. The piezoelectric performance is consistent with the applied stress in all the tests. For example, the freeform spiral-hexagon piezoelectric sensor has a peak-to-peak voltage output of 86.39 ± 1.145 mV when it is subjected to a cyclic compression force at 8 Hz for more than 800 cycles. The dimensional accuracy measurements using an optical microscope and the microstructure images taken using a scanning electron microscope (SEM) show that the fabricated 3D structures have good shape fidelity (with a maximum relative error of ~3.5%) when compared to the designed models. Our fabrication approach opens a new way to fabricate conformal and freeform piezoelectric structures with integrated electrodes from flexible composites for sensing and energy harvesting applications.