Schematic of the manufacturing process and images of the fabricated lattices and shoe sole, as well as the cyclic compression test conducted on the shoe sole (photo of the test setup and voltage output).A new flexible three-dimensional piezoelectric composite transforms an ordinary shoe into an energy harvesting device, according to researchers. Piezoelectric materials convert mechanical energy into useful electrical energy but while ceramics have better piezoelectric performance than polymers, they are intrinsically brittle.
To overcome this shortcoming, the researchers from Polytechnique Montreal, Research Center for High Performance Polymer and Composite Systems, and McGill University in Canada developed a flexible piezoelectric nanocomposite filament by incorporating ceramic piezoelectric lead zirconate titanate (PZT) nanoscale fillers into a thermoplastic polyurethane matrix (TPU). Flexible 3D piezoelectric devices with integrated electrodes can be created from the nanocomposite in a single manufacturing process using a multi-material fused filament fabrication (FFF) 3D printing technique [Tao et al., Applied Materials Today 29 (2022) 101596, https://doi.org/10.1016/j.apmt.2022.101596 ].
“Traditional piezoelectric structures are mainly simple 1D fiber or 2D film shapes,” explains Rui Tao, first author of the study. “[We wanted to see] if light-weight 3D piezoelectric lattices with designed configurations could have higher voltage outputs than their fully dense solid counterparts.”
Led by Daniel Therriault, the team fabricated four different polymer-PZT nanocomposite structures–from a simple cube-like lattice to more complex body-centered cubic, cubo-octahedral, and octet truss lattices–with integrated electrodes. They compared the voltage output from experimental results and finite element analysis (FEA) simulations of the different lattices and a fully dense solid counterpart under compression. The octet truss lattice generates twice as much voltage as the fully dense solid composite under the same compressive conditions. A nanocomposite of TPU/30 vol% PZT shows the best compromise between highly piezoelectric behavior, printability, and mechanical flexibility.
To demonstrate the energy-harvesting possibilities of the nanocomposite, the researchers used the octet truss lattice to fabricate a shoe sole, which generated a stable voltage output at a typical walking frequency (2 Hz) over 200 cycles in a compression test. With up to 20 V generated in a single ‘stomp’ by a 60 kg person, the shoe sole can power an LED.
“The multi-material 3D printed flexible piezoelectric nanocomposite devices could be conformally applied to soft, irregularly shaped surfaces and used in lightweight, battery-free wearable sensors and energy harvesters,” he says.
Therriault is confident that the printing quality can be further improved, other lattice configurations can be designed to optimize piezoelectric performance, and the range of smart materials used in the multi-material 3D printing process can be broadened to include shape memory and triboelectric materials for advanced multifunctional devices.