EH relies on smart materials, capable of directly transforming energy from one form to another. In Kinetic EH, motion is the primary source of energy and piezoelectric materials are the prime choice as they can convert mechanical work into electrical energy directly.
Piezoelectric ceramics like PZT have been extensively studied for the application to EH, including wearable EH . However, in this latter case of harvesting energy from the human body, ceramics are not ideal due to their high stiffness and density. Lighter and more compliant materials like the cellular polypropylene (cell-PP) shown in this SEM photograph have clear advantages in this application. Cell-PP becomes piezoelectrically active when it is exposed to large electric fields: the air within the cells suffers breakdown and electrical charges are embedded in the cells’ walls .
Currently, the production process of cell-PP does not permit the fine control of the size and dimensions of the cells.
 M. Pozzi, M. S. H. Aung, M. Zhu, R. K. Jones, and J. Y. Goulermas, ‘The pizzicato knee-joint energy harvester: characterization with biomechanical data and the effect of backpack load’, Smart Mater. Struct., vol. 21, no. 7, p. 075023, Jul. 2012.
 M. Paajanen, M. Wegener, and R. Gerhard-Multhaupt, ‘Understanding the role of the gas in the voids during corona charging of cellular electret films – a way to enhance their piezoelectricity’, J. Phys. Appl. Phys., vol. 34, no. 16, p. 2482, 2001.