Schematic diagrams of the fabrication process for the piezoelectric hemispheres embedded stretchable composites.A flexible, self-powered piezoelectric sensor for potential use by Alzheimer’s patients has been developed by a team of Korean researchers.
Large-scale energy harvesting technologies have played a major role in our energy landscape for over a decade. But in addition, there have been a number of extensive investigations into smaller-scale, ambient technologies, such as piezoelectric nanogenerators. Piezoelectricity can be used to convert mechanical energy to electricity - by applying a stress to a piezo-material, a voltage is produced inside the crystal, driving electron flow. But this process is also reversible, meaning that piezoelectric nanogenerators can act as motion sensors, sensitive to even the smallest displacements.
A paper published in Nano Energy [DOI: 10.1016/j.nanoen.2014.10.010] reports on the development of a new, stretchable piezoelectric film that could be used as a self-powered motion sensor for a range of biomechanical applications. A number of composite nanogenerators have appeared in the literature in the past five years, with most focusing on nanowires. In all cases, highly-flexible or stretchable substrates are used, in order to measure forces in the micronewton range. Jeong Min Baik’s device consists of a stretchy composite film of highly-ordered piezoelectric micro-hemispheres embedded in a polydimethylsiloxane (PDMS) matrix.
Polystyrene beads of varying diameter (0.5 – 10µm) were used as a template on which to deposit the piezoelectric zinc oxide (ZnO) or lead zirconate titante (PZT) films, and to produce the periodic hemispheres required. Once the PDMS solution was poured and dried around the template, ITO/PET films were attached on the top and bottom sides of the structures, to act as electrodes. In energy harvesting (power generation) mode, the composite containing 10 µm diameter particles of ZnO performed best (4 V and 0.13μA/cm2).
In motion sensor mode, the team attached a 15 x 15 mm piece of the composite film to the top side of a person’s wrist. They found that the output voltage of the composites varied with the bending direction - from 1.1 V with the wrist under convex bending, to 0.25 V under concave bending. This result demonstrates the potential for the use of piezoelectric nanogenerators as ultrasensitive strain sensors in smart skin applications. The team hope that their devices could help to monitor the instability and gait disturbances common in patients suffering from diseases such as Alzheimer's or Parkinson's.
Chun, J. et al, “Highly anisotropic power generation in piezoelectric hemispheres composed stretchable composite film for self-powered motion sensor”, Nano Energy (2015) 11, 1–10. DOI: 10.1016/j.nanoen.2014.10.010.