Flexible nanogenerator harvests energy from human motion

January 02, 2017 // By Julien Happich
In a Nano Energy paper titled "Flexible and biocompatible polypropylene ferroelectret nanogenerator (FENG): On the path toward wearable devices powered by human motion," researchers from Michigan State University (East Lansing, MI) have shared an innovative polymer-based ferroelectret doing away with magnets and ferroelectric materials altogether.

The researchers have built a thin polypropylene ferroelectret (PPFE) by creating a cheap polypropylene foam whose empty voids they charged through microplasma discharges (by applying a large electric field to the PP film). The artificial voids in the foam, spanning from the 1μm scale to the 100μm scale then form highly oriented giant dipoles across the 80μm thick film.

Two conductive silver layers sputtered on the surfaces of the PPFE film complete the device, turning it into a sandwich-like metal-insulator-metal (MIM) structure. Supporting their practical experimentations with finite element method (FEM) analysis, the researchers explain that as the charged voids change their thickness and thus their dipole moments under mechanical stress (compression for example), the change of dipole moments is capable of driving the electrons from the electrode with negative charge to the electrode with positive charge. 

This generates a voltage under open circuit conditions, or a current under short circuit condition (a flow of charge from one electrode to the other). They also highlight the PPFE films' piezoelectric coefficient (d33∼400 pC/N) as being significantly greater than that of typical piezopolymers like PVDF (d33∼15pC/N) or parylene-C (d33∼2 pC/N).

Exploded-view illustration of the encapsulated FENG consisting of a stacked metal-PPFE-metal structure without moving parts or micro-fabricated features. Source Michigan State University.

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