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An interesting property of this seemingly very simple to manufacture polypropylene ferroelectret developed by researchers from the Michigan State University is that it is not only robust and easily scalable in area, but stacking several polypropylene ferroelectrets (PPFEs) to increase voltage or current output is as simple as folding a single unit upon itself. In effect, the symmetric folding process keeps the surfaces of same polarity in electric contact, akin to electrically connecting single layers of PPFE in parallel.

Effectively, the researchers' experiments show that both the open-circuit voltage (Voc) and shortcircuit current (Isc) are doubled with each folding along an axis of symmetry (equivalent to doubling the piezoelectric coefficient d33 of the unfolded state).

MSU energyharvest PPFE 02 (cr) Figure 1: Output voltage and current signals are amplified with each folding action along an axis of symmetry. The performance follows a 2n behaviour, where n is the number of folds. (Source: Michigan State University)

Applying touch pressure, a non-folded 35mm × 25mm PPFE film would output about 1V in open circuit, or generate a current of about 0.1μA in short circuit. Opposite charge changes and signals are generated when releasing the pressure, the material exhibiting a pretty much symmetrical behaviour.

The charges could either be cumulated into a nearby capacitor for energy storage (connecting the PPFE through a Schottky bridge rectification circuit) or used to power small electronics.

To demonstrate the energy harvesting capability of the novel PPFE film, the researchers created a 2cm x 2cm FENG consisting of a stack of 7 PPFE film layers. Upon one press of the hand, the 40mm2 device provided enough energy to power a series of 20 commercial LEDs operating at around 3V (in this configuration Voc and Isc reached higher than 50V and 5μA, respectively).

In another demonstration, the researchers created a foldable PPFE-based self-powered keyboard (with stickers for the keys). Here the PPFE's top and bottom surfaces were uniformly coated with electrically conductive paint (through a simple bar-coating process). Key strokes were enough to power the individual signal traces for the corresponding characters to be sent to a nearby device, which could make the rollable and foldable keyboard an interesting alternative to today's rigid battery-powered designs.

Associate Professor of Mechanical Engineering at the Michigan State University and corresponding author Nelson Sepulveda envisages that the new FENG could be used in many energy harvesting applications, converting human motion to electrical energy in order to power wearables or even implantable electronics (the devices can be coated with bio-compatible polyimide).

Another tentative application showcased in the paper is the use of PPFE film to enable self-powered touchscreens, where the touch function would be powered by the actual touch action.

The researchers integrated their FENG with a 4-bit LCD screen, and a gentle tap was enough to drive the LCD to display characters without any rectifiers or charging circuits. "Such integration could increase the energy efficiency of smart phones and wearable devices by scavenging energy from the users' touch during regular operation", the researchers wrote in their paper, "thus reducing the frequency of required battery charges from external energy sources."

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