Fish scales extracted from food waste have been used to build tiny generators that can convert mechanical energy, such as a touch or sound vibrations, to electrical energy. The work was done by physicists in India, who say that the piezoelectric device could be used to develop environmentally friendly, self-powered electronics with a wide-range of applications.

Piezoelectric materials respond to mechanical stress by separating positive and negative electrical charge, and therefore can be used to convert the mechanical energy of vibrations into electrical energy. Piezoelectric generators that harvest energy from vibrations in the living environment would allow the development of fully independent, battery-free devices. These could be particularly useful for medical devices, such as pacemakers and insulin pumps, and targeted drug-delivery systems that consume little power, but need it continuously.

But, to realize the full potential of such devices, researchers need to develop new environmentally friendly piezoelectric materials. Dipankar Mandal, a physicist at Jadavpur University in Koltata, India, says this is "simply because most of the traditional piezoelectric materials contain toxic elements, such as lead and bismuth". As well as being useful for biomedical applications, Mandal adds that non-toxic and environmentally friendly piezoelectric materials would also reduce electronic waste and society's dependence on traditional energy sources, like batteries, which often contain toxic elements.

Tipping the scales

Fish is a popular food in India and one possible source of non-toxic piezoelectric materials in that country is the large quantity of fish scales that are disposed of as waste. The scales are composed of collagen nano-fibrils, which are known to have piezoelectric properties, and this inspired Mandal and colleague Sujoy Ghosh to see if they could use waste scales to produce a cost-effective, piezoelectric nano-generator.

Collagen consists of three polypeptide chains that twist together to form a triple-helical structure. Hydrogen bonds between the polypeptide chains all orientate in the same direction and act as molecular dipoles, resulting in spontaneous electrical polarization and piezoelectric properties.

Within fish scales, collagen nano-fibrils self-assemble and align. "We wanted to explore what happens to the piezoelectric yield when a bunch of collagen nanofibrils are hierarchically well aligned and self-assembled in the fish scales," Mandal explains. To build their piezoelectric device, the researchers washed and then treated fish scales – collected from a local fish market – with an acidic demineralizing solution to make them transparent and flexible. They then attached gold electrodes to these flexible, transparent scales and laminated them with a polypropylene film to create a robust "bio-piezoelectric nano-generator".

Tests showed that the device had an intrinsic piezoelectric response of around 5 pC/N. And it was able to harvest energy from various ambient motions, including body movements, machine and sound vibrations, and wind flow.

Green power source

When subjected to a repeated compressive stress of 0.17 MPa the nano-generator produced an output of 4 V with a current of 1.5 μA – or 6 μW of power. The researchers also linked four of the devices together and were able to produce a voltage of 14 V. By gently slapping this device with their hands, they were able to switch on more than 50 LEDs. According to the researchers, this demonstrates that it is "a sustainable green power source".

Mandal told "The piezoelectric output is really promising, particularly if we look at the instantaneous piezoelectric energy-conversion efficiency and similar-sized available biocompatible piezoelectric materials." He adds that the research could have enormous potential for "tiny electronic gadgets, health-care monitoring, self-powered implantable bio-medical devices, targeted drug delivery, national security and defence applications".

The researchers now plan to scale up the nano-generator and "test it in different bio-medical and self-powered devices".

The device is described in Applied Physics Letters.