PAPER POWER [+]Enlarge Credit: ACS Nano

A new transparent-paper device can generate electrical power from a user’s touch (ACS Nano 2015, DOI: 10.1021/acsnano.5b02414). The paper energy-harvester could be used to make disposable, self-powered touch screens that fold; interactive light-up books; touch-sensitive skin for prosthetics; and security systems for art and documents, the researchers say.

Research on paper electronics has taken off in the past decade given their promise for light, flexible, low-cost devices made from a renewable resource. The goal is to use paper as a substrate for circuits instead of glass or plastic.

For high-quality electronics, people have turned to nanopaper, a tangled mat made of cellulose fibers that are nanometers wide, instead of the micrometer-sized fibers found in regular paper. The tinier fibers make nanopaper transparent. They also make it smooth like plastic, ideal for depositing ultrathin layers of electronic materials for circuits. Various teams have made organic light-emitting diodes, transistors, and antennae on nanopaper. But these devices needed an external power source.

Liangbing Hu of the University of Maryland; Jun Zhou of Huazhong University of Science & Technology, in China; and their colleagues wanted to make nanopaper electronics self-powered. So they designed a nanopaper-based generator that converts mechanical energy into electrical power.

To make the device, the researchers used carbon nanotubes to coat two sheets of nanopaper, which served as a pair of electrodes. Then they coated one of the nanotube films with a 30-µm-thick film of polyethylene (PE) and pressed the edges of the two sheets together, sandwiching the PE in the middle and leaving a tiny air gap between the sheets. Before making the sandwich, they subjected the PE film to a high-voltage electric field, which injected it with extra electrons and made it negatively charged. This induces balanced positive charges on the nanotube film surfaces next to the PE.

The generator operates via electrostatic induction. Pressing the device narrows the air gap, bringing the PE-coated electrode closer to the other electrode, increasing the positive charge on that electrode. This change in charge balance between the nanotube electrodes results in a flow of current through the device. Releasing the pressure causes electrons to flow back, so repeated pressing and releasing creates continuous current.

The device is robust, producing a steady current for more than 54,000 press-release cycles. The researchers reported that pressing a 2- by 2-cm generator produced enough current to light up a small liquid-crystal display.

Because of the device’s transparency, the researchers propose its use in security and anticounterfeit systems for valuable paintings and documents. For instance, a sensor patch fixed on a painting could trigger an alarm when someone touches it. The team imagines other applications as well. “This device is invisible, and it’s powered by human finger,” Hu says. “So you could make any document, toy, or book interactive.” Zhou adds that his group plans to extend the technology for electronic skin.