Self-bending paper

South Korean researchers have invented a gold-plated cellophane that bends when charged. Thanks to this feature, it can act as a wing flap in a paper plane, insect robot, or smart wallpaper that emits sound.

This electric paper is being developed by Jaehwan Kim of South Korea's Inha University, which contains cellulose-like fibers. As a result, it is very light, cheap and biodegradable.

In the past, researchers have discovered a strange characteristic of cellulose: when charged under pressure. However, the charge called this piezoelectricity is too small to produce the necessary force to bend the paper, so its potential is not exploited.

Transparencies before setting voltage. Photo: Discovery

And after the voltage is set, it bends away. Photo: Discovery

Kim and his team find it possible to amplify the piezoelectricity by using charged particles (positive ions) in cellulose-based paper such as cellophane.

To create a "motor" for the paper, they covered one side of a 40-millimeter thin transparent sheet of paper. When placing a voltage on it, the gold layer becomes positively charged, while the other side of the paper is negatively charged.

Positively charged ions, attached to water molecules, become attracted to the negative side of the paper. As they migrate, they pull the water molecules along, creating a tension on one side of the paper, causing it to bend.

The combination of ionic motion and piezoelectric effect makes the paper curl faster even under low voltage. The shade is curved 10 millimeters in one direction with enough power to lift about 1 gram.

However, the mechanical strength of a piece of paper, though large, can not be enough for a variety of devices, says Paul Calvert, an expert in biomimetic materials at Dartmouth College in Massachusetts. know.

Kim and his collaborators are trying to add carbon nanotubes to the cellulose fiber to increase their charge and thereby increase muscle strength.

T. An