Linked liquid crystals move matter

By Kimberly Patch, Technology Research News

Picture a field of wheat with the force of wind causing the tops of the plants to bend closer to the ground. To harness the movement of the wheat to do the work of lifting a large piece of cardboard, you could get the plants to push on the cardboard when the wind dies and they spring back up. It might be tricky, though, because the wheat plants, especially if they are tall, might simply slip around the cardboard.

Researchers from Germany have solved a similar problem in producing a material that uses the liquid crystals commonly used in laptop computer screens to convert electrical energy into mechanical work. The material could eventually be used to power microscopic machines.

Liquid crystal displays sandwich liquid crystals between transparent electrodes; the screen changes when the liquid crystals reorient under the influence of an electric field.

The tricky part of using liquid crystals to do work is their slippery structure. Liquid crystals have a stable crystalline structure only in one dimension. In the other two dimensions they have a liquid structure, which prevents them from exerting mechanical force.

This makes them rather like that field of wheat. In the presence of an electric field, the long, rod-like, molecules that make up the liquid crystal tilt, compressing the material. "However, [they] could not be used for applications until now, because the liquid crystal molecules would just flow around any structure that should be actuated," said Friedrich Kremer, director of the Institute for Experimental Physics at the University of Leipzig.

The researchers solved the problem by chemically tying off the molecules so they would not have room to slip around what they were supposed to push. "The trick is to tie the tilting liquid crystal molecules to a long polymer backbone molecule via flexible spacer molecules," said Kremer. "This leaves enough mobility for the liquid crystal molecules to react to the electric field by tilting, but does not allow them to flow around. This chaining allows them to do work," he said.

Combining the properties of liquid crystals and the polymers made a material that required two orders of magnitude less electricity to change shape than a material made only of the polymers used in the backbone, said Kremer.

The chained liquid crystal material, ferroelectric liquid crystalline elastomer (FLCE) changes shape by four percent, according to the researchers.

Historically, crystals like quartz have been used in a similar fashion to translate electrical energy into the mechanical action of watch hands, but the amount a crystal can strain, or change shape and spring back, is less than 0.1 percent, said Kremer. The FLCE's four percent potential for change could be used to produce work in the burgeoning field of nanotechnology, he said. For example, "a hypothetical cubic film of 100 microns width and 100 microns thickness would decrease in thickness by four microns [using] 150 volts," Kremer said.

The material is "a step in the right direction in the technology of developing biomimetic or plastic like materials -- robotic materials that are controlled by an electric field," said Mohsen Shahinpoor, a professor of mechanical engineering and neurosurgery at the University of New Mexico, and director of its Artificial Muscle Research Institute.

However, the voltage needed for the new material is still relatively high, he said. "They added a liquid crystal elastomer to the ferroelectric [polymer] to reduce the voltage. But the voltage is still high," which is a disadvantage in medical applications he said. "The advantage is just that's another type of soft material, which is good. There may be applications for that," Shahinpoor said.

The researchers' next steps are to tune the materials performance, find better ways of manufacturing it, and find applications for it. "FLCE's could be directly used as sensors or actuators," said Kremer. Simple actuators could be made immediately, but it will take 10 years or longer to produce devices for specific applications, he said.

Eventually, the material could be used as a type of artificial muscle, he said. "The ultimate aim is the artificial muscle, with all its possibilities of creating nanoscale robots and other artificial creatures," he said.

Kremer's research colleagues were Walter Lehmann, Holger Skupin, Peter Krüger, and Mathias Lösche of the University of Leipzig in Germany, and Christian Tolksdorf, Elisabeth Gebhard, Rudolf Zentel of the University of Mainz in Germany.

They published the research in the March 22, 2001 issue of the journal Nature. The research was funded by The German Research Foundation (DFG).

Timeline:   Now, 10 years
Funding:   Government
TRN Categories:  Nanotechnology; Semiconductors
Story Type:   News
Related Elements:  Technical paper, "Giant Lateral Electrostriction in Ferroelectric Liquid-Crystalline Elastomers," Nature, March 22, 2001.


April 25, 2001

Page One

Shaped waves promise speed

Touch improves VR collaboration

Manners matter for the circuit-minded

Network tools handle hefty science files

Linked liquid crystals move matter


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