'Materials that move in response to light developed'

Scientists have developed new materials that move when exposed to light, an advance that may enable a wide range of products which perform simple to complex movements, from tiny engines to solar arrays that bend towards the sunlight.

The light actuated materials created by researchers at the Tufts University in the US are based on the principle of the Curie temperature - above which certain materials will change their magnetic properties.

By heating and cooling a magnetic material, one can turn its magnetism off and on.

"We could combine these simple movements into more complex motion, like crawling, walking, or swimming," said Fiorenzo Omenetto, a professor at Tufts University.

"And these movements can be triggered and controlled wirelessly, using light," Omenetto said.

The team demonstrated some of these complex movements by constructing soft grippers that capture and release objects in response to light illumination.

"One of the advantages of these materials is that we can selectively activate portions of a structure and control them using localised or focused light," said Meng Li, first author of the research paper published in the journal Proceedings of the National Academy of Sciences.

"And unlike other light actuated materials based on liquid crystals, these materials can be fashioned to move either towards, or away from the direction of the light. All of these features add up to the ability to make objects large and small with complex, coordinated movements," Li said.

The researchers constructed a simple "Curie engine". A light actuated film was shaped into a ring and mounted on a needle post.

Placed near a permanent magnet, when a laser was focused onto a fixed spot on the ring, it locally demagnetises that portion of the ring, creating an unbalanced net force that causes the ring to turn.

As it turns, the demagnetised spot regains its magnetisation and a new spot is illuminated and demagnetised, causing the engine to continuously rotate.

Materials used to create the light actuated materials include polydimethylsoloxane (PDMS), which is a widely used transparent elastomer often shaped into flexible films, and silk fibroin, a versatile biocompatible material with excellent optical properties that can be shaped into a wide range of forms.