Magnetic composites show off their moves under the spotlight

Researchers at Tufts University have developed magnetic elastomeric composites that move in different ways when exposed to light. These materials could form the basis for a wide range of products that perform simple to complex movements, from tiny engines and valves to solar arrays that bend toward the sunlight. The research is described in a paper in the Proceedings of the National Academy of Sciences.

In biology, there are many examples of light inducing movement or change – think of flowers and leaves turning toward sunlight. The light actuated materials created in this study are based on the principle of the Curie temperature – the temperature above which the magnetic properties of certain materials will change. This means that by heating and cooling certain magnetic materials, their magnetism can be turned off and on.

The researchers showed that biopolymers and elastomers doped with ferromagnetic chromium dioxide (CrO₂) will heat up when exposed to laser or sunlight, temporarily losing their magnetic properties until they cool down again. The basic movements of the material, shaped into films, sponges and hydrogels, are induced by nearby permanent magnets or electromagnets and can exhibit as bending, twisting and expanding.

"We could combine these simple movements into more complex motion, like crawling, walking or swimming," said Fiorenzo Omenetto, corresponding author of the paper and a professor of engineering in the School of Engineering at Tufts. "And these movements can be triggered and controlled wirelessly, using light."

Omenetto's team demonstrated some of these complex movements by constructing soft grippers that can 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 localized or focused light," explained Meng Li, the first author of the paper. "And unlike other light-actuated materials based on liquid crystals, these materials can be fashioned to move either toward 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."

To demonstrate this versatility, the researchers constructed a simple ‘Curie engine’. This involved shaping a light-actuated film into a ring, which is then mounted on a needle post and placed near a permanent magnet. When a laser is focused onto a fixed spot on the ring, it locally demagnetizes that portion of the ring, creating an unbalanced net force that causes the ring to turn. As it turns, the demagnetized spot regains its magnetization while a new spot is illuminated and demagnetized, causing the engine to rotate continuously.

The researchers used various materials to create their light-actuated composites. These included polydimethylsoloxane (PDMS), 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 – from films to gels, threads, blocks and sponges.

"With additional material patterning, light patterning and magnetic field control, we could theoretically achieve even more complicated and fine-tuned movements, such as folding and unfolding, microfluidic valve switching, micro and nano-sized engines and more," said Omenetto.

This story is adapted from material from Tufts University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.