Magnetic Materials Self-Actuate in Response to Light

Materials that move in response to light can be used in various new products, including tiny engines and valves as well as solar arrays that bend toward the sunlight.

One of the current trends in materials science is to design materials that can self-actuate or move using remote control in various ways. At Tufts University School of Engineering, for example, a team developed magnetic elastomeric composites that move in different ways when exposed to light. The magnetic aspect of the materials, which were developed by a team led by engineering professor Fiorenzo Omenetto, is key to the work. It is based on the principle of the Curie temperature—the temperature above which certain materials will change their magnetic properties.

light responsive material
Incident light (from right) causes an elastomer film to deflect in a magnetic field. The film is one of the materials developed by researchers at Tufts University that can actuate in various ways in response to light. (Image source: Tufts University School of Engineering)

Switched On

Researchers discovered that by heating and cooling a magnetic material, they can switch off and on its magnetism. Biopolymers and elastomers doped with ferromagnetic chromium oxide will heat up when exposed to laser or sunlight, they found. This causes them to temporarily lose their magnetic properties until they cool down again.

In this way, nearby permanent or electromagnets can affect the basic movements of the material—which can be shaped into films, sponges, and hydrogels—causing bending, twisting, and expansion, Omenetto said in a Tufts University news release.

“We could combine these simple movements into more complex motion, like crawling, walking, or swimming,” he explained. “And these movements can be triggered and controlled wirelessly, using light.” 

Common Materials

Specific materials used to create the light-actuated materials include polydimethylsoloxane (PDMS)—a widely used, transparent elastomer often shaped into flexible films. Researchers also used silk fibroin—a versatile, biocompatible material with significant optical properties that can be shaped into a wide range of forms, such as films, gels, threads, blocks, and sponges.

In experiments, the team demonstrated the material’s movements by creating soft grippers that capture and release objects in response to light illumination. Researchers selectively activated portions of a structure and controlled them using localized or focused light, paving the way for the potential ability to make objects in various sizes with complex, coordinated movements, they said in the release.

Another experiment the team performed was to construct what’s called a simple “Curie engine.” They shaped a light-actuated film into a ring and mounted it on a needle post. They then placed it near a permanent magnet. Next, they focused a laser on a fixed spot on the ring, which serves to locally demagnetize that portion of the ring—creating an unbalanced net force that causes the ring to turn, researchers said. As it turns, the demagnetized spot regains its magnetization, illuminating and demagnetizing a new spot and causing the engine to continuously rotate.

Researchers envision various applications for the materials, Omenetto said. These include a range of products that perform simple to complex movements, such as tiny engines and valves or solar arrays that bend toward the sunlight.

The team published a paper about the work in the journal Proceedings of the National Academy of Science.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York City. In her free time, she enjoys surfing, traveling, music, yoga, and cooking. She currently resides in a village on the southwest coast of Portugal.

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