Researchers Create Elastomeric Composites that Move in Response to Light

Scientists at Tufts University School of Engineering have created magnetic elastomeric composites that move in various ways when exposed to light, raising the probability that these materials could enable a broad range of products that perform simple-to-complex movements, from miniature engines and valves to solar arrays that tilt toward the sunlight. The study is illustrated in an article published on July 23^rd in the Proceedings of the National Academy of Sciences.

In biology, there are a number of instances where light triggers movement or change—think of leaves and flowers turning toward sunlight. The light actuated materials developed in this study are based on the principle of the Curie temperature—the temperature above which some materials will alter their magnetic properties. By heating and cooling a magnetic material, one can switch its magnetism off and on. Biopolymers and elastomers doped with ferromagnetic CrO₂ will heat up when laser or sunlight hits them, momentarily losing their magnetic properties until they cool down again. The simple movements of the material, shaped into sponges, films, and hydrogels, are triggered by adjacent permanent or electromagnets and can be seen as twisting, bending, and expansion.

“We could combine these simple movements into more complex motion, like crawling, walking, or swimming,” said Fiorenzo Omenetto, Ph.D., corresponding author of the study and the Frank C. Doble Professor of Engineering in the School of Engineering at Tufts. “And these movements can be triggered and controlled wirelessly, using light.”

Omenetto’s team showed a few of these complex movements by building soft grippers that capture and free objects in reaction 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. 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.

Meng Li, First Author

To show this flexibility, the scientists built a simple “Curie engine”. A light actuated film was shaped into a ring and placed on a needle post. Positioned near a permanent magnet, when a laser was aimed at a fixed spot on the ring, it locally demagnetizes that area of the ring, developing an unbalanced net force that causes the ring to turn. As it turns, the demagnetized area regains its magnetization and a new area is illuminated and demagnetized, causing the engine to uninterruptedly rotate.

Materials used to make the light actuated materials include polydimethylsoloxane (PDMS), which is an extensively used transparent elastomer mostly shaped into flexible films, and silk fibroin, which is an adaptable biocompatible material with exceptional optical properties that can be shaped into a wide variety of forms - from films to gels, blocks, threads, 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.

Fiorenzo Omenetto, Ph.D.

Other authors on the paper are: Graduate students Meng Li, Yu Wang, Arin Naidu, Carlos Lopez Rodrigues, Bradley Napier and Wenyi Li of the Tufts University SilkLab and the Department of Biomedical Engineering. Aiping Chen and Scott Crooker, Ph.D. of the National High Magnetic Field Laboratory in Los Alamos, NM, helped with measuring and characterizing the magnetic properties of the materials.

This research was aided by the National Science Foundation (#1541959).