In biology, there are many examples where light induces movement or change — think of flowers and leaves turning toward sunlight. Magnetic elastomeric composites were developed that move in different ways when exposed to light, raising the possibility that these materials could enable a wide range of products that perform simple to complex movements, from tiny engines and valves, to solar arrays that bend toward the sunlight. These simple movements could be combined into more complex motion like crawling, walking, or swimming, and can be triggered and controlled wirelessly using light.
The light-actuated materials are based on the principle of the Curie temperature — the 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. Biopolymers and elastomers doped with ferromagnetic 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 or electromagnets and can exhibit as bending, twisting, and expansion.
To demonstrate these complex movements, soft grippers were constructed that capture and release objects in response to light illumination. Portions of a structure can be selectively activated and controlled using localized or focused light. Unlike other light-actuated materials based on liquid crystals, the 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, a simple “Curie engine” was constructed. 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 demagnetized that portion of the ring, creating an unbalanced net force that causes the ring to turn. As it turned, the demagnetized spot regained its magnetization and a new spot was illuminated and demagnetized, 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 bio-compatible material with excellent optical properties that can be shaped into a wide range of forms including films, gels, threads, blocks, and sponges.
With additional material patterning, light patterning, and magnetic field control, even more-complicated and fine-tuned movements could theoretically be achieved, such as folding and unfolding, microfluidic valve switching, and micro- and nano-sized engines.