A fundamental advance in controlling soft robots involves using magnetic fields to remotely manipulate microparticle chains embedded in soft robotic devices. Several devices have been developed that make use of the new technique. Possible applications for these devices range from remotely triggered pumps for drug delivery, to the development of remotely deployable structures.
Magnetic field-directed self-assembly of magnetic particles in chains is useful for developing directionally responsive materials for applications in soft robotics. Using materials with greater complexity allows advanced functions, while still using simple device architectures. For this study, the researchers introduced iron microparticles into a liquid polymer mixture, and then applied a magnetic field to induce the microparticles to form parallel chains. The mixture was then dried, leaving behind an elastic polymer thin film embedded with the aligned chains of magnetic particles.
Elastomer films containing chained magnetic microparticles were prepared through solvent casting and formed into magnetically actuated lifters, accordions, valves, and pumps. Chaining both enhances actuation and imparts a directional response. Applying magnetic fields to folded accordion structures caused extension and compression, depending on the accordion’s orientation. In peristaltic pumps comprised of composite tubes containing embedded chains, magnetic fields caused a section of the tube to pinch closed where the field was applied.
Specifically, the direction of the magnetic field and its strength can be varied. The chains of iron microparticles respond by aligning themselves and the surrounding polymer in the same direction as the applied magnetic field. Using this technique, three kinds of soft robots were created. One device is a cantilever that can lift up to 50 times its own weight. The second device is an accordion-like structure that expands and contracts, mimicking the behavior of muscle. The third device is a tube designed to function as a peristaltic pump — a compressed section travels down the length of the tube, much like someone squeezing out the last bit of toothpaste by running their finger along the tube.
The researchers have also developed a metric for assessing the performance of magnetic lifters, such as the cantilever device. This is done by measuring the amount of weight being lifted, and taking into account both the mass of particles in the lifter and the strength of the magnetic field being applied.