Magnetic fields cause microcubes to respond like mini robots

Hours of practice enable marching band members to smoothly step through one orderly formation after another. Some colloidal particles can also do that fancy footwork, but they don’t need to practice. A team of researchers including Koohee Han and Orlin D. Velev of North Carolina State University made polymer cubes with 10-µm-long edges and selectively coated one face of each cube with a 100-nm-thick film of cobalt, which can be magnetized. Then they formed aqueous suspensions of the microcubes and showed that by controlling the way magnetic fields were applied to the suspensions, including switching the fields on and off and superimposing fields from multiple electromagnets, the cubes could be made to spontaneously and reversibly assemble in a variety of shapes and patterns (Sci. Adv. 2017, DOI: 10.1126/sciadv.1701108). In some cases, the cubes reversibly switched between a linear chain and ringlike configuration. In others, the cubes underwent complex folding, unfolding, and rotational motions. In yet another display of control, the team used a pattern of cubes to capture and transport a live cell and then release it. The researchers propose that this strategy may one day be used to develop microbots, artificial muscles, and other biomimetic devices.