While experimenting with superparamagnetic colloids as part of his research into how fluids are naturally transported in the body, Alfredo Alexander-Katz, an assistant professor of materials science and engineering at MIT (Cambridge, MA; www.mit.edu), was reminded of cilia. Found in nature clustered on epithelial surfaces, these hair-like organelles undulate in unison to propel the ovum to the uterus or to usher mucus with unwanted particles from the trachea, for example. Inspired by this coordinated motion, Alexander-Katz and his colleagues developed a self-assembling system that could someday impact microfluidic device design and medical diagnostics.
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In the presence of electric fields, superparamagnetic beads are attracted to one another to form rotating chains potentially suited for microfluidic applications.
At the root of the technology are micrometer-sized particles—dubbed superparamagnetic beads—that contain small amounts of magnetic material. Unlike typical paramagnetic materials, these beads do not exhibit hysteresis, which means that they immediately lose their magnetism when a magnetic field is removed. Alternately, they are instantaneously magnetized when exposed to a field.
When exposed to a magnetic field while immersed in liquid, the beads act as microscopic magnets. Quickly attracted to one another, they subsequently form chains. “We then rotated the magnetic field—that means we rotated the orientation of those dipoles and, in fact, the whole chain,” explains Alexander-Katz. “Since the chains are resting on the surface because the magnetic materials are a bit heavy, they begin to do what I describe as cartwheels.”
The rotating chains of magnetic beads essentially migrate across the surface, according to the scientists. As the beads rotate, whichever one is closest to the surface operates as a hinge, prompting the other beads in the chain to rotate around it. “It’s not exactly like that, though,” warns Alexander-Katz, “because friction is not infinite on the surface; [the chain] takes several cycles to move its own size.” The movement of the magnetic chains is contingent upon the presence of a surface.
Like cilia, this self-assembling system creates currents in the surrounding liquid, which can move other, nonmagnetic particles. Also like cilia, numerous self-assembled chains can effectively form a carpet of bead-based chains that beat in unison and create a sizeable flow field, Alexander-Katz says.
Supported by such attributes, the system could have a dramatic impact on the field of microfluidics. Alexander-Katz suggests that the system has the potential to enhance existing microfluidic designs; however, it could also someday replace the traditional use of etched channels to direct fluid flow. The self-assembling system provides a new technique for precisely controlling the direction of fluid and particle movement through the use of external magnetic fields. It also offers the advantage of cost-effective manufacturing processes. Furthermore, the use of software to dictate parameters for the system allows for on-the-fly reconfiguring and promotes design flexibility, states Alexander-Katz.
In the short term, the researchers are working on scaling up to more-complex assemblies, and even believe that advancements in microfluidic technology could be achieved within the next year or so. Down the road, the researchers speculate that the technology may eventually allow for the control of superparamagnetic particles to perform targeted tasks inside the body.