Alfredo Alexander-Katz, the Merton C. Flemings Assistant Professor of Materials Science and Engineering, his doctoral student Charles Sing, and researchers at Boston University and in Germany, devised a system that uses so-called superparamagnetic beads — tiny beads made of polymers with specks of magnetic material in them — suspended in liquid.

Due to the heavy magnetic material content, these beads sink to the bottom of the liquid. They placed the whole system inside two pairs of magnetic coils and used them to apply a rotating magnetic field, which caused the beads to spontaneously form short chains that began spinning. This motion created currents that could then carry along surrounding particles — even particles as much as 100 times larger than the beads themselves.

Alexander-Katz refers to the microscopic assembly of beads — each just a few microns (millionths of a meter) in size — as “micro-ants,” because of their ability to move along while “carrying” objects so much larger than themselves. A paper describing the research will appear the week of Dec. 14 in the Proceedings of the National Academy of Sciences.

Biological flows at the microscopic scale are important for the transport of nutrients, locomotion, and differentiation. Here, we present a unique approach for creating controlled, surface-induced flows inspired by a ubiquitous biological system, cilia. Our design is based on a collection of self-assembled colloidal rotors that “walk” along surfaces in the presence of a rotating magnetic field. These rotors are held together solely by magnetic forces that allow for reversible assembly and disassembly of the chains. Furthermore, rotation of the magnetic field allows for straightforward manipulation of the shape and motion of these chains. This system offers a simple and versatile approach for designing microfluidic devices as well as for studying fundamental questions in cooperative-driven motion and transport at the microscopic level.

The new method could provide a simpler, less-expensive alternative to present microfluidic devices, a technology involving the precise control of tiny amounts of liquids flowing through microscopic channels on a chip in order to carry out chemical or biological analysis of tiny samples. Now, such devices require precisely made channels, valves and pumps created on a silicon chip using microchip manufacturing methods, in order to control the movement of fluids through them. But the new system could offer such precise control over the movement of liquids and the particles suspended in them that it may be possible to dispense with the channels and other plumbing altogether, controlling the movements entirely through variations in the applied magnetic field.

In short, software rather than hardware could control the chip’s properties, allowing it to be instantly reconfigured through changes in the controlling software — an approach Alexander-Katz refers to as “virtual microfluidics.” This could reduce the cost and increase the flexibility of the devices, which might be used for such things as biomedical screening, or the detection of trace elements for pollution monitoring or security screening. It might also provide even finer spatial control than can presently be achieved using conventional channels on chips.