Microtubes Follow Directions

Fabricating microfluidic devices is generally a painstaking process that requires a unique mold or mask for each device configuration. Geoffrey J. T. Cooper and Leroy Cronin of the University of Glasgow, in Scotland, have now taken a step toward a more flexible approach to device fabrication by developing a way to control, in real time, the growth, direction, and diameter of self-fabricating polyoxometalate (POM) microtubes.

POMs are oxo-anion clusters of early transition metals. In a previous study, Cooper, Cronin, and colleagues observed spontaneous growth of micrometer-scale tubes from a tungstate POM crystal upon immersion in an aqueous solution of a polyaromatic organic cation (Nat. Chem. 2009, 1, 47). The interaction of POM anions with organic cations causes a semipermeable membrane to form around the crystal, and osmotic pressure within the membrane drives microtube growth. The microtubes are uniform in diameter and sufficiently robust to allow the flow of liquid, thereby raising the possibility of their use as channels in microfluidic devices. In their latest paper, Cooper and Cronin developed a method to precisely control, in real time, the direction of microtube assembly with the help of an applied electric field (J. Am. Chem. Soc., DOI: 10.1021/ja902684b).

To prepare microtube assemblies, the researchers introduce POM crystals to the center of a reaction vessel containing an organic cation solution. The vessel has four electrodes that are perpendicular to each other, and by varying the direction and duration of the applied field, the researchers produce complex patterns of microtubes, such as zigzags and 90° and 180° bends. They also control the diameters of microtubes by changing the concentration of the cation solution.

Because POMs have semiconducting, catalytic, and optical properties, "this method could provide the base material for numerous microreactor systems," says J. Tanner Nevill, applications engineer at Fluxion Biosciences, a manufacturer of microfluidic devices. "Also, the ability to change the tubing diameter during growth offers interesting potential for tapered, axially symmetric microfluidic channels, which are extremely difficult to achieve with conventional microfabrication techniques. The real challenge will be interfacing these tubes in a practical manner."