Small tubes with diameters ranging from one micrometre to just over a tenth of a millimetre can be grown from inorganic crystals immersed in water, reports a study online in Nature Chemistry this week. Controlling the growth process could lead to networks of tubes that may prove useful for making designer microfluidic ― so-called lab-on-a-chip ― devices.

Leroy Cronin and colleagues take crystals of inorganic compounds known as polyoxometallates ― large clusters made up of metal and oxygen atoms ― and drop them into water containing positively charged organic molecules. In a matter of seconds, a tubular structure begins to grow from each crystal, and the direction of a given tube can be controlled by either placing a physical object in its path or applying a voltage to the solution. In this way, tubes can be grown into each other and merged to form branched junctions. By flowing a fluorescent dye through the tubes, Cronin and co-workers demonstrate that the structures are hollow and robust enough not to leak.

The diameter of the growing tubes can be controlled by varying the quantity or type of the charged molecules dissolved in the water. Moreover, the recipe for growing the microtubes is a fairly general one and a range of different polyoxometallates can be used. This enables the chemical properties of the tubes to be tuned, and offers the possibility of making microfluidic channels that not only direct the flow of solutions, but can chemically interact with them for applications such as catalysis or sensing.