IMAGINE trying to suck molasses through a straw, and you get some idea of the big problem facing microchip-sized “labs” designed to perform experiments on tiny quantities of fluids or biomolecules. Liquids all too easily get stuck in their diminutive pipework and gum up their valves and pumps.
Now a team has developed a lab-on-a-chip that can move reactants around without using pipes. They even have their own equivalent of a Bunsen burner built in to speed reactions along.
“This lab-on-a-chip can move reactants around without using pipes”
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Standard lab-on-a-chip devices, which are used for drug discovery or diagnosing disease, work by pumping picolitre volumes of test liquids around microchannels and reaction chambers etched into a silicon or polymer substrate. However, their pumps and valves can jam up, and some fluid always sticks to the channel walls or soaks into them. This can be a big drawback when you need to do lots of tests on a limited volume of fluid, such as a blood sample.
So in 2004, Michael Sailor, a chemist from the University of California, San Diego, and his team published details on a novel way to construct a lab on a chip, eschewing microchannels and valves entirely. They have now revealed their progress and how they introduced heat into their device in the Journal of the American Chemical Society (DOI: 10.1021/ja0612854).
In their scheme the chip becomes a miniature oil bath, and the water-soluble reactants are suspended in the oil in 1-millimetre droplets (see Diagram). To move the suspended droplets and react them together, you need a way to grab hold of them, so the team’s answer is to add tiny flakes of a mixture of silicon and iron oxide, coated on one side with an oil-loving substance and on the other with a water-loving one. This makes the fragments self-assemble around the outsides of the droplets. Because iron oxide is magnetic, the team can then use an array of electromagnets in the base of the chip to position the droplets anywhere in the bath, including bringing them together.
Of course, reactions often need heat to get them going, so Sailor’s team plans to place a coil in the centre of the bath to heat the droplets magnetically.
Bench tests of the technology show it is all feasible, says Sailor, although he hasn’t yet built a complete chip. For instance, the team showed that a droplet magnetically steered close to a heater coil became hot because some DNA placed inside it unwound – and re-coiled when the heat was removed.
“The thing that’s really nice is that it doesn’t heat up the surroundings,” Sailor says. “This allows us to apply heat in precise regions.”
