A surface has been constructed and patented that, when shaken, moves drops along certain paths easily, move and mix small samples of liquid to conduct medical diagnostic tests.

The simple technology is a textured surface that tends to push drops along a given path and inspired by the lotus effect, a phenomenon in which a lotus leaf's almost fractal texture makes it appear to repel drops of water.

Medical scientists and engineers at the University of Washington (Seattle, WA, USA) team used nanotechnology-manufacturing techniques to build a surface with tiny posts of varying height and spacing. When a drop sits on this surface, it makes so little contact with the surface that it is almost perfectly round. That means even a small joggle can move it. The scientists used an audio speaker or machine to vibrate the platform at 50 to 80 times per second. The asymmetrical surface moves individual drops along predetermined paths to mix, modify, or measure their contents. Changing the vibration frequency can alter a drop's speed, or can target a drop of a certain size or weight.

The type of system is known as a "lab in a drop" as all the ingredients are inside the drop and surface tension acts as the container to keep everything together. Attempts were made to use a smartphone's speaker to vibrate the platform, but so far, a phone does not supply enough energy to move the drops. To better accommodate low-energy audio waves, the group will use an electron beam-lithography machine to build a surface with posts up to 100 times smaller. In testing, different versions of the system could move the drops uphill, downhill, in circles, upside down, or join two drops and then move the combined sample.

Karl Böhringer, PhD, a professor of electrical engineering and bioengineering, said, "All you need is a vibration, and making these surfaces is very easy. You can make it out of a piece of plastic. I could imagine this as a device that costs less than a dollar - maybe much less than that – can be used with saliva or blood or water samples." The study was published on March 22, 2012, in the journal Advanced Materials.

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