Dutch scientists have copied nature to develop a faster and more efficient method for mixing small volumes of liquid. They were inspired by micro-organisms that use tiny oscillating hairs (called cilia) to move through liquid, and made artificial cilia to use in microfluidics.

Artificial cilia (right) mimic real cilia such as those on a Paramecium micro-organism (left)

Microfluidics is the study of fluid flow in structures with dimensions smaller than a millimetre, with devices as small as a credit card. Key applications for these devices include biochips, especially the immediate point-of-care diagnosis of diseases. The problem with such a small device is that good mixing is almost impossible. Usually, mixing is done by etching grooves into the fluid channels or applying an electrical pulse or sound wave. However, there is no real control over the mixing in these cases, according to Jaap den Toonder of the Philips Research Laboratory in Eindhoven.

Toonder and colleagues have therefore made artificial cilia from a double layer of thin polymer, called polyimide, and a thin conductive layer of chromium. The artificial cilia look a lot like a curled-up leaf.

These tiny mixers are activated by applying a voltage. When applied, the rolled-up cilia uncurl. When the voltage is turned off, the cilia roll back up by elastic recovery. In this way, the mixing can be turned on or off by demand and in any location in the device, an advantage over other mixing methods.

Andreas Manz, head of the Institute for Analytical Sciences in Dortmund, Germany, commented that the use of the artificial cilia for mixing was intriguing, as 'in biological systems cilia are used for transport'.

When questioned about the future, den Toonder said the next steps were to find alternative ways to activate the artificial cilia, such as magnetically-driven systems. The team's final goal is to develop artificial cilia 'into a versatile method for medical diagnostic lab-on-a-chip devices'.

Rebecca Brodie