Inside animal cells, kinesin motor proteins travel along microtubules radiating out from the nucleus. The kinesins transport materials such as vesicles and viruses. In the Delft and other similar devices, it's the microtubules that travel while the kinesins remain attached to the walls of the channel.

Cees Dekker and colleagues employed 800 nm deep closed channels in a fused silica substrate. The fact that the channels were closed enabled the team to apply a strong electric field perpendicular to the direction of travel of the molecules in order to direct their transport along one arm of a Y. In this way, the scientists controlled the movement of individual microtubules for the first time.

Although each microtubule remains linked to the walls of the channel through the kinesins, the researchers believe that the application of an electric field makes the tip of the microtubule more likely to move to a kinesin motor in the desired direction. The team estimated that the microtubules had an effective charge of 12 e- per tubulin dimer.

The device had an additional intersecting channel near the junction of the Y through which the electric field was applied. It was able to transport fluorescently labelled microtubules around 1 to 15 µm long – the team was able to monitor microtubule progress through epifluorescence microscopy.

To demonstrate the sorting effect, the team used microtubules attached to either green fluorescent or red fluorescent molecules. They changed the polarity of the electric field according to the colour of each microtubule to direct the molecules down one of the arms of the Y.

The device directed 72% of the red (rhodamine)-labelled molecules into the red reservoir, with 3% going to the green reservoir and the remaining 25% moving into the perpendicular channel. Similarly, for fluorescein-labelled microtubules, 69% went to the green reservoir, 12% to the red reservoir and 19% to the perpendicular channel.

The researchers believe the biomolecular motor technique could replace existing pressure- or electroosmotic flow-driven microfluidic devices for sorting whole cells.

The researchers reported their work in Science.