Molecular Traffic Control
Creating order is not for everyone. For future nanotechnology, however, tiny helpers that move building block by building block into place are an essential prerequisite. Help may now come from the most successful micromachine of all time: the living cell.
The idyll is deceptive. The picture that schoolbooks give us of the biological cell - with wide, free spaces and molecules happily diffusing around - is long outdated and fundamentally wrong. New microscopy techniques and imaging processes have shown that the cell is like Sunday shopping in Scandinavian furniture stores. It's full, crowded, congested; there is jostling, pushing, shoving; and somehow the bulky things still get from A to B.
Various specialized molecules such as the combination of microtubules and kinesin take care of the transport in the cell. The tubular microtubules take on the role of the conduction system. These proteins run through the entire cell body in a practical modular design. The kinesins, heavily loaded with the payloads, move along them. Each step costs them chemical energy, which they use to squeeze through the crowd. Accurate and on time.
This is how nanotechnologists would like their construction helpers for tomorrow's breakthrough. Because what they have to show in the area of billionths of a meter of wheels, rods and motors is mostly the result of painstaking manual work. And it becomes even more difficult when two or even more such individual pieces are to be combined to form a complete apparatus. At the molecular level, tinkering is hyper-delicate, heavy-duty work.
Dutch scientists led by Cees Dekker from the Technical University of Delft have therefore decided that the job would best be carried out by the experts. The only problem is giving the microtubule kinesin team the right commands at the crucial moment. And this is exactly where the researchers have now taken a big step forward: In their demonstration experiments, they used electric fields to steer microtubules at a fork in the path to the left or right.
The trials first required appropriate test tracks. They were blasted in silicon dioxide using conventional lithographic techniques and etched to create a system of channels 800 billionths of a meter deep, capped with a disk of the same material. The walls were lined with kinesin molecules, whose "little legs" protruded towards the center and could wriggle vigorously. They transported individual microtubule tubes that floated like a pop star on the hands of his fans above the crowd. Fixed kinesin carries mobile microtubules – exactly the opposite constellation as in the cell, but just as functional.
The experiment reached its crucial point when a microtubule arrived at a Y-junction. Here, the scientists applied a strong electrical field of the order of up to 50 kilovolts per meter via two narrow additional channels. As an electrically charged molecule, the protein felt a pulling force that it could follow with the freely moving headpiece, while its long body was fixed by kinesin. The microtubule thereby bent towards the field and changed the direction of its migration. In this way, the researchers were able to sort color-coded microtubules with good efficiency.
There's certainly a long way to go from those early model railroad experiments using microtubule trains on kinesin rails and electric switches to nanofactories constructing tiny devices. But with the appropriate knowledge, he will one day revolutionize our technology. After all, our cells have shown us how – for ages.
