uni'wissen 01(3)-2011_ENG

square centimeter arranged like tin soldiers on a plate of glass. With the help of lithographic meth- ods from electrical engineering, in which struc- tures are applied to semiconductor chips, micro- systems engineers can now also construct mechanical or optical structures. Photolithogra- phy is a technique for casting shadows. Every- thing that can be drawn on a sheet of paper can be copied with the help of this technique. For in- stance, when one transfers a circle to a thin, light-sensitive layer a so-called resist it is copied as such. When one transfers it to a thick layer, it casts a cylindrical shadow. “If we want to create thick structures that are also three-dimensional, we produce one layer after the other,” explains Wallrabe. In this way, the researchers can stack up several layers and vary their thickness or the materials they are made of. Wrapped around the Cylinder The researchers use lithography to create a cylinder of resist and use it as a mount for a gold wire coil that starts at the bottom and moves up the cylinder. “The coil accomplishes something that is not possible with lithographic methods alone. It twists upward into the third dimension,” says Wallrabe. The basis for this method was Jan Korvink’s idea of converting a machine from microelectronics. “You might say we misused it to create the coils,” says the micro-electrical en- gineer. Like a high-tech sewing machine, the ma- chine takes thin gold wire and coils it around the cylinder with a wire bonding technique. The size and thickness of the coil depend on what it is to be used for. The coils are between 0.2 and 1 milli­meters in diameter. The coiling process takes half a second, after which the machine is ready for the next coil. This allows large amounts of the tiny structures to be produced in a short amount of time and at low costs. “If we hadn’t come up with this method, it would have been extremely expensive,” says Korvink. There was previously no standardized production process for three-dimensional microcoils. Once devel- oped, the coil technology can be used for other projects. Four to five coils are enough to conduct bio- logical research on cells using magnetic reso- nance imaging. The cells enter the resist cylin- der, which thus has a dual function: On the one hand, it holds the coil in place, and on the other hand, it replaces the petri dish. The necessary magnetic field is created by a magnet in Jürgen Hennig’s laboratory with a strength of 9.4 tesla (the unit for magnetic flux density). The strength of standard tubes in medical MRI scanners is only 1.5 tesla. The tiny cylinders with the gold coils are placed in this big machine and subject- ed to a constant magnetic field. “At first we only miniaturized the measuring coil; all of the other coils were still the same size,” says Wallrabe. This gives the scientists enough signals to ob- serve the cells living in water or in a nutrient so- lution, but not to look inside of the cells them- selves. “If the cells are living, they can also divide and biologists can observe the process,” says Henning. This gives the scientists com- pletely new insight into the processes of life. Big and small: The measuring coils of conventional MRI scanners are designed to scan human body parts, like the head, arms, or legs (left). The Freiburg research team produced coils so small they can be used to scan individual cells (right). Photos: Spiegelhalter/Freiburg University Medical Center, Badilita “If the cells are living, they can also divide and biologists can observe the process” 30 uni'wissen 03