Imagine a force barely strong enough to lift a protein molecule, and far too weak to budge a blood cell.
Until now forces this small have been virtually undetectable. But scientists from Stanford University and IBM's Almaden Research Center report that they have successfully measured forces of such an infinitesimal magnitude for the first time.
Speaking at the American Physical Society meeting in Kansas City on Monday, March 17, the researchers described the method that they have developed to measure "atto-newton" forces. A newton is about one-fifth of a pound, while an atto-newton is a billionth of a billionth of a newton. An analogy provides a sense of just how small such a force is: an atto-newton is to a feather as a feather is to Hoover Dam.
"As far as we know, this is the first time that a mechanical sensor has been demonstrated that is delicate enough to measure atto-newton forces," said Thomas Kenny, assistant professor of mechanical engineering at Stanford, and a member of the team who made the measurements.
The new measurements were presented at the meeting by Timothy Stowe, graduate student in applied physics at Stanford. Co-authors were Kenny, Dan Rugar, IBM Nanoscale Studies manager, IBM visiting scientists David Botkin and Koichi Wago, and Stanford applied physics graduate Kevin Yashimura.
The measurements were made using a microscopic cantilever being developed for a new instrument called a magnetic resonance force microscope (MRFM). The MRFM combines the scanning tunneling microscope's ability to image individual atoms with magnetic resonance imaging's capability of telling one kind of atom from another. The new instrument, which is still under development, holds promise of revolutionizing the study of biological processes at the molecular level and adding an entire new dimension to the study of electronic materials at the atomic level.
First suggested by John Sidles of the University of Washington, MRFM techniques have been pioneered by Rugar and Nino Yannoni at Almaden. Like standard MRI, the technique uses a radio-frequency (RF) coil to excite magnetic resonance in a sample. Like conventional MRI, the MRFM can be tuned to interact only with specific types of atoms by varying the frequency of the radio waves. Rather than using the same coil to detect the magnetic signals coming from the atoms, however, the MRFM relies on a microscopic cantilever with a magnetized tip. The force between the tip and sample oscillates as the RF coil periodically reverses the polarity of the atoms in the sample. This, in turn, causes the cantilever to vibrate. An optical fiber interferometer records the cantilever's movements. As the tip is scanned over a surface, it responds to magnetic signals coming from specific atoms within an annulus on the sample's surface. By recording variations in the amplitude of the cantilever's vibration at different positions, the scientists are able to make a three-dimensional map of the position of the resonant atoms
So far, the MRFM has taken images at the micron scale. "One key to extending MRFM capability to the atomic scale is the ability to detect forces at the atto-newton level," said Rugar. "This is the motivation for developing the new ultra-sensitive cantilevers."
It took several years of work before the Stanford/IBM team succeeded in making cantilevers with the required characteristics. Measuring 230 microns long, five microns wide and less than 600 angstroms thick, the new cantilevers are 1,000 times thinner than a human hair, and are invisible to the naked eye. The team made these micro-devices by starting with a material called silicon-on-insulator, which consists of an ultra-thin layer of silicon bonded onto a layer of silicon oxide coating a silicon wafer. They etched cantilever shapes out of the ultra-thin silicon layer and then chemically removed the underlying layers of oxide. They chose silicon in order to minimize the mechanical damping that occurs when the cantilever vibrates, and because it is compatible with semiconductor processing techniques.
As an illustration of the delicacy of this process, the researchers found that they could not use normal drying techniques after the final fabrication step. The surface tension of the water is strong enough so that as it dries it bends the cantilevers into "U" shapes, causing them to reattach to the underlying material. They circumvented the problem by employing an exotic technique used in biology called critical point drying, which uses liquid carbon dioxide at high pressures.