Direct Force Sensing at the Piconewton Level

The measurement of mass can be carried out at the 10^-21 (zeptogram) level (see PNU 725) and of force to the 10^-18 newton (attonewton) level (Arlett et al., in Nano Letters, 2006). But for many measurements in the cell biology world, this is too much sensitivity. Forces in this realm are typically at the piconewton (10^-12 newton) level. Examples include the force applied by the kinesin molecular motor protein to transport vesicles (6 piconewton), the force needed to unzip a DNA molecule at room temperature (9-20 piconewton), or the force needed to pull a DNA apart by pulling on opposite ends (65 piconewton).

Biophysicists need a cost-effective force sensor that works reliably in water at the piconewton level. Steven Koch and his colleagues at Sandia National Laboratories in Albuquerque, N.M., are well along on delivering the needed sensor. The core of the device is a spring one millimeter long but only a micron thick and is fabricated using a standard polysilicon micromachining process. This spring operates according to the classic experiment conducted by Robert Hooke in the 17th century: the force exerted on the spring equals the amount of the spring’s compression or extension multiplied by a spring constant, which in this case is about 1 piconewton per nanometer.

The spring, mounted on a substrate, can be used in a number of ways: it can be entrained to move with the push or pull of a biological sample or it can be made sensitive to magnetic fields and so function as a field sensor. The displacement of the spring is currently viewed by a video camera with precision of 2 nanometer, but faster and more precise methods are possible.

Koch (now at the University of New Mexico, skoch@chtm.unm.edu) says that the most likely applications of the new sensor will be in measuring forces on the kind of magnetic microspheres used in single-biomolecule experiments and to calibrate the electromagnets used in deploying microspheres in doing things such as stretch, twist, or unzip DNA. He also envisions direct mechanical force measurements, combined with other MEMS (microelectromechanical systems) implements, in biophysical experiments where optical tweezers (using laser beams to manipulate the microspheres attached to molecules) cannot be used.

The Sandia sensor could be adapted to apply an adjustable tension to single DNA molecules in order to study protein binding or enzymatic processes.

Koch et al., Applied Physics Letters, 23 October 2006
Contact Steven Koch