A piconewton force sensor

Many forces in the world of cell biology are at the piconewton (10⁻¹² N) level. Examples include the force applied by the kinesin molecular motor protein to transport vesicles (6 pN), the force needed to unzip a DNA molecule at room temperature (9–20 pN), and the force needed to rupture a DNA molecule by pulling on opposite ends (65 pN). Biophysicists need a cost-effective force sensor in this range that works reliably in water and other solvents, and Steven Koch and his colleagues at Sandia National Laboratories are working on one. The core of their device is a spring 0.5 mm long but only a micron thick, fabricated using a standard micromachining process and having a spring constant of about 1 pN/nm. (The figure shows a stiffer and shorter—186 μm long—spring between the combs.) Mounted on a substrate, the spring can be used in a number of ways: For example, 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 function as a field sensor. It is also relatively insensitive to its environment, including temperature and solvent. The displacement of the spring is currently viewed with 2-nm precision by a video camera, but faster and more precise methods are possible. Koch says the most likely applications of the new sensor will be to measure forces on magnetic microspheres used in single-biomolecule experiments and to calibrate the electromagnets used in deploying microspheres. The Sandia sensor could also be adapted to apply an adjustable tension to single DNA molecules in order to study protein binding or enzymatic processes. (S. Koch, G. E. Thayer, A. D. Corwin, M. P. de Boer, Appl. Phys. Lett. 89 , 173901, 2006 .)