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...