Direct Force Sensing at the Piconewton Level
http://www.aip.org/pnu/2006/split/807-1.html
Physics News Update -- Number 807 #1, December 28, 2006
by Phil Schewe, Ben Stein, and Davide Castelvecchi
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
