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Measuring a nematode with a piezoresistorStudying the mechanics of C. elegans is the first step in understanding the sense of touch.

Why would you want to measure the “springiness” of a worm that

is about half a millimeter long? And, having decided to make the measure-ment, how would you do it? The answer to “Why?” is easy. Important insight into the functions of neurons and force-sensing organs may be gained from such a study. “Understanding the mechanics of the nematode structures—that is, the internal linkages between the cuticle and the organs inside it—will enable understanding of force transmission and the underlying mechanisms for the sense of touch and locomotion,” says Beth L. Pruitt of Stanford University’s mechanical engi-neering department.

Pruitt and two Stanford col-leagues, Miriam Goodman in the molecular and cellular physi-ology department and gradu-ate student Sung-Jin Park, also have answered the question of “How?” (Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 17,376–17,381). Researchers will often choose optical tweezers and atomic force microscopy (AFM) to measure small displacements (usually, less than a few micrometers) or forces (typically in the 10–12 to 10–9 N range). Although these tools are useful at the biomolecular scale, they present some inherent problems when studying tissues and organs, which are larger. So, the team designed a force displacement measurement system, based on a mi-croelectromechanical silicon cantilever with piezoresistive (PR) sensing, that is a better match for the dimensions and material properties of small organisms. The tool can apply forces in the 10–8 to 10–3 N range and can measure displace-ments as great as 100 µm.

The PR cantilever system is built from single silicon crystals 2–6 mm long. A U-shaped piezoresistor (200 μm long and 20 μm wide with a 20

μm gap) is fabricated in the cantilever base, at the point of maximum bending. The velocity and position of the canti-lever are controlled and monitored by a piezoelectric actuator with a capacitive sensor. Resistance changes in the can-tilever are proportional to the applied force, and the force and actuator travel are proportional to voltage.

According to Pruitt, the cantilever design gives the device one particular advantage over optical measurements. “The sample interacts with the canti-lever tip, far from the base where some joule heating would occur,” she says. “This is particularly important in mea-surements that are sensitive to thermal effects. Although there is some energy added by mechanical interaction, it should not upset the system being mea-sured [that is, the nematodes].”

Another advantage of the PR can-tilever is that its working displacement is limited primarily by its stiffness, whereas that of an atomic force micro-scope is limited by the optical detector; the cantilever is, therefore, easy to tune across a dynamic range. In addition, PR cantilevers can be integrated into high-bandwidth feedback-control systems that incorporate high-resolution actua-tors. Furthermore, PR-cantilever-based

feedback systems can be integrated with other cellular measurements, such as patch-clamp electrophysiology and live-cell imaging of fluorescent proteins, more readily than optical methods can. Finally, the cantilever can operate in any orientation and can be made sensitive in two axes, allowing 2D measurements, whereas AFM and optical tweezers are

limited to 1D measurements.According to Daniel Fletcher

of the bioengineering department of the University of California Berkeley, the approach of Pruitt and colleagues “represents a use-ful new application of piezoresis-tive cantilevers that will be ap-plicable to a range of interesting biomechanical problems.” Taher Saif of the Micro and Nano-technology Laboratory at the University of Illinois Urbana–Champaign adds, “This paper is an example of how fundamental biological investigations can be facilitated by new, advanced engi-

neering tools—in this case, the piezore-sistive microcantilevers—that have ad-vantages over conventional techniques, such as optical tweezers and AFM.”

When the Stanford team analyzed the body mechanics of the nematode C. elegans, they found that the relation-ship between force and displacement is consistent with an elastic-shell-type model of the animal’s body. In addi-tion, hydrostatic pressure had a modest contribution to the stiffness of the outer shell compared with the contribution of cuticle mechanics. “The mechani-cal connection between the cuticle and touch-receptor cells may be important, and these internal linkages may be re-lated to the on–off responses observed in the ion channels,” says Pruitt. “This study is the first building block in de-termining how forces are transmitted to ion channels within the organism.” a

—Steve Miller

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Piezoresistor50 µm

Schematic of the PR cantilever. The inset shows an image of a cantilever with a glass bead (10 μm in diameter) at the tip. (Adapted with permission. Copyright 2007 National Academy of Sciences, U.S.A.)