instrumentals
Measuring a nematode with a piezoresistor Studying the mechanics of C. elegans is the first step in understanding the sense of touch.
W
hy would you want to measure μm gap) is fabricated in the cantilever feedback systems can be integrated with the “springiness” of a worm that base, at the point of maximum bending. other cellular measurements, such as is about half a millimeter long? And, The velocity and position of the cantipatch-clamp electrophysiology and livehaving decided to make the measurelever are controlled and monitored by a cell imaging of fluorescent proteins, ment, how would you do it? The answer piezoelectric actuator with a capacitive more readily than optical methods can. to “Why?” is easy. Important insight sensor. Resistance changes in the canFinally, the cantilever can operate in any into the functions of neurons and force- tilever are proportional to the applied orientation and can be made sensitive in sensing organs may be gained from such force, and the force and actuator travel two axes, allowing 2D measurements, a study. “Understanding the mechanics are proportional to voltage. whereas AFM and optical tweezers are of the nematode structures—that limited to 1D measurements. is, the internal linkages between According to Daniel Fletcher the cuticle and the organs inside of the bioengineering department it—will enable understanding of the University of California Piez ores of force transmission and the Berkeley, the approach of Pruitt isto r 50 µm underlying mechanisms for the and colleagues “represents a usesense of touch and locomotion,” ful new application of piezoresissays Beth L. Pruitt of Stanford tive cantilevers that will be apUniversity’s mechanical engiplicable to a range of interesting w t neering department. biomechanical problems.” Taher l Pruitt and two Stanford colSaif of the Micro and Nanoleagues, Miriam Goodman in technology Laboratory at the the molecular and cellular physi- Schematic of the PR cantilever. The inset shows an image University of Illinois Urbana– ology department and graduof a cantilever with a glass bead (10 μm in diameter) at the Champaign adds, “This paper is ate student Sung-Jin Park, also tip. (Adapted with permission. Copyright 2007 National an example of how fundamental have answered the question of Academy of Sciences, U.S.A.) biological investigations can be “How?” (Proc. Natl. Acad. Sci. facilitated by new, advanced engiU.S.A. 2007, 104, 17,376–17,381). According to Pruitt, the cantilever neering tools—in this case, the piezoreResearchers will often choose optical design gives the device one particular sistive microcantilevers—that have adtweezers and atomic force microscopy advantage over optical measurements. vantages over conventional techniques, (AFM) to measure small displacements “The sample interacts with the cantisuch as optical tweezers and AFM.” (usually, less than a few micrometers) or lever tip, far from the base where some When the Stanford team analyzed forces (typically in the 10 –12 to 10 –9 N joule heating would occur,” she says. the body mechanics of the nematode range). Although these tools are useful “This is particularly important in meaC. elegans, they found that the relationat the biomolecular scale, they present surements that are sensitive to thermal ship between force and displacement some inherent problems when studying effects. Although there is some energy is consistent with an elastic-shell-type tissues and organs, which are larger. So, added by mechanical interaction, it model of the animal’s body. In addithe team designed a force displacement should not upset the system being meation, hydrostatic pressure had a modest measurement system, based on a misured [that is, the nematodes].” contribution to the stiffness of the outer croelectromechanical silicon cantilever Another advantage of the PR canshell compared with the contribution with piezoresistive (PR) sensing, that is tilever is that its working displacement of cuticle mechanics. “The mechania better match for the dimensions and is limited primarily by its stiffness, cal connection between the cuticle and material properties of small organisms. whereas that of an atomic force microtouch-receptor cells may be important, The tool can apply forces in the 10 –8 to scope is limited by the optical detector; and these internal linkages may be re10 –3 N range and can measure displace- the cantilever is, therefore, easy to tune lated to the on–off responses observed ments as great as 100 µm. across a dynamic range. In addition, PR in the ion channels,” says Pruitt. “This The PR cantilever system is built cantilevers can be integrated into highstudy is the first building block in defrom single silicon crystals 2–6 mm bandwidth feedback-control systems termining how forces are transmitted to long. A U-shaped piezoresistor (200 that incorporate high-resolution actuaion channels within the organism.” a μm long and 20 μm wide with a 20 tors. Furthermore, PR-cantilever-based —Steve Miller 28
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