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Microdevices flex their muscles S
COURTESY OF CARLO MONTEMAGNO
for the measurement of forces exerted by plains Montemagno. “The problem is ome microdevices literally strut their the cells. When the cells were at rest, [that] the technology used for creating stuff. Carlo Montemagno and colthere was a constant deflection in the [a] sacrificial layer involves [acids] like leagues at the University of California, cantilever. When the muscle cells conHCl or HF, which aren’t conducive to Los Angeles, have developed one that tracted in a power stroke, the cantilever keeping cells alive,” he says. has muscle cells and moves along a surdeflected even further before returning The PNIPAAm layer was removed face. Not only do these tiny robotic devices have motion, they can also be used once the muscle cells had positioned and to its original position. The investigators could calculate the forces generated by anchored themselves on the skeleton. to measure the mechanical properties of the cells—and underthe muscle cells (Nat. Mater. stand their mechanical 2005, 4, 180–184). properties—by using The new devices are a the magnitude of the hybrid of a silicon skeleton deflection, the materiand rat cardiac muscle cells. al properties of silicon, “They appear to be the and the dimensions of first stand-alone devices inthe cantilever. volving cells as actuators. Montemagno and [This suggests] that it may colleagues are curbe possible to integrate rently involved in a cells and chips of similar project funded by the length scales,” says ChrisThe deflection of a silicon cantilever can be used to measure the forces generU.S. National Aerotopher Chen of the Uniated by muscle cells. nautics and Space Adversity of Pennsylvania. ministration (NASA) to use the miThe result was an autonomous micro“This demonstration raises the bar on crodevices for the analysis of muscle device made out of freely contracting, cell-device integration.” atrophy under low-gravity conditions. monolithic muscle structures attached Montemagno and colleagues used Montemagno explains that the devices only to the Cr/Au layer on the silicon. the single-crystal reactive etching and can be adapted into “a biotechnological “The cells aren’t just tissue,” states metallization (SCREAM) process (Miassay system for doing a direct correlaMontemagno. “They are actually fully croelectron. Eng. 1996, 32, 49–73) to organized muscles.” Because the fabrica- tion between proteomic expression and fabricate the silicon skeleton. The skelephysiological function.” tion of the silicon structure was indeton was incubated in a solution of a Many proteins are implicated, either pendent of the cell integration, a variety thermoresponsive polymer, poly-N-isoby their malfunction or by lack of exof cell types could potentially be used. propylacrylamide (PNIPAAm). The repression, in muscle atrophy. Because no The muscle cells contracted and resulting layer inhibited cell growth, but a direct evidence correlates the expression leased against the skeleton, using glulayer of Cr/Au sputtered on top of the and behavior of different proteins to PNIPAAm promoted cell adherence. By cose in the solution as the energy muscle function, Montemagno and colsource, and caused the device to inch selectively patterning the silicon surface leagues are using the microdevices to with PNIPAAm and Cr/Au, the investi- along the surface. Henry Hess of the University of Washington explains, “The track the expression of different proteins gators were able to direct the attachwith fluorescent markers. “By having a most exciting aspect is the self-assembly ment and growth of the cells. series of different markers, you’re able of a motile structure, where the beating The PNIPAAm acted as a sacrificial to look at simultaneously the expression of muscle tissue is transduced by metal layer. When cooled to lower temperaof a large number of proteins and legs into directed motion.” The microtures, the polymer underwent a solid– equate them to the amount of work devices can take steps of up to ~25 µm liquid phase transition and dissolved in muscles are able to do,” says Montean aqueous solution. “The real key [was at a frequency of ~1.8 Hz. magno. a The fabrication of the microdevices that] we had to figure out a way to —Rajendrani Mukhopadhyay could be modified to produce cantilevers make these [cells] grow in thin air,” ex© 2005 AMERICAN CHEMICAL SOCIETY
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