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ANALYTICAL CURRENTS Cell migration along gradients Coiled nanotube resonators W. Monty Reichert and colleagues at Duke University and the University of Delaware have developed a new technique for quantitatively characterizing the movement of cells along gradients of surface-bound ligands. Cell migration in response to chemical gradients is essential for processes such as wound healing and immune regulation.
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Although carbon nanotubes (CNTs) have been used as mechanical resonant sensors, complicated sample preparation techniques and sophisticated electron microscope measurements are typically required. To overcome these limitations, Alexander Volodin and colleagues at Katholieke Universiteit Leuven, Facultés Universitaires Notre-Dame de la Paix (both in Belgium), and the Helsinki University of Technology (Finland) have turned to intrinsically coiled multiwalled CNTs. They have discovered that coiled CNTs with gold contacts make convenient resonators that can detect mass changes down to a few tens of attograms. The researchers attached CNTs to a silicon substrate and deposited narrow gold electrodes on top of them. The CNTs retained their 3-D structure with sections of free-standing helical windings.
A schematic of migration assays on gradients of fibronectin: (a) Teflon restraint assay prior to removal of the restraint and (b) after removal of the restraint. (c) Free cell migration assay.
These sections of windings revealed characteristic resonances,
The researchers used two different migration assays —the Teflon restraint assay and the free cell assay—to monitor the movement of cells. In the Teflon restraint assay, cells were plated behind a Teflon restraint and allowed to reach a confluence in growth media for a minimum of 40 h. The barrier was then removed, and the cells were allowed to migrate until they moved out of the field of view or moved away from the rest of the cells and became free cells. In the free cell assay, cells were positioned at a much lower density so that no cell touched another cell. This assay was used to determine the effect of the gradient on cell migration without cell–cell interactions. To demonstrate the migration assays, the researchers measured the drift speed of bovine aortic endothelial cells on linear gradients of the cell-adhesion molecule fibronectin. Surfaces with a uniform concentration of fibronectin served as controls. The fibronectin gradients were generated by cross-diffusion of alkanethiols on a gold substrate and characterized by a combination of X-ray photoelectron spectroscopy and surface plasmon resonance. The results establish reproducible conditions for studying cell motility on gradients functionalized with cell-adhesive proteins. The researchers suggest that the same technique could also be used to study other cells in other signaling environments. (Langmuir 2004, 20, 8279–8286)
The gold electrodes were 80 nm thick.
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which depended on their shape and size. The CNTs had a diameter of 15–30 nm, and the windings typically had a diameter of 200 nm.
When excited by either a radio-frequency electrical field or an ultrasonic transducer, the CNTs showed a piezoresistive-like response (i.e., they were sensitive to vibrations). Using the coiled CNTs, the researchers could detect frequencies of 100–400 MHz. Additional studies are needed to understand the response of the electrical contacts and to improve their reproducibility. To enhance the piezoresistive response, Volodin and colleagues suggest including an artificial barrier between the CNTs and the gold contacts. (Nano Lett. 2004, 4, 1775–1779)
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Atomic force microscopy image of a coiled carbon nanotube, which can serve as a mechanical resonator.
