When a cell meets its demise, so too does its fine-tuned system for regulating nutrient uptake and waste excretion. As a result, cell death is typically marked by a sharp increase in electrical conductivity, as various ions become free to pass through newly opened pores in the cell membrane. That telltale change affords a convenient way to sort live cells from dead ones: In what’s known as dielectrophoresis, electric-field gradients induce cells to migrate with a conductivity-dependent velocity. Exploiting the effect typically calls for fashioning tiny electrodes inside a microfluidic channel, but a group led by Xiangchun Xuan of Clemson University, South Carolina, has now devised a simpler alternative. The key was recognizing that a severe flow constriction is often sufficient, by itself, to distort an otherwise uniform electric field and create the strong gradients needed for dielectrophoresis. The essential component of the Clemson team’s device is pictured here. Yeast cells generally flow from left to right, from a reservoir into a narrow microchannel, but near the constriction, dielectrophoretic forces oppose the bulk flow. Since those forces act more strongly on dead yeast than live yeast, they can be tailored to trap dead cells in the reservoir while allowing live ones—fluorescent green in the image—to enter the channel. The researchers anticipate that their design can be integrated into lab-on-a-chip devices to aid in biomedical diagnostics and drug screening. (S. Patel et al., Biomicrofluidics, in press.)—Ashley G. Smart
