To adapt the system to measure density, the researchers needed to flow each cell through the channel twice, each time in a different fluid. A cell's buoyant mass (its mass as it floats in fluid) depends on its absolute mass and volume, so by measuring two different buoyant masses for a cell, its mass, volume and density can be calculated.
The new device rapidly exchanges the fluids in the channel without harming the cell, and the entire measurement process for one cell takes as little as five seconds.
Changes in density
The researchers tested their system with several types of cells, including red blood cells and leukemia cells. In the leukemia study, the researchers treated the cells with an antibiotic called staurosporine, then measured their density less than an hour later. Even in that short time, a change in density was already apparent. (The cells grew denser as they started to die.) The treated leukemia cells increased their density by only about 1 percent, a change that would be difficult to detect without a highly sensitive device such as this one. Because of that rapid response and sensitivity, this method could become a good way to screen potential cancer drugs.
"It was really easy, by the density measurement, to identify cells that had responded to the drug. If we had looked at mass alone, or volume alone, we never would have seen that effect," Bryan says.
The researchers also demonstrated that malaria-infected red blood cells lose density as their infection progresses. This density loss was already known, but this is the first time it has been observed in single cells.
Being able to detect changes in red-blood-cell density could also offer a new way to test athletes who try to cheat by "doping" their blood that is, by removing their ow
|Contact: Caroline McCall|
Massachusetts Institute of Technology