December 6, 2004, Ann Arbor, MI--A multidisciplinary team from the University of Michigan is combining nanoparticles and ultrafast pulsed laser to "see" individual cells as they zip past in the bloodstream. The researchers are using a $3 million grant from NASA to determine a way of detecting radiation exposure on the fly by looking for individual cells that have been harmed. At present, such cell counting is only achieved by drawing blood and using a cytometer.
A certain amount of cell death is normal and expected, so there is always some background fluorescence. What the researchers are looking for is a sudden increase in the population of dead white blood cells, which could indicate radiation poisoning. NASA is particularly concerned with radiation exposure as one of the leading health risks in long-term space travel. Radiation�sub-atomic particles moving at tremendous speeds�careens in all directions in space. It can kill cells and damage the DNA within them, causing long-term health problems, especially cancers.
Individual cells in the bloodstream are tagged with a nanoparticle called a dendrimer, which is much smaller than a blood cell and is grown in layers of branching molecules that resemble a tree. At the tips of these "branches" scientists can attach biomolecules that have specific affinity for the white blood cells. Other arms of the dendrimer carry a fluorescent material that will light up if the white blood cell dies.
The idea of using dendrimers for real-time cell counting came from James Baker Jr., professor of biologic nanotechnology and director of the university's Center for Biologic Nanotechnology. His research group is also exploring the use of dendrimers for drug delivery and improved medical imaging.
To see cells as they flow, the researchers are using a pulsed laser developed by physicist Theodore Norris of the university's Center for Ultrafast Optical Science that can be focused down to a spot smaller than a cell. The laser allows researchers to watch a capillary blood vessel just a few blood cells in diameter and to count individual spots of fluorescence as they zip past. The focal area of the near-infrared laser is so tight that they can be sure that each flash of fluorescence represents just a single cell, said Norris, professor of electrical engineering and applied physics.
So far, studies on living mice have shined the light through their semi-translucent ears to see the fluorescing dendrimers within capillaries. The proposal to NASA was for using the capillaries on the retina at the back of the eye, but human ears might work just as well.
"We just need to see a capillary," Norris said. "It doesn't have to be in the eye."