Present methods allow cells to be sorted based on whether or not they
emit fluorescent light when mixed with a marker that responds to a
particular protein or other compound. The new system allows more
precise sorting, separating out cells based not just on the overall
average fluorescent response of the whole cell but on responses that
occur in specific parts of the cell, such as the nucleus. The system
can also pick up responses that vary in how fast they begin or how
long they last.
�We�ve been interested in looking at things inside the cell that
either change over time, or are in specific places,� Voldman says.
Separating out cells with such characteristics �can�t be done with
traditional cell sorting.�
For example, if cells differ in how quickly they respond to a
particular compound used in the fluorescent labeling, the new system
would make it possible to �select out the ones that are faster or
slower, and see what�s different,� says Voldman, who also has
appointments in MIT�s Research Laboratory of Electronics and the
Microsystems Technology Laboratories.
�It seems like that should be easy, but it isn�t,� he said. There are
other ways of accomplishing the same kind of cell separation, but they
require complex and expensive equipment, or are limited in the number
of cells they can process.
The new system uses a simple transparent silicone layer bonded to a
conventional glass microscope slide. Fabricated in the layer are a
series of tiny cavities, or traps, in which cells settle out after
being added to the slide in a solution. Up to 10,000 cells could be
sorted on a single slide.
Looking through the microscope, either a technician or a computerized
system can check each cell to determine whether it has fluorescence in
the right area or at the right time to meet the selection criteria. If
so, its position is noted by the computer. At the end of the selection
process, all of the cells whose positions were recorded are then
levitated out of their traps using the pressure of a beam of targeted
light from a low-cost laser. A flowing fluid then sweeps the selected
cells off to a separate reservoir.
The laser levitation of the cells acts like �a fire hose pushing up a
beach ball,� Voldman says. But the laser method is gentle enough that
the living cells remain viable after the process is complete, allowing
further biological testing.
Voldman and Kovac are continuing to refine the system, working on
making it easier to use and on improving its ability to keep samples
sterile. Voldman says that unlike expensive separation techniques such
as optical tweezers, the new system could cost only a few thousand
dollars. As a result, it could be employed in a variety of biological
research laboratories or clinical settings, not just in big,
centralized testing facilities.
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