MIT researchers designed this tiny microfluidic chip that can measure the mass and density of single cells. Photo: Manalis Lab |
More
than 2,000 years after Archimedes found a way to determine the density
of a king’s crown by measuring its mass in two different fluids, MIT
scientists have used the same principle to solve an equally vexing
puzzle—how to measure the density of a single cell.
“Density
is such a fundamental, basic property of everything,” says William
Grover, a research associate in MIT’s Department of Biological
Engineering. “Every cell in your body has a density, and if you can
measure it accurately enough, it opens a whole new window on the biology
of that cell.”
The
new method, described in the Proceedings of the National Academy of
Sciences, involves measuring the buoyant mass of
each cell in two fluids of different densities. Just as measuring the
crown’s density helped Archimedes determine whether it was made of pure
gold, measuring cell density could allow researchers to gain biophysical
insight into fundamental cellular processes such as adaptations for
survival, and might also be useful for identifying diseased cells,
according to the authors.
Grover
and recent MIT PhD recipient Andrea Bryan are lead authors of the
paper. Both work in the lab of Scott Manalis, a professor of biological
engineering, member of the David H. Koch Institute for Integrative
Cancer Research and senior author of the paper.
Going with the flow
Measuring
the density of living cells is tricky because it requires a tool that
can weigh cells in their native fluid environment, to keep them alive,
and a method to measure each cell in two different fluids.
In
2007, Manalis and his students developed the first technique to measure
the buoyant mass of single living cells. Their device, known as a
suspended microchannel resonator, pumps cells, in fluid, through a
microchannel that runs across a tiny silicon cantilever, or diving-board
structure. That cantilever vibrates within a vacuum; when a cell flows
through the channel, the frequency of the cantilever’s vibration
changes. The cell’s buoyant mass can be calculated from the change in
frequency.
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%, 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 own blood and storing it until just before their
competition, when it is transfused back into the bloodstream. This
boosts the number of red blood cells, potentially enhancing athletic
performance.
Storing
blood can alter the blood’s physical characteristics, and if those
include changes in density, this technique may be able to detect blood
doping, Grover says.
Researchers
in Manalis’ lab are now investigating the densities of other types of
cells, and are starting to work on measuring single cells as they grow
over time—specifically cancer cells, which are characterized by
uncontrolled growth.
“Understanding
how density of individual cancer cells relates to malignant progression
could provide fundamental insights into the underlying cellular
processes, as well as lead to clinical strategies for treating patients
in situations where molecular markers don’t yet exist or are difficult
to measure due to limited sample volumes,” Manalis says.
