Biomedical Engineering Reference
In-Depth Information
THE SIMPLEST OUTSMARTS THEM ALL
The Di Carlo cell counter
sets a very high bar for improvement, and it sends a
humbling note as well. How can it be that the brightest minds in the ield spent so many
years investigating complicated designs for low cytometry—didn't it seem like a compli-
cated problem?—and then a irst-year professor, in one of his irst articles, presented the
optimal
solution to the problem, which simply consists of using
straight
channels? Indeed,
the device contains straight single-layer channels, uses fairly simple camera equipment to
capture the data, and yet it is capable of the highest throughputs ever achieved in the ield.
Moving forward, it is rather unlikely that anyone will want to perform low cytometry
at rates higher than a million cells per second (or using less than one inlet per device), so
the challenge now will be the integration of inexpensive detection modules, such as opto-
luidics, lensless microscopy, and electrical detectors to acquire the data without the need
of microscopes such that the samples can be analyzed in home or ield settings.
5.2 Cell Sorting
If low cytometry (counting cells) is the simplest operation one can do with cells, the next-simplest
is to try to sort them (i.e., route them into separate paths using low) for analysis. Next, we will
consider the challenge of trapping cells, a particular case of sorting that involves immobilizing
the cells, usually so that a static assay such as microscopy can be performed.
5.2.1 Red Blood Cell Assays
Blood is our most abundant luid; hence, the diagnostics of many diseases take advantage of blood
analysis. Automating and miniaturizing blood analysis into a microluidic format is very attrac-
tive because it reduces required sample size, operator expertise, patient discomfort, assay time,
reagents, and cost. As early as 1989, a team of researchers led by Kazuo Sato at Hitachi in Tokyo
(Japan) presented in the journal
Biorheology
a microluidic device for measuring mechanical
properties of blood cells. Albeit simple, this oten-forgotten article deserves the honorable men-
tion of having pioneered the introduction of
any type of
live cells into microchannels. he device
consisted of a large chamber with a central inlet hole through which blood was injected into the
chamber; as blood entered the chamber loor, it was radially forced to exit through one of the 2600
silicon microgrooves (each 6 μm deep, 9 μm wide at the top, and 10 μm long) etched in one of the
four walls of the chamber. Both preparations of white blood cells (before and ater activation with
fMLP
) and of erythrocytes were pushed through the device to study diferences in the suspension
passage time; however, the results are diicult to interpret because of the triangular cross-section
of the channels. In 1995, Adrian Barnes and colleagues from Hertfordshire University in England
(
Figure 5.9
) designed a similar device consisting of eight square, cross-section microchannels
connecting two glass-capped silicon-pit reservoirs. he microchannels were smaller than the red
blood cells (mimicking the size of microcapillaries), which forced the cells to deform in the direc-
tion of the low and allowed for discerning anemic cells from healthy cells by the speed at which
they traveled through the microchannel (anemic cells are more deformable, so they travel faster).
Dan Chiu's group at the University of Washington in Seattle elaborated on this idea sev-
eral years later to model malaria infection using PDMS microchannels that contained constric-
tions (see
Figure 5.10
). Indeed, in people alicted by malaria, organ failure is observed due
to an increase in the rigidity of red blood cells and the blockage of capillaries. Uninfected red
blood cells (6 μm wide and highly elastic) were able to traverse channels of widths 2, 4, 6, and 8
μm. In their tropozoite stage, the cells could traverse only channels 6 μm wide or larger. Also,
uninfected cells were able to squeeze through the blockages formed by immobile schizonts in a
6-μm-wide microchannel.
