Biomedical Engineering Reference
In-Depth Information
counterparts, the collected concentrations are higher (which contributes to increase the signal-
to-noise ratio of the measurement), but the collected volumes are smaller (which oten makes the
luid collection and the detection more challenging). hus, depending on the sensitivity of the
assay, the microluidic collection of luids for use in a macroluidic assay may not be beneicial nor
practical, and a microengineered version of the assay may have to be developed. Whenever pos-
sible, assays based on microscopic observation (e.g., calcium imaging using luorescent markers,
time-lapse imaging of changes in cell morphology), which can be done in situ, should prove more
practical. However, microscopy assays have inherently low throughput (the microscope probes
only a small ield of view), so adopting a microscopy assay in a microluidic cell culture hinders the
high-throughput beneits of microluidic perfusion; to achieve high throughputs, a programmable
motorized stage can be used to automate image acquisition from multiple ields of view.
5.5 Gene Expression Cellular Microarrays (“Cellomics”)
For many applications in biotechnology and cell biology, it is desirable to create microarrays
of cells in which each array unit (either a single cell or a group of cells) is made genetically
unique, either by the addition of a plasmid containing a bioluminescent protein, a
GFP
reporter
system, or a plasma-permeable dye. In all three cases, the response of the cell(s) to the addition
of an analyte can be detected optically and depends on their (artiicially-added) genetic iden-
tity. David Sabatini's group from the Whitehead Institute (MIT) in Cambridge (Massachusetts)
printed cDNA on a glass slide, coated it with the standard transfecting reagents, and observed
that (surprisingly) cells cultured on top of the cDNA get spontaneously transfected by it (it is
surprising that this “surface-bound transfection” works at all, because the reagents that package
the cDNA were designed to work in solution, not on a surface). Because it is straightforward to
print cDNA in microarrays using robotic pin arrayers, the technique efectively allows for pro-
ducing “gene expression arrays” (
Figure 5.38
).
(a)
(b)
30-80 cells
DNA/gelatin
(c)
Slide
Lipid
Cells
100 µm
Assay
Co-expression
(f )
(d)
(e)
30
25
20
15
10
5
0
Cy3
GFP
Merged
10
20
30
40
50
GFP cDNA (pg)
FIGURE 5.38
Gene. expression. cell. arrays.. (From. Junaid. Ziauddin. and. David. M.. Sabatini,.
“Microarrays.of.cells.expressing.deined.cDNAs,”.
Nature
.411,.107-110,.2001..Adapted.with.per-
mission.from.the.Nature.Publishing.Group.)
