Biomedical Engineering Reference
In-Depth Information
a
b
δ
F
PDMS
Cell
traction
d
L
3EI
L
3
F =
δ
20 nN
5 µm
FIGURE 6.23
Measuring. cell. traction. using. PDMS. microneedles.. (From. John. L.. Tan,. Joe. Tien,.
Dana.M..Pirone,.Darren.S..Gray,.Kiran.Bhadriraju,.and.Christopher.S..Chen,.“Cells.lying.on.a.bed.
of.microneedles:.An.approach.to.isolate.mechanical.force,”.
Proc. Natl. Acad. Sci. U. S. A.
.100,.
1484-1489,.2003..(a).Figure.contributed.by.Chris.Chen;.Copyright.(2003).National.Academy.of.
Sciences,.U..S..A;.(b).Figure.contributed.by.Nate.Sniadecki.)
6.4.2 Chemotaxis
Chemotaxis
is the phenomenon in which cells direct their movement through the action of cer-
tain chemicals. he irst observation of chemotaxis is credited to the German microbiologist,
heodow W. Engelmann, who in 1881 observed that bacteria move toward the chloroplasts in
algae—and he correctly hypothesized that the chemoattractant is the oxygen produced by the
chloroplasts. In general, if a cell senses a diference in chemoattractant concentration across the
length of its cell body, then it mounts a migration response, typically (but not always) toward the
increasing chemoattractant concentration. (A parallel deinition is used for chemorepellent sub-
stances.) Exceptions to this rule occur when the chemoattractant concentration saturates all the
receptors on the cell surface (making the cell insensitive to chemoattractant spatial variations)
and when the chemoattractant concentration ield changes so fast that the signal transduction
machinery cannot adapt to the changes fast enough. Because chemotaxis is essentially a phenom-
enon in which the cell needs to sense chemical gradients on a cellular scale with high temporal
resolution, many groups have seen a great opportunity for using microluidic gradient generators
to control the gradients to which the cells are exposed. Nonmicroluidic methods to create gradi-
ents, such as glass pipettes, the
Boyden chamber
, the
Zigmond chamber
, and the
Dunn cham-
ber
, have been traditionally used by biologists (and are commercially available) but are slowly
being displaced by the more quantitative microluidic assays described in the next section.
6.4.2.1 Neutrophil Chemotaxis
Neutrophils are the body's irst line of defense against bacterial infections and they are constantly
roaming the blood, which they exit in response to bacterial or macrophage signals when there is an
infection (
Figure 6.24
). hey are a convenient source of cells for cell culture experiments, as they are
straightforwardly extracted by pricking a human volunteer's inger. he various neutrophil chemoat-
tractants, such as
interleukin-8
(
IL-8
) or
fMLP
(
N
-formyl-methionine-leucine-phenylalanine), a
peptide chain produced by some bacteria, are commercially available. Hence, in addition to their
intrinsic scientiic interest, neutrophils have become a popular cell model for studying cell migration
and, historically, they were the irst cell type to be used in a microluidic chemotaxis assay.
In 2002, a team led by Mehmet Toner at Harvard Medical School used the Dertinger gradi-
ent generator (see
Figure 3.74
in Section 3.9.2) to expose neutrophils to various gradients of
IL-8 (
Figure 6.25
). he use of the Dertinger generator allowed for exposing the neutrophils to
complex (linear as well as nonlinear) gradients. As expected, the cells migrate toward increasing
IL-8 concentrations in linear gradients and stop abruptly when they ind a sudden “clif” gradi-
ent (drop to zero concentration). However, they keep migrating without reversing direction if
they ind a “hill” (a maximum) in chemoattractant concentration, despite the fact that they are
migrating toward decreasing concentrations. It is important to stress that the cells react quickly
around zero concentrations (they do not overshoot the minimum concentration point), but they
