Biomedical Engineering Reference
In-Depth Information
and rabbits) has a remarkable capacity for processing motion information, a function previously
thought to be restricted to the neurons of the visual cortex.
With hindsight, the success of the retina-on-MEA system is not surprising because it is a
combination of good choices: (1) As neural circuits go, the retina is extremely well character-
ized and its general function is also very well known; (2) the circuit can be removed from its
organ without damaging its internal connections; (3) as the supporting cells are removed with
the circuit and the layer of tissue is so thin that nutrients can be delivered to the cells by difu-
sion through the tissue, the circuit can be kept alive in simple saline solution for many hours;
(4) there is no need to engineer complicated interfaces to provide an input to the circuit because
the natural input is light (which can be projected in patterns through the objective of a micro-
scope); and (5) the retina's output (the action potentials in ganglion cell axons) is clearly deined
and organized in a single lat layer, which can be recorded with the planar MEA.
Note that the impedance of the cell-electrode contact is expected to vary from cell to cell,
depending on how closely the cell is attached to the electrode, and to vary with time because
focal attachments to the substrate are inherently dynamic and the electrode's adhesiveness (e.g.,
given by its protein coverage) may also be changing.
Recently, Bruce Wheeler's laboratory, formerly at the University of Illinois at Urbana-
Champaign, devised a clever variation of MEAs designed for detecting action potential propa-
gation from isolated axons in culture (
Figure 5.46
). he neurons are seeded in an open well that
has microfabricated tunnels (10 μm wide, 3 μm high, and 750 μm long) through which the axons
Electrode
(75-µm width)
Electrode
(25-µm)
Electrode
(50-µm)
Electrode
(75-µm)
a
b
Microtunnels
Neurons
PDMS
Microtunnels
Electrode
(50-µm width)
100 µm
Neurons
Electrode
(25-µm width)
Axons
150
c
Electrode A (75-µm)
Electrode B (50-µm)
Electrode C (25-µm)
100
50
0
-50
-100
-150
A
C
C
B
Reverse propagation
A
B
-200
-250
0
Forward propagation
2
4
6
8
10
12
Time/ms
FIGURE 5.46
MEAs. combined. with. microtunnels. to. measure. axonal. signal. conduction. velocity..
(From. Bradley. J.. Dworak. and. Bruce. C.. Wheeler,. “Novel. MEA. platform. with. PDMS. microtunnels.
enables.the.detection.of.action.potential.propagation.from.isolated.axons.in.culture,”.
Lab Chip
.9,.
404-410,.2009..Reproduced.with.permission.from.The.Royal.Society.of.Chemistry.)
