Biomedical Engineering Reference
In-Depth Information
Spurred by the biopharmaceutical interest in overcoming the drug-screening bottleneck,
several laboratories have developed various procedures to build planar patch clamp-on-a-chip
devices, whereby cells are deposited on top of a micron-sized hole on a thin insulating substrate
over a cavity or channel illed with electrolyte solution (i.e., the hole and the cavity act as a
“microfabricated, prepositioned pipette,” see conceptual rendering in
Figure 5.51
).
he evolution of the ield provides a very interesting lesson for materials scientists and biolo-
gists, as the device material (glass) was known by biologists to be critical for device function yet
engineers oten compromised on the material of choice due to fabrication challenges (building
the structure of
Figure 5.51b
exactly as shown in glass is, to this day, impossible!)—until a new
material (PDMS) was found to be suitable. hese are the main pioneering contributions:
In 2000, Horst Vogel's group from the Swiss Federal Institute of Technology made
apertures (0.6-7 μm diameter) in silicon nitride membranes (0.1-1 μm thick) sus-
pended over a silicon pit; they used a combination of anisotropic KOH silicon etch-
ing and reactive-ion etching (RIE), followed by building a SiO
2
layer with deposition
and thermal oxidation. Lipid vesicles were electrophoretically positioned onto the
aperture and seals with the membrane patches could be obtained with resistances
up to 200 GΩ if the chip was coated with polylysine or chemically modiied with
4-aminobutyl-dimethyl-methoxysilane.
In 2002, James Heath and colleagues at UCLA reconstituted K
+
ion channels in lipid
bilayers suspended in the 100- and 200-μm-diameter pores of silanized (hydropho-
bic) SiO
2
membranes created by RIE. Two years later, this group was able to demon-
strate good seals with RAW 264.7, CHO-K1, HIT-T15, and RIN m5F cells, although
only approximately 5% of the attempts yielded a giga-seal (with CHO-K1 and RIN
m5F cells; with HIT-T15 and RAW 264.7 the resistances were ~100 MΩ).
In 2002, a collaboration between James Klemic and Fred Sigworth's groups at Yale
University produced micromolded arrays of planar PDMS patch electrodes with
apertures of 2 to 20 μm. he apertures were formed by point-contact replication of
epoxy pyramids (in contact with a hard surface), but the pyramids were fabricated
from crystalline-silicon molds, so the points of the pyramids were necessarily formed
by the coincidence of four crystalline planes (which never forms a perfect point—a
point is deined by the intersection of three planes) and small irregularities in the
etching conditions produced diferences in the height of the pyramids (so during
replication, some point-contacts received more pressure than others, resulting in
irregular holes, as can be seen in
Figure 5.52a
). Nevertheless, by plasma oxidation of
PDMS, they could temporarily improve the hydrophilicity of PDMS and obtain seals
on oocytes expressing potassium channels. Two years later, the same group proposed
a clever molding strategy based on a focused stream of pressurized air as the PDMS
Nonadhesive
coating
a
Glass
micropipette
Cell
membrane
K
+
Giga seal
Giga seal
b
Aperture
Na
+
Ion
channel
Microchannel
FIGURE 5.51
Conceptual.comparison.between.(a).the.traditional.approach.and.(b).the.planar.chip.
approach.to.patch.clamp.recording..The.nonadhesive.coating.in.(b).is.optional..(Figure.contributed.
by.the.author.)
