Biomedical Engineering Reference
In-Depth Information
FIGURE 5.15
Cell.sorting.using.acoustic.streaming..(From.T..Franke,.S..Braunmüller,.L..Schmid,.A..
Wixforth,.and.D..A..Weitz,.“Surface.acoustic.wave.actuated.cell.sorting.(SAWACS),”.
Lab Chip
.10,.
789-794,.2010..Reproduced.with.permission.from.The.Royal.Society.of.Chemistry.)
5.3 Cell Trapping
Here we consider technologies for trapping cells, which can be considered a special case of sort-
ing whereby the cells end up being immobilized such that they can be observed or assayed for
a long time.
5.3.1 Neuro-Cages
Since the late 1990s, Jerome Pine's group at Caltech has used micromachined “neuro-cages,” ini-
tially made in silicon, to conine neurons. he idea behind conining the neurons in cages is that
neurons tend to migrate onto chemically untreated silicon, so it becomes necessary to physi-
cally constrain their soma in a “cage” with “tunnels” that allow the cells to grow processes out
of the cage to synapse with other (also conined) neurons. In the initial silicon cages (produced
by KOH etching), the cages were actually wells and the cells were supported by a metalized
20-μm-thick membrane at the bottom of the well, which allowed for extracellular recordings.
In a more recent design, the cages have been constructed on an electrode-patterned wafer by
two-layer photolithography in parylene, a biocompatible photosensitive polymer (
Figure 5.16
).
he device, however, is nontransparent (although, in principle, it could be built on quartz), so
it is incompatible with many microscopy modalities based on light transmission such as phase-
contrast microscopy.
5.3.2 PDMS Microwells
he author's group at the University of Washington in Seattle has demonstrated an inexpensive
alternative to the problem of trapping cells based on PDMS molding. A master made by photoli-
thography contains an array of cylindrical posts, each slightly wider than the size of a single cell.
he PDMS replica thus contains an array of circular wells. he cell-trapping procedure exploits
the high apparent viscosity of water on the microscale: once a cell falls into a well, it is very
hard for luid to dislodge the cell out of the well. With repeated seedings, it is possible to obtain
more than 90% microwell occupancy rates even with low seeding densities (
Figure 5.17
). In
principle, the microwells can be designed to accommodate more than one cell (of more than one
cell type) for studies of cell-cell communication. Adherent cells conform to the curved walls or
edges of the bottom of the microwells, adopting shapes that may produce confounding results.
With a large-chip cooled-CCD camera and an optimized array, it is possible to image more than
100,000 cells (in particular, calcium transients in olfactory sensory neurons) simultaneously at
4× magniication.
