and pressure sources, one can focus the cells to the desired location. Single cells
were isolated in high-density arrays composed of two channel height levels
(Fig.8.6B). The larger 40-μm channel height served as the main fluid conduits
for cell solutions while the 2-μm-height regions were used to form elevated
trapping regions. Having a 2-μm gap allows a fraction of fluid streamlines carrying
cells to enter a trap. Once the cell enters a trap and partially occludes the 2-μm gap,
then there is a reduction in the fraction of fluid streamlines (and cells) entering the
trap region which leads to a high quantity of single-cell isolation [ 59 ]. The prob-
ability of trapping was found to be dependent on the number of cells previously
entrapped. For on-chip valves based focusing PDMS-based valves and pumps
configured within the device a pressure-driven, feedback-based dynamic trap
using on-chip sieve valves that are positioned at both ends of the sensing channel
and can selectively capture a cell without replacing the solution (Fig.8.6C)
[ 63 ]. The small channel dimensions and laminar flow in MF channels also enable
a variety of immiscible droplet-based generation technologies to be developed
[ 65 ]. Droplets are generated by shearing a dispersed aqueous phase with a constant
Fig. 8.6 (A) Schematic of the various types of methods reported for transporting and manipulating
cells. (a) Cells can be hydrodynamically focused at an intersection so that they all flow past an
interrogation region in single file. (b) Cells can be hydrodynamically transported to an intersection,
where they can be lysed either electrically or chemically and the lysate injected into a separation
channel. (c) Cells can be incorporated into aqueous droplets within an immiscible fluid and
hydrodynamically transported to an interrogation region. Cells can be trapped in an interrogation
region geometrically using pillars (d), weirs (e), or hydrodynamically balanced flows (f). Arrays of
cells can also be trapped using filters (g). Dielectrophoretic trap for cells (h). The gray circles
represent cells and the arrows indicate the direction of fluid and cell flow [ 62 ]. (B)(a–c)Hydrody-
namic cell sorting method [ 59 ]and(C) cell sorting based on pumps and valves on chip [ 63 ]
8 Biological Applications of Microfluidics System 207