liver, brain, gut, kidney, lung, and heart [ 28 ]. For analysis of kidney transport
barrier function, a simplified kidney model was created by stacking two
microfabricated PDMS chambers, separated by a porous thin membrane [ 29 ]. The
apical channel is separated from a bottom reservoir by an ECM-coated porous
membrane. The presence of the physiological level of apical fluid shear stress helps
in culturing the primary human proximal tubule epithelial cells on the membrane.
The second compartment (basolateral compartment) is readily accessible for fluid
sampling and addition of test compounds to study the active and passive epithelial
transport. The dimensions of the MF channel were 1 mm wide, 1 cm long and
100 μm high. The bottom structure beneath the MF channel is rectangular 1 mm
wide and 0.6 cm long which is made from a cured PDMS slab to serve as a medium
reservoir directly. This fabricated design mimics the living kidney proximal tubule
in terms of natural architecture, tissue–tissue interface and dynamically active
mechanical microenvironment. The top channel mimics the urinary lumen and
has fluid flow, whereas the bottom chamber mimics interstitial space and is filled
with media. Primary human proximal tubular epithelial cells were cultured under
flow conditions in the MF chip device. On reaching the confluence, the cells are
exposed to circulating culture medium at a fluid shear stress of 0.2 dyne cm^2 for
3 days using a syringe pump. After 3 days of culture under both fluidic and static
conditions the immunofluorescence, microscopic analysis of the proximal tubular
epithelial cells revealed a well defined confluent epithelial monolayers which are
continuously lined by a linear distribution of the tight junction (Fig.8.4A). In
contrast, exposure of cells to physiological fluid shear stress (0.2 dyne cm^2 ) results
in restoring the cells to normal columnar form and the height is increased by almost
twofold (Fig.8.4B). In a typical MF “alveoli-on-a-chip” setup there is a cyclic
propagation of a meniscus over a flexible PDMS membrane which recreates a
combined stresses and provides a more physiologic recreation of the stresses,
including cyclic distribution of air-liquid interface and wall stretch. During the
process, the alveolar chamber can be partially filled with fluid and positioned in the
vertical configuration to establish a meniscus at the interface of fluid and air. By
withdrawing fluid from the “actuation channel”, the membrane can be forced to
deform and relax stretching cells and propagating the meniscus over a specified cell
region (Fig.8.4C)[ 30 ]. The MF system provides the first in vitro technique to study
the role of both solid and fluid mechanical forces in ventilator-induced lung injury
systematically.
In another example of lung-on-a-chip a microfabricated lung mimic device was
created which uses compartmentalized PDMS microchannels to form an alveolar-
capillary barrier on a thin, porous, flexible PDMS membrane coated with ECM
[ 31 ]. Soft lithography techniques were used to fabricate these hollow
microchannels which are conjugated with a new method that uses chemical etching
of PDMS to form the vacuum chambers. The fabrication starts with the alignment
and permanent bonding of a 10-μm thick porous PDMS membrane (10-μm wide;
pentagonal pores) and two PDMS layers containing recessed microchannels
(Fig.8.4D). The entire integrated device was only 1–2 cm in length, with the
central channels only millimeters in width, thus making it entirely amenable to
200 S. Solanki and C.M. Pandey