Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 226

a b

electrode

z

1

membrane

y

x 4

2

x

PDMS

3

Fig. 8.14 ( a ) Side view (cross section) and ( b ) top view of a detector of three-dimensional forces

process that takes place first at high temperatures (higher than the Curie temperature
of the polymer) in a DC field, stage at which only the nanoparticles are poled, and
then at low temperatures in an AC field, which poles the polymer matrix and can
be interrupted when the polarization of the matrix becomes parallel or antiparallel
to that of the nanoparticles. An appropriate formulation and poling minimizes the
cross-sensitivities between these subcells. Electrical transduction of the signals is
achieved by aligning each subcell on the frontplane foil with an amorphous-silicon
thin-film transistor placed on the flexible backplane.
Animal skin contains very sensitive tactile sensors. Such sensors based on ionic
membranes and capable of detecting three-dimensional forces, i.e., can detect the
direction of the applied forces, are described in Wang et al. ( 2009 ). In the skin of a
cucumber tendril, tactile sensors of three-dimensional forces are shaped as extruded
papillae positioned on the inner surface. A similarly shaped tactile sensor has been
fabricated in a light and flexible ionic polymer metal composite ( Wang et al. 2009 ).
The induced stress is proportional to the charge density accumulated at the polymer
surface as a result of water uptake at the interface between the polymer and metal
electrode at actuation, while the direction of applied force is determined by a special,
dome-shaped design of the sensor. More precisely, the dome, with a diameter of
9 mm and a height of 3 mm, consists of a sheet of Flemion polymer sandwiched
between 8- and 10-m-thick gold electrodes and is positioned over a layer of soft
elastomer polydimethylsiloxane (PDMS). Viewed from above, the sensor consists
of four segments, denoted by 1, 2, 3, and 4 in Fig. 8.14 , the comparison of the
current/voltage signals of all segments determining the direction of the applied
force. For instance, for a force applied along z , all four signals are equal, whereas
for a force applied along the positive x axis, only sensor 2 has a significant response.
Even such delicate organs as lungs can be mimicked in microdevices that
reproduce the critical functionality of the alveolar-capillary interface of the human
organ. Such a microdevice is, by necessity, a microfluidic system, which con-
sists of two microchannels separated by a porous and flexible membrane of
poly(dimethylsiloxane) with a thickness of only 10m( Huh et al. 2010 ). If the
membrane is coated with extracellular matrix such as collagen or fibronectin, which
provides structural support to animal cells, and if human pulmonary microvascular
endothelial and alveolar epithelial cells are cultured on opposite sides of the mem-
brane, the resulting structure closely resembles a human lung, the microchambers