Microelectronic Devices Based on Nanomaterial-Carrier Cells - Cell Surface Engineering: Fabrication of Functional Nanoshells - page 134

type of charge carriers and their densities are easily modulated by external
stimuli, also function on similar principles. The two basic modulations in
such nonmetallic interfaces are the density of charge carriers or their
mobility. 8,9,16,18,19
Devices using doped Si nanowires have been demonstrated. 3,15,19 The
devices in these cases were interfaced with neurons and myocyte cells and
their modulation was recorded in response to the changes in the cell
membrane potential. In the case of neurons this was due to the generation
and propagation of neural impulses and for myocytes it corresponds to their
rhythmic beating. In both cases, modulations of the cell membrane poten-
tial lead to gating of the nanowire and the dynamics of this modulation are
recorded in the current across the nanowire. The interface-cell interaction is
similar to a field effect transistor (FET). Similar configurations using semi-
conducting CNTs, other inorganic materials and organic semiconductors
have been demonstrated. 7-9,16
Recently, the FET type devices have been configured where the inside of
the cell is probed using similar concept (Figure 7.2a). In this configuration,
one end of a nanotube penetrates and is in contact with the cells interior,
while its other end is linked to a nanowire (active wire) that translates be-
tween anode and cathode electrode. This configuration is similar to an in-
verted T-junction. 17 The nanotube in contact with the cell brings the cell's
cytoplasmic fluid in contact with the active wire. The potential (chemical) of
the cells interior relative to the outside is effectively translated by the gating
of the active wire due to the contacting cellular fluid. The cells dynamics are
reflected in the current modulation of the active wire. The FET-type devices
rely on the gating effect due to the potential of the cell. Nanomaterials, due
to their size constraints are able to act as effective transistors and also
translate the cell dynamics into current modulations. 17,22
Changes in a cell's shape, size and surface stresses are linked to its
cytoskeleton. It is a dynamic system that also responds to a cell's interaction
with its environment. Translation of changes in the mechanical state of
the cell requires an electromechanically active nanomaterial interface. The
nanomaterial should be able to respond to mechanical changes and its
electrical properties have to be sensitive to such changes. In this respect,
nanowire and nanoparticles have limitations as stresses in cellular systems
are on the scale of kPa. The solid cross section of the nanowires and
nanoparticles limits their deformation in compliance with the changes in a
cell's mechanics. Further, unless the material has a high peizoresistive co-
ecient, the modulation in the current due to these stresses will be limited.
Graphene sheets with nanometer-scale thickness are like paper and hence
respond to inplane stresses by formation of wrinkles. Stresses in the range of
kPa are sucient to produce this response. 23,24 The thin cross section of the
sheets also makes their charge-carrier dynamics sensitive to the applied
stresses. The formation of wrinkles leads to local modulations in the band
structure and also scattering of charge carriers and hence results in in-
creased resistance with stress on the sheets. Devices with cells partially

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