Biomedical Engineering Reference
In-Depth Information
monitored-water should probably be larger than the partial charge on each hydrogen
atom (e.g., 0.417
e
in TIP3P water model). This is consistent with our numerical
observation.
On the other hand, for the practical applications, it is very important to output
the water dipole signal. As we have shown above, there is a hydrogen (oxygen)
atom with a partial positive (negative) charge at the end of the nanochannel when
the water dipole chain is pointing outward (inward). This partial charge can trigger
(or control) a neighboring polar molecule or charge outside the channel (e.g., see
Fig.
1.43
, for the yellow-colored first water molecule outside the channel). Since all
the water molecules share the same dipole orientation as the first water molecule
inside the nanochannels, the overall water dipole moment might be large enough to
be detected. This dipole moment has a value of several Debyes [
59
] and can interact
with the electric field. We note that the semiconductor or conductor properties
of some nanotubes may screen the dipole moment interacting with electric field.
Insulator nanochannels, which may be fabricated in the future, might be better for
this application.
Finally, via dipole-dipole interaction, with relevant interference at nanoscale, we
can achieve signal transmission, conversion, and multiplication. As an important
view, since water is not the only polar molecule with a dipole moment, we expect
that other polar molecules, such as urea or ethanol, might have the same capability
to transmit and multiply signals inside nanotubes. These small molecules, especially
water, have some unusual and important physical and chemical properties, including
their interactions with proteins [
99
,
100
,
107
,
119
]. In fact, as a fundamental property
of all cells, cell polarity has already played a central role in signal transmission and
controlled a variety of polarized cell behaviors [
120
-
124
]. Similar to the monitored
charge that we have used in this chapter, a polarizing signal initiates polar distri-
bution of signaling molecules and leads to polarity establishment and maintenance
through the cytoskeleton and vesicular trafficking [
121
]. For example, during planar
cell polarity (PCP) signaling, core PCP proteins are sorted asymmetrically along
the polarization axis, where PCP refers to coordinated polarization of cells within
the plane of a cell sheet. This sorting is thought to direct coordinated downstream
morphogenetic changes across the entire tissue [
123
-
126
]. We also note that there
have been other modes to achieve signal transmission, such as using the injection of
finite-duration vibrational signals encoding information into a biomolecular wire of
the polypeptide glycine1000 [
127
].
For this open field, there are still much more to be thought and to be done. Future
directions might include, but not limited to, the following forefronts.
(1). The concerted orientations of the dipole molecule chains inside the nanochan-
nels are maintained by the dipole-dipole interactions. It is expected that the “in
phase” strength between those dipole orientations becomes weak as the number
of dipole molecules increases. This is very important in the practice of the long-
range transmission of the dipole signals. It has been shown that the correlation
of the motion of the water molecules in a one-dimensional water chain confined
