Biomedical Engineering Reference
In-Depth Information
Fig. 1.1
(
a
) A water molecule, where the
red
and
gray balls
represent one oxygen atom and two
hydrogen atoms. (
b
) A water molecule together with four neighboring water molecules. The
arrows
denote a hydrogen bond and the head and the end of an
arrow
shows the acceptor and the donor,
respectively
dynamics of water molecules confined in nanochannels is also of great importance in
designing novel molecular devices/machines/sensors, which have wide applications
in nanotechnology [
1
-
19
], including lab-on-a-chip technology [
20
]. In this decade,
the lack of clean water, particularly in developing countries, has become one of the
main challenges. The desalination of seawater and filtration of polluted water [
5
]
are regarded as two main methods to solve the clean water shortage. Considering
the difficulty in the existing techniques, the nanotechnologies based on the water
permeation across the nanochannels have become the most promising direction.
There have been many discussions on the water across the nanochannels [
17
,
21
-
28
]. In 2001, Hummer et al., using molecular dynamics (MD) simulations,
showed that water confined in very narrow nanochannels with appropriate radii
displayed single-filed structure [
21
]. Later, Holt et al. [
6
] experimentally found
that the water flow rate through a carbon nanotube (CNT) with a radius of
1-2 nm, a comparable radius to the CNTs used by Hummer et al. in the numerical
simulations, was more than three orders of magnitude faster than the conventional
nonslip hydrodynamic flow.
Biological systems always inspire us with new idea. In 1980s, Peter Agre discov-
ered the aquaporins, which are proteins embedded in the cell membrane that regulate
the flow of water but stop the protons [
29
,
30
]. Later, the structure, the function,
and the physics of aquaporins have been extensively studied both from experiments
and from numerical simulations [
31
-
39
]. In 2001, Groot and Grubmuller used
MD simulations to study the water permeation across the biological membrane
proteins: aquaporin-1 (AQP1) and Glpf [
32
]. They found that the water molecules
in those biological channels were single-filed. Later, Tajkhorshid et al. examined the
apparently paradoxical property, facilitation of efficient permeation of water while
excluding protons [
34
]. We note that people usually take the biological systems as
