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Today,
biology implements by far the most advanced
nanomachines on the
planet. It is tempting to think that biology
must be efficient, and that
we can't hope to design nanomachines with
higher performance. But we
already know some techniques that biology
has never been able to try.
This essay discusses several of them and
explains why biology could not
use them, but manufactured nanomachines
will be able to.
Low
Friction Via Superlubricity
Imagine
you're pulling a toy wagon with square wheels.
Each time a wheel
turns past a corner, the wagon lurches forward
with a thump. This would
waste substantial amounts of energy. It's
as though you're continually
pulling the wagon up tiny hills, which it
then falls off of. There's no
way to avoid the waste of energy.
At
the molecular scale, static friction is
like that. Forces between the
molecules cause them to stretch out of position,
then snap into a new
configuration. The snap, or clunk, requires
energy--which is immediately
dissipated as heat.
In
order for a sliding interface to have low
friction, there must be an
extremely small difference in energy between
all adjacent positions or
configurations. But between most surfaces,
that is not the case. The
molecular fragments at the surface are springy
and adhesive enough that
they grab hold, get pulled, and then snap
back, wasting energy.
There
are several ways in which a molecule can
be pulled or pushed out
of position. If the interface is rough or
dirty, the surfaces can be
torn apart as they move. This of course
takes a lot of energy, producing
very high friction. Even apparently smooth
surfaces can be sources of
friction. If the surface is coated with
molecular bumps, the bumps may
push each other sideways as they go past,
and then spring back, wasting
energy. Even if the bumps are too short
and stiff to be pushed sideways
very far, they can still interlock, like
stacking egg cartons or ice
cube trays. (Thanks to [Wikipedia] for this
analogy.) If the bumps
interlock strongly, then it may take a lot
of force to move them past
each other--and just as they pass the halfway
point, they will snap into
the next interlocking position, again wasting
energy.
http://en.wikipedia.org/wiki/Superlubricity
One
way to reduce this kind of friction is to
separate the surfaces. A
film of water or oil can make surfaces quite
slippery. But another way
to reduce friction is to use stiff surfaces
that don't line up with each
other. Think back to the egg-carton image.
If you turn one of the
cartons so that the bumps don't line up,
then they can't interlock; they
will simply skim past each other. In fact,
friction too low to measure
has been observed with graphite sheets that
were turned so as to be out
of alignment. Another way to prevent alignment
is to make the bumps have
different spacing, by choosing different
materials with different atoms
on their surfaces.
This
low-friction trick, called superlubricity,
is difficult to achieve
in practice. Remember that the surfaces
must be very smooth, so they can
slip past each other; and very stiff, so
the bumps don't push each other
sideways and spring back; and the bumps
must not line up, or they will
interlock. Biological molecules are not
stiff enough to use the
superlubricity trick. Superlubricity may
be counterintuitive to people
who are accustomed to the high friction
of most hard dry surfaces. But
[experiments have shown] that superlubricity
works. A variety of
materials that have been proposed for molecular
manufacturing should be
stiff enough to take advantage of superlubricity.
http://www.physics.leidenuniv.nl/sections/cm/ip/group/PDF/Phys.rev.lett/2004/92(2004)12601.pdf
Electric
Currents
The
kind of electricity that we channel in wires
is made up of vast
quantities of electrons moving through the
wire. Electrons can be made
to move by a magnetic field, as in a generator,
or by a chemical
reaction, as in a battery. Either way, the
moving electrons can be sent
for long distances, and can do useful work
along the way. Electricity is
extremely convenient and powerful, a foundation
of modern technology.
With
only a few exceptions like electric eels,
biological organisms do
not use this kind of electricity. You may
know that our nerve cells use
electricity. But instead of moving electrons,
biology uses ions--the
“charged” atoms that remain when one or
more electrons are removed. Ions
can move from place to place, and can do
work just like electrons.
Bacteria use ions to power their flagella
“tails.” Ions moving suddenly
through a nerve cell membrane cause a change
that allows more ions,
further along the cell, to be able to move,
creating a domino effect
that ripples from one end of the cell to
the other.
Ions
are convenient for cells to handle. An ion
is much larger than an
electron, and is therefore easier to contain.
But ions have to move
slowly, bumping through the water they are
dissolved in. Over long
distances, electrons in a wire can deliver
energy far more rapidly than
ions in a liquid. But wires require insulation.
It
is perhaps not surprising that biology hasn't
used electron currents.
At cellular scales, ions diffuse fast enough
to do the job. And the same
membranes that keep chemicals properly in
(or out of) the cell can also
keep ions contained where they can do useful
work. But if we actually
had “nerves of steel,” we could react far
more quickly than we do.
To
use electron currents, all that's needed
is a good conductor and a
good insulator. Carbon nanotubes can be
both conductors and insulators,
depending on how they are constructed. Many
organic molecules are
insulating, and some are conductive. There
is a lot of potential for
molecular manufacturing to build useful
circuits, both for signaling and
for power transmission.
Deterministic
Machines
Cells
have to reconfigure themselves constantly
in response to changing
conditions. They are built out of individual
molecules, loosely
associated. And the only connection between
many of the molecular
systems is other molecules diffusing randomly
through the cell's
interior. This means that the processes
of the cell will happen
unpredictably, from molecules bumping into
each other after a random
length of time. Such processes are not deterministic:
there's no way to
know exactly when a reaction or process
will happen. This lack of tight
connection between events makes the cell's
processes more adaptable to
change, but more difficult to engineer.
Engineered
nanosystems can be designed, and then built
and used, without
needing to be reconfigured. That makes it
easier to specify mechanical
or signal linkages to connect them and make
them work in step, while a
constantly changing configuration would
be difficult to accommodate. Of
course, no linkage is absolutely precise,
but it will be possible to
ensure that, for example, an intermediate
stage in a manufacturing
process always has its input ready at the
time it begins a cycle. This
will make design quite a bit easier, since
complex feedback loops will
not be required to keep everything running
at the right relative speed.
This also makes it possible to use standard
digital logic circuits.
Digital
Logic
Digital
logic is general-purpose and easy to engineer,
which makes it
great for controlling almost any process.
But it requires symbolic codes
and rapid, reliable computation. There is
no way that the diffuse
statistical chemical signaling of biology
could implement a high-speed
microprocessor (CPU). But rapid, lock-stepped
signals make it easy.
Biology, of course, doesn't need digital
logic, because it has complex
control loops. But complex things are very
difficult to engineer. Using
digital logic instead of complexity will
allow products to be designed
much more quickly.
Rapid
Transport and Motion
Everything
in a cell is flooded with water. This means
that everything
that moves experiences high drag. If a nanomachine
can be run dry, its
parts can move more efficiently and/or at
higher speeds.
Things
that move by diffusion are not exempt from
drag: it takes as much
energy to make objects diffuse from point
A to point B in a certain time
as it does to drag it there. Although diffusion
seems to happen “by
itself,” to work as a transportation system
it requires maintaining a
higher concentration of particles (e.g.
molecules) at the source than at
the destination. This requires an input
of work.
In
a machine without solvent, diffusion can't
work, so particles would
have to be transported mechanically. (In
theory, certain small molecules
could be released into vacuum and bounce
around to their destination,
but this has practical difficulties that
probably would make it not
useful.) Mechanical transportation sounds
inefficient, but in fact it
can be more efficient than diffusion. Because
the particle is never
released, energy is not required to find
and recapture it. Because
nothing has to move through fluid, frictional
forces can be lower for
the same speed, or speeds can be higher
for the same energy consumption.
The use of machinery to move nanoparticles
and molecules may seem
wasteful, but it replaces the need to maintain
a pathway of solvent
molecules; it may actually require less
mass and volume. The increased
design complexity of the transport machinery
will be more or less
balanced by the reduced design complexity
of the receiving stations for
particles.
It
is not only transport that can benefit from
running without solvent.
Any motion will be subject to drag, which
will be much higher in liquid
than in gas or vacuum. For slow motions,
this is not so important. But
to obtain high power density and processing
throughput, machines will
have to move quickly. Drying out the machines
will allow greater
efficiency than biology can attain. Biology
has never developed the
ability to work without water. Engineered
machines can do so.
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