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Molecular
manufacturing is the use of programmable chemistry to build
exponential manufacturing systems and high-performance products.
There are several ways this can be achieved, each with its
own benefits and drawbacks. This essay analyzes the definition
of molecular manufacturing and describes several ways to achieve
the requirements.
Exponential
Manufacturing Systems
An
exponential manufacturing system is one that can, within broad
limits, build additional equivalent manufacturing systems.
To achieve that, the products of the system must be as intricate
and precise as the original. Although there are ways to make
components more precise after initial manufacture, such as
milling, lapping, and other forms of machining, these are
wasteful and add complications. So the approach of molecular
manufacturing is to build components out of extremely precise
building blocks--molecules and atoms, which have completely
deterministic structures. Although thermal noise will cause
temporary variations in shape, the average shape of two components
with identical chemical structures will also be identical,
and products can be made with no loss of precision relative
to the factories.
The
intricacy of a product is limited by its inputs. Self-assembled
nanotechnology is limited by this: the intricacy of the product
has to be built into the components ahead of time. There are
some molecular components such as DNA that can hold quite
a lot of information. But if those are not used--and even
if they are--the manufacturing system will be much more flexible
if it includes a programmable manipulation function to move
or guide parts into the right place.
Programmable
Chemistry: Mechanosynthesis
Chemistry
is extremely flexible, and extremely common; every waft of
smoke contains hundreds or thousands of carbon compounds.
But a lot of chemistry happens randomly and produces intricate
but uncontrolled mixtures of compounds. Other chemistry, including
crystal growth, is self-templating and can be very precise,
but produces only simple results. It takes special techniques
to make structures using chemistry that are both intricate
and well-planned.
There
are several different ways, at least in theory, that atoms
can be joined together in precise chemical structures. Individual
reactive groups can be fastened to a growing part. Small molecules
can be strung together like beads in a necklace. It's been
proposed that small molecules can be placed like bricks, building
3D shapes with the building blocks fastened together at the
edges or corners. Finally, weak parts can be built by self-assembly--subparts
can be designed to match up and fall into the correct position.
It may be possible to strengthen these parts chemically after
they are assembled.
Mechanosynthesis
is the term for building large parts by fastening a few atoms
at a time, using simple reactions repeated many times in programmable
positions. So far, this has been demonstrated for only a few
chemical reactions, and no large parts have been built yet.
But it may not take many reactions to complete a general-purpose
toolbox that can be used in the proper sequence and position
to build arbitrary shapes with fairly small feature sizes.
See http://foresight.org/stage2/project1A.html.
The
advantage of a mechanosynthetic approach is that it allows
direct fabrication of engineered shapes, and very high bond
densities (for strength). There are two disadvantages. First,
the range of molecular patterns that can be built may be small,
at least initially--the shapes may be quite programmable,
but lack the molecular subtlety of biochemistry. This may
be alleviated as more reactions are developed.
Second, mechanosynthesis will require rather intricate and
precise machinery--of a level that will be hard to build without
mechanosynthesis. This creates a “bootstrapping” problem--how
to build the first fabrication machine. Scanning probe microscopes
have the required precision, or one of the lower-performance
machine-building alternatives described in this essay may
be used to build the first mechanosynthesis machine.
Programmable
Chemistry: Polymers and Possibilities
Biopolymers
are long heterogeneous molecules borrowed from biology. They
are formed from a menu of small molecules called monomers
stuck end-to-end in a sequence that can be programmed. Different
monomers have different parts sticking out the sides, and
some of these parts are attracted to the side parts of other
monomers. Because the monomer joining is flexible, these attractive
parts can pull the whole polymer molecule into a “folded”
configuration that is more or less stable. Thus the folded
shape can be indirectly programmed by choosing the sequence
of monomers. Nucleic acid shapes (DNA and RNA) are a lot easier
to program than protein shapes.
Biopolymers
have been studied extensively, and have a very flexible
chemistry: it's possible to build lots of different features
into one molecule. However, protein folding is complex (not
just complicated, but inherently hard to predict), so it's
only recently become possible to design a sequence that will
produce a desired shape. Also, because there's only one chemical
bond between the monomers, biopolymers can't be much stronger
than plastic. And because the folded configurations hold their
shapes by surface forces rather than strong bonds, the structures
are not very stiff at all, which makes engineering more difficult.
Biopolymers are constructed (at least to date) with bulk chemical
processes, meaning that it's possible to build lots of copies
of one intricate shape, but harder to build several different
engineered versions. (Copying by bacteria, and construction
of multiple random variations, don't bypass this limitation.)
Also, reactants have to be flushed past the reaction site
for each monomer addition, which takes significant time and
leads to a substantial error rate.
A
new kind of polymer has just been developed (see http://www.foresight.org/Conferences/AdvNano2004/Abstracts/Schafmeister/index.html).
It's based on amino acids, but the bonds between them are
stiff rather than floppy. This means the folded shape can
be directly engineered rather than emerging from a complex
process. It also means the feature size should be smaller
than in proteins, and the resulting shapes should be stiffer.
This appears to be a good candidate for designing near-term
molecular machine systems, since relatively long molecules
can be built with standard solution chemistry. At the moment,
it takes about an hour to attach each monomer to the chain,
so a machine with many thousands of features would not be
buildable.
There's
a theorized approach that's halfway between mechanosynthesis
and polymer synthesis. The idea is to use small homogeneous
molecules that can be guided into place and then fastened
together. Because this requires lower precision, and may use
a variety of molecules and fastening techniques, this may
be a useful bootstrapping approach. Ralph Merkle wrote a paper
on it a few years ago (http://www.zyvex.com/nanotech/mbb/mbb.html).
A
system that uses solution chemistry to build parts can probably
benefit from mechanical control of that chemistry. Whether
by deprotecting only selected sites to make them reactive,
or mechanically protecting some sites while leaving others
exposed, or moving catalysts and reactants into position to
promote reactions at chosen sites, a fairly simple actuator
system may be able to turn bulk chemistry into programmable
chemistry.
Living
organisms provide one possible way to use biopolymers. If
a well-designed stretch of DNA is inserted into bacteria,
then the bacteria will make the corresponding protein; this
can either be the final product, or can work with other bacterial
systems or transplanted proteins. (The bacteria also duplicate
the DNA, which may be the final
product.) However, this is only semi-controlled due to complex
interactions within the bacterial system. Living organisms
dedicate a lot of structure and energy to dealing with issues
that engineered systems won't have to deal with, such as metabolism,
maintaining an immune system, food-seeking, reproduction,
and adapting to environmental perturbations. The use of bacteria
as protein factories has already been accomplished, but the
use of bacteria-produced biopolymers for engineered-shape
products has only been done in a very small number of cases
(e.g. Shih's recent octahedra, see http://www.nature.com/nature/journal/v427/n6975/pdf/nature02307.pdf;
in this case it was DNA, not protein), and only for relatively
simple shapes.
Manufacturing
Systems, Again
Now
that we have some idea of the range of chemical manipulations,
we can look at how those chemical shapes can be joined into
machines.
Machines are important because some kind of machine will be
necessary to translate programmed information into mechanical
operations. Also, the more functions that can be implemented
by nano-fabricated machines, the fewer will have to be implemented
by expensive, conventionally manufactured hardware.
A
system with the ability to build intricate parts by mechanosynthesis
or small building blocks probably will be able to use the
same equipment to move those shapes around to assemble machines,
since the latter function is probably simpler and doesn't
require much greater range of motion. A system based on biopolymers
could in theory rely on self-assembly to bring the molecules
together. However, this process may be slow and error-prone
if the molecules are large and many different ones have to
come together to make the product. A bit of mechanical assistance,
grabbing molecules from solution and putting them in their
proper places while protecting other places from incorrect
molecules dropping in, would introduce another level of programmability.
Any
of these operations will need actuators. For simple systems,
binary actuators working ratchets should be sufficient. Several
kinds of electrochemical actuators have been developed in
recent months. Some of these may be adaptable for electrical
control. For initial bootstrapping, actuators controlled by
flushing through special chemicals (e.g. DNA strands) may
work, although quite slowly. Magnetic and electromagnetic
fields can be used for quite precise steering, though these
have to be produced by larger external equipment and so are
probably only useful for initial bootstrapping. Mechanical
control by varying pressure has also been proposed for intermediate
systems.
In
order to scale up to handle large volumes of material and
make large products, computational elements and eventually
whole computers will have to be built. The nice thing about
computers is that they can be built using anything that makes
a decent switch. Molecular electronics, buckytube transistors,
and interlocking mechanical systems are all candidates for
computer logic.
High
Performance Products
The
point of molecular manufacturing is to make valuable products.
Several things can make a product valuable. If it's a computer
circuit, then smaller component size leads to faster and more
efficient operation and high circuit density. Any kind of
molecular manufacturing should produce very small feature
sizes; thus, almost any flavor of molecular manufacturing
can be expected to make valuable computers. A molecular manufacturing
system that can make all the expensive components of its own
machinery should also drive down manufacturing cost, increasing
profit margins for manufacturers and/or allowing customers
to budget for more powerful computers.
Strong
materials and compact motors can be useful in applications
where weight is important, such as aerospace hardware. If
a kilowatt or even just a hundred watt motor can fit into
a cubic millimeter, this will be worth quite a lot of money
for its weight savings in airplanes and space ships. Even
if raw materials cost $10,000 a kilogram, as some biopolymer
ingredients do, a cubic millimeter weighs about a milligram
and would cost about a penny. Of course this calculation is
specious since the mounting hardware for such a motor would
surely weigh more than the motor itself. Also, it's not clear
whether biopolymer or building-block styles of molecular manufacturing
can produce motors with anywhere near this power density;
and although the scaling laws are pretty straightforward,
nothing like this has been built or even simulated in detail
in carbon lattice.
Once
a process is developed that can make strong programmable shapes
out of simple cheap chemicals, then product costs may drop
precipitously.
Mechanosynthesis is expected to achieve this, as shown by
the preliminary work on closed-cycle mechanosynthesis starting
with acetylene. No reaction cycle of comparable cost has been
proposed for solution chemistry, but it seems likely that
one can be found, given that some polymerizable molecules
such as sugar are quite cheap.
Future
Directions
This
essay has surveyed numerous options for molecular manufacturing.
Molecular manufacturing requires the ability to inject programmability
for engineering, but this can be done at any of several stages.
For scalability, it also requires the ability to build nanoscale
machines capable of building their duplicates. There are several
options for machines of various compositions and in various
environments.
At
the present time, no self-duplicating chemical-building molecular
machine has been designed in detail. However, given the range
of options, it seems likely that a single research group could
tackle this problem and build at least a partial proof of
concept device--perhaps one that can do only limited chemistry,
or a limited range of shapes, but is demonstrably programmable.
Subsequent
milestones would include:
1) Not relying on flushing sequences of chemicals past the
machine
2) Machines capable of general-purpose manufacturing
3) Structures that allow several machines to cooperate in
building large products
4) Building and incorporating control circuits
Once
these are achieved, general-purpose molecular manufacturing
will not be far away. And that will allow the pursuit of more
ambitious goals, such as machines that can work in gas (instead
of solution) or vacuum for greater mechanical efficiency.
Working in inert gas or vacuum also provides a possible pathway
(one of several) to what may be the ultimate performer: products
built by mechanosynthesis out of carbon lattice.
The Center for Responsible Nanotechnology(TM) (CRN) is
an affiliate of World Care(R), an international, non-profit,
501(c)3 organization. All donations to CRN are handled through
World Care. The opinions expressed by CRN do not necessarily
reflect those of World Care
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