The
term “molecular manufacturing” has been associated with
all sorts of futuristic stuff, from bloodstream robots to
gray goo to tabletop factories that can make a new factory
in a few hours. This can make it hard for people who want
to understand the field to know exactly what's being claimed
and studied. This essay explains what the term originally
meant, why the approach is thought to be powerful enough
to create a field around, why so many futuristic ideas are
associated with it, and why some of those ideas are more
plausible than they may seem.
Original
Definition
Eric
Drexler defined the term “molecular manufacturing” in his
1992 technical work Nanosystems. His definition used some
other terms that need to be considered first.
Mechanochemistry
In this volume, the chemistry of processes in which mechanical
systems operating with atomic-scale precision either guide,
drive, or are driven by chemical transformations.
In
other words, mechanochemistry is the direct, mechanical
control of molecular structure formation and manipulationto
form atomically precise products. (It can also mean the
use of reactions to directly drive mechanical systems—a
process that can be nearly 100% efficient, since the energy
is never thermalized.) Mechanochemistry has already been
demonstrated: [Oyabu] has used atomic force microscopes,
acting purely mechanically, to remove single silicon atoms
from a covalent lattice and put them back in the same spot.
http://focus.aps.org/story/v11/st19
Mechanosynthesis
Chemical synthesis controlled by mechanical systems operating
with atomic-scale precision, enabling direct positional
selection of reaction sites; synthetic applications of mechanochemistry.
Suitable mechanical systems include AFM mechanisms, molecular
manipulators, and molecular mill systems.
In
other words, mechanosynthesis is the use of mechanically
guided molecular reactions to build stuff. This does not
require that every reaction be directly controlled. Molecular
building blocks might be produced by ordinary chemistry;
products might be strengthened after manufacture by crosslinking;
molecular manufactured components might be joined into products
by self-assembly; and building blocks similar to those used
in self-assembly might be guided into chosen locations and
away from alternate possibilities. Drexler’s definition
continues:
Processes
that fall outside the intended scope of this definition
include reactions guided by the incorporation of reactive
moieties into a shared covalent framework (i.e., conventional
intramolecular reactions), or by the binding of reagents
to enzymes or enzymelike catalysts.
The
point of this is to exclude chemistry that happens by pure
self-assembly and cannot be controlled from outside. As
we will see, external control of the reactions is the key
to successful molecular manufacturing. It is also the main
thing that distinguishes molecular manufacturing from other
kinds of nanotechnology.
The
principle of mechanosynthesis—direct positional control—can
be useful with or without covalent bonding. Building blocks
like those used in self assembly, held together by hydrogen
bonding or other non-covalent interactions, could also be
joined under mechanical control. This would give direct
control of the patterns formed by assembly, rather than
requiring that the building blocks themselves encode the
final structure and implement the assembly process.
Molecular
manufacturing The production of complex structures via
nonbiological mechanosynthesis (and subsequent assembly
operations).
There
is some wiggle room here, because “complex structures” is
not defined. Joining two molecules to make one probably
doesn't count. But joining selected monomers to make a polymer
chain that folds into a predetermined shape probably does.
Machine-phase
chemistry The chemistry of systems in which all potentially
reactive moieties follow controlled trajectories (e.g.,
guided by molecular machines working in vacuum).
This
definition reinforces the point that machine-phase chemistry
is a narrow subset of mechanochemistry. Mechanochemistry
does not require that all molecules be controlled; it only
requires that reactions between the molecules must be controlled.
Mechanochemistry is quite compatible with “wet” chemistry,
as long as the reactants are chosen so that they will only
react in the desired locations. A ribosome appears to fit
the requirement; Drexler specified that molecular manufacturing
be done by nonbiological mechanosynthesis, because otherwise
biology would be covered by the definition.
Although
it has not been well explored, machine-phase chemistry has
some theoretical advantages that make it worth further study.
But molecular manufacturing does not depend on a workable
machine-phase chemistry being developed. Controversies about
whether diamond can be built in vacuum do not need to be
settled in order to assess the usefulness of molecular manufacturing.
Extending
molecular manufacturing
As
explained in the first section, the core of molecular manufacturing
is the mechanical control of reactions so as to build complex
structures. This simple idea opens up a lot of possibilities
at the nanoscale. Perhaps the three most important capabilities
are engineering, blueprint delivery, and the manufacture
of manufacturing tools. These capabilities reinforce each
other, each facilitating the others.
It
is often thought that the nanoscale is intractably complex,
impossible to analyze. Nearly intractable complexity certainly
can be found at the nanoscale, for example in the prediction
of protein folding. But not everything at the nanoscale
is complex. DNA folding, for example, is much simpler, and
the engineering of folded structures is now pretty straightforward.
Crystals and self-assembled monolayers also have simple
aspects: they are more or less identical at a wide range
of positions. The mechanical properties of nanoscale structures
change as they get extremely small, but even single-nanometer
covalent solids (diamond, alumina, etc) can be said to have
a well-defined shape.
The
ability to carry out predictable synthesis reactions at
chosen sites or in chosen sequences should allow the construction
of structures that are intricate and functional, but not
intractably complex. This kind of approach is a good fit
for engineering. If a structure is the wrong shape or stiffness,
simply changing the sequence of reactions used to build
it will change its structure—and at least some of its properties—in
a predictable way.
It
is not always easy to control things at the nanoscale. Most
of our tools are orders of magnitude larger, and more or
less clumsy; it's like trying to handle toothpicks with
telephone poles. Despite this, a few techniques and approaches
have been developed that can handle individual molecules
and atoms, and move larger objects by fractions of nanometers.
A separate approach is to handle huge numbers of molecules
at once, and set up the conditions just right so that they
all do the same thing, something predictable and useful.
Chemistry is an example of this; the formation of self-assembled
monolayers is another example. The trouble with all of these
approaches is that they are limited in the amount of information
that can be delivered to the nanoscale. After a technique
is used to produce an intermediate product, a new technique
must be applied to perform the next step. Each of these
steps is hard to develop. They also tend to be slow to use,
for two reasons: big tools move slowly, and switching between
techniques and tools can take a lot of time.
Molecular
manufacturing has a big advantage over other nanoscale construction
techniques: it can usefully apply the same step over and
over again. This is because each step takes place at a selected
location and with selected building blocks. Moving to a
different location, or selecting a different building block
from a predefined set, need not insert enough variation
into the process to count as a new step that must be developed
and characterized separately.
A
set of molecular manufacturing operations, once worked out,
could be recombined like letters of an alphabet to make
a wide variety of predictable products. (This benefit is
enhanced because mechanically guided chemistry can play
useful games with reaction barriers to speed up reactions
by many orders of magnitude; this allows a wider range of
reactants to be used, and can reduce the probability of
unwanted side
reactions.) The use of computer-controlled tools and computer-aided
translation from structure to operation sequence should
allow blueprints to be delivered directly to the nanoscale.
Although
it is not part of the original definition of molecular manufacturing,
the ability to build a class of product structures that
includes the manufacturing tools used to build them may
be very useful.
If the tools can be engineered by the same skill set that
produces useful products, then research and development
may be accelerated. If new versions of tools can be constructed
and put into service within the nanoscale workspace, that
may be more efficient than building new macro-scale tools
each time a new design is to be tested. Finally, if a set
of tools can be used to build a second equivalent set of
tools, then scaleup becomes possible.
The
idea of a tool that can build an improved copy of itself
may seem
counterintuitive: how can something build something else
that's more complex than itself? But the inputs to the process
include not just the structure of the first tool, but the
information used to control it.
Because of the sequential, repetitive nature of molecular
manufacturing, the amount of information that can be fed
to the process is essentially unlimited. A tool of finite
complexity, controlled from the outside, can build things
far more physically complex than itself; the complexity
is limited by the quality of the design. If engineering
can be applied, then the design can be quite complex indeed;
computer chips are being designed with a billion transistors.
From the mechanical engineering side, the idea of tools
building tools may be suspect because it seems like precision
will be lost at each step. However, the use of covalent
chemistry restores precision.
Covalent reactions are inherently digital: in general, either
a bond is formed which holds the atoms together, or the
bond is missing and the atoms repel each other. This means
that as long as the molecules can be manipulated with enough
precision to form bonds in the desired places, the product
will be exactly as it was designed, with no loss of precision
whatsoever. The precision required to form bonds reliably
is a significant engineering requirement that will require
careful design of tools, but is far from being a showstopper.
Scaleup
The
main limitation of molecular manufacturing is that molecules
are so small. Controlling one reaction at a time with a
single tool will produce astonishingly small masses of product.
At first sight, it may appear that there is no way to build
anything useful with this approach.
However, there is a way around this problem, and it’s the
same way used by ribosomes to build an elephant: use a lot
of them in parallel. Of course, this requires that the tools
must be very small, and it must be possible to build a lot
of them and then control them all. Engineering, direct blueprint
injection, and the use of molecular manufacturing tools
to build more tools can be combined to achieve this.
The
key question is: How rapidly can a molecular manufacturing
tool create its own mass of product? This value, which I'll
call “relative productivity,” depends on the mass of the
tool; roughly speaking, its mass will be about the cube
of its size. For each factor of ten shrinkage, the mass
of the tool will decrease by 1,000. In addition, small things
move faster than large things, and the relationship is roughly
linear. This means that each factor of ten shrinkage of
the tool will increase its relative productivity by 10,000
times; relative productivity increases as the inverse fourth
power of the size.
A
typical scanning probe microscope might weigh two kilograms,
have a size of about 10 cm, and carry out ten automated
operations per second.
If each operation deposits one carbon atom, which masses
about 2x10^-26 kg, then it would take 10^26 seconds or six
billion billion years for that scanning probe microscope
to fabricate its own mass. But if the tool could be shrunk
by a factor of a million, to 100 nm, then its relative throughput
would increase by 10^24, and it would take only 100 seconds
to fabricate its own mass. This assumes an operation speed
of 10 million per second, which is about ten times faster
than the fastest known enzymes (carbonic anhydrase and superoxide
dismutase). But a relative productivity of 1,000 or even
10,000 seconds would be sufficient for a very worthwhile
manufacturing technology. (An inkjet printer takes about
10,000 seconds to print its weight in ink.) Also, there
is no requirement that a fabrication operation deposit only
one atom at a time; a variety of molecular fragments may
be suitable.
To
produce a gram of product will take on the order of a gram
of nanoscale tools. This means that huge numbers of the
tools must be controlled in parallel: information and power
must be fed to each one.
There are several possible ways to do this, including light
and pressure. If the tools can be fastened to a framework,
it may be easier to control them, especially if they can
build the framework and include nanoscale structures in
it. This is the basic concept of a nanofactory.
Nanofactories
and their products
A
nanofactory is (will be) an integrated manufacturing system
containing large numbers of nanoscale molecular manufacturing
workstations (tool systems). This appears to be the most
efficient and engineerable way to make nanoscale productive
systems produce large products. With the workstations fastened
down in known positions, their nanoscale products can more
easily be joined. Also, power and control signals can be
delivered through hardwired connections.
The
only way to build a nanofactory is with another nanofactory.
However, the product of a nanofactory may be larger than
itself; it does not appear conceptually or practically difficult
to build a small nanofactory with a single molecular manufacturing
tool, and build from there to a kilogram-scale nanofactory.
The architecture of a nanofactory must take several problems
into account, in addition to the design of the individual
fabrication workstations. The mass and organization of the
mounting structure must be included in the construction
plans. A small fraction (but large number) of the nanoscale
equipment in the nanofactory will be damaged by background
radiation, and the control algorithms will have to compensate
for this in making functional products. To make heterogeneous
products, the workstations and/or the nanoproduct assembly
apparatus must be individually controlled; this probably
requires control logic to be integrated into the nanofactory.
It
may seem premature to be thinking about nanofactory design
before the first nanoscale molecular manufacturing system
has been built. But it is important to know what will be
possible, and how difficult it will be, in order to estimate
the ultimate payoff of a technology and the time and effort
required to achieve it. If nanofactories were impossible,
then molecular manufacturing would be significantly less
useful; it would be very difficult to make large products.
But preliminary studies seem to show that nanofactories
are actually not very difficult to design, at least in broad
outline. I have written an [80-page paper] that covers error
handling, mass and layout, transport of feedstock, control
of fabricators, and assembly and design of products for
a very primitive nanofactory design. My best estimate is
that this design could produce a duplicate nanofactory in
less than a day. Nanofactory designs have been proposed
that appear to be much more flexible in how the products
are formed, but they have not yet been worked out in as
much detail.
http://www.jetpress.org/volume13/Nanofactory.htm
If
there is a straightforward path from molecular manufacturing
to nanofactories, then useful products will not be far behind.
The ability to specify every cubic nanometer of an integrated
kilogram product, filling the product with engineered machinery,
will at least allow the construction of extremely powerful
computers. If the construction material is strong, then
mechanical performance may also be extremely good; scaling
laws predict that power density increases as the inverse
of machine size, and nanostructured materials may be able
to take advantage of almost the full theoretical strength
of covalent bonds rather than being limited by propagating
defects.
Many
products have been imagined for this technology. A few have
been designed in sufficient detail that they might work
as claimed. Robert Freitas's [Nanomedicine Vol. I] contains
analyses of many kinds of nanoscale machinery. However,
this only scratches the surface. In the absence of more
detailed analysis identifying quantitative limits, there
has been a tendency for futurists to assume that nano-built
products will achieve performance close to the limits of
physical law. Motors three to six orders of magnitude more
powerful than today's; computers six to nine orders of magnitude
more compact and efficient; materials at least two orders
of magnitude stronger—all built by manufacturing systems
many orders of magnitude cheaper—it's not hard to see why
futurists would fall in love with this field, and skeptics
would dismiss it. The solution is threefold: open-minded
but quantitative investigation of the theories and proposals
that have already been made; constructive attempts to fill
in missing details; and critical efforts to identify unidentified
problems with the application of the theories.
http://nanomedicine.com/NMI.htm
Based
on a decade and a half of study, I am satisfied that some
kind of nanofactory can be made to work efficiently enough
to be more than competitive with today's manufacturing systems,
at least for some products. In addition, I am satisfied
that molecular manufacturing can be used to build simple,
high-performance nanoscale devices that can be combined
into useful, gram-scale, high-performance products via straightforward
engineering design. This is enough to make molecular manufacturing
seem very interesting, well worth further study; and in
the absence of evidence to the contrary, worth a measure
of preliminary concern over how some of its possible products
might be used.
