| This
month's essay is adapted from a paper I recently wrote for
my NIAC
grant, explaining why planar assembly, a new way to build
large products
from nano-sized building blocks, is better and simpler than
convergent
assembly. http://wise-nano.org/w/Planar_vs_Convergent_Assembly
History
Molecular manufacturing promises to
build large quantities of
nano-structured material, quickly and cheaply. However, achieving
this
requires very small machines, which implies that the parts
produced will
also be small. Combining sub-micron parts into kilogram-scale
machines
will not be trivial.
In Engines of Creation (1986), [Drexler
suggested] that large products
could be built by self-contained micron-scale “assembler”
units that
would combine into a scaffold, take raw materials and fuel
from a
special fluid, build the product around themselves, and then
exit the
product, presumably filling in the holes as they left. This
would
require a lot of functionality to be designed into each assembler,
and a
lot of software to be written.
http://www.foresight.org/EOC/EOC_Chapter_4.html#section02of03
In Nanosystems (1992), Drexler developed
a simpler idea: convergent
assembly. Molecular parts would be fabricated by mechanosynthesis,
then
placed on assembly lines, where they would be combined into
small
assemblages. Each assemblage would move to a larger line,
where it would
be combined with others to make still larger concretions,
and so on
until a kilogram-scale product was built. This would probably
be a lot
simpler than the self-powered scaffolding of Engines, but
implementing
automated assembly at many different scales for many different
assemblages would still be difficult.
In 1997, Ralph Merkle published [a
paper], “Convergent Assembly,”
suggesting that the parts to be assembled could have a simple,
perhaps
even cubical shape. This would make the assembly automation
significantly less complex. In 2003, I published a [very long
paper]
analyzing many operational and architectural details of a
kilogram-per-hour nanofactory. However, despite 80 pages of
detail, my
factory was limited to joining cubes to make larger cubes.
This imposed
severe limits on the products it could produce.
http://www.zyvex.com/nanotech/convergent.html
http://www.jetpress.org/volume13/Nanofactory.htm
In 2004, a collaboration between Drexler
and former engineer John Burch
resulted in the resurrection of an idea that was touched on
in
Nanosystems: instead of joining small parts to make bigger
parts through
several levels, [add small parts] directly to a surface of
the
full-sized product, extruding the product from the assembly
plane. It
turns out that this does not take as long as you'd expect;
in fact, the
[speed of deposition] (about a meter per hour) should not
depend on the
size of the parts, even for parts as small as a micron in
size.
http://www.foresight.org/lizardfire/nanofactorySS.html
http://www.foresight.org/animation_challenge/nanofactory_360x240copyright_sor3.mov
(38 MB movie)
Problems with Earlier Methods
In studying molecular manufacturing,
it is common to find that problems
are easier to solve than they initially appeared. Convergent
assembly
requires robotics in a wide range of scales. It also needs
a large
volume of space for the growing parts to move through. In
a simple
cube-stacking design, every large component must be divisible
along cube
boundaries. This imposes constraints on either the design
or the
placement of the component relative to the cube matrix.
Another set of problems comes from
the need to handle only cubes. Long
skinny components have to be made in sections and joined together,
and
supported within each cube. Furthermore, each face of each
cube must be
stiff, so as to be joined to the adjacent cube. This means
that products
will be built solid: shells or flimsy structures would require
interior
scaffolding.
If shapes other than cubes are used,
assembly complexity quickly
increases, until a nanofactory might require many times more
programming
and design than a modern “lights-out” factory.
However, planar assembly bypasses all
these problems.
Planar Assembly
The idea of planar assembly is to take
small modules, all roughly the
same size, and attach them to a planar work surface, the working
plane
of the product under construction. In some ways, this is similar
to the
concept of 3D inkjet-style prototyping, except that there
are billions
of inkjets, and instead of ink droplets, each particle would
be
molecularly precise and could be full of intricate machinery.
Also,
instead of being sprayed, they would be transported to the
workpiece in
precise and controlled trajectories. Finally, the workpiece
(including
any subpieces) would be gripped at the growing face instead
of requiring
external support.
Small modules supplied by any of a
variety of fabrication technologies
would be delivered to the assembly plane. The modules would
all be of a
size to be handled by a single scale of robotic placement
machinery.
This machinery would attach them to the face of a product
being extruded
from the assembly plane. The newly attached modules would
be held in
place until yet newer modules were attached. Thus, the entire
face under
construction serves as a "handle" for the growing
product. If blocks are
placed face-first, they will form tight parallel-walled holes,
making it
hard to place additional blocks; but if the blocks are placed
corner-first, they will form pyramid-shaped holes for subsequent
blocks
to be placed into. Depending on fastening method, this may
increase
tolerance of imprecision and positional variance in placement.
The speed of this method is counterintuitive;
one would expect that the
speed of extrusion would decrease as the module size decreased.
But in
fact, the speed remains constant. For every factor of module
size
decrease, the number of placement mechanisms that can fit
in an area
increases as the square of that factor, and the operation
speed
increases by the same factor. These balance the factor-cubed
increase in
number of modules to be placed. This analysis breaks down
if the modules
are made small enough that the placement mechanism cannot
scale down
along with the modules. However, sub-micron kinematic systems
are
already being built via both MEMS and biochemistry, and robotics
built
by molecular manufacturing should be better. This indicates
that
sub-micron modules can be handled.
Advantages of Planar Assembly
This approach requires only one level
of modularity from nanosystems to
human-scale products, so it is simpler to design. Blocks (modules)
built
by a single fabrication system can be as complex as that system
can be
programmed to produce. Whether the feedstock producing system
uses
direct covalent deposition or guided self-assembly to build
the
nanoblocks, the programmable feature size will be sub-nanometer
to a few
nanometers. Since a single fabrication system can produce
blocks larger
than 100 nanometers, a fair amount of complexity (several
motors and
linkages, a sensor array, or a small CPU) could be included
in a single
module.
Programmable, or at least parameterized,
(or at worst case,
limited-type) modules would then be aggregated into large
systems and
"smart materials." Because of the molecular precision
of the nanoblocks,
and because of the inter-nanoblock connection, these large-scale
and
multi-scale components could be designed without having to
worry about
large-scale divisions and fasteners, which are a significant
issue in
the convergent assembly approach (and also in contemporary
manufacturing).
Support of large structures will be
much easier in planar assembly than
in convergent assembly. In simplistic block-based convergent
assembly,
each structure (or cleaved subpart thereof) must be embedded
in a block.
This makes it impossible to build a long thin structure that
is not
supported along each segment of its length, at least by scaffolding.
In planar assembly, such a structure
can be extruded and held at the
base even if it is not held anywhere else along its length.
The only
constraint is the strength of the holding mechanism vs. the
forces
(vibration and gravity) acting on the system; these forces
are
proportional to the cube of size, and rapidly become negligible
at
smaller scales. In addition, the part that must be positioned
most
precisely--the assembly plane--is also the part that is held.
Positional
variance at the end of floppy structures usually will not
matter, since
nothing is being done there; in the rare cases where it is
a problem,
collapsible scaffolds or guy wires can be used. (The temporary
scaffolds
used in 3D prototyping have to be removed after manufacture,
so are not
the best design for a fully automated system.)
This indicates that large open-work
structures can be built with this
method. Unfolding becomes much less of an issue when the product
is
allowed to have major gaps and dangling structures. The only
limit on
this is that extrusion speed is not improved by sparse structures,
so
low-density structures will take longer to build than if built
using
convergent assembly.
Surface assembly of sub-micron blocks
places a major stage of product
assembly in a very convenient realm of physics. Mass is not
high enough
to make inertia, gravity, or vibration a serious problem.
(The mass of a
one-micron cube is about a picogram, which under 100 G acceleration
would experience a nanoNewton of force. This is comparable
to the force
required to detach 1 square nanometer of van der Waals adhesion
(tensile
strength 1 GPa, Nanosystems 9.7.1). Resonant frequencies will
be on the
order of MHz, which is easy to isolate/damp.) Stiffness, which
scales
adversely with size, is significantly better than at the nanoscale.
Surface forces are also not a problem: large enough to be
convenient for
handling--instead of grippers, just put things in place and
they will
stick--but small enough that surfaces can easily be separated
by
machinery. (The problems posed by surface forces in MEMS manipulation
are greatly exacerbated by the crudity of surfaces and actuation
in
current technology. Nanometer-scale actuators can easily modulate
or
supplement surface forces to allow convenient attachment and
release.)
Sub-micron blocks are large enough
to contain thousands or even millions
of features: dozens to thousands of moving parts. But they
are small
enough to be built directly out of molecules, benefiting from
the
inherent precision of this approach as well as nanoscale properties
including superlubricity. If blocks can be assembled from
smaller parts,
then block fabrication speed can improve.
Centimeter-scale products can benefit
from the ability to directly build
large-scale structures, as well as the fine-grained nature
of the
building blocks (note that a typical human cell is 10,000-20,000
nm
wide). For most purposes, the building blocks can be thought
of as a
continuous smooth material. Partial blocks can be placed to
make the
surfaces smoother--molecularly smooth, except perhaps for
joints and
crystal atomic layer steps.
Modular Design Constraints
Although there is room for some variability
in the size and shape of
blocks, they will be constrained by the need to handle them
with
single-sized machinery. A multi-micron monolithic subsystem
would not be
buildable with this manufacturing system: it would have to
be built in
pieces and assembled by simple manipulation, preferably mere
placement.
The "expanding ridge joint" system, described in
my above-referenced
Nanofactory paper, appears to work for both strong mechanical
joints and
a variety of functional joints.
Human-scale product features will be
far too large to be bothered by
sub-micron grain boundaries. Functions that benefit from miniaturization
(due to scaling laws) can be built within a single block.
Even at the
micron scale, where these constraints may be most troublesome,
the
remaining design space is a vast improvement over what we
can achieve
today or through existing technology roadmaps.
Sliding motion over a curved unlubricated
surface will not work well if
the surface is composed of blocks with 90 degree corners,
no matter how
small they are. However, there are several approaches that
can mitigate
this problem. First, there is no requirement that all blocks
be
complete; the only requirement is that they contain enough
surface to be
handled by assembly robotics and joined to other blocks. Thus
an
approximation of a smooth curved surface with no projecting
points can
be assembled from prismatic partial-cubes, and a better approximation
(marred only by joint lines and crystal steps) can be achieved
if the
fabrication method allows curves to be built. Hydrodynamic
or molecular
lubrication can be added after assembly; some lubricant molecules
might
be built into the block faces during fabrication, though this
would
probably have limited service life. Finally, in clean joints,
nanoscale
machinery attached to one large surface can serve as a standoff
or
actuator for another large surface, roughly equivalent to
a forest of
traction drives.
The grain scale may be large enough
to affect some optical systems. In
this case, joints like those between blocks can be built at
regular
intervals within the blocks, decreasing the lattice spacing
and
rendering it invisible to wave propagation.
See the [original NIAC paper] for discussion
of factory architecture and
extrusion speed.
http://wise-nano.org/w/Planar_vs_Convergent_Assembly
Conclusion and Further Work
Surface assembly is a powerful approach
to constructing meter-scale
products from sub-micron blocks, which can themselves be built
by
individual fabrication systems implementing molecular manufacturing
or
directed self-assembly. Surface assembly appears to be competitive
with,
and in many cases preferable to, all previously explored systems
for
general-purpose manufacture of large products. It is hard
to find an
example of a useful device that could not be built with the
technique,
and the expected meter-per-hour extrusion rate means that
even large
products could be built in their final configuration (as opposed
to folded).
What this means is that, once we have
the ability to build billion-atom
(submicron) blocks of nanomachinery, it will be straightforward
to
combine them into large products. The opportunities and problems
of
molecular manufacturing can develop even faster than was previously
thought.
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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