...read the wave
Guest Writer - Gastautor - Gast Schrijver


Planar Assembly:
A better way to build large nano-products


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


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.

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.

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.
(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

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

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.

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


Chris has studied nanotechnology and molecular manufacturing for more than a decade. After successful careers in software engineering and dyslexia correction, Chris co-founded the Center for Responsible Nanotechnology in 2002, where he is Director of Research. Chris holds an MS in Computer Science from Stanford University.

Copyright © 2004 Chris Phoenix


Chris Phoenix

CRN Director of Research