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Guest Writer - Gastautor - Gast Schrijver
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Information Delivery for Nanoscale Construction

 

A widely acknowledged goal of nanotechnology is to build intricate,
useful nanoscale structures. What usually goes unstated is how the
structures will be specified. Simple structures can be created easily: a
crystal is an atomically precise structure that can be created from
simple molecules and conditions. But complex nano-products will require
some way to deliver large quantities of information to the nanoscale.

A key indicator of a technology's usefulness is how fast it can deliver
information. A kilobyte is not very much information--less than a page
of text or a thumbnail image. A dialup modem connection can transfer
several kilobytes per second. Today's nanoscale manufacturing techniques
can transfer at most a few kilobytes per second. This will not be enough
to make advanced products--only simple materials or specialized components.

The amount of information needed to specify a product is not directly
related to the size of the product. A product containing repetitive
structures only needs enough information to specify one of the
structures and control the placement of the rest. The amount of
information that needs to be delivered also depends on whether the
receiving machine must receive an individual instruction for every
operation, or whether it can carry out a sequence of operations based on
stored instructions. Thus, a primitive fabrication system may require a
gigabyte of information to place a million atoms, while a gigabyte may
be sufficient to specify a fairly simple kilogram-scale product built
with an advanced nanofactory.

There are several ways to deliver information to the nanoscale so as to
construct things. Information can either be encoded materially, in a
stable pattern of atoms or electrons, or it can be in an ephemeral form
such as an electric field, a pattern of light, a beam of charged
particles, the position of a scanning probe, or an environmental
condition like temperature. The goal of manufacturing is to embody the
information, however it is delivered, into a material product. As we
will see, different forms of delivery have different advantages and
limitations.

Today's Techniques

To create a material pattern, it is tempting to start with materially
encoded information. This is what self-assembly does. A molecule can be
made so that it folds on itself or joins with others in quite intricate
patterns. An example of this that is well understood, and has already
been used to make nanoscale machines, is DNA. (See our previous science
essay, “Nucleic Acid Engineering.”) Biology uses DNA mainly to store
information, but in the lab it has been used to make polyhedra, grid
structures, and even a programmable machine that can synthesize DNA
strands.
http://crnano.org/essays04.htm#nucleic

One problem with self-assembly is that all the information in the final
structure must be encoded in the components. In order to make a
complicated structure, a lot of information must be programmed into the
component molecules. There are only a few ways to get information into
molecules. One is to make the molecules a piece at a time. In a long
linear chain like DNA, this can be done by repeating a few operations
many times--specifically, by changing the chemical environment in a way
that adds one selected block to the chain in each operation. (This can
be viewed either as chemistry or as manufacturing.) Automated machines
exist that will do this by cycling chemicals through a reactor, but they
are relatively slow, and the process is expensive. The information rate
can be greatly increased by controlling the process with light; by
shining light in programmed sequence on different regions of a surface,
DNA can be grown in many different patterns in parallel. This can create
a large “library” of different DNA molecules with programmed sequences.

Another problem with self-assembly is that when the building blocks are
mixed together, it is hard to impose long-range order and to build
heterogeneous engineered structures. This limitation may be partially
alleviated by providing a large-scale template, either a material
structure or an ephemeral spatial pattern. Adding building blocks in a
programmed sequence rather than mixing them all together all at once
also may help. A combination of massively parallel programmable molecule
synthesis and templated or sequenced self-assembly may be able to
deliver kilobytes per second of information to the nanoscale.

A theoretical possibility should be mentioned here. Information can be
created by starting with a lot of random codes, throwing away all the
ones that don't work, and duplicating the ones that do. One problem with
this is that for all but the simplest criteria, it will be too difficult
and time-consuming to implement tests for the desired functionality.
Another problem is that evolved solutions will require extra work to
characterize, and unless characterized, they will be hard to integrate
into engineered systems. Although evolution can produce systems of great
subtlety and complexity, it is probably not suitable for producing
easily characterized general-purpose functional modules. Specific
molecular bio-designs such as molecular motors may be worth
characterizing and using, but this will not help with the problem of
controlling the construction of large, heterogeneous, information-rich
products.

Optical lithography of semiconductors now has the capability to generate
nanoscale structures. This technique creates a pattern of light using a
mask. The light causes chemical changes in a thin surface layer; these
changes can then be used to pattern a substrate by controlling the
deposition or removal of material. One drawback of this approach is that
it is not atomically precise, since the pattern of light is far too
coarse to resolve individual atoms. Another drawback is that the masks
are pre-built in a slow and very expensive process. A computer chip may
embody billions of bytes of information, but the masks may take weeks to
make and use; again, this limits the data rate to kilobytes per second.
There has been recent talk of using MEMS (micro electro mechanical
systems) technology to build programmable masks; if this works out, it
could greatly increase the data rate.

Several tools can modify single points in serial fashion with atomic or
near-atomic resolution. These include scanning probe microscopes and
beams of charged particles. A scanning probe microscope uses a large but
sensitive positioning and feedback system to bring a nanoscale point
into controlled physical contact with the surface. Several thousand
pixels can be imaged per second, so in theory an automated system could
deliver kilobytes per second of changes to the surface. An electron beam
or ion beam can be steered electronically, so it can be relatively fast.
But the beam is not as precise as a scanning probe can be, and must work
in vacuum. The beam can be used either to remove material, to chemically
transform it, or to deposit any of several materials from low-pressure
gas. It takes a fraction of a millisecond to make a shallow feature at a
chosen point. Again, the information delivery rate is kilobytes per second.

Nanoscale Tools

To deliver information at a higher rate and use the information for more
precise construction, new technology will be required. In most of the
techniques surveyed above, the nanoscale matter is inert and is acted on
by outside forces (ephemeral information) created by large machines. In
self-assembly, the construction material itself encodes static patterns
of information--which probably were created by large machines doing
chemistry. By contrast, nanoscale tools, converting ephemeral
information to concrete operations, could substantially improve the
delivery rate of information for nanoscale construction. Large tools
acting on inert nanoscale objects could never come close to the data
rates that are theoretically possible with nanoscale tools.

One reason why nanoscale tools are better is that they can move faster.
To a first approximation, the operating frequency of a tool increases in
direct proportion as its linear size shrinks. A 100-nm tool should be
about a million times faster than a 10-cm tool.

The next question is how the information will be delivered. There are
several candidates for really fast information delivery. Light can be
switched on and off very rapidly, but is difficult to focus tightly.
Another problem is that absorption of light is probabilistic, so a lot
of light would have to be used for reliable information delivery.
Perhaps surprisingly, mechanical signals may be useful; megahertz
vibrations and pressure waves can be sent over useful distances.
Electrical signals can be sent along nanoscale wires so that multiple
independent signals could be delivered to each tool. In principle, the
mechanical and electrical portions of the system could be synchronized
for high efficiency.

Nanoscale computing elements can help with information handling in two
ways. First, they can split up a broadcast signal, allowing several
machines receiving the same signal to operate independently. This can
reduce the complexity of the macro-to-nano interface. Second, nanoscale
computation can be used to implement some kinds of error handling at a
local level.

A final advantage of nanoscale tools, at least the subset of tools built
from molecules, is that they can be very precise. Precision is a serious
problem in micron-sized tools. A structure built by lithography looks
like it has been whittled with a pocket knife--the edges are quite
ragged. This has made it very difficult to build complex, useful
mechanical devices at the micron scale using lithography. Fortunately,
things get precise again at the very bottom, because atoms are discrete
and identical. Small and simple molecular tools have been built, and
work is ongoing to build larger and more integrated systems. The
structural precision of molecular tools promises several advantages,
including predictable properties and low-friction interfaces.

Several approaches could be used, perhaps in combination, to build a
nanoscale fabrication system. If a simple and repetitive system can be
useful, then self-assembly might be used to build it. A repetitive
system, once fabricated, might be made less repetitive (programmed
heterogeneously) by spatial patterns such as an array of light. If it
contains certain kinds of electronics, then signals could be sent in to
uniquely reconfigure the circuitry in each repeating sub-pattern.

Of course, the point of the fabrication system is to build stuff, and a
particularly interesting kind of system is one that can build larger or
better fabrication systems. With information supplied from outside, a
manufacturing system of this sort could build a larger and more complex
version of itself. This approach is one of the goals of molecular
manufacturing. It would allow the first tiny system to be built by a
very expensive or non-scalable method, and then that tiny system can
build larger ones, rapidly scaling upward and drastically reducing cost.
Or if the initial system was built by self-assembly, then subsequent
systems could be more complex than self-assembly could easily achieve.

The design of even a tabletop general-purpose manufacturing system could
be relatively simple, heterogeneous but hierarchical and repetitive.
Once the basic capabilities of nanoscale actuation, computation, and
fabrication are achieved in a way that can be engineered and recombined,
it may not take too long to start developing nanoscale tools that can do
this in parallel, using computer-supplied blueprints to build larger
manufacturing systems and a broad range of products.

 

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 © 2004-2005 Chris Phoenix

 


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

 

 

The contents of this page, including the views expressed above, are the responsibility of the author. They do not represent the views or policies of Nano Tsunami Dot Com, except where explicitly stated.

 

 

Chris Phoenix

CRN Director of Research

 

 
 


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