An Overview of Nanotechnology
INTRODUCTION
Nanotechnology is an anticipated manufacturing technology giving
thorough, inexpensive control of the structure of matter. The term has sometimes
been used to refer to any technique able to work at a submicron scale; Here on
sci.nanotech we are interested in what is sometimes called molecular
nanotechnology, which means basically "A place for every atom and every atom in
its place." (other terms, such as molecular engineering, molecular
manufacturing, etc. are also often applied).
Molecular manufacturing will enable the construction of giga-ops
computers smaller than a cubic micron; cell repair machines; personal
manufacturing and recycling appliances; and much more.
NANOTECHNOLOGY
Broadly speaking, the central thesis of nanotechnology is that
almost any chemically stable structure that can be specified can in fact be
built. This possibility was first advanced by Richard Feynman in 1959 [4] when
he said: "The principles of physics, as far as I can see, do not speak against
the possibility of maneuvering things atom by atom." (Feynman won the 1965 Nobel
prize in physics).
This concept is receiving increasing attention in the research
community. There have been two international conferences directly on molecular
nanotechnology[30,31] as well as a broad range of conferences on related
subjects. Science [23, page 26] said "The ability to design and manufacture
devices that are only tens or hundreds of atoms across promises rich rewards in
electronics, catalysis, and materials. The scientific rewards should be just as
great, as researchers approach an ultimate level of control - assembling matter
one atom at a time." "Within the decade, [John] Foster [at IBM Almaden] or some
other scientist is likely to learn how to piece together atoms and molecules one
at a time using the STM [Scanning Tunnelling Microscope]."
Eigler and Schweizer[25] at IBM reported on "...the use of the STM
at low temperatures (4 K) to position individual xenon atoms on a single-crystal
nickel surface with atomic precision. This capacity has allowed us to fabricate
rudimentary structures of our own design, atom by atom. The processes we
describe are in principle applicable to molecules also. ..."
ASSEMBLERS
Drexler[1,8,11,19,32] has proposed the "assembler", a device
having a submicroscopic robotic arm under computer control. It will be capable
of holding and positioning reactive compounds in order to control the precise
location at which chemical reactions take place. This general approach should
allow the construction of large atomically precise objects by a sequence of
precisely controlled chemical reactions, building objects molecule by molecule.
If designed to do so, assemblers will be able to build copies of themselves,
that is, to replicate.
Because they will be able to copy themselves, assemblers will be
inexpensive. We can see this by recalling that many other products of molecular
machines--firewood, hay, potatoes--cost very little. By working in large teams,
assemblers and more specialized nanomachines will be able to build objects
cheaply. By ensuring that each atom is properly placed, they will manufacture
products of high quality and reliability. Left-over molecules would be subject
to this strict control as well, making the manufacturing process extremely
clean.
Ribosomes
The plausibility of this approach can be illustrated by the
ribosome. Ribosomes manufacture all the proteins used in all living things on
this planet. A typical ribosome is relatively small (a few thousand cubic
nanometers) and is capable of building almost any protein by stringing together
amino acids (the building blocks of proteins) in a precise linear sequence. To
do this, the ribosome has a means of grasping a specific amino acid (more
precisely, it has a means of selectively grasping a specific transfer RNA, which
in turn is chemically bonded by a specific enzyme to a specific amino acid), of
grasping the growing polypeptide, and of causing the specific amino acid to
react with and be added to the end of the polypeptide[9].
The instructions that the ribosome follows in building a protein
are provided by mRNA (messenger RNA). This is a polymer formed from the four
bases adenine, cytosine, guanine, and uracil. A sequence of several hundred to a
few thousand such bases codes for a specific protein. The ribosome "reads" this
"control tape" sequentially, and acts on the directions it provides.
Assemblers
In an analogous fashion, an assembler will build an arbitrary
molecular structure following a sequence of instructions. The assembler,
however, will provide three-dimensional positional and full orientational
control over the molecular component (analogous to the individual amino acid)
being added to a growing complex molecular structure (analogous to the growing
polypeptide). In addition, the assembler will be able to form any one of several
different kinds of chemical bonds, not just the single kind (the peptide bond)
that the ribosome makes.
Calculations indicate that an assembler need not inherently be
very large. Enzymes "typically" weigh about 10^5 amu (atomic mass units). while
the ribosome itself is about 3 x 10^6 amu[9]. The smallest assembler might be a
factor of ten or so larger than a ribosome. Current design ideas for an
assembler are somewhat larger than this: cylindrical "arms" about 100 nanometers
in length and 30 nanometers in diameter, rotary joints to allow arbitrary
positioning of the tip of the arm, and a worst-case positional accuracy at the
tip of perhaps 0.1 to 0.2 nanometers, even in the presence of thermal noise.
Even a solid block of diamond as large as such an arm weighs only sixteen
million amu, so we can safely conclude that a hollow arm of such dimensions
would weigh less. Six such arms would weigh less than 10^8 amu.
Molecular Computers
The assembler requires a detailed sequence of control signals,
just as the ribosome requires mRNA to control its actions. Such detailed control
signals can be provided by a computer. A feasible design for a molecular
computer has been presented by Drexler[2,11]. This design is mechanical in
nature, and is based on sliding rods that interact by blocking or unblocking
each other at "locks." This design has a size of about 5 cubic nanometers per
"lock" (roughly equivalent to a single logic gate). Quadrupling this size to 20
cubic nanometers (to allow for power, interfaces, and the like) and assuming
that we require a minimum of 10^4 "locks" to provide minimal control results in
a volume of 2 x 10^5 cubic nanometers (.0002 cubic microns) for the
computational element. (This many gates is sufficient to build a simple 4-bit or
8-bit general purpose computer, e.g. a 6502).
An assembler might have a kilobyte of high speed (rod-logic based)
RAM, (similar to the amount of RAM used in a modern one-chip computer) and 100
kilobytes of slower but more dense "tape" storage - this tape storage would have
a mass of 10^8 amu or less (roughly 10 atoms per bit - see below). Some
additional mass will be used for communications (sending and receiving signals
from other computers) and power. In addition, there will probably be a "toolkit"
of interchangable tips that can be placed at the ends of the assembler's arms.
When everything is added up a small assembler, with arms, computer, "toolkit,"
etc. should weigh less than 10^9 amu.
Escherichia coli (a common bacterium) weigh about 10^12 amu[9,
page 123]. Thus, an assembler should be much larger than a ribosome, but much
smaller than a bacterium.
Self-Replicating Systems
It is also interesting to compare Drexler's architecture for an
assembler with the Von Neumann architecture for a self replicating device. Von
Neumann's "universal constructing automaton"[21] had both a universal Turing
machine to control its functions and a "constructing arm" to build the
"secondary automaton." The constructing arm can be positioned in a
two-dimensional plane, and the "head" at the end of the constructing arm is used
to build the desired structure. While Von Neumann's construction was theoretical
(existing in a two dimensional cellular automata world), it still embodied many
of the critical elements that now appear in the assembler.
Should we be concerned about runaway replicators? It would be hard
to build a machine with the wonderful adaptability of living organisms. The
replicators easiest to build will be inflexible machines, like automobiles or
industrial robots, and will require special fuels and raw materials, the
equivalents of hydraulic fluid and gasoline. To build a runaway replicator that
could operate in the wild would be like building a car that could go off-road
and fuel itself from tree sap. With enough work, this should be possible, but it
will hardly happen by accident. Without replication, accidents would be like
those of industry today: locally harmful, but not catastrophic to the biosphere.
Catastrophic problems seem more likely to arise though deliberate misuse, such
as the use of nanotechnology for military aggression.
Positional Chemistry
Chemists have been remarkably successful at synthesizing a wide
range of compounds with atomic precision. Their successes, however, are usually
small in size (with the notable exception of various polymers). Thus, we know
that a wide range of atomically precise structures with perhaps a few hundreds
of atoms in them are quite feasible. Larger atomically precise structures with
complex three-dimensional shapes can be viewed as a connected sequence of small
atomically precise structures. While chemists have the ability to precisely
sculpt small collections of atoms there is currently no ability to extend this
capability in a general way to structures of larger size. An obvious structure
of considerable scientific and economic interest is the computer. The ability to
manufacture a computer from atomically precise logic elements of molecular size,
and to position those logic elements into a three- dimensional volume with a
highly precise and intricate interconnection pattern would have revolutionary
consequences for the computer industry.
A large atomically precise structure, however, can be viewed as
simply a collection of small atomically precise objects which are then linked
together. To build a truly broad range of large atomically precise objects
requires the ability to create highly specific positionally controlled bonds. A
variety of highly flexible synthetic techniques have been considered in [32]. We
shall describe two such methods here to give the reader a feeling for the kind
of methods that will eventually be feasible.
We assume that positional control is available and that all
reactions take place in a hard vacuum. The use of a hard vacuum allows highly
reactive intermediate structures to be used, e.g., a variety of radicals with
one or more dangling bonds. Because the intermediates are in a vacuum, and
because their position is controlled (as opposed to solutions, where the
position and orientation of a molecule are largely random), such radicals will
not react with the wrong thing for the very simple reason that they will not
come into contact with the wrong thing.
Normal solution-based chemistry offers a smaller range of
controlled synthetic possibilities. For example, highly reactive compounds in
solution will promptly react with the solution. In addition, because positional
control is not provided, compounds randomly collide with other compounds. Any
reactive compound will collide randomly and react randomly with anything
available. Solution-based chemistry requires extremely careful selection of
compounds that are reactive enough to participate in the desired reaction, but
sufficiently non-reactive that they do not accidentally participate in an
undesired side reaction. Synthesis under these conditions is somewhat like
placing the parts of a radio into a box, shaking, and pulling out an assembled
radio. The ability of chemists to synthesize what they want under these
conditions is amazing.
Much of current solution-based chemical synthesis is devoted to
preventing unwanted reactions. With assembler-based synthesis, such prevention
is a virtually free by-product of positional control.
To illustrate positional synthesis in vacuum somewhat more
concretely, let us suppose we wish to bond two compounds, A and B. As a first
step, we could utilize positional control to selectively abstract a specific
hydrogen atom from compound A. To do this, we would employ a radical that had
two spatially distinct regions: one region would have a high affinity for
hydrogen while the other region could be built into a larger "tip" structure
that would be subject to positional control. A simple example would be the
1-propynyl radical, which consists of three co-linear carbon atoms and three
hydrogen atoms bonded to the sp3 carbon at the "base" end. The radical carbon at
the radical end is triply bonded to the middle carbon, which in turn is singly
bonded to the base carbon. In a real abstraction tool, the base carbon would be
bonded to other carbon atoms in a larger diamondoid structure which provides
positional control, and the tip might be further stabilized by a surrounding
"collar" of unreactive atoms attached near the base that would prevent lateral
motions of the reactive tip.
The affinity of this structure for hydrogen is quite high. Propyne
(the same structure but with a hydrogen atom bonded to the "radical" carbon) has
a hydrogen-carbon bond dissociation energy in the vicinity of 132 kilocalories
per mole. As a consequence, a hydrogen atom will prefer being bonded to the
1-propynyl hydrogen abstraction tool in preference to being bonded to almost any
other structure. By positioning the hydrogen abstraction tool over a specific
hydrogen atom on compound A, we can perform a site specific hydrogen abstraction
reaction. This requires positional accuracy of roughly a bond length (to prevent
abstraction of an adjacent hydrogen). Quantum chemical analysis of this reaction
by Musgrave et. al.[41] show that the activation energy for this reaction is
low, and that for the abstraction of hydrogen from the hydrogenated diamond
(111) surface (modeled by isobutane) the barrier is very likely zero.
Having once abstracted a specific hydrogen atom from compound A,
we can repeat the process for compound B. We can now join compound A to compound
B by positioning the two compounds so that the two dangling bonds are adjacent
to each other, and allowing them to bond.
This illustrates a reaction using a single radical. With
positional control, we could also use two radicals simultaneously to achieve a
specific objective. Suppose, for example, that two atoms A1 and A2 which are
part of some larger molecule are bonded to each other. If we were to position
the two radicals X1 and X2 adjacent to A1 and A2, respectively, then a bonding
structure of much lower free energy would be one in which the A1-A2 bond was
broken, and two new bonds A1-X1 and A2-X2 were formed. Because this reaction
involves breaking one bond and making two bonds (i.e., the reaction product is
not a radical and is chemically stable) the exact nature of the radicals is not
critical. Breaking one bond to form two bonds is a favored reaction for a wide
range of cases. Thus, the positional control of two radicals can be used to
break any of a wide range of bonds.
A range of other reactions involving a variety of reactive
intermediate compounds (carbenes are among the more interesting ones) are
proposed in [32], along with the results of semi-empirical and ab initio quantum
calculations and the available experimental evidence.
Another general principle that can be employed with positional
synthesis is the controlled use of force. Activation energy, normally provided
by thermal energy in conventional chemistry, can also be provided by mechanical
means. Pressures of 1.7 megabars have been achieved experimentally in
macroscopic systems[43]. At the molecular level such pressure corresponds to
forces that are a large fraction of the force required to break a chemical bond.
A molecular vise made of hard diamond-like material with a cavity designed with
the same precision as the reactive site of an enzyme can provide activation
energy by the extremely precise application of force, thus causing a highly
specific reaction between two compounds.
To achieve the low activation energy needed in reactions involving
radicals requires little force, allowing a wider range of reactions to be caused
by simpler devices (e.g., devices that are able to generate only small force).
Further analysis is provided in [32].
Feynman said: "The problems of chemistry and biology can be
greatly helped if our ability to see what we are doing, and to do things on an
atomic level, is ultimately developed - a development which I think cannot be
avoided." Drexler has provided the substantive analysis required before this
objective can be turned into a reality. We are nearing an era when we will be
able to build virtually any structure that is specified in atomic detail and
which is consistent with the laws of chemistry and physics. This has substantial
implications for future medical technologies and capabilities.
Cost
One consequence of the existence of assemblers is that they are
cheap. Because an assembler can be programmed to build almost any structure, it
can in particular be programmed to build another assembler. Thus, self
reproducing assemblers should be feasible and in consequence the manufacturing
costs of assemblers would be primarily the cost of the raw materials and energy
required in their construction. Eventually (after amortization of possibly quite
high development costs), the price of assemblers (and of the objects they build)
should be no higher than the price of other complex structures made by
self-replicating systems. Potatoes - which have a staggering design complexity
involving tens of thousands of different genes and different proteins directed
by many megabits of genetic information - cost well under a dollar per pound.
PATHWAYS TO NANOTECHNOLOGY
The three paths of protein design (biotechnology), biomimetic
chemistry, and atomic positioning are parts of a broad bottom up strategy:
working at the molecular level to increase our ability to control matter.
Traditional miniaturization efforts based on microelectronics technology have
reached the submicron scale; these can be characterized as the top down
strategy. The bottom-up strategy, however, seems more promising. INFORMATION
More information on nanotechnology can be found in these books
(all by Eric Drexler (and various co-authors)):
Engines of Creation (Anchor, 1986) ISBN: 0-385-19972-2
This book was the definition of the original charter of
sci.nanotech. Popularly written, it introduces assemblers, and discusses the
various social and technical implications nanotechnology might have.
Unbounding the Future (Morrow, 1991) 0-688-09124-5
Essentially an update of Engines, with a better low-level
description of how nanomachines might work, and less speculation on space
travel, cryonics, etc.
Nanosystems (Wiley, 1992) 0-471-57518-6
This is the technical book that grew out of Drexler's PhD thesis.
It is a real tour de force that provides a substantial theoretical background
for nanotech ideas.
The Foresight Institute publishes on both technical and
nontechnical issues in nanotechnology. For example, students may write for their
free Briefing #1, "Studying Nanotechnology". The Foresight Institute's main
publications are the Update newsletter and Background essay series. The Update
newsletter includes both policy discussions and a technical column enabling
readers to find material of interest in the recent scientific literature. These
publications can be found at Foresight's web page.
email address: foresight@cup.portal.com
A set of papers and the archives of sci.nanotech can be had by
standard anonymous FTP to nanotech.rutgers.edu. /nanotech
Sci.nanotech is moderated and is intended to be of a technical
nature.
--JoSH (moderator)
REFERENCES
[Not all of these are referred to in the text, but they are of
interest nevertheless.]
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2. "Nanotechnology: wherein molecular computers control tiny
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pages 100 to 103.
3. "Foresight Update", a publication of the Foresight Institute,
Box 61058, Palo Alto, CA 94306.
4. "There's Plenty of Room at the Bottom" a talk by Richard
Feynman (awarded the Nobel Prize in Physics in 1965) at an annual meeting of the
American Physical Society given on December 29, 1959. Reprinted in
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Adapted
by J.Storrs Hall from papers by Ralph C. Merkle and K. Eric Drexler