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A publication of the Foresight Institute
Dendritic polymers -- also called "dendrimers" --
are molecules that branch successively from a central core. They
have engaged the interest of chemists in recent years because,
being a generalization of traditional polymers, they are a huge
class of chemical substances that could have valuable new
properties. And, being polymeric, a given dendrimer can be built
up from a small number of kinds of simple components (monomers)
through repetition of a few basic chemical reactions. Traditional
polymers are usually simple chains because they are made of
monomers having just two reactive sites. Dendrimers, however, are
made of "branched" monomers having three or more
reactive sites.
Until now chemists have sought to synthesize dendrimers by
starting at the core of the molecule and working outward.
Monomers are bonded to the growing molecular surface in layers
until the branching chains become too dense for further layers to
be added. At least that was what was supposed to happen. In
practice there were problems that led to inaccuracies and low
yields of pure dendrimers with the desired structure.
Dendrimers possess some of the properties we expect of
nanodevices: they are eutactic (i.e., have atomically precise
structure), are relatively large (about 105 atoms),
and can have nontrivial functionality. They may play a role in
developing nanotechnology: for example, they could be worth
investigating as tips for atomic force microscopes, or as
platforms for attaching functional protein and nucleic acid
structures to make multifunctional nanosystems.
It is even possible that dendrimers represent the beginnings of a
new approach to nanotechnology. A suitably designed set of
dendrimers might serve as a set of structural components for
building larger devices, much as the parts of an Erector set do.
More news on the construction set front: a new family of polymers called "carborods" has been developed by M.F. Hawthorne and J. Michl and their coworkers. Carborods are chains of para-carborane monomers -- molecules consisting of a cage of boron atoms with a carbon atom at either end. Carbon-carbon bonds form the links between the monomers.
For some of us, at least, it comes as a surprise that boron
might serve better than carbon as the basic element in
nanosystems. A systematic study could turn up still more
possibilities.
Another type of cage molecule that has been in the news a lot
lately is the fullerene family. These generally have a tubular
framework of carbon atoms capped at both ends. (In the special
case of a tubule of zero length, the molecule consists only of
the end-caps and is called C60 or
buckminsterfullerene.) Chemists have been exploring the
properties of fullerenes in hopes of turning them into something
useful.
From the standpoint of nanotechnology, the instability of the end-caps relative to the cylindrical part of the tubules is of interest. Unlike the cylindrical body of a tubule, the end-caps contain a number of five-membered rings of carbon atoms which apparently make the structure less stable.
Foresight Update 16 - Table of Contents
Nanotechnology evokes images of tiny robots building things
out of atoms, and that seems like a radical departure from the
past. However, nanotechnology can equally well be viewed as an
improved version of a familiar chemical tool: catalysis. The
basic underlying process in both cases is the act of forming
chemical bonds by guiding, orienting, and distorting the reacting
molecular groups. Catalysts are molecular manipulators --
primitive predecessors of tomorrow's molecular assemblers.
The search for new catalysts is a central part of chemistry. If
only we had a good specific catalyst for each chemical reaction
we want to carry out, chemistry would be simple and
nanotechnology would be a straightforward development.
Unfortunately, existing catalysts are far too few and their
specificities leave much to be desired. Even enzymes, biology's
catalysts, will generally act upon a range of different molecules
rather than on a single type.
Catalysts vary greatly in size, from hydrogen ions to enzymes
consisting of thousands of atoms. As one would expect, there is a
correlation between size and specificity. The smallest catalysts,
being single atoms, have few appendages with which to manipulate
their substrates: they have only a few fuzzy lobes of electric
charge -- their electron clouds. Large catalysts, in contrast,
have complex shapes that trap and hold their substrates; they
bring groups of atoms to bear that probe and distort the
substrates, enhancing their reactivity; many catalysts even have
moving parts that drive the substrates along optimum paths during
the reaction.
Current work focuses mainly on catalytic molecules in two size
ranges -- small and large.
How is this result to be explained? After all, generations of
chemistry students have been taught that catalysts accelerate
chemical reactions but do not change their eventual outcome --
i.e., the relative proportions of the various possible products
and byproducts. At first take, this seems to imply that catalysts
should be helpless to selectively enhance or suppress one
reaction path over another, for a given set of reactants.
In the long run (which might mean billions of years), a set of
reactants will indeed interact to give the most energetically
favored set of products -- i.e., the set with the lowest overall
energy -- with or without catalysis. But in the short run,
matters are different. Catalysts can create new reaction paths to
products otherwise impeded by high activation energy barriers. In
effect, they can selectively lower these barriers while raising
barriers along the paths that otherwise would be favored by low
activation energies. If the products are removed from the system
before equilibrium is reached, a disfavored reaction will have
become the favored one. This is not to say that the catalysts
make the reactions "run uphill" from lower to higher
energy, but rather that the reaction path promoted by a given
catalyst need not be the one that originally had the lowest
activation energy barrier.
Greater selectivity generally requires more intricate catalysts.
Couched in mechanical terms: objects with lots of springs and
gears can't be assembled with a hammer. A molecular assembly
robot represents the ultimate catalyst because it is intricate
enough to perform the manipulations needed to assemble a
complicated product.
About a year ago, J.A.
Sidles of the University of Washington proposed a method for
using magnetic resonance to detect and locate single protons on a
surface. Now he
and D. Rugar and C.C. Yannoni of IBM have built a device to
prove the validity of the basic concept. In this initial effort
their goals were modest: to detect electrons rather than protons,
and to measure position with low resolution in only one
dimension. These goals were were achieved last fall.
In the device, a few grains of a crystalline sample are glued to
a small cantilever. The cantilever is then subjected to the
combined effects of three different magnetic fields: a field A
varying strongly in space, a field B oscillating at the resonant
frequency of the cantilever, and a field C to excite the electron
spins in the sample. The forces generated by these fields cause
the cantilever to vibrate at its resonant frequency. The strength
of the field B is systematically varied while the vibrational
amplitude of the cantilever is measured optically. In a plot of
the amplitude versus field strength, peaks occur where there is
resonance with the electron spins. Because the field A is
spatially inhomogeneous, different parts of the sample may be
subject to different field strengths and may resonate at
different values of field B. Thus, the peaks in the resonance
curve can be interpreted as images of these different parts of
the sample.
The researchers believe that these techniques can be used to
build magnetic resonance microscopes capable of forming images of
molecules in three dimensions at atomic resolution. [Science
News 27Mar93:199; Nature 360:563-566,10Dec92]
E. Henderson, et al., at Iowa State University have imaged actin filaments in living cells with an atomic force microscope (AFM). Glial cells adhering to a coated glass surface have thin flat borders in which actin filaments lie close to the cell membrane. It unclear whether the AFM was detecting bulges in the membrane caused by the underlying actin filaments or was penetrating the surface and imaging the filaments directly. [Science 257:1944-1946,25Sep92 --MEDLINE Abstract]
AFMs are routinely used to image hard samples with atomic
resolution, but soft samples are often problematic. The mechanism
of soft sample imaging has been studied by M. Radmacher, and
others, at the Technische Universität München. Their analysis
helps to explain why AFM images of soft materials have lower
resolution and spurious contrast. For example, when an AFM tip
crosses a region of higher friction, the resulting deflection of
the AFM cantilever will produce an erroneous value for the height
of the region. However, the authors show that these effects give
AFMs the ability to measure local properties of surfaces, such as
viscosity and elasticity. [Science 257:1900-1905,25Sep92
--MEDLINE
Abstract]
Russell Mills is research director at a company in
California.
Due to lack of space in this issue, coverage of media articles will be postponed. Briefly, nanotechnology has been discussed in a myriad publications including Technology Review, Nature, Popular Science, Chemical & Engineering News, New York Times, World Monitor, Government Technology, Utne Reader, Hemispheres (United Airlines), Develop: Apple Technical Journal, Pittsburgh Post-Gazette. As usual, the level of accuracy varied tremendously.
Foresight Update interviewed Chairman Eric Drexler on the
subject of his spring visit to Japan. Watch for further news
after his next trip this fall.
Update: What was the purpose of your most
recent trip to Japan?
Drexler: MITI organized a meeting to launch its
new nanotechnology research program and invited me as the foreign
keynote speaker for the molecular machine workshop. This was the
third time a Japanese research institution brought me to Japan
and the second time by MITI. I enjoyed the trip in part because
it was an opportunity to find out where Japanese researchers are
going.
Update: Tell us a little bit about the meeting
and the bigger picture of what MITI is doing in nanotechnology.
Drexler: The meeting had three sections lasting
a total of five days. The first concentrated on technologies
relevant to mechanosynthesis, primarily the use of scanning
tunneling microscopes to manipulate atoms and molecules and the
use of quantum chemistry programs to model the behavior of
reacting atoms and molecules.
The second concentrated on experiments and experimental
techniques for characterizing a class of nanometer scale objects
made with present technologies, termed "clusters." The
third was divided into two sections; one on biological processes
that lay down solid materials, called
"biocrystallization," and the other concentrating on
molecular machine systems.
This series of meetings mixed talks from Japanese and foreign
researchers including several Americans and several Europeans.
Since mechanosynthesis and molecular machine systems are basic to
molecular manufacturing, I found the combinations of these two
topics in one meeting quite striking. US research into molecular
machines systems -- at least, research with an engineering
orientation -- is almost absent.
The broader context of this meeting was the inauguration of
MITI's ten-year $200 million nanotechnology research program.
This will be housed in a building just a short walk away from
this meeting site, on the campus of the Agency for Industrial
Science and Technology, a large complex of MITI research
laboratories at the Tsukuba science city north of Tokyo.
Update: Is your new book, Nanosystems,
available in Japan yet? Did you find that researchers are
familiar with those concepts?
Drexler: Researchers in Japan seem quite
familiar with the concept of molecular manufacturing, at least,
relative to researchers in the United States. Maybe it was just
an excess of Japanese politeness, but I was repeatedly told by
different researchers that efforts including the Protein
Engineering Research Institute in Osaka and the new MITI
initiative were in significant measure inspired by my papers, and
books published during the 1980s.
Nanosystems at that time had been on the US market
for only a few months, and I saw no evidence that it had yet
reached Japan, though one researcher had ordered it through his
company's technical awareness service, which had brought it to
his attention.
In Books Sanseido, a large bookstore in downtown Tokyo, I found
several new nanotechnology titles in Japanese on the same shelves
with the Japanese translation of Engines
of Creation.
Update: Did you visit any other laboratories on
this trip?
Drexler: I visited several groups and gave a
number of talks, but the most interesting laboratory visit was at
the Aono Atomcraft Project, which has its main research
laboratory in Tsukuba. They are working to use a scanning
tunneling microscope to lay down individual metal atoms on a
semiconductor surface, with the goal of making quantum wires and
a four-point probe for measuring the resistance of these wires.
They have a novel and promising approach that they liken to
dipping a quill pen in ink and then writing with it. This should
be tested experimentally sometime this year. Their laboratory has
already succeeded in removing individual silicon atoms and moving
silicon atoms on a surface with considerable control.
After the workshop I also went on to Matsushita, a company that
makes everything from washing machines to next-generation
computer chips. After a reorganization this fall, two of their
six R&D divisions will be working in areas on the path to
nanotechnology, One is aimed at the manipulation of atoms and
molecules, and the other has a biological focus. When I asked for
more details on the latter, I was told that it would work with
molecular machines, such as the bacterial flagellar motor.
Foresight member Tom McKendree, who is also president of the
National Space Society's Molecular
Manufacturing Shortcut Group, will be in Tsukuba this summer
as part of an NSF program, I expect he can give us a good report
this fall.
Update: You and the Foresight Institute advocate
international development of nanotechnology. When you picture
this, do you picture many independent efforts all of which are
international and collaborative, or do you picture one national
program which is then done jointly, and what kind of European
participation do you picture?
Drexler: At this point I think the sensible
objective is to have multiple efforts pursuing multiple
directions in multiple countries. The international goal should
be to pursue this research in an atmosphere in which there is a
consensus that extensive international exchange of information is
appropriate and actively encouraged by national governments and
their funding agencies. As we draw closer to real breakthrough
capabilities, we may be able to build a consensus that a more
closely coupled effort is appropriate.
Copies of four articles are still available from Foresight in
the form of offprints:
"Molecular Directions in Nanotechnology," K.E. Drexler, Nanotechnology
2 (1991) 113-118.
"Molecular Manufacturing for Space Systems: an
Overview," K.E. Drexler, JBIS 45
(1992) 401-405.
"Self
Replicating Systems and Molecular Manufacturing," R. Merkle, JBIS 45
(1992) 407-413.
"Nanotechnology: Evolution of the Concept," C.L.
Peterson, JBIS 45 (1992) 395-400.
To obtain these, send $3 each to Foresight Institute, PO Box
61058, Palo Alto, CA 94306 USA. (To conserve our supply, please
request only one copy of each paper.)
From Foresight Update16,
originally published 1 July 1993.