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One
of the most amazing revelations that the scientific world-view
has brought us is the number of different scales in the
universe. Pre-civilized cultures had the concept of a scale
bigger than the everyday – associated with the “heavens,”
and usually with gods of some sort. The ancient Greeks conceived
of a smaller “atomic” scale, and other ancient
cultures like the Indians had similar ideas; but these were
not fleshed out into detailed pictures of the microworld,
because they were pure conceptual speculations, ungrounded
in empirical observation. When Leeuwenhoek first looked
through his microscope, he discovered a whole other world
down there, with a rich dynamic complexity no one had imagined
in any detail. Galileo and the other pioneers of astronomy
had the same experience with the telescope. Yes, everyone
had known the stars were up there: but so many of them!
… so much variety! And the planets – the moon
-- the shapes and markings on them -- !
Now we take for granted that the world we daily see and
act in is just a very small scale slice taken out of all
the scales of the cosmos. There are vast complex dynamic
processes happening on the galactic scale, in which our
sun and planet are just tiny insignificant pieces. And there
are bafflingly complex things going on in our bodies –
antibodies collaborating with each other to fight germs,
enzymatic networks carrying out subtle computations …
quantum wave functions interfering mysteriously to produce
protein dynamics that interpret obscure DNA code sequences….
Until recently our participation in non-human scale processes
has been basically limited to observation and experimentation.
We look at the planets; we don’t create new planets,
or even move around existing ones. As of this century, we
do create new molecules, but it’s a hellishly difficult
process. We probe particles and their interactions in our
particle accelerators, but to measure or understand anything
at this tiny scale we need very carefully controlled conditions.
But this is going to change. In time, technology will allow
us to act as well as observe on the full range of scales.
And it seems likely that we’re going to master the
realm of the very small well before the realm of the very
large. Manufacturing has been pushing in this direction
for decades now: machines just keep getting smaller and
smaller and smaller. The circuitry controlling the computer
I’m typing these words on, is far too small for me
to see with the naked eye.
The next step along this path, the purposeful construction
of molecular-level machinery, has been christened as nanotechnology.
The most ambitious goals of nanotechnology still live in
the future: the construction of molecular assemblers, machines
that are able to move molecules into arbitrary configurations,
playing with matter like a child plays with Play-doh and
Legos. But there are concrete scientific research programs
moving straight in this direction; most ambitiously at the
Zyvex Corporation, which has hired an impressive staff of
nanotechnology research pioneers. And there are more specialized
sorts of nanomachines in production right now: from teeny
tiny transistors to DNA computers to human-engineered proteins
the likes of which have never been seen in nature. Exactly
how many years it will be until we can cure diseases by
releasing swarms of millions of germ-killing nanobots into
the bloodstream, is hard to say. But the door to the nanorealm
has been opened -- we’ve stepped through the doorway
-- and bit by bit, year by year, we’re learning our
way around inside….
It’s
hard for the human mind to grasp the tiny scales involved
here. One way to conceptualize these scales is to look at
the metric system’s collection of prefixes for dealing
with very small quantities. We all learned about centi and
micro in school. Computer technology has taught us about
the scales beyond kilo: mega, giga, tera and soon peta.
Over the next few decades we’ll be developing more
and more commonsense collective intuition for nano, pico,
femto and perhaps even the scales below.
| prefix |
abbreviation
(upper and
lower case
are important) |
meaning |
example |
a sense of scale
(for some)
Most are approximate. |
| yotta |
Y |
1024
|
yottagram,
1 Yg = 1024 g |
mass of water in
Pacific Ocean ~ 1 Yg
energy given off by the sun in 1 second ~ 400
YJ
volume of earth ~ 1 YL
mass of earth ~ 6000 Yg |
| zetta
|
Z
|
1021
|
zettameter,
1 Zm = 1021 m |
radius of Milky
Way galaxy ~ 1 Zm
volume of Pacific Ocean ~ 1 ZL
world energy production per year, ~ 0.4 ZJ |
| exa
|
E
|
1018
|
exasecond,
1 Es = 1018 s |
age of universe
~ 0.4 Es (12 billion yr) |
| peta
|
P
|
1015
|
petameter,
1 Pm = 1015 m |
1 light-year (distance
light travels in one year) ~ 9.5 Pm |
| tera
|
T
|
1012
|
terameter,
1 Tm = 1012 m |
distance from sun
to Jupiter ~ 0.8 Tm |
| giga |
G |
109 |
gigasecond,
1 Gs = 109 s |
human life expectancy
~ 1 century ~ 3 Gs
1 light-second (distance light travels in one
second) ~ 0.3 Gm |
| mega
|
M
|
106
|
megasecond,
1 Ms = 106 s |
1 Ms ~ 11.6 days
|
| kilo |
k |
103 |
kilogram,
1 kg = 103 g |
|
| hecto
|
h
|
102
|
hectogram,
1 hg = 102 g |
|
| deka (or deca) |
da
|
10
= 101 |
dekaliter,
1 daL = 101 L |
|
| deci
|
d
|
10-1
|
deciliter,
101 dL = 1 L |
|
| centi |
c |
10-2
|
centimeter,
102 cm = 1 m |
|
| milli |
m |
10-3
|
millimole,
103 mmol = 1 mol |
|
| micro
|
µ
|
10-6
|
microliter,
106 µL = 1 L |
1 µL ~ a very
tiny drop of water |
| nano
|
n
|
10-9
|
nanometer,
109 nm = 1 m |
radius of a chlorine
atom in Cl2 ~ 0.1 nm or 100 pm |
| pico
|
p
|
10-12
|
picogram,
1012 pg = 1 g |
mass of bacterial
cell ~ 1 pg |
| femto |
f |
10-15
|
femtometer,
1015 fm = 1 m |
radius of a proton
~ 1 fm |
| atto |
a |
10-18
|
attosecond,
1018 as = 1 s |
time for light
to cross an atom ~ 1 as
bond energy for one C=C double bond ~ 1 aJ |
| zepto
|
z
|
10-21
|
zeptomole,
1021 zmol =
1 mol |
1 zmol ~ 600 atoms
or molecules |
| yocto |
y |
10-24
|
yoctogram,
1024 yg = 1 g |
1.7 yg ~ mass of
a proton or neutron |
|
| Metric
prefixes from yotta- to yocto- |
Technology
at the scale of a micrometer, a millionth of a meter, isn’t
even revolutionary anymore. Micromachinery, machines the
size of a miniscule water droplet, are relatively easily
achievable using current engineering technology. The nanometer
scale, a billionth of a meter, is where today’s most
pioneering tiny-scale research aims.
Nano is atomic/molecular scale. Think about this: Everything
we see around us is just a configuration of molecules. But
out of the total number of possible molecular configurations,
existing matter represents an incredibly small percentage
– basically zero percent. Nearly all possible types
of matter are as yet unknown to us. If we create machines
that can piece molecules together in arbitrary ways, total
control over matter is ours. This is the power and the wonder
of nanotechnology.
As the above scale chart shows, it’s quite possible
to get smaller than nanotechnology. A protein is about one
femtometer across. Femtotechnology, the arbitrary reconfiguration
of particles, may hold even greater rewards one day. The
construction of novel forms of matter via the assemblage
of known molecules may one day be viewed as limiting. But
femtotechnology today is where nanotechnology was 50 years
ago – abstract though enticing; a pipe dream for the
moment. One may even nurse dreams beyond femtotechnology
– for the sci-fi-fueled imagination, it’s not
so hard to imagine dissecting quarks and intermediate vector
bosons, perhaps reconfiguring being and time themselves.
But what’s fascinating about nanotechnology right
now is that, more and more rapidly each year, it’s
exiting the domain of wild dreams and visions -- and becoming
fantastically real.
The
amazing thing, from a 2002 perspective, is how ridiculously
long it took the mainstream of science and industry to understand
that the future lies in smaller, smaller, smaller. Physicist
Richard Feynman laid out the point very clearly in a 1959
lecture, “There’s Plenty of Room at the Bottom”.
Today the lecture is a legend; at the time it was pretty
much ignored. The whole text of the talk is online at http://www.zyvex.com/nanotech/feynman.html,
and it makes for pretty fascinating reading.
Feynman laid out his vision clearly and simply:
They
tell me about electric motors that are the size of the nail
on your small finger. And there is a device on the market,
they tell me, by which you can write the Lord's Prayer on
the head of a pin. But that's nothing; that's the most primitive,
halting step…. It is a staggeringly small world that
is below. In the year 2000, when they look back at this
age, they will wonder why it was not until the year 1960
that anybody began seriously to move in this direction.
Why cannot we write the entire 24 volumes of the Encyclopedia
Brittanica on the head of a pin?
Though 1959 was well before the structure of DNA was understood,
molecular biology was already a growing field, and many
physicists were looking to it for one sort of inspiration
or another. Quantum pioneer Schrodinger wrote a lovely,
speculative little book What Is Life?, describing DNA as
an “aperiodic crystal,” which motivated many
physicists to give biology a careful look. And Feynman had
the open-mindedness to look at the barely-newly-charted
world of proteins and enzymes, and view it not merely as
biology but as molecular machinery:
This fact---that enormous amounts of information can be
carried in an exceedingly small space---is, of course, well
known to the biologists, and resolves the mystery which
existed before we understood all this clearly, of how it
could be that, in the tiniest cell, all of the information
for the organization of a complex creature such as ourselves
can be stored. All this information---whether we have brown
eyes, or whether we think at all, or that in the embryo
the jawbone should first develop with a little hole in the
side so that later a nerve can grow through it---all this
information is contained in a very tiny fraction of the
cell in the form of long-chain DNA molecules in which approximately
50 atoms are used for one bit of information about the cell.
Well before PC’s and the like existed, Feynman proposed
molecular computers. “The wires should be 10 or 100
atoms in diameter, and the circuits should be a few thousand
angstroms across….” This, he foresaw, would
easily enable AI of a sort: computers that could “make
judgments” and meaningfully learn from experience.
Feynman didn’t give engineering details, but he did
give an overall conceptual approach to nanoscale engineering,
one that is still in play today. Long before nanotechnology,
Jonathan Swift wrote:
So,
naturalists observe, a flea
Hath smaller fleas that on him prey;
And these have smaller fleas to bite 'em,
And so proceed ad infinitum
More
recently, Dr. Seuss, in the classic children’s book
The Cat in the Hat Comes Back, told the tale of the Cat
in the Hat, in whose hat resides yet a smaller cat, in whose
hat resides yet a smaller cat… and so on for 26 layers
until the final, invisibly small cat contains a mystery
substance called Voom! Feynman’s proposal was similar.
Build a small machine, whose purpose is to build a smaller
machine, whose purpose is to build a smaller machine, whose
purpose is to build a smaller machine, and so on. The Voom!
happens when one reaches the molecular level. Dr. Seuss’s
Voom! merely cleaned up all the red gunk that the Cat in
the Hat and his little cat friends had smeared all across
the snow in the yard of their young human friends. Feynman’s
Voom!, on the other hand, promises the ability to reconfigure
the molecular structure of the universe, or at least a small
portion of it.
Today this kind of nanofuturist rhetoric is commonplace.
In 1959 it was the kind of eccentricity that could be tolerated
only in a scientist whose greatness was established for
more conventional achievements. The technology wasn’t
there to make nanotechnology real back then; but a little
more attention to the idea would surely have gotten us more
rapidly to where we are today.
After
Feynman, the next major nanovisionary to come along was
Eric Drexler. His fascination with the nanotech idea grew
progressively as he did his interdisciplinary undergrad
and graduate work at MIT during the 1970’s and 80’s.
Drexler coined the term “nanotechnology,” and
he presented the basic concepts of molecular manufacturing
in a scientific paper in 1981, published in the Proceedings
of the National Academy of Sciences. His 1986 book Engines
of Creation introduced the notion of nanotechnology to the
scientific world and the general public, giving due credit
to Feynman and other earlier conceptual and practical pioneers.
His 1992 work Nanosystems went into far more detail, presenting
for a scientific audience a dizzying array of detailed designs
for nanoscale systems: motors, computers, and more and more
and more. Most critically, he proposed designs for molecular
assemblers: machines that could create new molecular structures
to order, allowing the creation of new forms of matter to
order.
Drexler’s design work as presented in Nanosystems
was a kind of science fiction for the professional scientist.
He described what plausibly could be built in the future,
consistently with physical law, given various reasonably
assumptions about future engineering technology. Whether
things will ever be built exactly according to Drexler’s
designs is doubtful. How close future technology will come
to a detailed implementation of his visions is unknown.
But the conceptual and inspirational power of his work is
undeniable.
A taste of his thinking is conveyed in the following chart,
which shows how Drexler maps conventional manufacturing
technologies into molecular structures and dynamics. The
analogies given in this chart were, of course, just the
starting-point for his vast exploration into the domain
of speculative nanosystem design. This is high-level, big-picture
scientific vision-building at its best.
| Technology |
Function |
Molecular example(s) |
| Struts, beams,
casings |
Transmit force,
hold positions |
Microtubules, cellulose,
mineral structures |
| Cables |
Transmit tension |
Collagen |
| Fasteners, glue |
Connect parts |
Intermolecular
forces |
| Solenoids, actuators |
Move things |
Conformation-changing
proteins, actin/myosin |
| Motors |
Turn shafts |
Flagellar motor |
| Drive shafts |
Transmit torque |
Bacterial flagella |
| Bearings |
Support moving
parts |
Sigma bonds |
| Containers |
Hold fluids |
Vesicles |
| Pipes |
Carry fluids |
Various tubular
structures |
| Pumps |
Move fluids |
Flagella,
membrane proteins |
| Conveyor belts |
Move components |
RNA moved by fixed
ribosome (partial analog) |
| Clamps |
Hold workpieces |
Enzymatic binding
sites |
| Tools |
Modify workpieces |
Metallic complexes,
functional groups |
| Production lines |
Construct devices |
Enzyme systems,
ribosomes |
| Numerical control
systems |
Store and read
programs |
Genetic system |
|
| Drexler’s Analogies
Between Mechanical and Molecular Devices |
In
the years since his initial publications, Drexler has made
a name for himself as a general-purpose futurist, and a
thinker on the social and ethical implications of nanotechnology.
In fact, he says, ethical concerns delayed his publication
of his initial ideas by several years. "Some of the
consequences of the potential abuse of this technology frankly
scared the hell out of me. I wasn't sure I wanted to talk
about it publicly. After a while, though, I realized that
the technology was headed in a certain direction whether
people were paying attention to the long-term consequences
or not. It then made sense to publish and to become more
active in developing these ideas further." He founded
the Foresight Institute, which deals with general social
issues related to the future of nanotechnology; and in recent
years has extended its scope a bit further, dealing with
future technology issues in general.
Along with his move into a role as a general-purpose futurist,
Drexler has consistently been helpful to others in the general
techno-futurist community. For instance, when the Alcor
Life Extension Foundation, a California cryonics lab, got
into legal trouble for allegedly removing and freezing a
client's head before she was declared legally dead, Drexler
came to their rescue. He supplied a deposition in their
defense, arguing that in the future nanotechnology quite
plausibly would allow a person’s mind to be reconstructed
from their frozen head. His position at Stanford at the
time gave him a little more clout with the court system
than the Alcor staff, who were viewed by some as crazy head-freezing
eccentrics. Alcor survived the lawsuit.
As the story of nanotech is still unfolding, the real nature
of Eric Drexler’s legacy isn’t yet clear. Perhaps
history will view him merely as having done what Feynman
tried and failed to do in his 1959 lecture: woken up the
world to the engineering possibilities of the very small.
This is the perspective taken by some contemporary nanoresearchers,
whose pragmatic ideas have little to do in detail with Drexler’s
more science-fictional Nanosystems proposals. On the other
hand, it may well happen that as nanotechnology matures,
more and more aspects of Drexler’s ambitious designs
will become relevant to practical work. Only time will tell
– and given the general acceleration of technological
development, perhaps not as much time as you think.
Very
broadly speaking, there are two ways to make a larger physical
object out of a set of smaller physical objects. You can
do it manually, step by step; or you can somehow coax the
smaller objects to self-organize into the desired larger
form.
More specifically, in the molecular domain, the two techniques
widely discussed are self-assembly and positional assembly.
Self-assembly is how biological systems do it: there is
no engineer building organisms out of DNA and other biomolecules,
the organism builds itself. Eric Drexler’s work, on
the other hand, is largely inspired by analogies to conventional
manufacturing, and tends to take a positional assembly oriented
view.
In self-assembly, a bunch of molecules are allowed to move
around randomly, jouncing in and out of various configurations.
Over time, more stable configurations will tend to persist
longer, and a high-level structure will self-assemble. For
instance, two complementary strands of DNA, if left to drift
around in an appropriate solution for long enough, will
end up bumping into each other and then grasping together
in a double-helix. And a protein, a one-dimensional strand
of amino acids, will wiggle around for a while, and then
eventually curl up into a 3D structure that is determined
by its 1D amino acid sequence. There can be a lot of subtlety
here; for instance, the protein-folding process tends to
be nudged along by special “helper” molecules
that enable a protein to more rapidly find the right configuration.
But we don’t build a TV by throwing the parts into
a soup and waiting for them to lock together in the appropriate
overall configuration. Rather, we hold the partially-constructed
TV in a fixed position and then attach a new part to it
– and repeat this process over and over. This is positional
assembly. The problem with doing this on the molecular scale
is that it requires us to have the ability to: a) hold a
molecule or molecular structure in place, recording its
precise location and b) grab a molecule or molecular structure
and move it from one precise location to another. Neither
of these things is easy to do using current technology.
However, approaches to both problems are under active development.
Drexler’s design for a molecular assembler is based
on positional assembly. It is assumed that the molecules
in question are in vacuum, so there are no other molecules
bouncing into them and jouncing them around. It assumes
that, unlike in traditional chemistry and biology, highly
reactive intermediate structures can be constructed without
reacting with one another willy-nilly, because they’re
held in place.
There is a lot of power in self-assembly: each one of us
is evidence of this! On the other hand, positional assembly
can achieve a degree of precision that self-assembly lacks,
at least in the biological domain. Each person is a little
different, because of the flexibility of the self-assembly
process, and this is good from an evolutionary perspective
– without this diversity, new forms wouldn’t
evolve. On the other hand, it’s also good that, if
we have to have nuclear missiles, each one of them is close
to exactly identical: in this context, variation ensuing
from biological-style self-assembly could lead to terrible
consequences. Less dramatically, we probably wouldn’t
want this kind of variation in our auto engines either.
Crystals, if grown under appropriate conditions, are examples
of self-assembled structures that have the precision of
positionally-constructed machines. But they are much simpler
than the biological molecules that Schroedinger called “aperiodic
crystals,” let alone than whole organisms, computers
or motors. Whether new forms of self-assembly combining
precision with complexity will emerge in the future, is
hard to say. One suspects that some combination of positional
and self-assembly will one day become the standard. Positionally
constructed components may self-assemble into larger structures;
and self-assembled components may be positionally joined
together in specific ways. Contemporary manufacturing technology
is astoundingly diverse, and we can expect nanomanufacturing
methods to be even more so.
Dozens
of companies today are involved in nanotechnology in one
way or another (see http://www.homestead.com/nanotechind/companies.html
for a partial list). It’s also a serious pursuit of
major research labs such as Los Alamos and Sandia. Perhaps
the most ambitious nanotech business concern, however, is
the Texas firm Zyvex. Zyvex makes no bones about its goal:
it wants to construct a Drexler-style molecular assembler,
and play a leading role in the revolution that will ensue
from this. The Zyvex R&D group has outlined a program
that it believes will lead to this goal. And unlike some
other contemporary nanotech approaches that we’ll
discuss below, Zyvex’s approach is pretty much squarely
in the positional assembly camp.
Zyvex has two distinct research groups, called Top-Down
and Bottom-Up. The Bottom-Up team deals explicitly with
molecule manipulation. The Top-Down team, on the other hand,
works largely with micromanufacturing. They’re currently
prototyping assemblers using microsopic but non-nanoscale
components that can be built for relatively low cost.
Using relatively straightforward contemporary technologies,
one can create devices measuring tens or hundreds of microns,
with individual feature sizes of about one micron. After
prototypes at the micron scale have been achieved, extension
to the nanoscale will be attempted. Of course, the micron-scale
machinery developed along the way to true nanotechnology
may also have significant practical applications, which
Zyvex may commercialize.
One approach the Zyvex Top-Down team is pursuing is called
exponential assembly. Basically this means small machines
building more and more small machines. If each machine builds
two other machines, then the total number of machines will
grow exponentially. This is currently being applied specifically
in the context of robot arms. Robot arm technology is by
far the most mature aspect of modern robotics; factories
worldwide now use robot arms to automate assembly processes.
In Zyvex’s plan, the first robot arm picks up miniature
parts carefully laid out for it, and assembles them into
a second robot arm. The two robot arms then build two more
robot arms, when then build four more robot arms, and so
forth.
This
is a first step toward Feynman’s “small machines
building yet smaller machines” idea. Once exponential
assembly of same-sized robot arms has been completed, the
next step is having robot arms assemble slightly smaller
robot arms. And so proceed ad infinitum.
On the Bottom-Up side, long-time nanotechnologist and Zyvex
principal scientist Ralph Merkle has created a set of impressively
detailed designs for components of molecular engines, computers
and related devices. For instance, the following figure
shows one of Merkle’s designs for a molecular bearing.
Ring-shaped molecules are plentiful in nature. Just create
two rings, one slightly smaller than another – insert
the small one in the big one – and presto, molecular
bearing!
Merkle
has put a fair amount of effort into figuring out how to
make a molecular computer that doesn’t create too
much heat. This involves the notion of reversible computing.
Thermodynamics teaches us that the creation of heat is connected
with the execution of irreversible operations – operations
that lose information, so that after they’re completed,
you can’t accurately “roll back” and figure
out what the state of the world was like before the operation
was carried out. Ordinary computers are based on irreversible
operations; for instance a basic OR gate outputs TRUE if
either of its two inputs is TRUE. From the output of the
OR gate, one can’t roll back and figure out which
of the two inputs was TRUE. This kind of irreversible logical
operation, when implemented directly on the physical level,
creates heat. Fortunately, it’s possible to implement
logical operations in a fully reversible manner, at the
cost of using up more memory space than is required for
normal irreversible computing.
Maverick scientist Ed Fredkin pioneered the theory of reversible
computing years ago, in the context of his radical theory
that the universe is a giant computer (the theory of universe
as an irreversible computer has fatal flaws, but the theory
of the universe as a reversible computer is not so obviously
false). Fredkin’s posited universal computer would
operate below the femto scale, giving rise to quarks, leptons,
protons and the whole gamut of particles as emergent patterns
in its computational activity. Merkle’s proposed reversible
computer components are conservative by comparison, merely
involving novel configurations of molecules. The details
of molecular reversible computing are not yet known, but
it seems likely that they’ll involve carrying out
operations at very low temperatures, and that true reversibility
won’t be achieved – we don’t need zero
heat, if we can come close enough.
Zyvex’s
work in tiny-scale engineering is explicitly aimed at Drexler-ish
long-term goals. But loads of other similar fascinating
work is being carried out by other firms, without such clearly
stated grandiose ambitions. As a single example, consider
Bell Labs’ researchers’ work on tiny transistors.
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Transistors
are based on semiconductors – materials like silicon
and germanium that conduct electricity better than insulators
but not as well as really good conductors. A transistor
is a small electronic device that contains a semiconductor,
has at least three electrical contacts, and used in a circuit
as an amplifier, detector, or switch. Transistors are key
components of nearly all modern electronic devices, and
some have called them the most important invention of the
century. The original transistor was invented at Bell Labs
in 1947 (12 years before Feynman’s original nanotech
lecture). In 1999, a team of scientists at this same institution
succeeded in creating a transistor 50 nanometers across.
This is about 1/2000 the width of a human hair. All of its
components are built on top of a silicon wafer.
This kind of innovation is essential to the continuation
of Moore’s Law, the empirical observation that computer
performance doubles roughly every 18 months. Moore’s
Law has survived several technology revolutions already.
Sometime in the next decade, conventional transistor technology
will hit a brick wall, a point at which further incremental
improvements will be more and more difficult to come by.
At this point, quite probably, nanotransistors – successors
to today’s experimental ones -- will be ready to kick
in and keep the smaller, faster, better computers coming.
Positional
assembly based nanotech is powerful, in that it allows us
to extend our vast knowledge of macro-scale manufacturing
and engineering to the nanoworld. On the other hand, the
only known examples of complex nano-level machines –
biological systems – make use of the self-assembly
approach. And there has been some impressive recent progress
in using biological self-assembly to create systems with
nonbiological purposes.
An excellent example of the self-assembly approach is the
construction of stick figures out of DNA, as in the following
figure of a DNA truncated octahedron, depicting a real molecular
compound constructed in Ned Seeman's lab at New York University.
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In
Seeman’s lab, DNA cubes have also been constructed,
along with 2D DNA-based crystals and, most ambitiously,
simple DNA-based computing circuits. Computer scientists
have known for a while how to make simple lattice-based
devices that carry out complex computational functions,
via the state of each part of the lattice affecting the
states of the nearby parts of the lattice. Seeman’s
group constructed a system of this nature out of a lattice
of DNA.
So far there’s no advanced computation involved
here: what they implemented was XOR (“exclusive
or”), a simple logic function embodied in a computer
by a single logic gate. An XOR gate has two inputs, and
gives a TRUE output if one or the other, but not both,
of its outputs, are TRUE. Simple, sure -- but it’s
out of large numbers of very simple (but much bigger!)
gates of this sort that computer chips are constructed.
A DNA solution contains a vast number of molecules, and
if these can be harnessed for parallel processing, we
have a lot more logic-gate-equivalents than there are
neurons in the brain.
Work is also underway creating DNA cages, in which cubic
lattices of DNA are used to trap other molecules at specific
positions and specific angles.
These
cages can potentially be used to assemble a complex biocomputer,
with DNA structures embodying logic operations feeding
data to each other along specified paths. Many noncomputational
types of matter could also be synthesized this way.
Arranging
DNA molecules in novel configurations is exciting and may
lead to such things as molecular computers and new forms
of matter. But for some purposes one wants to go a little
further – not accepting Nature’s biomolecules
as given, but rather creating new ones from scratch.
At first, designing new proteins seems like an incredibly
difficult problem. The problem is, no one knows how to look
at the sequence of amino acids defining a protein molecule’s
one-dimensional structure, and predict from it the three-dimensional
structure that the protein will fold up into. IBM is currently
building the world’s largest supercomputer, Blue Gene,
with the specific intention of taking a stab at this fabulously
difficult “protein folding” problem.
But the problem of protein engineering is a rare case of
something that’s actually easier than it looks. Because
some proteins are easier to understand than others. To engineer
new proteins, you don’t need to understand all proteins,
only those that you want to build. It turns out that there
are many unnatural proteins whose folding is more predictable
than that of proteins found in nature. Protein engineers
strategically place special bonds and molecular groups in
their designed proteins, so as to be able to predict how
they’ll fold up.
example Jonathan Blackburn at the University of Cambridge
is working on the creation of new proteins that bind to
DNA, and whose binding can be controlled by small molecules.
The potential value of this is enormous. Ultimately, it
could allow us to create an explicitly controllable genetic
process, with the small molecules used as interactive “control
switches.” In the shorter run, there are fantastic
implications for genetic medicine and pharmacology in general.
The
likely long-term applications of nanotechnology are far
too numerous to list here. If you can configure arbitrary
forms of matter, heck, what can’t you do? All current
forms of manufacturing and engineering are ridiculously
pathetic by comparison.
One of the more exciting medium-term applications of nanotech,
however, is in the medical domain. Robert Freitas, in his
1999 book Nanomedicine, has explored this space in great
detail, taking a Drexler-like approach of exploring what
will likely be possible given future technologies.
For
example, one of his inventions is the respirocyte, a bloodborne
1-micron-diameter spherical nanomedical device that acts
as an artificial mechanical red blood cell. This will basically
be a little tiny vessel full of oxygen pressurized to 1000
atmospheres. The pumping out of oxygen will be accomplished
chemically by endogenous serum glucose, and if it’s
built as currently designed, it should be able to deliver
236 times more oxygen to the tissues per unit volume than
natural red cells. In addition to numerous medical applications,
pumping your body full of some of these would allow you
to hold your breath for hours underwater.
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Another
envisioned medical nanobot is the “cell rover.”
This one would be built by piecing together traditional
and/or custom-designed biomolecules. It would zoom through
the bloodstream, performing special medical tasks like removing
toxins, delivering drugs, or simple cell repair. The internal
frame would be made of keratin, chitin or calcium carbonate;
the skin panels would be made of lipids (fats). Movement
could be achieved by various biological mechanisms such
as bacterial cilia or flagella, or more original nanotech
designs. A submillimeter band single-molecule radio antenna
could be supplied to allow communication with a control
device and/or other cell rovers. If the device were made
to look like a native cell, the immune system would leave
it alone, allowing it to happily zip about the body doing
its business. The implications of a device like this for
medical science would, obviously, be more than tremendous.
Positional
assembly or self-assembly, or some combination therebetween?
Cascades of machines, each one producing smaller machines
– or subtle modifications of existing biomolecular
processes? General-purpose molecular assemblers, or a host
of different but overlapping nanoscale assembly processes?
The questions are many. But what’s clear from the
vast amount of work doing on is: This isn’t science
fiction anymore. Nanotechnology is here right now. At the
moment it’s mainly in the research lab, but within
the next 5-10 years it will be productized, at least in
its simpler incarnations. And the practical experience obtained
with things like DNA lattices and micromachines and tiny
transistors, will help us flesh out the difficult issues
involved in realizing Feynman’s and Drexler’s
bigger dreams. The tendency toward miniaturization, that
we see all around us with everyday technology like radios,
calculators and computers, is going to be far more profound
than these examples suggest. The possibilities achievable
by reconfiguring matter at will are mind-boggling, but within
the 21’st century they may well be within our reach.
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