Nanotechnologies
TERM
PAPER
“NANOTECHNOLOGIES”
Coal and diamonds, sand and computer chips, cancer and healthy tissue:
throughout history, variations in the arrangement of atoms have distinguished the cheap from the cherished, the
diseased from the healthy. Arranged one way, atoms make up soil, air, and water;
arranged another, they make up ripe strawberries. Arranged one way, they make
up homes and fresh air; arranged another, they make up ash and smoke.
Our ability to
arrange atoms lies at the foundation of technology. We have come far in our
atom arranging, from chipping flint for arrowheads to machining aluminum for
spaceships. We take pride in our technology, with our lifesaving drugs and
desktop computers. Yet our spacecraft are still crude, our computers are still
stupid, and the molecules in our tissues still slide into disorder, first
destroying health, then life itself. For all our advances in arranging atoms,
we still use primitive methods. With our present technology, we are still
forced to handle atoms in unruly herds. But the laws of nature leave plenty of
room for progress, and the pressures of world competition are even now pushing
us forward. For better or for worse, the greatest technological breakthrough in
history is still to come.
Two Styles Of Technology
Our modern technology builds on an
ancient tradition. Thirty thousand years ago, chipping flint was the high
technology of the day. Our ancestors grasped stones containing trillions of
trillions of atoms and removed chips containing billions of trillions of atoms
to make their axheads; they made fine work with skills difficult to imitate
today. They also made patterns on cave walls in France
with sprayed paint, using their hands as stencils. Later they made pots by
baking clay, then bronze by cooking rocks. They shaped bronze by pounding it.
They made iron, then steel, and shaped it by heating, pounding, and removing
chips. We now cook up pure ceramics and stronger steels, but we still shape
them by pounding, chipping, and so forth. We cook up pure silicon, saw it into
slices, and make patterns on its surface using tiny stencils and sprays of
light. We call the products "chips" and we consider them exquisitely
small, at least in comparison to axheads. Our microelectronic technology has
managed to stuff machines as powerful as the room-sized computers of the early
1950s onto a few silicon chips in a pocket-sized computer. Engineers are now
making ever smaller devices, slinging herds of atoms at a crystal surface to
build up wires and components one tenth the width of a fine hair. These
microcircuits may be small by the standards of flint chippers, but each
transistor still holds trillions of atoms, and so-called
"microcomputers" are still visible to the naked eye. By the standards
of a newer, more powerful technology they will seem gargantuan. The ancient
style of technology that led from flint chips to silicon chips handles atoms
and molecules in bulk; call it bulk technology. The
new technology will handle individual atoms and molecules with control and
precision; call it molecular technology. It will change our world in more ways
than we can imagine. Microcircuits have parts measured in micrometers - that
is, in millionths of a meter - but molecules are measured in nanometers (a
thousand times smaller). We can use the terms "nanotechnology" and
"molecular technology" interchangeably to describe the new style of
technology. The engineers of the new technology will build both nanocircuits
and nanomachines.
Molecular Technology Today
One dictionary
definition of a machine is "any system, usually of rigid bodies, formed
and connected to alter, transmit, and direct applied forces in a predetermined
manner to accomplish a specific objective, such as the performance of useful
work." Molecular machines fit this definition quite well. To imagine these
machines, one must first picture molecules. We can picture atoms as beads and
molecules as clumps of beads, like a child's beads linked by snaps. In fact,
chemists do sometimes visualize molecules by building models from plastic beads
(some of which link in several directions, like the hubs in a Tinkertoy set).
Atoms are rounded like beads, and although molecular bonds are not snaps, our
picture at least captures the essential notion that bonds can be broken and
reformed. If an atom were the size of a small marble, a fairly complex molecule would be the
size of your fist. This makes a useful mental image, but atoms are really about
1/10,000 the size of bacteria, and bacteria
are about 1/10,000 the size of mosquitoes. (An atomic nucleus, however, is
about 1/100,000 the size of the atom itself; the difference between an atom and
its nucleus is the difference between a fire and a nuclear reaction.) The
things around us act as they do because of the way their molecules behave. Air
holds neither its shape nor its volume because its molecules move freely,
bumping and ricocheting through open space. Water molecules stick together as
they move about, so water holds a constant volume as it changes shape. Copper
holds its shape because its atoms stick together in regular patterns; we can
bend it and hammer it because its atoms can slip over one another while
remaining bound together. Glass shatters when we hammer it because its atoms
separate before they slip. Rubber consists of networks of kinked molecules,
like a tangle of springs. When stretched and released, its molecules straighten
and then coil again. These simple molecular patterns make up passive
substances. More complex patterns make up the active nanomachines of living cells. Biochemists
already work with these machines, which are chiefly made of protein, the main
engineering material of living cells. These molecular machines have relatively
few atoms, and so they have lumpy surfaces, like objects made by gluing
together a handful of small marbles. Also, many pairs of atoms are linked by
bonds that can bend or rotate, and so protein machines are unusually flexible.
But like all machines, they have parts of different shapes and sizes that do
useful work. All machines use clumps of atoms as parts. Protein machines simply
use very small clumps. Biochemists dream of designing and building such
devices, but there are difficulties to be overcome. Engineers use beams of
light to project patterns onto silicon chips, but chemists must build much more
indirectly than that. When they combine molecules in various sequences, they
have only limited control over how the molecules join. When biochemists need
complex molecular machines, they still have to borrow them from cells.
Nevertheless, advanced molecular machines will eventually let them build
nanocircuits and nanomachines as easily and directly as engineers now build
microcircuits or washing machines. Then progress will become swift and
dramatic. Genetic engineers are already showing the way. Ordinarily, when
chemists make molecular chains - called "polymers" - they dump
molecules into a vessel where they bump and snap together haphazardly in a
liquid. The resulting chains have varying lengths, and the molecules are strung
together in no particular order. But in modern gene synthesis machines,
genetic engineers build more orderly polymers - specific DNA molecules - by
combining molecules in a particular order. These molecules are the nucleotides
of DNA (the letters of the genetic alphabet) and genetic engineers don't dump
them all in together. Instead, they direct the machine to add different
nucleotides in a particular sequence to spell out a particular message. They
first bond one kind of nucleotide to the
chain ends, then wash away the leftover material and add chemicals to prepare
the chain ends to bond the next nucleotide. They grow chains as they bond on
nucleotides, one at a time, in a programmed sequence. They anchor the very
first nucleotide in each chain to a solid surface to keep the chain from
washing away with its chemical bathwater. In this way, they have a big clumsy
machine in a cabinet assemble specific molecular structures from parts a
hundred million times smaller than itself. But this blind assembly process
accidentally omits nucleotides from some chains. The likelihood of mistakes grows
as chains grow longer. Like workers discarding bad parts before assembling a
car, genetic engineers reduce errors by discarding bad chains. Then, to join
these short chains into working genes (typically thousands of nucleotides
long), they turn to molecular machines found in bacteria. These protein
machines, called restriction enzymes, "read" certain DNA sequences as
"cut here." They read these genetic patterns by touch, by sticking to
them, and they cut the chain by rearranging a few atoms. Other enzymes splice pieces
together, reading matching parts as "glue here" - likewise
"reading" chains by selective stickiness and splicing chains by
rearranging a few atoms. By using gene machines to write, and restriction
enzymes to cut and paste, genetic engineers can write and edit whatever DNA
messages they choose. But by itself, DNA is a fairly worthless molecule. It is
neither strong like Kevlar, nor colorful like a dye, nor active like an enzyme,
yet it has something that industry is prepared to spend millions of dollars to
use: the ability to direct molecular machines called ribosomes. In cells,
molecular machines first transcribe DNA, copying its information to make RNA "tapes."
Then, much as old numerically controlled machines shape metal based on
instructions stored on tape, ribosomes build proteins based on instructions
stored on RNA strands. And proteins are useful. Proteins, like DNA, resemble
strings of lumpy beads. But unlike DNA, protein molecules fold up to form small
objects able to do things. Some are enzymes, machines that build up and tear
down molecules (and copy DNA, transcribe it, and build other proteins in the
cycle of life). Other proteins are hormones, binding to yet other proteins to
signal cells to change their behavior. Genetic engineers can produce these
objects cheaply by directing the cheap and efficient molecular machinery inside
living organisms to do the work. Whereas engineers running a chemical plant
must work with vats of reacting chemicals (which often misarrange atoms and
make noxious byproducts), engineers working with bacteria can make them absorb
chemicals, carefully rearrange the atoms, and store a product or release it
into the fluid around them. Genetic engineers have now programmed bacteria to
make proteins ranging from human growth hormone to rennin, an enzyme used in
making cheese. The pharmaceutical company Eli Lilly (Indianapolis) is now marketing Humulin, human insulin
molecules made by bacteria.
Existing Protein Machines
These protein
hormones and enzymes selectively stick to other molecules. An enzyme changes
its target's structure, then moves on; a hormone affects its target's behavior
only so long as both remain stuck together. Enzymes and hormones can be
described in mechanical terms, but their behavior is more often described in
chemical terms. But other proteins serve basic
mechanical functions. Some push and pull, some act as cords or
struts, and parts of some molecules make excellent bearings. The machinery of
muscle, for instance, has gangs of proteins that reach, grab a "rope"
(also made of protein), pull it, then reach out again for a fresh grip;
whenever you move, you use these machines. Amoebas and human cells move and
change shape by using fibers and rods that act as molecular muscles and bones.
A reversible,
variable-speed motor drives bacteria through water by turning a
corkscrew-shaped propeller. If a hobbyist could build tiny cars around such
motors, several billions of billions would fit in a pocket, and 150-lane
freeways could be built through your finest capillaries. Simple
molecular devices combine to form systems resembling industrial machines. In
the 1950s engineers developed machine tools that cut metal under the control of
a punched paper tape. A century and a half earlier, Joseph-Marie Jacquard had
built a loom that wove complex patterns under the control of a chain of punched
cards. Yet over three billion years before Jacquard, cells had developed the
machinery of the ribosome. Ribosomes
are proof that nanomachines built of protein and RNA can be programmed to build
complex molecules. Then consider viruses. One kind, the T4 phage,
acts like a spring-loaded syringe and looks like something out of an industrial
parts catalog. It can stick to a bacterium, punch a hole, and inject viral DNA
(yes, even bacteria suffer infections). Like a conqueror seizing factories to
build more tanks, this DNA then directs the cell's machines to build more viral
DNA and syringes. Like all organisms, these viruses exist because they are
fairly stable and are good at getting copies of themselves made. Whether in
cells or not, nanomachines obey the universal laws of nature. Ordinary chemical
bonds hold their atoms together, and ordinary chemical reactions (guided by
other nanomachines) assemble them. Protein molecules can even join to form
machines without special help, driven only by thermal agitation and chemical
forces. By mixing viral proteins (and the DNA they serve) in a test tube,
molecular biologists have assembled working T4 viruses. This ability
is surprising: imagine putting automotive parts in a large box, shaking it, and
finding an assembled car when you look inside! Yet the T4 virus is but one of
many self-assembling structures.
Molecular biologists have taken the machinery of the ribosome apart into over
fifty separate protein and RNA molecules, and then combined them in test tubes
to form working ribosomes again. To see how this happens, imagine different T4
protein chains floating around in water. Each kind folds up to form a lump with
distinctive bumps and hollows, covered by distinctive patterns of oiliness,
wetness, and electric charge.
Picture them
wandering and tumbling, jostled by the thermal vibrations of the surrounding
water molecules. From time to time two bounce together, then bounce apart.
Sometimes, though, two bounce together and fit, bumps in hollows, with sticky
patches matching; they then pull together and stick. In this way protein adds
to protein to make sections of the virus, and sections assemble to form the
whole. Protein engineers will not need nanoarms and nanohands to assemble
complex nanomachines. Still, tiny manipulators will be useful and they will be
built. Just as today's engineers build machinery as complex as player pianos
and robot arms from ordinary motors, bearings, and moving parts, so tomorrow's
biochemists will be able to use protein molecules as motors, bearings, and
moving parts to build robot arms which will themselves be able to handle
individual molecules.
Designing with Protein
How far off is
such an ability? Steps have been taken, but much work remains to be done.
Biochemists have already mapped the structures of many proteins. With gene machines
to help write DNA tapes, they can direct cells to build any protein they can design.
But they still don't know how to design chains that will fold up to make
proteins of the right shape and function. The forces that fold proteins are
weak, and the number of plausible ways a protein might fold is astronomical, so
designing a large protein from scratch isn't easy. The forces that stick
proteins together to form complex machines are the same ones that fold the
protein chains in the first place. The differing shapes and kinds of stickiness
of amino acids - the
lumpy molecular "beads" forming protein chains - make each protein
chain fold up in a specific way to form an object of a particular shape.
Biochemists have learned rules that suggest how an amino acid chain might fold,
but the rules aren't very firm. Trying to predict how a chain will fold is like
trying to work a jigsaw puzzle, but a puzzle with no pattern printed on its
pieces to show when the fit is correct, and with pieces that seem to fit
together about as well (or as badly) in many different ways, all but one of
them wrong. False starts could consume many lifetimes, and a correct answer might
not even be recognized. Biochemists using the best computer programs now
available still cannot predict how a long, natural protein chain will actually
fold, and some of them have despaired of designing protein molecules soon. Yet
most biochemists work as scientists, not as engineers. They work at predicting
how natural proteins will fold, not at designing proteins that will fold
predictably. These tasks may sound similar,
but they differ greatly: the first is a scientific challenge, the second is an engineering challenge.
Why should natural proteins fold in a way that scientists will find easy to
predict? All that nature requires is that they in fact fold correctly, not that
they fold in a way obvious to people. Proteins could be designed from the start
with the goal of making their folding more predictable. Carl Pabo, writing in the journal Nature,
has suggested a design strategy based on this insight, and some biochemical
engineers have designed and built short chains of a few dozen
pieces that fold and nestle onto the surfaces of other molecules as
planned. They have designed from scratch a
protein with properties like those of melittin, a toxin in bee
venom. They have modified existing enzymes, changing their behaviors in
predictable ways. Our understanding of proteins is growing daily. In
1959, according to biologist Garrett
Hardin, some geneticists called genetic engineering impossible;
today, it is an industry. Biochemistry and computer-aided design are now
exploding fields, and as Frederick Blattner wrote in the
journal Science, "computer chess programs have already reached
the level below the grand master. Perhaps the solution to the protein-folding
problem is nearer than we think." William Rastetter of Genentech,
writing in Applied Biochemistry and
Biotechnology asks, "How far off is de novo enzyme design and
synthesis? Ten, fifteen years?" He answers, "Perhaps not that
long." Forrest Carter of the U.S. Naval Research Laboratory,
Ari Aviram and Philip Seiden of IBM, Kevin Ulmer of
Genex Corporation, and other researchers in university and industrial
laboratories around the globe have already begun theoretical work and
experiments aimed at developing molecular switches, memory devices, and other
structures that could be incorporated into a protein-based computer. The U.S.
Naval Research Laboratory has held two international workshops on
molecular electronic devices, and a meeting sponsored by the U.S.
National Science Foundation has recommended support for basic
research aimed at developing molecular computers. Japan has
reportedly begun a multimillion-dollar program aimed at developing
self-assembling molecular motors and computers, and VLSI
Research Inc., of San Jose, reports that "It
looks like the race to bio-chips [another term for molecular electronic
systems] has already started. NEC, Hitachi, Toshiba, Matsushita, Fujitsu, Sanyo-Denki and Sharp have commenced
full-scale research efforts on bio-chips for bio-computers." Biochemists
have other reasons to want to learn the art of protein design. New enzymes
promise to perform dirty, expensive chemical processes more cheaply and
cleanly, and novel proteins will offer a whole new spectrum of tools to
biotechnologists. We are already on the road to protein engineering, and as
Kevin Ulmer notes in the quote from Science that heads this chapter, this road
leads "toward a more general capability for molecular engineering which
would allow us to structure matter atom by atom."
Second-Generation Nanotechnology
Despite its
versatility, protein has shortcomings as an engineering material. Protein
machines quit when dried, freeze when chilled, and cook when heated. We do not
build machines of flesh, hair, and gelatin; over the centuries, we have learned
to use our hands of flesh and bone to build machines of wood, ceramic, steel,
and plastic. We will do likewise in the future. We will use protein machines to
build nanomachines of tougher stuff than protein. As nanotechnology moves
beyond reliance on proteins, it will grow more ordinary from an engineer's
point of view. Molecules will be assembled like the components of an erector
set, and well-bonded parts will stay put. Just as ordinary tools can build
ordinary machines from parts, so molecular tools will bond molecules together
to make tiny gears, motors, levers, and casings, and assemble them to make
complex machines. Parts containing only a few atoms will be lumpy, but
engineers can work with lumpy parts if they have smooth bearings to support
them. Conveniently enough, some bonds between atoms make fine bearings; a part
can be mounted by means of a single chemical
bond that will let it turn freely and smoothly. Since a bearing can
be made using only two atoms (and since moving parts need have only a few
atoms), nanomachines can indeed have mechanical components of molecular size.
How will these better machines be built? Over the years, engineers have used
technology to improve technology. They have used metal tools to shape metal
into better tools, and computers to design and program better computers. They
will likewise use protein nanomachines to build better nanomachines. Enzymes
show the way: they assemble large molecules by "grabbing" small
molecules from the water around them, then holding them together so that a bond
forms. Enzymes assemble DNA, RNA, proteins, fats, hormones, and chlorophyll in
this way - indeed, virtually the whole range of molecules found in living
things. Biochemical engineers, then, will construct new enzymes to assemble new
patterns of atoms. For example, they might make an enzyme-like machine which
will add carbon atoms to a small spot, layer on layer. If bonded correctly, the
atoms will build up to form a fine, flexible diamond fiber having
over fifty times as much strength as the same weight of aluminum. Aerospace
companies will line up to buy such fibers by the ton to make advanced
composites. (This shows one small reason why military competition will drive
molecular technology forward, as it has driven so many fields in the past.) But
the great advance will come when protein machines are able to make structures
more complex than mere fibers. These programmable protein machines will
resemble ribosomes programmed by RNA, or the older generation of automated
machine tools programmed by punched tapes. They will open a new world of
possibilities, letting engineers escape the limitations of proteins to build
rugged, compact machines with straightforward designs. Engineered proteins will
split and join molecules as enzymes do. Existing proteins bind a variety of
smaller molecules, using them as chemical tools; newly engineered proteins will
use all these tools and more. Further, organic chemists have shown that
chemical reactions can produce remarkable results even without nanomachines to
guide the molecules. Chemists have no direct control over the tumbling motions
of molecules in a liquid, and so the molecules are free to react in any way
they can, depending on how they bump together. Yet chemists nonetheless coax reacting molecules
to form regular structures such as cubic and dodecahedral molecules, and to
form unlikely-seeming structures such as molecular rings with highly strained
bonds. Molecular machines will have still greater versatility in bondmaking,
because they can use similar molecular motions to make bonds, but can guide
these motions in ways that chemists cannot. Indeed, because chemists cannot yet
direct molecular motions, they can seldom assemble complex molecules according
to specific plans. The largest molecules they can make with specific, complex
patterns are all linear chains. Chemists form these patterns (as in gene
machines) by adding molecules in sequence, one at a time, to a growing chain.
With only one possible bonding site per chain, they can be sure to add the next
piece in the right place. But if a rounded, lumpy molecule has (say) a hundred
hydrogen atoms on its surface, how can chemists split off just one particular
atom (the one five up and three across from the bump on the front) to add
something in its place? Stirring simple chemicals together will seldom do the
job, because small molecules can seldom select specific places to react with a
large molecule. But protein machines will be more choosy. A flexible,
programmable protein machine will grasp a large molecule (the workpiece) while
bringing a small molecule up against it in just the right place. Like an
enzyme, it will then bond the molecules together. By bonding molecule after
molecule to the workpiece, the machine will assemble a larger and larger
structure while keeping complete control of how its atoms are arranged. This is
the key ability that chemists have lacked. Like ribosomes, such nanomachines
can work under the direction of molecular tapes. Unlike ribosomes, they will
handle a wide variety of small molecules (not just amino acids) and will join
them to the workpiece anywhere desired, not just to the end of a chain. Protein
machines will thus combine the splitting and joining abilities of enzymes with
the programmability of ribosomes. But whereas ribosomes can build only the
loose folds of a protein, these protein machines will build small, solid
objects of metal, ceramic, or diamond - invisibly small, but rugged. Where our
fingers of flesh are likely to bruise or burn, we turn to steel tongs. Where
protein machines are likely to crush or disintegrate, we will turn to
nanomachines made of tougher stuff.
Universal
Assemblers
These
second-generation nanomachines - built of more than just proteins - will do all that proteins can do,
and more. In particular, some will serve as improved devices for
assembling molecular structures. Able to tolerate acid or vacuum, freezing or
baking, depending on design, enzyme-like second-generation machines will be
able to use as "tools" almost any of the reactive molecules used by
chemists - but they will wield them with the precision of programmed machines.
They will be able to bond atoms together in virtually any stable pattern,
adding a few at a time to the surface of a workpiece until a complex structure
is complete. Think of such nanomachines as assemblers. Because
assemblers will let us place atoms in almost any reasonable arrangement
(as discussed in the Notes), they will let us build almost anything
that the laws of nature allow to exist. In particular, they will let us build
almost anything we can design - including more assemblers. The consequences of
this will be profound, because our crude tools have let us explore only a small
part of the range of possibilities that natural law permits. Assemblers will
open a world of new technologies. Advances in the technologies of medicine,
space, computation, and production - and warfare - all depend on our ability to
arrange atoms. With assemblers, we will be able to remake our world or destroy
it. So at this point it seems wise to step back and look at the prospect as
clearly as we can, so we can be sure that assemblers and nanotechnology are not
a mere futurological mirage.
Nailing
Down Conclusions
In everything
I have been describing, I have stuck closely to the demonstrated facts of
chemistry and molecular biology. Still, people regularly raise certain
questions rooted in physics and biology. These deserve more direct answers. ° Will
the uncertainty principle of quantum physics make molecular machines
unworkable? This principle states (among other things) that particles can't be
pinned down in an exact location for any length of time. It limits what
molecular machines can do, just as it limits what anything else can do.
Nonetheless, calculations show that the uncertainty principle places few
important limits on how well atoms can be held in place, at least for the
purposes outlined here. The uncertainty principle makes electron positions
quite fuzzy, and in fact this fuzziness determines the very size and structure
of atoms. An atom as a whole, however, has a comparatively definite position
set by its comparatively massive nucleus. If atoms didn't stay put fairly well,
molecules would not exist. One needn't study quantum mechanics to trust these
conclusions, because molecular machines in the cell demonstrate that molecular
machines work. Will the molecular vibrations of heat make molecular machines
unworkable or too unreliable for use? Thermal vibrations will cause greater
problems than will the uncertainty principle, yet here again existing molecular
machines directly demonstrate that molecular machines can work at ordinary
temperatures. Despite thermal vibrations, the DNA-copying machinery in some
cells makes less than one error in 100,000,000,000 operations. To
achieve this accuracy, however, cells use machines (such as the enzyme DNA
polymerase I) that proofread the copy and correct errors. Assemblers may well
need similar error-checking and error-correcting abilities, if they are to
produce reliable results. ° Will radiation disrupt molecular machines and
render them unusable? High-energy radiation can break chemical bonds and
disrupt molecular machines. Living cells once again show that solutions exist:
they operate for years by repairing and replacing
radiation-damaged parts. Because individual machines are so tiny,
however, they present small targets for radiation and are seldom hit. Still, if
a system of nanomachines must be reliable, then it will have to tolerate a
certain amount of damage, and damaged parts must regularly be repaired or
replaced. This approach to reliability is well known to designers of aircraft
and spacecraft. ° Since evolution has failed
to produce assemblers, does this show that they are either impossible or
useless? The earlier questions were answered in part by pointing to the working
molecular machinery of cells. This makes a simple and powerful case that
natural law permits small clusters of atoms to behave as controlled machines,
able to build other nanomachines. Yet despite their basic resemblance to
ribosomes, assemblers will differ from anything found in cells; the things they
do - while consisting of ordinary molecular motions and reactions - will have
novel results. No cell, for example, makes diamond fiber. The idea that new
kinds of nanomachinery will bring new, useful abilities may seem startling: in
all its billions of years of evolution, life has never abandoned its
basic reliance on protein machines. Does this suggest that
improvements are impossible, though? Evolution progresses through small
changes, and evolution of DNA cannot easily replace DNA. Since the
DNA/RNA/ribosome system is specialized to make proteins, life has had no real
opportunity to evolve an alternative. Any production manager can well
appreciate the reasons; even more than a factory, life cannot afford to shut
down to replace its old systems. Improved molecular machinery should no more
surprise us than alloy steel being ten times stronger than bone, or copper
wires transmitting signals a million times faster than nerves. Cars outspeed
cheetahs, jets outfly falcons, and computers already outcalculate
head-scratching humans. The future will bring further examples of improvements
on biological evolution, of which second-generation nanomachines will be but
one. In physical terms, it is clear enough why advanced assemblers will be able
to do more than existing protein machines. They will be programmable like
ribosomes, but they will be able to use a wider range of tools than all the
enzymes in a cell put together. Because they will be made of materials far more
strong, stiff, and stable than proteins, they will be able to exert greater
forces, move with greater precision, and endure harsher conditions. Like an
industrial robot arm - but unlike anything in a living cell - they will be able
to rotate and move molecules in three dimensions under programmed control,
making possible the precise assembly of complex objects. These advantages will
enable them to assemble a far wider range of molecular structures than living
cells have done. ° Is there some special magic about life, essential to making
molecular machinery work? One might doubt that artificial nanomachines could
even equal the abilities of nanomachines in the cell, if there were reason to
think that cells contained some special magic that makes them work. This idea
is called "vitalism."
Biologists have abandoned it because they have found chemical and physical
explanations for every aspect of living cells yet studied, including their
motion, growth, and reproduction. Indeed, this knowledge is the very foundation
of biotechnology. Nanomachines floating in sterile test tubes, free of cells,
have been made to perform all the basic sorts of activities that they perform
inside living cells. Starting with chemicals that can be made from smoggy air,
biochemists have built working protein machines without help from cells. R. B. Merrifield, for example,
used chemical techniques to assemble simple amino acids to make
bovine pancreatic ribonuclease, an
enzymatic device that disassembles RNA molecules. Life is special in structure,
in behavior, and in what it feels like from the inside to be alive, yet the
laws of nature that govern the machinery of life also govern the rest of the
universe. ° The case for the feasibility of assemblers and other nanomachines
may sound firm, but why not just wait and see whether they can be developed?
Sheer curiosity seems reason enough to examine the possibilities opened by
nanotechnology, but there are stronger reasons. These developments will sweep
the world within ten to fifty years - that is, within the expected lifetimes of
ourselves or our families. What is more, the conclusions of the following
chapters suggest that a wait-and-see policy would be very expensive - that it
would cost many millions of lives, and perhaps end life on Earth. Is the case
for the feasibility of nanotechnology and assemblers firm enough that they
should be taken seriously? It seems so, because the heart of the case rests on
two well-established facts of science and engineering. These are (1) that
existing molecular machines serve a range of basic functions, and (2) that
parts serving these basic functions can be combined to build complex machines.
Since chemical reactions can bond atoms together in diverse ways, and since
molecular machines can direct chemical reactions according to programmed
instructions, assemblers definitely are feasible.
Nanocomputers
Assemblers will bring one breakthrough
of obvious and basic importance: engineers will use them to shrink the size and cost
of computer circuits and speed their operation by enormous factors. With
today's bulk technology, engineers make patterns on silicon chips by throwing
atoms and photons at them, but the patterns remain flat and molecular-scale
flaws are unavoidable. With assemblers, however, engineers will build circuits
in three dimensions, and build to atomic precision. The exact limits of
electronic technology today remain uncertain because the quantum behavior of
electrons in complex networks of tiny structures presents complex problems,
some of them resulting directly from the uncertainty principle. Whatever the
limits are, though, they will be reached with the help of assemblers. The
fastest computers will use electronic effects, but the smallest may not. This
may seem odd, yet the essence of computation has nothing to do with
electronics. A digital computer is a collection of switches able to turn one
another on and off. Its switches start in one pattern (perhaps representing 2 +
2), then switch one another into a new pattern (representing 4), and so on.
Such patterns can represent almost anything. Engineers build computers from
tiny electrical switches connected by wires simply because mechanical switches
connected by rods or strings would be big, slow, unreliable, and expensive,
today. The idea of a purely mechanical computer is scarcely new. In England
during the mid-1800s, Charles Babbage invented a
mechanical computer built of brass gears; his co-worker Augusta Ada, the Countess of
Lovelace, invented computer programming. Babbage's endless
redesigning of the machine, problems with accurate manufacturing, and
opposition from budget-watching critics (some doubting the usefulness of
computers!), combined to prevent its completion. In this tradition, Danny
Hillis and Brian Silverman of the MIT Artificial Intelligence
Laboratory built a special-purpose mechanical computer able to play
tic-tac-toe. Yards on a side, full of rotating shafts and movable frames that
represent the state of the board and the strategy of the game, it now stands in
the Computer Museum in Boston. It looks much like a large
ball-and-stick molecular model, for it is built of Tinkertoys. Brass gears and
Tinkertoys make for big, slow computers. With components a few atoms wide,
though, a simple mechanical computer would fit within 1/100 of a cubic micron,
many billions of times more compact than today's so-called microelectronics.
Even with a billion bytes of storage, a
nanomechanical computer could fit in a box a micron wide, about the
size of a bacterium. And it would be fast. Although mechanical signals
move about 100,000 times slower than the electrical signals in today's
machines, they will need to travel only 1/1,000,000 as far, and thus will face
less delay. So a mere mechanical computer will work faster than the electronic
whirl-winds of today. Electronic nanocomputers will likely be thousands of
times faster than electronic microcomputers - perhaps hundreds of thousands of
times faster, if a scheme proposed by Nobel
Prize-winning physicist Richard Feynman works out. Increased speed
through decreased size is an old story in electronics.
Disassemblers
Molecular
computers will control molecular assemblers, providing the swift flow of
instructions needed to direct the placement of vast numbers of atoms.
Nanocomputers with molecular memory devices will also store data generated by a
process that is the opposite of assembly.
Assemblers
will help engineers synthesize things; their relatives, disassemblers, will
help scientists and engineers analyze things.
The case for
assemblers rests on the ability of enzymes and chemical reactions to form
bonds, and of machines to control the process. The case for disassemblers rests
on the ability of enzymes and chemical reactions to break bonds, and of
machines to control the process. Enzymes, acids, oxidizers, alkali metals, ions, and reactive
groups of atoms called free radicals - all
can break bonds and remove groups of atoms.
Because
nothing is absolutely immune to corrosion, it seems that molecular tools will
be able to take anything apart, a few atoms at a time.
What is more,
a nanomachine could (at need or convenience) apply mechanical force as well, in
effect prying groups of atoms free.
A nanomachine
able to do this, while recording what it removes layer by layer, is a disassembler.
Assemblers, disassemblers, and nanocomputers will work together.
For example, a
nanocomputer system
will be able to direct the disassembly of an object, record its structure, and
then direct the assembly of perfect copies, And this gives some hint of the
power of nanotechnology.
The World Made New
Assemblers
will take years to emerge, but their emergence seems almost inevitable: Though
the path to assemblers has many steps, each step will bring the next in reach,
and each will bring immediate rewards. The first steps have already been taken,
under the names of "genetic engineering" and
"biotechnology." Other paths to assemblers seem possible. Barring
worldwide destruction or worldwide controls, the technology race will continue
whether we wish it or not. And as advances in computer-aided design speed the
development of molecular tools, the advance toward assemblers will quicken. To
have any hope of understanding our future, we must understand the consequences
of assemblers, disassemblers, and nanocomputers. They promise to bring changes
as profound as the industrial revolution, antibiotics, and nuclear weapons all
rolled up in one massive breakthrough. To understand a future of such profound
change, it makes sense to seek principles of change that have survived the
greatest upheavals of the past. They will prove a useful guide.
|
|
|