Self Replication and Nanotechnology
Source: Nanotechnology Industries A crucial objective of nanotechnology is the ability to
make products inexpensively. While the ability to make a few very
small, very precise molecular machines very expensively would clearly
be a major scientific achievement, it would not fundamentally change
how we make most products. Fortunately, we are surrounded and inspired
by products that are
marvelously complex and yet very inexpensive. Potatoes, for example,
are made by intricate molecular machines involving tens of thousands
of genes, proteins, and other molecular components; yet the result
costs so little that we think nothing of mashing this biological
wonder and eating it.
It's easy to see why potatoes and other agricultural
products are so cheap: put a potato in a little moist dirt, provide it
with some air and sunlight, and we get more potatoes. In short,
potatoes are self replicating.
Just as the early pioneers of flight took inspiration by
watching birds soar effortlessly through the air, so we can take
inspiration from nature as we develop molecular manufacturing systems.
Of course, "inspired by" does not mean "copied from." Airplanes are
very different from birds: a 747 bears only the smallest resemblance
to a duck even though both fly. The artificial self replicating
systems that have been envisioned for molecular manufacturing bear
about the same degree of similarity to their biological counterparts
as a car
might bear to a horse.
Horses and cars both provide transportation. Horses,
however, can get their energy from potatoes, corn, sugar, hay, straw,
grass, and countless other types of "fuel." A car uses only a single
artifical and carefully refined source of energy: gasoline. Putting
sugar or straw into its gas tank is not recommended!
The machines that people make tend to be inflexible and
brittle in response to changes in their environments. By contrast,
living biological systems are wonderfully flexible and adaptable.
Horses can pick their way along a narrow trail or jump over shrubs;
they get "parts" (from their food) in the same flexible way they get
energy; and they have a remarkable self repair ability.
Cars, on the other hand, need roads on which to travel;
have to be provided with odd and very unnatural parts; are often
difficult to repair (let alone self repairing!); and in general are
simply unable to cope with a complex environment. They work because we
want them to work, and because we can fairly inexpensively provide
carefully controlled conditions under which they can perform as we
desire.
In the same way, the artifical self replicating systems
that are being proposed for molecular manufacturing are inflexible and
brittle. It's difficult enough to design a system able to self
replicate in a controlled environment, let alone designing one that
can approach the marvelous adaptibility that hundreds of millions of
years of evolution have given to living systems. Designing a system
that uses a single source of energy is both much easier to do and
produces a much more efficient system: the horse pays for its ability
to eat potatoes when
grass isn't available by being less efficient at both. For artificial
systems where we wish to decrease design complexity and increase
efficiency, we'll design the system so that it can handle one source
of energy, and handle that one source very well.
Horses can manufacture the many complex proteins and
molecules they need from whatever food happens to be around. Again,
they pay for this flexibility by having an intricate digestive system
able to break down food into its constituent molecules, and a complex
intermediary metabolism able to synthesize whatever they need from
whatever they've got. Artificial self replicating systems will be both
simpler and more
efficient if most of this burden is off-loaded: we can give them the
odd compounds and unnatural molecular structures that they require in
an artifical "feedstock" rather than forcing the device to make
everything itself -- a process that is both less efficient and more
complex to design.
The mechanical designs proposed for nanotechnology are
more reminiscent of a factory than of a living system. Molecular scale
robotic arms able to move and position molecular parts would assemble
rather rigid molecular products using methods more familiar to a
machine shop than the complex brew of chemicals found in a cell.
Although we are inspired by living systems, the actual designs are
likely to owe more to design constraints and our human objectives than
to living systems. Self replication is but one of many abilities that
living systems exhibit. Copying that one ability in an artificial
system will be challenge enough without attempting to emulate their
many other remarkable abilities.
Complexity of self replicating systems
If our designs are going to be very different from the
living systems that inspired us, what approach are we going to follow?
The study of artificial self replicating systems was first pursued by
von Neumann in the 1940's. Subsequent work, including a study by NASA
in 1980, confirmed and extended the basic insights of von Neumann.
More recent work by Drexler continued this trend and applied the
concepts to molecular scale systems. The author has also contributed a
few articles, including: Self Replicating Systems and Low Cost
Manufacturing, Self Replicating Systems and Molecular Manufacturing
and Design Considerations for an Assembler. (A web page on artificial
self replication maintained by Moshe Sipper has links to and
information on other references). One conclusion from this body of
work is that the design complexity of artificial self replicating
systems need not be excessive. One of the simplest "self replicating
systems" (when executed, it prints itself out on the standard output)
is the following one line C program:
main(){char q=34,n=10,*a="main(){char
q=34,n=10,*a=%c%s%c;printf(a,q,a,q,n);}%c";printf(a,q,a,q,n);}
(From Self-reproducing programs, Byte magazine, August 1980, page 74.
Those interested in a deeper understanding of the recursion theorem
and its applications are referred to Introduction to the Theory of
Computation by Michael Sipser, 1996, PWS Publishing Company, chapter
6.)
The following table illustrates the design complexity of
several other systems:
The estimate of the complexity of the internet worm is
simply an approximation to the number of bits in the C source code.
For the biological systems, the complexity is derived by multiplying
the number of base pairs in the DNA times 2. For humans, the number of
base pairs is for the haploid, rather than diploid, system. The
complexity for the the NASA proposal was taken from Advanced
Automation for Space Missions.
Mycoplasma genitalium is the simplest natural living system that can
survive on a well defined chemical medium. Its genomic complexity of
1,160,140 bits (twice the 580,070 base pairs sequenced by TIGR) is
less than 150 kilobytes -- about one tenth of a typical floppy disk.
TIGR is pursuing the Minimal Genome Project to reduce to a minimum the
number of genes required for a simple living system. (While viruses
are simpler they require a living system to infect: they need
additional molecular machinery provided in their environment. For this
reason, we exclude them from the table).
The primary observation to be drawn from this data is
that simpler designs and proposals for self replicating systems both
exist and are well within current design capabilities. The engineering
effort required to design systems of such complexity will be
significant, but should not be greater than the complexity involved in
the design of such existing systems as computers, airplanes, etc. A
recent proposal
is "Exponential growth of large self-reproducing machine systems," by
Klaus S. Lackner and C. H. Wendt, Mathl. Comput. Modelling Vol. 21,
No. 10, pages 55-81, 1995.
One last point: self replication is used here as a means
to an end, not as an end in itself. A system able to make copies of
itself but unable to make much of anything else would not be very
useful and would not satisfy our objectives. The purpose of self
replication in the context of manufacturing is to permit the low cost
replication of a flexible and programmable manufacturing system -- a
system which can be reprogrammed to make a very wide range of
molecularly precise
structures. This lets us economically build a very wide range of
products.
Systems that function in a complex environment
If artificial self replicating systems will only
function in carefully controlled artificial environments, how can we
develop applications of nanotechnology that function in complex
environments, such as the inside of the human body or a (rather messy)
factory floor? While self replicating systems are the key to low cost,
there is no need (and little desire) to have such systems function in
the outside world. Instead, in an artificial and controlled
environment they can manufacture simpler and more rugged systems that
can then be transferred to their final destination. Medical devices
designed to operate in the human body don't have to self replicate: we
can manufacture them in a controlled environment and then inject them
into
the patient as needed. The resulting medical device will be simpler,
smaller, more efficient and more precisely designed for the task at
hand than a device designed to perform the same function and self
replicate. This conclusion should hold generally: optimize device
design for the desired function, manufacture the device in an
environment optimized for manufacturing, then transport the device
from the manufacturing environment to the environment for which it was
designed. A single device able to do everything would be harder to
design and less efficient.
Conclusions
Self replication is an effective route to truly low cost
manufacturing. Our intuitions about self replicating systems, learned
from the biological systems that surround us, are likely to seriously
mislead us about the properties and characteristics of artificial self
replicating systems designed for manufacturing purposes. Artificial
systems able to make a wide range of non-biological products (like
diamond) under programmatic control are likely to be more brittle and
less adaptable in their response to changes in their environment than
biological systems. At the same time, they should be simpler and
easier to design. The complexity of such systems need not be excessive
by present engineering standards.
http://onward.to/inventions/
Complexity of self replicating systems (bits)
Von Neumann's universal constructor ~500,000
Internet worm (Robert Morris, Jr., 1988) ~500,000
Mycoplasma genitalium 1,160,140
E. Coli
9,278,442
Drexler's assembler ~100,000,000
Human ~6,400,000,000
NASA Lunar Manufacturing Facility over 100,000,000,000