For centuries, humans watched birds flying and wished they could do the same. However, today's airplanes look and function very differently from birds. We have learned that rigid wings, engines, and propellers or turbines can be used to build much larger and faster flying machines than nature ever created. Still, it was the birds that showed us it was possible.
In the same way, biology shows us that it is possible to build molecular machines--constructions of atoms that can perform intricate and useful operations at the atomic scale. An example of a machine in biology is the flagellar motor: an arrangement of protein molecules that turns chemical energy into rotating motion to help bacteria swim. Another example is the ribosome, which is made of protein and another molecule type called RNA, and is used by all living cells to make more protein by sticking amino acids together in a pattern specified by yet more molecules of RNA.
Protein, the building block of biology, has some significant limitations. A steel knife can cut even the toughest steak. Just as airplanes of aluminum and steel can fly faster than birds, a molecular machine, or nanobot, made out of stronger and stiffer materials such as diamond should be able to do things that biology cannot. Can we build such a thing? The answer is a qualified Yes--qualified only by the fact that we haven't done it yet.
In 1980, the scanning tunneling microscope (STM), which can make pictures of individual atoms, had not yet been invented. In 1990, buckytubes (incredibly strong rolled-up sheets of graphite) had not yet been discovered. Here in 2001, STMs have been used not only to make pictures of the atoms in buckytubes, but to cause a variety of chemical reactions at specific points in molecules. Buckytubes themselves, in addition to being incredibly strong, have been used to build electronic circuits. Researchers have recently developed a process for making three-dimensional plastic shapes with a resolution of 120 nanometers (nm), about 600 atoms wide, and several different techniques exist for making 2-D structures even smaller than that. Several trends indicate that by the end of this decade, we will be able to build small intricate structures while controlling the placement of each atom.
One use for small intricate structures is as machine parts. Soon after we create molecular gears and motors, we will be able to assemble them into robots ("nanomachines" or "nanobots"). A nanobot, for the purposes of this chapter, is a robot with parts of atomic scale and precision. A typical nanobot, smaller than a cell, may contain billions of atoms. But nanobots need not be small. Sometimes it's useful to fit a lot of functionality into a single device. Some nanobots may have structural application, such as reinforcing bone or replacing muscle--such devices could be quite large. The important point about a nanobot is that it can make efficient use of space, with functionality as densely packed as a bacterium.
Building structures one atom at a time will be very slow. There are 10,000,000,000,000,000,000,000 atoms in a one-carat diamond, and many times that in a potato. Every molecule in the potato was built one atom at a time (many of them by ribosomes). How can potatoes grow so fast? They contain trillions of cells, and each cell contains hundreds of ribosomes and thousands of enzymes dedicated to sticking molecules together. A simple bacterial cell can make a complete copy of itself in as little as fifteen minutes, and then both copies can duplicate themselves, and so on; one trillion cells requires only 40 replication cycles. A potato cell is more complicated than a bacterium, but with cells working in parallel to create more cells, a potato can grow in just a few weeks at very low cost.
If biology can create self-replicating devices, why can't humans design them? The answer, again speculative-but-likely, is that we can. A self-contained factory made with today's macroscopic (large-sized) technology, that can make a copy of itself from simple materials with no outside help, might be as small as 100 tons(see references). On the macroscopic scale, it's easier and cheaper to distribute the manufacturing processes and put some humans in the loop, since our hands and brains are far more capable than any robot. Nanoscale factories must be self-contained, since human hands cannot directly manipulate nanoscale objects. However, there are advantages to working with individual atoms and molecules. There are only a few dozen types of atoms that need to be manipulated to build almost any desired nanobot; the vast array of manufacturing techniques that would be needed to create a macroscopic factory from scratch will be unnecessary. Also, macroscopic parts have flaws and inaccuracies; nanoscale parts, being essentially large molecules, will be precisely identical and thus predictable enough for automated assembly. A self-replicating factory has the potential to make large quantities of product at extremely low cost.
Thus, although we have not yet built a self-replicating diamond nanobot, this chapter assumes that such a thing will exist within a few decades. Many medical problems may be solved without the use of fully-developed nanotech; the intent of this chapter is to show that nanotech can solve the rest of them, thus allowing a lifespan unlimited by disease.
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