The ability to manipulate matter on the scale of individual atoms, by means of microscopic machines known as assemblers, is the field of nanotechnology.
The power of nanotechnology is ultimately limited by the laws of thermodynamics. The increase of entropy means that every time a bond is made or broken, or a molecular switch changes state, or a motor powers a gear system, waste heat is produced. Even worse, systems in thermal equilibrium are continuously vibrating, and atoms are jumping out their assigned lattice sites. This means that molecular systems must have a complex set of self-repair systems, and that even the most carefully engineered materials contain flaws.
Almost all nanomachines built thus far have been made to depend on certain complex globular proteins, which do not occur naturally, for certain stages of replication or dependent on laser light of a narrow frequency range as a power supply (the system used is dependent on the operating conditions). The proteins or laser power supply are always present in the manufacturing cell, but on escape the assemblers are unable to replicate.
An assembler is a molecular-scale robot arm which is able to hold and position reactive atoms and molecules so that they react at precisely determined locations to build or dismantle a structure a few atoms at a time. A typical assembler is built of about a million atoms, making up an arm that is roughly one hundred atoms long, together with devices to modify the tool mounted on the arm, a control system that can interpret instructions from a nanocomputer and a receiver for accepting molecules from a conveyor system. An average assembler arm is capable of positioning approximately a million atoms per second.
Systems consisting of assemblers, nanocomputers, storage devices, chemical processors and conveyors which can build copies of themselves without external help are known as replicators. A typical replicator has several assembler arms mounted on a molecular framework controlled by a single nanocomputer. Such a device consists of about 200 million atoms, intermediate in size between an organelle and a cell.
The time needed for a replicator depends on its supply of molecular feedstock. Floating in a nutrient solution, or in the bloodstream, a replicator can build a copy of itself in about five minutes, which is several times faster than the replication of a bacterium under optimal conditions. Given a more efficient source, such as a conveyor network connected to an ordered reservoir, this time can be reduced to as little as 30 seconds.
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