Kinematic Self-Replicating Machines
© 2004 Robert A. Freitas Jr. and Ralph C. Merkle. All Rights Reserved.
Robert A. Freitas Jr., Ralph C. Merkle, Kinematic Self-Replicating Machines, Landes Bioscience, Georgetown, TX, 2004.
4.9.2 Drexler Extruding Tube Assembler (1991)
In the “unit replication” or organismic model, the design most faithful to von Neumann’s original kinematic concept, each replicator is an independent unit which employs the surrounding substrate to directly produce an identical copy of itself. Both the original and the copy remain fertile and may replicate again, thus exponentiating their numbers. Figure 4.30 shows the unit replicator design described informally by Drexler et al , taken from a Stanford University course (CS 404) that Drexler taught in the spring of 1988. Drexler’s unit replicator design is a two-armed mechanical molecular assembler in an extruding tube architecture ~200 nm in diameter and ~650 nm in length, having a total molecular volume of ~20,000,000 nm3 (equivalent to a cube ~270 nm on an edge), containing ~1 billion atoms with a molecular weight of ~10 gigadaltons – very roughly the mass and scale of a very small bacterium (Section 5.3). For comparison, the average enzyme (a biochemical crimping tool) weighs perhaps ~0.1 megadalton, while a typical ribosome (a primitive “protein assembler”) weighs 2.5-4.2 megadaltons (Section 4.2). Both parent and daughter machines are tubular in shape, and the daughter extrudes axially from the parent.
Drexler’s unit replicator receives its control signals from an onboard nanocomputer , which can accept stored instructions that are sequentially executed to direct the manipulator arm to place the correct moiety or nanopart in the desired position and orientation, thus giving precise control over the timing and locations of chemical reactions or assembly operations. Most of the interior of Drexler’s device is taken up by a tape memory system that tells how to move the arm to build all the parts of the replicator, except the tape itself. The tape gets made by a special tape-copying machine. At the right-hand end of the replicator are pores for bringing in fuel and raw-material molecules, and machinery for processing them. The replicator would be able to build copies of itself when supplied with fuel and raw materials. In the middle are computer-controlled arms that do most of the actual construction. In Figure 4.30, (A) contains a nanocomputer, (B) a library of stored instructions, (C) contains machinery that takes in fuel and produces electric power, (D) is a motor, and (E) contains machinery that prepares raw materials for use. The lower diagrams illustrate various stages in a replication cycle, showing how the working space is kept isolated from the external liquid which provides the needed fuel and raw-material molecules.
The steps in the replication cycle – using a copy to block the tube, beginning a fresh copy, then releasing the old one – show one way that a machine could build a copy of itself while floating in a liquid, yet doing all of its construction work inside itself, in a better-controlled vacuum environment. Since the replicator contains about a billion atoms and each arm can handle about a million atoms per second, the whole construction cycle could be completed in less than 15 minutes. At that rate, one device can double and double again to make trillions in little more than ten hours. The replicators sit in a special chemical bath, absorbing what they need and making more replicators. Eventually, either the special chemicals run out or new chemicals are added to signal the replicator population to do something else. At that point, as suggested schematically by Figure 3.39, they can be reprogrammed for a production phase that will allow them to produce any other product that is within their physical capacity to manufacture, so long as it can be extruded from the front. These products may be long, and can unfold or be pieced together to make larger objects, so the small-bacterium size of the initial replicators need only be a temporary limitation.
Drexler  notes that pure replicators of this sort “are useful as thought experiments to show how nanomachines can product more nanomachines, but specialized manufacturing equipment would be more efficient in practice.”
Last updated on 1 August 2005