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.
5.4 Efficient Replicator Scaling Conjecture
The first simple molecular assembler is likely to be a macroscale device, perhaps a modified SPM system as was being pursued by Zyvex starting in 1998 (Section 4.14). Multiple SPM heads could be equipped with a small number of nanoscale tool tips. In some scenarios, nanoparts fabricated using either bulk chemistry or positional mechanosynthesis techniques would be inspected and selected by the SPM, then assembled one by one into working nanomachines (e.g., the desired useful nanoscale products). Such assembly operations will be very slow, because the placement of each new component may require simultaneous rotations and translations of large macroscale SPM components. Assembly time may scale roughly linearly with assembler size  because smaller assembler components moving at a given velocity need to travel less distance to accomplish a given physical operation, consuming less time and energy per physical operation. Hence an important early developmental goal would then be to design and fabricate nanoscale molecular assemblers whose manipulatory components are closer in size to the scale of the parts which must be assembled.
In 1996, Freitas  informally proposed the “Efficient Replicator Scaling Conjecture” which holds that “the most efficient replicator will operate on a substrate consisting of parts that are approximately of the same size scale as the parts with which it is itself constructed. Hence a robot made of ~1 cm parts will operate most efficiently in an environment in which ~cm-scale parts (of appropriate types) are presented to it for assembly. Such a robot would be less efficient if it was forced to build itself out of millimeter or micron-scale parts, since the robot would have to preassemble these smaller parts into the 1-cm parts it needed for the final assembly process. Similarly, input parts much larger than 1 cm would have to be disassembled or milled down to the proper size before they could be used, consuming additional time, information, and physical resources – also reducing replicative efficiency. If this conjecture is correct, then it follows that to most efficiently replicate from an atomic or molecular substrate, you would want to use atomic or molecular-scale parts – that is, nanotechnology.”
This conjecture seems broadly consistent with Drexler’s scaling analysis of an exemplar manufacturing system architecture using 10 stages of convergent assembly in which product scale is commensurate with mechanism scale at all but the earliest stages of input ordering and reagent preparation involving simple molecular inputs (Nanosystems , Table 14.1; see also Section 4.9.3). It also seems consistent with Merkle’s analysis of the convergent assembly approach to molecular manufacturing (Section 5.9.4) using a progression of manufacturing stages at different scales . As Fearing  noted in connection with the efficient construction of microdevices: “There are many advantages to shrinking robots and mechanical actuators to the same size as the parts to be manipulated.....”
Last updated on 1 August 2005