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.
4.10.2 Merkle Cased Hydrocarbon Assembler (1998-2000)
After abandoning his 1995-1997 “replicating brick” assembler design (see Section 4.11.2) because of several inconvenient features, in 1998 Merkle began work on a more technically detailed third assembler design  which he called a “cased hydrocarbon assembler.” Reminiscent of peas-in-a-pod (Figure 4.37), Merkle’s new design included an assembler device that floats in a solution feedstock and is completely enclosed within a flexible cylindrical graphene casing, allowing the gaseous interior environment to be kept separate from the liquid exterior environment. During a single replication cycle, the parent assembler device first inflates its casing, builds two copies of itself inside the casing, then ruptures the original casing (which is thrown away), pushing or releasing the two daughters out into the exterior environment. When fully unfurled, the assembler device is 200 nm in diameter and 1000 nm in length. The parent is destroyed during replication, but the assemblers increase their numbers during each replicative cycle and the number of daughters that each parent produces per generation can be made larger than two* to modestly improve replicative efficiency.
* C. Phoenix  observes that the maximum efficiency of this replicative strategy cannot be significantly increased because the optimum number of daughters per generation appears to be approximately three. A complete quantitative discussion is in Section 5.9.2.
Because evacuated single-walled carbon nanotubes with a radius exceeding ~2.7 nm are more stable when collapsed  and because the graphene casing of Merkle’s exemplar assembler is 100 nm in radius, it is necessary to pressurize the casing to prevent this collapse. Inert neon gas pressurized to 50-600 atm should be sufficient to inflate but not rupture the casing, but makes a near-liquid internal environment since the compressed neon will be 4-40% the density of liquid water according to van der Waals’ equation . During construction, the casings of inactive daughter devices are built rolled-up (like a sleeping bag). The volume of each rolled-up casing is 210,000 nm3, or just 0.7% of the 0.031 micron3 volume of the fully inflated casing – hence both daughter devices can easily fit inside the parent. The interior manufacturing systems are much the same as in the replicating brick proposal (Section 4.11.2), except that in the cased assembler proposal Merkle refers to “robot arm” manipulators and cites both Drexler’s telescoping molecular manipulator arm and his own Stewart platform designs as examples, but without specifying which is to be used in this design.
Both control signals and mechanical power enter the device through two large (~104 nm3) externally-mounted pistons, rather than the few dozen internally-positioned pistons in Merkle’s previous replicating brick design. This eliminates the need for pressure equilibration (between internal and external environment) evident in Merkle’s replicating brick design – allowing high internal pressurization which prevents casing collapse – but adds a requirement for two new subsystems. First, a signal demultiplexor is needed to convert the force from two pistons into corresponding force on any one of several molecular control cables. Second, a system of interior cables to carry power and control signals is needed, which must be strung in a way which does not interfere with assembler operation. The cables are needed because in the cased assembler architecture, the source of power is physically separated from the manufacturing mechanisms, whereas in the replicating brick architecture the acoustic power is broadcast throughout the gas-filled interior of the device and thus is directly available to all interior mechanisms able to receive these signals. The cables are envisioned as a flexible rod inside a flexible sheath analogous to a bicycle derailleur cable, as for example a polyyne rod encased in a (9,0) carbon nanotube sheath. Pulling on the rod while pushing on the sheath produces a force that can be transmitted along the length of the cable.
The two pistons, each measuring 12 nm in radius and 20 nm in length, have different threshold actuation pressures. This allows either piston to be selectively addressed by adjusting the pressure of the feedstock solution. The pistons are operated at 10 MHz using a two-band signaling approach, one band for each piston. In this design, the lower pressure piston cycles the demultiplexor through its possible outputs (selecting which among the many output lines is to be driven), while the higher pressure piston drives the currently selected output, allowing the outputs of the demultiplexor to be selected and driven in any desired sequence using only two pistons. An operating pressure of 2 atm, with excursions to 1 atm to operate the low pressure piston and excursions to 3 atm to operate the high pressure piston, should suffice for reliable operation. The maximum pressure is significantly smaller than that proposed for the Drexler minimal assembler (Section 4.11.1) for two major reasons. First, by adopting a two-band signaling system (instead of Drexler’s many-band signaling system) the number of pressure ranges that must be accommodated is reduced from many (perhaps 20 or more) to only two. Second, by reducing the number of pistons and locating them externally, their size and hence their sensitivity to pressure changes is increased. Fewer narrower pressure ranges reduces the maximum pressure that is required. The cased hydrocarbon assembler is composed of ~109 atoms. Assuming that on average the addition of one atom to an assembler will require 1000 pressure cycles, then a total of 1012 pressure cycles are needed to complete one replication cycle. At 10 MHz, this is ~28 hours.
Reviewing the rationale for his design efforts , Merkle explained: “Today, we are unable to build any assembler. We are, therefore, searching among the space of simpler designs for systems which might be easier to make. The present design is not among the simplest because it is constrained by the requirement that it be able to fabricate stiff hydrocarbons. Designs that might be appropriate targets for direct synthesis with existing technologies are unlikely to be able to make diamond, but will probably work with molecular building blocks whose assembly is simpler and which impose fewer constraints on the environment. The present design is neither the next experimental target nor the final destination. It is a way point along the route from present capabilities to the future.”
Last updated on 1 August 2005