4.11.3.4

Kinematic Self-Replicating Machines

Robert A. Freitas Jr., Ralph C. Merkle, Kinematic Self-Replicating Machines, Landes Bioscience, Georgetown, TX, 2004.

4.11.3.4 Hydrocarbon Assembler Subsystems

The Merkle-Freitas Hydrocarbon Molecular Assembler consists of 12 major subsystems, as illustrated schematically in Figure 4.45, quantified in Table 4.2, and summarized below:

(1) Acetylene Binding Site and Import Mechanism. The acetylene binding site consists of a (9,0) carbon nanotube of open inside diameter 0.7 nm and 2 nm in length, long enough to contain up to 4 molecules of acetylene. The binding site is embedded in a diamond block drawer that can be mechanically slid back and forth through a rectangular hole in the I/O (input-output) wall of the hull while maintaining a tight seal. Once conveyed inside, an acetylene molecule is forced into the receiving chamber by a polyyne rod inserted into the binding site. A di-silicon radical (or other group IV element, possibly Sn [2322] or Ge [2325]; further modeling work is required to validate a specific molecular tool) aligned to the acetylene molecule inside the receiving chamber binds the acetylene covalently, whereupon the captured feedstock molecule can be safely transported elsewhere inside the device for further mechanosynthetic processing.

(2) Vitamin Binding Site and Import Mechanism. When product objects consisting entirely of carbon and hydrogen atoms are to be fabricated, no vitamin imports are required during the manufacturing cycle. However, during the fabrication of a daughter device (e.g., self-replication) it is necessary to import 30 atoms each of Si and Sn (or other group IV element) so that the mechanosynthetic tool tips of the daughter may be constructed by the assembler. Chemically inert vitamin molecules consisting of an easily disassembled Si- and Sn-containing hydrocarbon complex are added at a parts-per-billion trace concentration to the external solvent fluid, but only a dozen of these molecules need be imported during the entire 10⁶-sec replication cycle of the device.

(3) Octane Assembly Site and Export Mechanism. n-Octane molecules are mechanosynthesized by combining excess hydrogen atoms with acetylenic precursors in a narrow hydrogen-passivated reaction bay that opens to the external solvent through an exhaust tube (terminating at the I/O wall) just beyond a narrow throat. After final synthesis and release of the n-octane molecule, a carbene rod is inserted into the bay and irreversibly forces the n-octane molecule through the resistive energy barrier at the throat and into the exhaust tube.

(4) Acoustic Transducer. Externally generated pressure pulses between 1-3 atm drive the acoustic transducer through a 15 nm throw, producing a linear rod motion that provides information and mechanical power to the assembler device. The transducer has 3 components: (A) a piston plate (a hydrogen-terminated 10-nm thick rectangular diamond block with 45^o-clipped corners to accommodate the legs of the van der Waals cage; see (11) below); (B) a piston rod 4-nm thick attached to the center of the piston plate and extending back through the elevator plate to engage the control chain mechanism that is attached to the other side of the elevator plate, in the assembler interior; and (C) the piston spring, attached to the exterior face of the elevator plate at one end and to the piston rod at the other end, to provide a restoring force. During replication, the daughter device is manufactured with its piston mechanically locked; slowly cycling through 4 atm unlocks this piston, synchronizing daughter(s) and parent (whose piston is already unlocked) in preparation for the next replicative cycle.

(5) X-Axis Elevator. The 10-nm thick diamond elevator block permits motion of the entire piston-manipulator assembly along the X-axis of the assembler. Movement of the elevator is achieved by four capstan-driven worm screws that engage a linear sequence of shallow slots on the inner face of the hull wall. The control chain mechanism and Stewart platform struts are attached to the interior face of the elevator plate.

(6) Stewart Platform Manipulators. All mechanical manipulations in the interior of the device are performed by either or both of a pair of Stewart platform manipulators [208, 2321]. Each manipulator consists of a tool chuck mounted via universal joints to 6 telescoping struts mounted at their base to universal joints affixed to the interior face of the elevator block. Each strut is a matched pair of internally-threaded and externally-threaded tubes, whose capstan-driven relative helical motion extends or retracts the strut. An additional 7th control strut allows interchangeable tool tips to be grasped or released by the tool chuck.

(7) Control Chain. The control chain consists of a single flexible molecular ribbon with a number of graphitic knobs integrally affixed to known positions along its length. A latchlike mechanism at the end of the piston rod engages the chain, converting piston-driven axial rod motion to transverse chain motion and forcing the chain to move. All normal operations use two pressure bands – an upper band from 2-3 atm and a lower band from 1-2 atm. The chain engages latching mechanisms at the control capstans of all elevator worm screws and Stewart platform struts. Cycling across the upper band steps the control chain forward, applying power to successive control capstans. Cycling across the bottom band steps the control chain backwards, but without applying power to any control capstans. Cycling across both bands (from 1-3 atm) causes the control chain to step back and forth over the same control capstan, repeatedly applying power only to that one control capstan.

(8) Tool Garage, Tools, and Tool Tips. A complete set of molecular tool tips [1, 216, 2322-2325] sufficient to perform all required mechanosynthetic functions during replication and product manufacturing, including diamond fabrication and deconstruction, hydrogenation and dehydrogenation of surfaces, and tool tip recharge and fabrication are stored in the garage. Graspable jigs for operating molecular import/export mechanisms, controlling extrusion, or extending van der Waals cages are stored in the tool garage, all within reach of the Stewart platform manipulators.

(9) Subassembly Stations. Several areas designated as subassembly stations include wall-mounted active tips, fixed jigs, and other mechanisms by which intermediate structures can be temporarily parked during fabrication, either to allow more efficient execution of manufacturing sequences or to allow more complex assembly operations such as insertion motions.

(10) Extrusion Control System. Manufacture of product object proceeds downward from the extrusion hole towards the opposite floor. When sufficient fabrication has been completed, the workpiece is ratcheted upward a controlled distance by sequentially pulling spring-loaded locking pins out of their staggered complementary pinholes in the exterior face of the workpiece and re-inserting them into the next-lowest pinhole in the workpiece exterior face. This provides a controlled extrusion process, provides mechanical resistance to hydrostatic backpressure and oscillatory acoustic forcing, and ensures that extrusion takes place even if resisted by significant external forces.

(11) Van der Waals Cages and Pegs. Multiple devices whose smooth, flat parallel surfaces approach too closely will be attracted to each other by van der Waals forces, in some cases noncovalently bonding into randomly-ordered aggregates. To prevent such surface bonding and consequent blockage of the I/O wall and the piston wall, two cages of rods configured as wire-frame boxes are extended into the external environment from the I/O and piston walls, as soon as physically possible. Short spring-loaded pegs also deploy from the outer face of the locked piston on extruding daughters and granddaughter devices (or the equivalent face for other product objects) as soon as physically possible, thus geometrically lessening surface attractive forces and reducing any attractive interdevice forces that might resist extrusion.

(12) Passive Hull System. The vast majority (~90%) of all carbon atoms present in the Merkle-Freitas Hydrocarbon Molecular Assembler constitute the 10-nm thick solid diamond hull. This passive hull is penetrated by three openings for materials import/export (I/O wall), by the piston plate at the opposite end (piston wall), by van der Waals cages at either end, and by the extrusion hole in which the daughter device or product object resides while it is being fabricated and extruded.

Last updated on 1 August 2005