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.
4.5.4 Positional Assembly Using Other Biological Means
Drum and Gordon  note that “diatom nanotechnology,” first proposed in 1988 , might be used to produce useful structures “by compustat selection experiments (i.e., forced evolution of development or evodevo). Their exponential growth in suspension cultures could compete with the lithography techniques of present day nanotechnology, which have limited 3-D capabilities. Alternatively, their fine detail could be used for templates for MEMS, or their silica deposition systems isolated for guiding silica deposition.” Artificial diatom patterns were first synthesized via 3-D vapor deposition of silica by Shultze  and others . Diatoms in the shapes of ~10-20 mm rectangular and triangular prisms are well-known . Diatom shells placed in a magnesium gas atmosphere at 900 oC for 4 hours undergo an atom for atom substitution of Mg for Si with no change in 3-D shape, and the authors list nine other “shape preserving gas/solid reactions” that are thermodynamically favored, and should “replicate” silica diatom shells in other oxide materials .
Drum and Gordon see two major advantages: “Large numbers of components are available via the exponential reproduction of the organisms that produce them, and mutation permits us to selectively evolve organisms with the components we want.” Diatoms have a ~24 hour cell cycle, so their numbers can double every day. Gordon’s “compustat” technique is “basically a selective breeding and culling of microorganisms based on shape. The idea was to do it visually by growing diatoms in a chamber with an image coming through a microscope to a computer. The computer would find each cell in turn and compare it with an ideal that you’d like to achieve. If the cell was anywhere near that ideal you’d leave it alone and if it wasn’t, you’d kill it by zapping it with a laser.”  Projected uses include sensors, optical gratings, catalytic substrates, and even nanoscale comb actuators .
Sponges can also form solid silica structures with precisely controlled morphologies, directed by proteins and polysaccharides. For example, silicatein-a protein found in the silica spicules of the sponge Tethya aurantia can hydrolyze and condense the precursor molecule tetraethoxysilane to form silica structures with controlled shapes, including hard silica spheres and well-defined columns of amorphous silica .
Last updated on 1 August 2005