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.
3.3 Penrose Block Replicators (1957-1962)
The first known physical implementation of simple machine replication was reported by the British geneticist Lionel S. Penrose at University College, London, and his son, the physicist Roger Penrose at Bedford College, London, in 1957 . They started by defining some ground rules  for such efforts: “A structure may be said to be [self-replicating] if it causes the formation of two or more new structures similar to itself in every detail of shape and also the same size, after having been placed in a suitable environment. One of the new structures may be identical with the original one; alternatively, the original structure may be destroyed in the process of forming two new replicas. Certain conditions are added which exclude all well-known types of physical or chemical chain reactions. First, the replicating structure must be built by assembling simpler units present in the environment. Secondly, more than one design can be built from the same set of units though the only replicating structure that can be automatically assembled will be one exactly copying a previously existing structure. The pre-existing structure is known as a seed.”
The first  in the Penrose family of replicators was a set of “A” and “B” type tilt-blocks, cut from plywood or vulcanite, and placed on a track where they could slide freely but not pass each other. When single parts are placed on the track and subjected to horizontal agitation, they do not join together. However, when an interlocked, two-block “AB” or “BA” unit is placed in the box and shaken, a simple form of 1-dimensional replication takes place (Figure 3.3). Collisions between the two-block unit and other lone parts in the box cause new two-block units to form, each identical to the original, demonstrating self-replication as a simple form of mechanical autocatalysis.
L. Penrose [681-685] went on to devise an ingenious interlocking column design which allowed the replication of a multipart machine which is free to move about in a 2-dimensional agitation environment strewn with a random assortment of premanufactured constitutive columns. The replicating machine consists of two interlocking columns of five blocks each, with clever arrangements of springs, levers, hooks, dovetails, and ratchets on each of the parts. Each column includes five blocks stacked vertically: two double-hook blocks which act as neutral “food” for a fully automatic replicating structure of any required length (Figure 3.4), one activating cam lever block that transmits activation but does not link (Figure 3.5), one blocking device that prevents more than four units from coming close together (Figure 3.6), and a footing block consisting of interdigitating bases upon which superstructures capable of activation, hooking and release could be mounted, permitting machines to operate in 2 dimensions and to orient themselves to one another for the purposes of self-replication (Figure 3.7).
The complete 2-D self-replicating machine (Figure 3.8) is the double-column seed unit at center (a) – linked by double hooks, incorporating the tilted cam-lever activating principle, and protected by the blocking device in its base (interdigitating footing blocks not shown). Explains Penrose : “When the neutral unit at left joins the seed (b), it disengages one of the hooks holding the seed together and sets the blocking mechanism so that only one more neutral unit can be added. When the fourth unit joins the triple group (c), it disengages the second hook in the original seed, causing it to come apart in the middle and form two replicas of itself (d).”
An 11-minute movie was made of the replicating blocks in action by H.A. Cresswell in 1958 . Interestingly, in the finest scientific tradition, the simplest of Penrose’s machine replicator designs was independently “replicated” (confirmed) in 1962 by another well-known scientist, Edward F. Moore, who wrote : “After constructing an exact copy of Penrose’s basic model, I have found that it not only operates mechanically with reasonable satisfaction, but is very useful in suggesting to audiences some of the problems and possibilities of self-reproducing machines. If the reader attempts the problem of how to design the shapes of the units A and B so as to have the specified properties, the difficulties he will encounter in his attempt will cause him to more readily appreciate the ingenuity of Penrose’s very simple solution to this problem.”
New generations of similar conformation-switched blocks are currently being pursued by Griffith (Section 3.28) and by Saitou  in the context of the self-assembly of microscale mechanical parts (Section 4.1.5). Prions (Section 4.3.1) also represent a crude biological analog of the Penrose block system.
Last updated on 1 August 2005