Although Drexler did not provide a detailed design for an assembler—such a design has still not been fully specified—his thesis did provide extensive feasibility arguments for each of the principal components of a molecular assembler, which include the following subsystems:
The computer: to provide the intelligence to control the assembly process. As with all of the device’s subsystems, the computer needs to be small and simple. As I described in chapter 3, Drexler provides an intriguing conceptual description of a mechanical computer with molecular “locks” instead of transistor gates. Each lock would require only sixteen cubic nanometers of space and could switch ten billion times per second. This proposal remains more competitive than any known electronic technology, although electronic computers built from three-dimensional arrays of carbon nanotubes appear to provide even higher densities of computation (that is, calculations per second per gram).75
The instruction architecture: Drexler and his colleague Ralph Merkle have proposed an SIMD (single instruction multiple data) architecture in which a single data store would record the instructions and transmit them to trillions of molecular-sized assemblers (each with its own simple computer) simultaneously. I discussed some of the limitations of the SIMD architecture in chapter 3, but this design (which is easier to implement than the more flexible multiple-instruction multiple-data approach) is sufficient for the computer in a universal nanotechnology assembler. With this approach each assembler would not have to store the entire program for creating the desired product. A “broadcast” architecture also addresses a key safety concern: the self-replication process could be shut down, if it got out of control, by terminating the centralized source of the replication instructions.
However, as Drexler points out, a nanoscale assembler does not necessarily have to be self-replicating.76 Given the inherent dangers in self-replication, the ethical standards proposed by the Foresight Institute (a think tank founded by Eric Drexler and Christine Peterson) contain prohibitions against unrestricted self-replication, especially in a natural environment.
As I will discuss in chapter 8, this approach should be reasonably effective against inadvertent dangers, although it could be circumvented by a determined and knowledgeable adversary.
Instruction transmission: Transmission of the instructions from the centralized data store to each of the many assemblers would be accomplished electronically if the computer is electronic or through mechanical vibrations if Drexler’s concept of a mechanical computer were used.
The construction robot: The constructor would be a simple molecular robot with a single arm, similar to von Neumann’s kinematic constructor but on a tiny scale. There are already examples of experimental molecular-scale systems that can act as motors and robot legs, as I discuss below.
The robot arm tip: Drexler’s Nanosystems provided a number of feasible chemistries for the tip of the robot arm to make it capable of grasping (using appropriate atomic-force fields) a molecular fragment, or even a single atom, and then depositing it in a desired location. In the chemical-vapor deposition process used to construct artificial diamonds, individual carbon atoms, as well as molecular fragments, are moved to other locations through chemical reactions at the tip. Building artificial diamonds is a chaotic process involving trillions of atoms, but conceptual proposals by Robert Freitas and Ralph Merkle contemplate robot arm tips that can remove hydrogen atoms from a source material and deposit them at desired locations in the construction of a molecular machine. In this proposal, the tiny machines are built out of a diamondoid material. In addition to having great strength, the material can be doped with impurities in a precise fashion to create electronic components such as transistors. Simulations have shown that such molecular-scale gears, levers, motors, and other mechanical systems would operate properly as intended.77 More recently attention has been focused on carbon nanotubes, comprising hexagonal arrays of carbon atoms assembled in three dimensions, which are also capable of providing both mechanical and electronic functions at the molecular level. I provide examples below of molecular-scale machines that have already been built.
The assembler’s internal environment needs to prevent environmental impurities from interfering with the delicate assembly process. Drexler’s proposal is to maintain a near vacuum and build the assembler walls out of the same diamondoid material that the assembler itself is capable of making.
The energy required for the assembly process can be provided either through electricity or through chemical energy. Drexler proposed a chemical process with the fuel interlaced with the raw building material. More recent proposals use nanoengineered fuel cells incorporating hydrogen and oxygen or glucose and oxygen, or acoustic power at ultrasonic frequencies.78
Although many configurations have been proposed, the typical assembler has been described as a tabletop unit that can manufacture almost any physically possible product for which we have a software description, ranging from computers, clothes, and works of art to cooked meals.79 Larger products, such as furniture, cars, or even houses, can be built in a modular fashion or using larger assemblers. Of particular importance is the fact that an assembler can create copies of itself, unless its design specifically prohibits this (to avoid potentially dangerous self-replication). The incremental cost of creating any physical product, including the assemblers themselves, would be pennies per pound—basically the cost of the raw materials. Drexler estimates total manufacturing cost for a molecular-manufacturing process in the range of ten cents to fifty cents per kilogram, regardless of whether the manufactured product were clothing, massively parallel supercomputers, or additional manufacturing systems.80
The real cost, of course, would be the value of the information describing each type of product—that is, the software that controls the assembly process. In other words, the value of everything in the world, including physical objects, would be based essentially on information. We are not that far from this situation today, since the information content of products is rapidly increasing, gradually approaching an asymptote of 100 percent of their value.
The design of the software controlling molecular-manufacturing systems would itself be extensively automated, much as chip design is today. Chip designers don’t specify the location of each of the billions of wires and components but rather the specific functions and features, which computer-aided design (CAD) systems translate into actual chip layouts. Similarly, CAD systems would produce the molecular-manufacturing control software from high-level specifications. This would include the ability to reverse engineer a Product by scanning it in three dimensions and then generating the software needed to replicate its overall capabilities.
In operation, the centralized data store would send out commands simultaneously to many trillions (some estimates as high as 1018) of robots in an assembler, each receiving the same instruction at the same time. The assembler would create these molecular robots by starting with a small number and then using these robots to create additional ones in an iterative fashion, until the requisite number had been created. Each robot would have a local data storage that specifies the type of mechanism it’s building. This storage would be used to mask the global instructions being sent from the centralized data store so that certain instructions are blocked and local parameters are filled in. In this way, even though all of the assemblers are receiving the same sequence of instructions, there is a level of customization to the part being built by each molecular robot. This process is analogous to gene expression in biological systems. Although every cell has every gene, only those genes relevant to a particular cell type are expressed. Each robot extracts the raw materials and fuel it needs, which include individual carbon atoms and molecular fragments, from the source material.
The Biological Assembler
Nature shows that molecules can serve as machines because living things work by means of such machinery. Enzymes are molecular machines that make, break, and rearrange the bonds holding other molecules together. Muscles a
re driven by molecular machines that haul fibers past one another. DNA serves as a data-storage system, transmitting digital instructions to molecular machines, the ribosomes, that manufacture protein molecules. And these protein molecules, in turn, make up most of the molecular machinery.
—ERIC DREXLER
The ultimate existence proof of the feasibility of a molecular assembler is life itself. Indeed, as we deepen our understanding of the information basis of life processes, we are discovering specific ideas that are applicable to the design requirements of a generalized molecular assembler. For example, proposals have been made to use a molecular energy source of glucose and ATP, similar to that used by biological cells.
Consider how biology solves each of the design challenges of a Drexler assembler. The ribosome represents both the computer and the construction robot. Life does not use centralized data storage but provides the entire code to every cell. The ability to restrict the local data storage of a nanoengineered robot to only a small part of the assembly code (using the “broadcast” architecture), particularly when doing self-replication, is one critical way nanotechnology can be engineered to be safer than biology.
Life’s local data storage is, of course, the DNA strands, broken into specific genes on the chromosomes. The task of instruction masking (blocking genes that do not contribute to a particular cell type) is controlled by the short RNA molecules and peptides that govern gene expression. The internal environment in which the ribosome is able to function is the particular chemical environment maintained inside the cell, which includes a particular acid-alkaline equilibrium (pH around 7 in human cells) and other chemical balances. The cell membrane is responsible for protecting this internal environment from disturbance.
Upgrading the Cell Nucleus with a Nanocomputer and Nanobot. Here’s a conceptually simple proposal to overcome all biological pathogens except for prions (self-replicating pathological proteins). With the advent of full-scale nanotechnology in the 2020s we will have the potential to replace biology’s genetic-information repository in the cell nucleus with a nanoengineered system that would maintain the genetic code and simulate the actions of RNA, the ribosome, and other elements of the computer in biology’s assembler. A nanocomputer would maintain the genetic code and implement the gene-expression algorithms. A nanobot would then construct the amino-acid sequences for the expressed genes.
There would be significant benefits in adopting such a mechanism. We could eliminate the accumulation of DNA transcription errors, one major source of the aging process. We could introduce DNA changes to essentially reprogram our genes (something we’ll be able to do long before this scenario, using gene-therapy techniques). We would also be able to defeat biological pathogens (bacteria, viruses, and cancer cells) by blocking any unwanted replication of genetic information.
With such a nanoengineered system the recommended broadcast architecture would enable us to turn off unwanted replication, thereby defeating cancer, autoimmune reactions, and other disease processes. Although most of these disease processes will already have been vanquished by the biotechnology methods described in the previous section, reengineering the computer of life using nanotechnology could eliminate any remaining obstacles and create a level of durability and flexibility that goes beyond the inherent capabilities of biology.
The robot arm tip would use the ribosome’s ability to implement enzymatic reactions to break off an individual amino acid, each of which is bound to a specific tRNA, and to connect it to its adjoining amino acid using a peptide bond. Thus, such a system could utilize portions of the ribosome itself, since this biological machine is capable of constructing the requisite string of amino acids.
However, the goal of molecular manufacturing is not merely to replicate the molecular-assembly capabilities of biology. Biological systems are limited to building systems from protein, which has profound limitations in strength and speed. Although biological proteins are three-dimensional, biology is restricted to that class of chemicals that can be folded from a one-dimensional string of amino acids. Nanobots built from diamondoid gears and rotors can also be thousands of times faster and stronger than biological cells.
The comparison is even more dramatic with regard to computation: the switching speed of nanotube-based computation would be millions of times faster than the extremely slow transaction speed of the electrochemical switching used in mammalian interneuronal connections.
The concept of a diamondoid assembler described above uses a consistent input material (for construction and fuel), which represents one of several protections against molecular-scale replication of robots in an uncontrolled fashion in the outside world. Biology’s replication robot, the ribosome, also requires carefully controlled source and fuel materials, which are provided by our digestive system. As nanobased replicators become more sophisticated, more capable of extracting carbon atoms and carbon-based molecular fragments from less well-controlled source materials, and able to operate outside of controlled replicator enclosures such as in the biological world, they will have the potential to present a grave threat to that world. This is particularly true in view of the vastly greater strength and speed of nanobased replicators over any biological system. That ability is, of course, the source of great controversy, which I discuss in chapter 8.
In the decade since publication of Drexler’s Nanosystems, each aspect of Drexler’s conceptual designs has been validated through additional design proposals,81 supercomputer simulations, and, most important, actual construction of related molecular machines. Boston College chemistry professor T. Ross Kelly reported that he constructed a chemically powered nanomotor out of seventy-eight atoms.82 A biomolecular research group headed by Carlo Montemagno created an ATP-fueled nanomotor.83 Another molecule-sized motor fueled by solar energy was created out of fifty-eight atoms by Ben Feringa at the University of Groningen in the Netherlands.84 Similar progress has been made on other molecular-scale mechanical components such as gears, rotors, and levers. Systems demonstrating the use of chemical energy and acoustic energy (as originally described by Drexler) have been designed, simulated, and actually constructed. Substantial progress has also been made in developing various types of electronic components from molecular-scale devices, particularly in the area of carbon nanotubes, an area that Richard Smalley has pioneered.
Nanotubes are also proving to be very versatile as a structural component. A conveyor belt constructed out of nanotubes was demonstrated recently by scientists at Lawrence Berkeley National Laboratory.85 The nanoscale conveyor belt was used to transport tiny indium particles from one location to another, although the technique could be adapted to move a variety of molecule-sized objects. By controlling an electrical current applied to the device, the direction and velocity of movement can be modulated. “It’s the equivalent of turning a knob … and taking macroscale control of nanoscale mass transport,” said Chris Regan, one of the designers. “And it’s reversible: we can change the current’s polarity and drive the indium back to its original position.” The ability to rapidly shuttle molecule-sized building blocks to precise locations is a key step toward building molecular assembly lines.
A study conducted for NASA by General Dynamics has demonstrated the feasibility of self-replicating nanoscale machines.86 Using computer simulations, the researchers showed that molecularly precise robots called kinematic cellular automata, built from reconfigurable molecular modules, were capable of reproducing themselves. The designs also used the broadcast architecture, which established the feasibility of this safer form of self-replication.
DNA is proving to be as versatile as nanotubes for building molecular structures. DNA’s proclivity to link up with itself makes it a useful structural component. Future designs may combine this attribute as well as its capacity for storing information. Both nanotubes and DNA have outstanding properties for information storage and logical control, as well as for building strong three-dimensional structures.
A research team at Ludw
ig Maximilians University in Munich has built a “DNA hand” that can select one of several proteins, bind to it, and then release it upon command.87 Important steps in creating a DNA assembler mechanism akin to the ribosome were demonstrated recently by nanotechnology researchers Shiping Liao and Nadrian Seeman.88 Grasping and letting go of molecular objects in a controlled manner is another important enabling capability for molecular nanotechnology assembly.
Scientists at the Scripps Research Institute demonstrated the ability to create DNA building blocks by generating many copies of a 1,669-nucleotide strand of DNA that had carefully placed self-complementary regions.89 The strands self-assembled spontaneously into rigid octahedrons, which could be used as blocks for elaborate three-dimensional structures. Another application of this process could be to employ the octahedrons as compartments to deliver proteins, which Gerald F. Joyce, one of the Scripps researchers, called a “virus in reverse.” Viruses, which are also self-assembling, usually have outer shells of protein with DNA (or RNA) on the inside. “With this,” Joyce points out, “you could in principle have DNA on the outside and proteins on the inside.”
A particularly impressive demonstration of a nanoscale device constructed from DNA is a tiny biped robot that can walk on legs that are ten nanometers long.90 Both the legs and the walking track are built from DNA, again chosen for the molecule’s ability to attach and detach itself in a controlled manner. The nanorobot, a project of chemistry professors Nadrian Seeman and William Sherman of New York University, walks by detaching its legs from the track, moving down it, and then reattaching its legs to the track. The project is another impressive demonstration of the ability of nanoscale machines to execute precise maneuvers.