CRN's Chris Phoenix was asked by the U.S. National Academy of Sciences to prepare briefing papers for their recent committee sessions investigating molecular manufacturing. So far, we've posted Concepts of Molecular Manufacturing, Current Status of Molecular Manufacturing, and Applications of Molecular Theory.
Today's entry is Molecular Manufacturing Challenges:
The core of molecular manufacturing is the mechanical control of molecular structure building. This has almost been achieved today by several methods, both "top-down" and "bottom-up." Seeman's DNA-building machine can be programmed to create any of several sequences. Scanning probe microscopes have been used to break and form selected covalent bonds between molecules, and in Japan both STM and AFM have removed and replaced single atoms at selected locations in covalent crystals.
A major goal of molecular manufacturing is building nanoscale tools that are small and flexible enough to build duplicate tools rapidly. Bottom-up technologies appear to be closer to that goal. Their speed and flexibility would need to be increased, but a polymer-based tool that could (perhaps assisted by self-assembly) build its polymer components appears to be within reach.
Top-down approaches should not be ruled out. Scanning probe systems have been built in MEMS technologies. If a nanoscale version could be designed, it might be built by any of several nanoscale fabrication methods. Further work with covalent solids (e.g. silicon, silica, diamond, alumina, cubic boron nitride) in either solvent or vacuum might show a way to build general 3D structures with scanning probe molecular manufacturing; this could allow one scanning probe to build a duplicate.
Bottom-up tools include protein design, DNA design, and several related polymer systems (see work by Seeman, and Schafmeister), which can make precise engineered 3D structures. Top-down tools include electron beam induced deposition, two-photon polymerization, ion beam milling, and e-beam lithography. Typical feature size is 10 nm, though ion beam may approach single-atom precision.
Barriers to advancement of molecular manufacturing are largely perceptual at this point. The use of mechanical nanoscale systems for exponential manufacturing has been prematurely criticized, resulting in widespread perception that the approach is a dead end. General acceptance that the technology may have significant potential would substantially accelerate its development.
Basic molecular manufacturing—building molecular structures under mechanical control—has nearly been achieved, though not developed. Exponential manufacturing could be demonstrated in a polymer system with modest effort. Several research directions could extend the utility of that. Nanoscale mechanical engineering, especially friction and classical approximations, should be better understood. Faster actuators could be developed and integrated. Stronger materials could be achieved either by crosslinking or by building covalent solids. The ultimate step is to integrate multiple nanoscale manufacturing systems. This would require research into nanoscale computers; fastening systems for nanoscale products; nanoscale and mesoscale structure; reliability and control; additional mechanical engineering (for ease of design); and product design methodologies.
Tomorrow: "Impacts of Molecular Manufacturing"