We define mechanochemistry as the enabling or enhancement of site-specific chemical reactions by positional control of the reactants, with localized mechanical (or electrical) force applied to overcome reaction barriers. This definition parallels that of Drexler's Nanosystems, and Ralph Merkle.
There are two regimes of mechanochemistry that are likely to be important to Zyvex; ultra-high vacuum, and solution. In both, the object being built and the reactant being added are held in precise alignment. Pushing the two together overcomes the reaction barrier and causes bonding of the reactant precisely where desired in the object being built.
In the UHV case, the reactants typically would have unterminated bonds which react when positioned close to one another, but not while the two components are held apart (since there are no other reactants in a good quality UHV). For example, a hydrocarbon assembler building hydrogen-terminated diamond surfaces would require a way to remove a specific hydrogen atom from a specific site on the object being built, leaving a dangling bond in the underlying carbon. Bringing an unterminated carbon atom down to the surface, the assembler then allows that carbon to bond to the underlying carbon and pulls away, leaving the new carbon attached to the object being built. If desired, it will then terminate the new carbon's bond by adding back hydrogen, so it can work elsewhere. Merkle has proposed a hydrocarbon metabolism which has a complete set of reactions for building objects in this manner.
In the solution case, the reactants must be non-reactive while in solution, but be able to react with one another when forced together mechanically or via electric field. In this case, it is likely that the reaction would have leaving groups that would go into solution. A Diels-Alder type reaction is one possible candidate for such a building block.
Mechanochemistry is not seen much in typical chemical processes. As an example, conventional explosives operate in the conventional realm of chemical bond breaking and making via diffusion. High explosives, which have a shock wave moving faster than the speed of sound, operate in the realm of mechanochemistry, where the reaction is driven by the force of the shock wave, not diffusion.
Nature does chemistry in living systems by exquisitely balanced reactions happening very near equilibrium. While this is a nice existence proof that atomically precise manufacturing is both possible and practical, the types of structures built by living systems are limited. By controlling reactions under conditions far from equilibrium, and with a broader choice of compounds, we should be able to build a much broader range of materials.
Initially, we expect to do mechanochemistry with molecular building blocks rather than atoms. Using larger blocks reduces the precision and holding mechanism stiffness needed during assembly, and means a smaller number of joining operations is required for making macroscopic objects. See Michelsen for a discussion of the possibility of using carbon analogs for mechanochemistry, as well as an exposition on how we might get to the point of doing mechanochemistry.
Zyvex is working with university researchers to develop candidate mechanochemistry systems. Ê
