Assemblers have been proposed as general purpose manufacturing devices, able to build a wide range of useful products as well as copies of themselves. Most proposals for assemblers involve an inside and an outside, separated by a barrier or casing. The outside is a relatively uncontrolled environment (a liquid solvent with dissolved feedstock molecules and impurities) while the inside environment is both better controlled (no impurities or undesired contaminants) and unreactive (a vacuum, or a noble gas such as neon).
A graphite casing would be strong, flexible, relatively unreactive, and largely impenetrable. The use of a graphite casing raises a number of design issues: how to manufacture the new casing within the confines of the old casing, what shape the casing should have and what maintains that shape, the conditions under which the casing might fail, how to eject the new casing into the outside solvent, etc.
We also discuss an improved method of using acoustic signals to power and control the assembler. In particular, the (possibly bulky) pressure-actuated pistons that were located internally to the casing in previous proposals are moved outside the casing. This lets us use pressurized neon internally to maintain the shape of the casing, much as the shape of an inflated balloon is maintained by internal air pressure. It also makes it easier to use larger pistons which can be driven by smaller pressure changes.