Casing an assembler

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).