I once saw a video about attempts to make a flying machine, before we actually had one. Nobody tried to build one out of concrete, but there was *a lot* of flapping going on. Of course the flying machines that worked in the end were more like a fish than a bird, but my point is that the immagination of the average person completely failed to predict the nature of man-made aviation before it existed. While the average nanotech fan might be looking forward to machine-phase titanium-terminated diamondoid nanites, I'm looking forward to the video of failed designs for "assembler arms".

With nanotechnology being such an interdisciplinary subject, the terminology involved can often be confusing. Words relating to assembly seem especially problematic. Since the concept behind nanotechnology is to assemble atomic and molecular components into more complex, functional devices it is necessary to address this issue. To begin, consider what it means to assemble a macroscopic device, for instance an automobile. In modern times, a typical automobile is assembled by a mostly-robotic assembly line. Each component is attached to the growing assembly, eventually resulting in a functional product. Such assembly requires the picking up and placing of each component into its appropriate position. When this method is applied to the nanoscale, it is known as positional assembly.

However, a more commonly employed approach to nanoscale assembly is to strategically select molecules for the assembly such that when they are mixed together in the appropriate environment, they spontaneously assemble to form the desired product. This method has been termed self-assembly. For instance, biological systems commonly use self-assembly to assemble biomolecular components such as proteins, nucleic acids and lipids. The mechanism behind self-assembly is governed by thermodynamics, that is the assembled state is of lower energy than the unassembled state, and the pathway from the starting material to the assembled product is "downhill" on the energy landscape. Thermodynamically speaking, directed assembly is somewhere between positional assembly and self-assembly, with the assembly being directed by an external source of information or energy, for example templates or fields.

The terminology used above is more or less established. However, the trouble begins when...