79 Essays on Infinite Lifespans   Robert A. Freitas Jr. or (if run backwards) as a motor to convert neon gas pressure into  rotary  power.  The  researchers  reported  that  prelimi- nary molecular dynamics simulations of the device showed that it could indeed function as a pump, although “structural deformations of the rotor can cause instabilities at low and high rotational frequencies”. The motor was not particularly energy efficient – but it worked. The  ultimate  goal  of  molecular  nanotechnology  is  to develop  a  manufacturing  technology  able  to  inexpensively manufacture most arrangements of atoms that can be speci- fied in molecular detail. Building medical nanorobots, each made of millions or billions of atoms, cheaply enough to be practical for medical therapies requires a new kind of manu- facturing  technology.  Molecular  manufacturing  will  be  the ultimate mechanized production in terms of its precision and flexibility. Two central mechanisms have been proposed to achieve these goals at the molecular scale: (1) programmable positional assembly including, for example, fabrication of dia- mond structures, using molecular feedstock, and (2) massive parallelism of all fabrication and assembly processes. As machine structures become more complex, getting all the parts to spontaneously self-assemble in the right sequence is increasingly difficult. To build complex structures, it makes more sense to design a mechanism that can assemble a molec- ular structure by what is called ‘positional assembly’ – that is, picking and placing molecular parts. A device capable of posi- tional assembly at the molecular scale would work much like the robot arms that manufacture cars on automobile assembly lines in Detroit, or which insert electronic components onto computer circuit boards with blinding speed in Silicon Valley. Using the positional assembly approach, the robot manipula- tor picks up a part, moves it to the workpiece, and installs it. The robot then repeats the procedure over and over with many different parts until the final product is fully assembled.