(This is an expansion of some comments I made in a recent thread. --Chris)
Rapid prototyping machines keep getting better. There are even some projects such as RepRap aimed at making manufacturing systems that can make more manufacturing systems. RepRap is already working on hardware. So why do we think molecular manufacturing will be revolutionary compared to rapid prototyping?
For those just joining us, molecular manufacturing plans to build nanoscale components by joining molecules together under programmable control. The goal is to build a machine that can join molecules--which can then build two side-by-side, which can work in parallel build four, and so on. Add nano-built computers and a little bit of micron-scale structure, and you've got a nanofactory: a tabletop machine that can make any of a wide range of products.
The following are some reasons why a nanofactory will be far more powerful and transformative than a present-day rapid prototyping system:
1) Maintenance of precision. By adding just a bit of energy to remove entropy, atom-built objects can be essentially perfect to N generations. Because atoms of an isotope are all exactly the same, the products can be exactly the same.
2) Low friction. Perfect atom-built objects can take advantage of superlubricity and zero wear. Micro-scale objects have serious problems with wear due to scaling laws.
3) High performance. For many machines, including electrostatic motors, performance increases with decreasing size. Aggregations of nanoscale machinery should be more powerful than microscale machinery by several orders of magnitude.
4) More applications. Especially medical applications.
5) More flexibility of construction. By controlling how the atoms and molecules are joined, you can make different materials as well as different shapes. Also, with improved functional density, it's easier to design products.
6) Chemistry in products. It'd be hard to build stuff like fuel cell membranes with micro-fabricators. It should be easier with molecular manufacturing.
There are probably other advantages as well.
RepRap is a great project. And it's worth noting that an inkjet printer can print its weight in ink in about a day. Micro-scale exponential manufacturing is neither impossible nor worthless. But nano-scale exponential manufacturing will be a whole lot better.
So, will there be a continuum between micro and nano fabrication? Only in the sense that there was a continuum between ENIAC and a modern Web-enabled PC with a Google homepage.
In some ways, there was a continuum. But there were also a lot of breakthroughs and sudden leaps in capability. The average person never saw a $20,000 PC, and never used BITNET. ENIAC simply couldn't run a GUI (though the first video game dates back to the 50's, if I remember correctly).
In some ways, there may be a continuum between micro fabs and nano fabs. Micro fabs already make products like shoes; RepRap includes rapid-prototyped wires; eventually micro-fab products may even include fab-built electronics and actuators and sensors.
So in the sense that they both make products, there's a continuum, just as ENIAC and Pentium both could crunch numbers. But for the reasons listed above, molecular manufacturing will be able to do a whole lot of stuff that micro-fabs simply can't do. RepRap would be able to build a lot of the things you find in the typical 99-cent store. But it's hard to imagine building an airplane, or a rifle, or an artificial kidney, or a supercomputer, with RepRap. All these things, and many more, should be buildable in unlimited quantity with a nanofactory. That will be truly revolutionary.