Friday, January 20. 2012
Of Geckos and Fat Fingers: Why We Don't Have 3D Printing Replicators
In Star Trek: The Next Generation Captain Picard is often seen ordering "Tea, Earl Grey. Hot" from the ship's computer. Flawlessly delivering his order every time, this replicator is everything we could hope for in 3D printing: on-demand, customized, localized, rapid manufacturing. Unfortunately, the high profile rollouts of consumer level 3D printers at this month's Consumer Electronic Show only serve as a reminder of how far we are from true in-home replicator reality. Crudely fabricating small parts by extruding hot plastic through a nozzle is a long way from replicating an iPhone or Earl Grey. So what's the holdup?
The Perfect 3D Replicator
In a 1959 presentation to the American Physical Society titled There's Plenty of Room at the Bottom Richard Feynman famously postulated on the future of fabrication in asking:
... whether, ultimately – in the great future – we can arrange the atoms the way we want; the very atoms, all the way down! What would happen if we could arrange the atoms one by one the way we want them ...
This presentation is credited with being the inspiration for today's nanotechnology, launched by Eric Drexler in his groundbreaking 1986 book Engines of Creation. With the ability to manipulate individual atoms using what he calls a universal assembler, Drexler expounds on many future possibilities, including a new industrial revolution as we change the way we 'make' things at the most fundamental level.
But how to create a 'universal assembler'? Our natural tendency is to start with what we already know. If, for example, we can build cars and buildings by picking up materials and placing them into a predetermined position why not the same thing for atoms? Why not build a nano-scale robotic arm which could pickup one atom at a time and place that atom in a pre-programmed position? And heck, while we're at it, why don't we use our arm to precisely position carbon atoms so that everything we make is out of incredibly strong diamond? With such an arm we would be well on our way to atomically precise manufacturing and transforming the science fiction of a Star Trek replicator-style 3D printing into reality.
The problem is, this universal assembler has proved extremely difficult to build. In fact some think it's impossible. Why?
Fat Fingers
Imagine a bin of beach balls that we wish to move to another bin. Also imagine that our hands are encased in ... beach balls. Of course our dexterity would suffer and our ability to perform the task would be severely compromised since our tools - our hands - would be essentially the same size as our materials. This is what we are faced with at the atomic level. Atom sizes do vary but the size of those with potential as structural materials tend to cluster around an angstrom or two in diameter. Building a universal assembler will be like typing on a small keyboard with 'fat fingers'.
While working with 'fat fingers' might be mechanistically difficult, one can imagine different scenarios where it may be possible. Unfortunately that is not the end of our difficulties.
Gecko Force
For years scientists were puzzled by the geckos' ability to climb on just about any surface. They ruled out any secreted glues or simple water adhesion before finally concluding in 2002 that the millions of tiny 'hairs' (setae) on the surface of a geckos' feet adhered to other surfaces through a weak atomic interaction known as van der Waals forces.
Although stable atoms are electrically neutral their electrons can vary in position causing minute electric field fluctuations. 
These fluctuations polarize the atom and, like the opposite poles of a magnet, other atoms can be attracted. This van der Waals force is very small in magnitude especially compared to common covalent, ionic and metallic bonds. It's also very dependent on the geometry of the interaction. This allows the gecko to adhere or peel off by simply changing the angle of its foot. Still, we normally don't come across this at a macroscopic scale; put a book on a table and it doesn't stick.
At an atomic scale it's a different story. This force is very real and of strength to throw a wrench into our atom handling. Quite simply, our tools will have a tendency to stick to our materials. Once we 'pick up' an atom, we'll have difficulty putting it down. Imagine trying to type with fingers that are both fat and covered with glue.
We will be faced with still more problems such as atom delivery and assembler programming and speed (self-replicating nanobots anyone?). But having to work with fat and sticky fingers are perhaps the most fundamental problems of all. (Richard Smalley - the originator of the fat, sticky finger idea - and Eric Drexler famously debated the issue in 2003.)
So Where Does That Leave Us?
The holy grail of 3D printing would be a universal assembler. With it we could precisely place whatever atoms we would like, in whatever fashion we would like, whenever we would like in order to exactly replicate anything we have a design for. Unfortunately, the problems of fat and sticky fingers mean that simply scaling down our macroscopic machines to an atomic level will likely be difficult if not impossible.
However, 3D replication already does take place at the atomic level, most significantly in biological systems. Ribosomes, for example, use a sophisticated interplay of mechanical and chemical properties to create macroscopic structures out of individual atoms and molecules. We've also shown that we can, under specialized conditions, nudge atoms around, make small 'machines' and a patent was even issued in 2010 for a simple diamond mechanosynthesis tool. But while the idea that we can fabricate objects with atomic precision is by no means dead, all of these examples are for specialized end products under specialized conditions. Eventually we may truly be able to fabricate replicator-style, but it will likely involve customized methods of manufacture for each end product. And it likely won't happen anytime soon. The simplest most elegant solution, a universal assembler may forever elude us simply because of fat and sticky fingers.
For more on the fascinating debate between Eric Drexler and Richard Smalley see here and here. Drexler has backed away from the universal assembler but still insists on the feasibility of atomically precise manufacturing.
3D Additive Fabrication, Inc. (3dAddFab) is a start up company located in Colorado, USA. 3dAddFab provides high quality 3D printing that is easy to price and order, at a lower cost than existing fabricators.
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