The nanomachinery builds diamondoid bacteria, that replicate with solar power and atmospheric CHON, maybe aggregate into some miniature rockets or jets so they can ride the jetstream to spread across the Earth's atmosphere, get into human bloodstreams and hide, strike on a timer.

— Eliezer Yudkowsky

To control these atoms you need some sort of molecular chaperone that can also serve as a catalyst. You need a fairly large group of other atoms arranged in a complex, articulated, three-dimensional way to activate the substrate and bring in the reactant, and massage the two until they react in just the desired way. You need something very much like an enzyme.

— Richard Smalley

My understanding is that anyone who can grasp what "orthos wildly attacking the heterodox without reading their stuff and making up positions to attack" looks like, considers that this is what Smalley did with Drexler - made up an unworkable approach and argued against it.

— Eliezer Yudkowsky

In this post, I use "nanobots" to mean "self-replicating microscopic machines with some fundamental mechanistic differences from all biological life that make them superior". Various specific differences from biological cells have been proposed. I've organized this post by those proposed differences.

1. localized melting

Most 3d printers melt material to extrude it through a nozzle. Large heat differences can't be maintained on a small scale.

2. rare materials

If a nanobot consists largely of something rare, getting more of that material to replicate is difficult outside controlled environments.

Growth of algae and bacteria is often limited by availability of iron, which is more common than most elements. Iron is the active catalytic site of many enzymes, and is needed by all known life. The growth of something made mostly of iron would be far more limited, and other metals have more limited availability than that.

3. metal surfaces

Melting material isn't feasible per (1), so material must be built up by adding to the surface. Since that's the case, the inside of structures must be chemically the same as what was the exterior.

Metal objects have a protective oxide layer. In an air or water environment, there's no way to add individual (eg) aluminum atoms to a metal surface and end up with metallic aluminum inside; the whole thing will typically be aluminum oxide or hydroxide.

Corrosion is also a ...

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