Nanobots are miniature molecular machines. So far they are just an idea as no one knows how to design or build one. But there is discussion of them, and certain properties be considered. For example, how big is a nanobot?
A ‘simple’, dumb miniature machine could be quite small, for example an antibody attached to a viral-like particle that binds to particular cells, get absorbed by the cell and opens to release the DNA into the cell. But that’s not a very interesting machine, the really interesting nanobots are miniature robots with sensors, computer logic to make decisions, and hands to grab or manipulate things.
So what’s the minimum size for a smart nanobot? It has 100,000 bytes of memory, 1,000,000 bits, and 10 atoms/bit. Let’s figure the same number of atoms for the computer logic. Add an equivalent number for energy storage and generation, structure, sensors, and manipulators. Thirty million atoms in total.
If it is mostly carbon, atoms will be 1 Å apart. Assume a spherical shape, and look at protein structures to estimate packing of atoms in a compact structure. From this, the core of the nanobot will be an estimated 1000 Å or roughly 100 nm.
An E. coli is roughly 1 µm long, so a nanabot would be a about a 1/10 the size of a bacteria. This is small, about the size of an average virus particle, small enough to exist inside cells. A nanobot is large enough to be recognized and engulfed by immune cells, and to need a specific mechanism to enter cells.