Utility fog

In the original application as a replacement for seatbelts, the swarm of robots would be widely spread out, and the arms loose, allowing air flow between them.

The foglets' bodies would be made of aluminum oxide rather than combustible diamond to avoid creating a fuel air explosive.

[4] Hall and his correspondents soon realized that utility fog could be manufactured en masse to occupy the entire atmosphere of a planet and replace any physical instrumentality necessary to human life.

Virtual buildings could be constructed and dismantled within moments, enabling the replacement of existing cities and roads with farms and gardens.

While molecular nanotech might also replace the need for biological bodies, utility fog would remain a useful peripheral with which to perform physical engineering and maintenance tasks.