Wait until that propeller has controllable pitch and direction, commanded by the on board chip. 20 um x20 um is enough area, in a 2nm process, to accommodate roughly 100,000 transistors. That's about 5x the Apollo guidance computer.

So you would supply bulk ultrasound energy to the organ or area you are treating, and these tiny machines would start to have complex interactions, communicate and locate themselves relative to one another, and coordinate to attack the tumor, deliver the drug, destroy amyloid plaque etc.

That's starting to sound like an RFID device, but with sound instead of radio waves. In this framework I guess the propeller-thing is analogous to a Crookes radiometer.[0] I wonder what would be the Great Seal Bug[1]?

[0]: https://en.wikipedia.org/wiki/Crookes_radiometer [1]: https://en.wikipedia.org/wiki/The_Thing_(listening_device)

20um x 20um is still a bit impractical -- it gets close to the practical limits of wafer dicing, and you need support circuitry. That said, we've made useful payloads in as little as 100um x 100um; here's an example of our (published) work in 200um x 200um:

A 200µm x 200µm x 100µm, 63nW, 2.4GHz Injectable Fully-Monolithic Wireless Bio-Sensing System

http://www.travisdeyle.com/publications/pdf/2017_rfic_implan...

But did they look like tiny-ass boat propellers? See, not nearly as cool.

No it's not. It's an external force propelling it. Clickbait article and headline. It's literally a piece of plastic that they move with noise. It's neither self propelling or a robot.

This kind of stuff has also already been done with magnetic fields.