Is self propelled the right wording when you blast a loose propellor with ultrasonic waves? If so I have created a large variety of self propelled robots in my ultrasonic cleaning machine.
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.
[0]: https://en.wikipedia.org/wiki/Crookes_radiometer [1]: https://en.wikipedia.org/wiki/The_Thing_(listening_device)
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...
This kind of stuff has also already been done with magnetic fields.