  Microscopic robots in the body could harvest energy from ultrasound to
provide on-board control of autonomous behaviors such as measuring and
communicating diagnostic information and precisely delivering drugs. This paper
evaluates the acoustic power available to micron-size robots that collect
energy using pistons. Acoustic attenuation and viscous drag on the pistons are
the major limitations on the available power. Frequencies around 100kHz can
deliver hundreds of picowatts to a robot in low-attenuation tissue within about
10cm of transducers on the skin, but much less in high-attenuation tissue such
as a lung. However, applications of microscopic robots could involve such large
numbers that the robots significantly increase attenuation, thereby reducing
power for robots deep in the body. This paper describes how robots can
collectively manage where and when they harvest energy to mitigate this
attenuation so that a swarm of a few hundred billion robots can provide tens of
picowatts to each robot, on average.
