  A major hurdle in brain-machine interfaces (BMI) is the lack of an
implantable neural interface system that remains viable for a lifetime. This
paper explores the fundamental system design trade-offs and ultimate size,
power, and bandwidth scaling limits of neural recording systems built from
low-power CMOS circuitry coupled with ultrasonic power delivery and backscatter
communication. In particular, we propose an ultra-miniature as well as
extremely compliant system that enables massive scaling in the number of neural
recordings from the brain while providing a path towards truly chronic BMI.
These goals are achieved via two fundamental technology innovations: 1)
thousands of 10 - 100 \mu m scale, free-floating, independent sensor nodes, or
neural dust, that detect and report local extracellular electrophysiological
data, and 2) a sub-cranial interrogator that establishes power and
communication links with the neural dust.
