] SAN DIEGO, Calif., August 28 -- Chemists have developed
] minute grains of silicon, or "smart dust," that
] spontaneously assemble, orient and sense their local
] environment, a first step toward the development of
] robots the size of sand grains that could be used in
] medicine, bioterrorism surveillance and pollution
] monitoring.

Each grain of dust is a single "moving" part which aligns itself according to the material sensed. The self-assembly comes from a whole bunch of them doing the same thing and clumping together in the process.

'Smart Dust' Could Lead to Tiny Robots - Photonics.com Feature Articles