Fig. 3. Actuation principles of artificial cilia. (a) Magnetic actuation with (i) electromagnets and (ii) a moving permanent magnet; the cilia orientation follows the magnetic field direction. (b) Light driven motion of artificial cilia; two segments of the cilia body can be made to deflect under two different wavelengths of light irradiation, creating an asymmetrical motion with an appropriate sequence. (c) Electrostatic cilia can be made to deflect with an electric field, and they relax elastically when the field is removed; these cilia can be actuated relatively fast (up to hundreds of Hz). (d) Pneumatic or hydraulic actuation. Hollow, elastic cilia with asymmetric structures can be made to deflect and relax back when gas or liquid is pumped in a pulsating manner; when individually addressed, they can create a metachronal wave.