Illustration for self-propelled mechanism of different particles (A) Chiral bent-wire (Adapted with permission from Ref. [52]. Copyright c 2005, American Physical Society). (B) Vibrating vibrot (Adapted with permission from Ref. [53]. Copyright c 2016, IOP Publishing Ltd. and Deutsche Physikalische Gesellschaft). (C) 3D printed walkers, in which structures in top row have connected (rigid) legs and structures in bottom row have free (flexible) legs (Adapted with permission from Ref. [51]. Copyright c 2016, IOP Publishing Ltd and Deutsche Physikalische Gesellschaft). (D) Self-propelled dimer made of two grain-filled balls. Upper panel, instantaneous horizontal velocity vs. time. Lower panel, vertical position of both balls and the vibrating plate as a function of time [57] (Adapted with permission from Ref. [57]. Copyright c 2017, Spring Nature).