  Ratchet effects can arise for single or collectively interacting Brownian
particles on an asymmetric substrate when a net dc transport is produced by an
externally applied ac driving force or by periodically flashing the substrate.
Recently, a new class of active ratchet systems has been realized through the
use of active matter, which are self-propelled units that can be biological or
non-biological in nature. When active materials such as swimming bacteria
interact with an asymmetric substrate, a net dc directed motion can arise even
without external driving, opening a wealth of possibilities such as sorting,
cargo transport, or micromachine construction. We review the current status of
active matter ratchets for swimming bacteria, cells, active colloids, and
swarming models, focusing on the role of particle-substrate interactions. We
describe ratchet reversals produced by collective effects and the use of active
ratchets to transport passive particles. We discuss future directions including
deformable substrates or particles, the role of different swimming modes,
varied particle-particle interactions, and non-dissipative effects.
