Many different propulsions mechanisms exist and are studied to reproduce the behavior of active matter. Most of the time it involves the catalytic degradation of a fuel (i.e. hydrogen peroxide) that creates a local electrolyte gradient that drives the particles. In other systems, the propulsion is initiated by living cells such as bacteria or sperm cells that can be conjugated with inorganic particles. In any case, many challenges remain before including these systems into real applications. For example, avoid the use of toxic fuels and increase the biocompatibility. What is also lacking in most experiments is the presence of an external flow and boundaries around the particles that can affect the behavior of the active matter.