Self-propelling capsules as artificial microswimmers

The mimicry of natural microswimmers by artificial nano- and micro-devices is extremely challenging because it is hard to achieve and control nanoscale actuation reproducibly and reversibly. In the context of recent developments, we shall review the basic phenomena of artificial swimming objects in the micrometer scale. Typically, these swimming devices were rigid, and up to now, the mechanisms of self-propulsion have only rarely been adapted to soft particles as microcapsules. The high flexibility of capsules is an important feature for more realistic descriptions of the basic swimming processes of biological cells. Additionally, micro- and nanocapsules show the advantage that they can store a defined amount of chemical or biological compounds in their core regions. This offers a high potential for the realization of diverse biological or medical applications (e.g. cargo transport and controlled drug delivery). The discussed phenomena are based on different chemical reactions or flow and diffusion principles, including bulk- and surface rheology, and they can be used to develop new ideas concerning the construction of advanced types of self-propelling microcapsules.