  Active systems -- including sperm cells, living organisms like bacteria,
fish, birds, or active soft matter systems like synthetic ''microswimmers'' --
are characterized by motility, i.e., the ability to propel using their own
''engine''. Motility is the key feature that distinguishes active systems from
passive or externally driven systems. In a large ensemble, motility of
individual species can vary in a wide range. Selecting active species according
to their motility represents an exciting and challenging problem. We propose a
new method for selecting active species based on their motility using an
acoustofluidic setup where highly motile species escape from the acoustic trap.
This is demonstrated in simulations and in experiments with self-propelled
Janus particles and human sperm. The immediate application of this method is
selecting highly motile sperm for medically assisted reproduction (MAR). Due to
the tunable acoustic trap, the proposed method is more flexible than the
existing passive microfluidic methods. The proposed selection method based on
motility can also be applied to other active systems that require selecting
highly motile species or removing immotile species.
