Design clues for motility and rheotaxis-based microfluidic chips for sperm sorting

Microfluidic-based sperm selection is an essential tool recently introduced into assisted reproductive technologies. Conventional approaches such as swim-up and density-gradient centrifugation (DGC) are widely used, however, they lack selectivity and limit the necessary sperm amount in the sample. Moreover, the DGC method has been reported to damage the sperm's DNA, whilst the emerging microfluidic devices offer a soft and flexible way to selectively sort various volumes of raw sperm samples. The flexibility of the discussed technology is associated with the channel architectures based on different sorting mechanisms. In particular, motility-based sorting devices are generally applied for rapid sperm selection without cell damaging by reactive oxygen species. Non-motile sperm samples can be separated from non-sperm cells by inertial microfluidics. The most promising approach to sperm selection has been presented by rheotaxis-based chips, shown to closely mimic the female reproductive tract. In this review, we discuss the key aspects of the chip design according to the underlying mechanisms. The microfluidic chips' fabrication issues and challenges have also been highlighted.