  Sperm motility is a prerequisite for male fertility. Enhancing the
concentration of motile sperms in assisted reproductive technologies - for
human and animal reproduction - is typically achieved through aggressive
methods such as centrifugation. Here we propose a passive technique for the
amplification of motile sperm concentration, with no externally imposed forces
or flows. The technique is based upon the disparity between probability rates,
for motile cells, of entering in and escaping from complex structures. The
effectiveness of the technique is demonstrated in microfluidic experiments with
microstructured devices, comparing the trapping power in different geometries.
In these micro-traps we observe an enhancement of cells concentration close to
10, with a contrast between motile and non-motile increased by a similar
factor. Simulations of suitable interacting model sperms in realistic
geometries reproduce quantitatively the experimental results, extend the range
of observations and highlight the ingredients that are key to optimal trap
design.
