  Active swimmers are ubiquitous in nature, found in many diverse biological
systems ranging from bacteria to vertebrate fish. Of particular importance are
sperm cells which are swimmers that are crucial for the survival of many
species including humans. Despite decades of work, the fluid physics of sperm
in complex micro-environments such as the cervical tract, or the microfluidic
devices used in assistive reproductive technologies remains illusive. Recently,
a novel microfluidic device featuring periodic post arrays has been developed,
and shown to select sperm cells with better motility, morphology and DNA
integrity, more efficiently than existing approaches. Motivated by this, here
we present a multi-scale model that aims to provide insight physical insight
into the motility behavior of sperm in such periodic geometries. Our model
combines a fluctuating hydrodynamic model of sperm with a probabilistic
discrete-time lattice approach, and we show how hydrodynamic and boundary
interactions facilitate both the enhancement of speed and persistence length of
sperm cells in this post array. We then discuss how this enhancement of
flagellar transport is related to its propensity, and develop a phase diagram.
Our findings not only shed light into the fluid physics of flagellar swimmers
in periodic arrays, but also have direct implications in a broad range of areas
beyond fertility, including bio-inspired robotics, disease detection and drug
delivery.
