Sperm-Driven Micromotors Moving in Oviduct Fluid and Viscoelastic Media

Biohybrid micromotors propelled by motile cells are fascinating entities for autonomous biomedical operations on the microscale. Their operation under physiological conditions, including highly viscous environments, is an essential prerequisite to be translated to in vivo settings. In this work, a sperm-driven microswimmer, referred to as a spermbot, is demonstrated to operate in oviduct fluid in vitro. The viscoelastic properties of bovine oviduct fluid (BOF), one of the fluids that sperm cells encounter on their way to the oocyte, are first characterized using passive microrheology. This allows to design an artificial oviduct fluid to match the rheological properties of oviduct fluid for further experiments. Sperm motion is analyzed and it is confirmed that kinetic parameters match in real and artificial oviduct fluids, respectively. It is demonstrated that sperm cells can efficiently couple to magnetic microtubes and propel them forward in media of different viscosities and in BOF. The flagellar beat pattern of coupled as well as of free sperm cells is investigated, revealing an alteration on the regular flagellar beat, presenting an on-off behavior caused by the additional load of the microtube. Finally, a new microcap design is proposed to improve the overall performance of the spermbot in complex biofluids.

Effects of surrounding fluid on motility of hyperactivated bovine sperm

Mammalian spermatozoa in organisms with internal fertilization are required to swim in the cervical and oviductal mucus, whose rheological properties differ substantially from those of water. Moreover, on the way to the oviduct, a change in sperm motility called hyperactivation may occur. In the present study, we focused on the motion characteristics of hyperactivated bovine sperm and investigated the effect of the surrounding fluid on motility. We prepared two kinds of polyacrylamide with high-viscosity non-Newtonian fluid properties, similar to the actual cervical and oviductal mucus. Using semen from Japanese cattle, we evaluated curvilinear velocity (VCL), straight-line velocity (VSL), and average path velocity (VAP). Additionally, we estimated linearity (LIN), straightness (STR), and wobble (WOB) as sperm motility parameters for several surrounding fluids. We successfully induced hyperactivation of bovine sperm in high-viscosity non-Newtonian fluid. Hyperactivation resulted in an increase in VCL and a decrease in VSL. In the high-viscosity non-Newtonian fluid, the hyperactivated sperm moved in a zig-zag pattern with regularity, different from the movement observed in a diluted solution. The increase in WOB in the non-Newtonian fluid suggests that hyperactivated sperm efficiently progress along the groove that exists on the oviductal mucus wall. These results improve our understanding of the motility of bovine sperm when they undergo hyperactivation in the actual cervical and oviductal mucus.