Label-free 3D computational imaging of spermatozoon locomotion, head spin and flagellum beating over a large volume

Abstract

We report a high-throughput and label-free computational imaging technique that simultaneously measures in three-dimensional (3D) space the locomotion and angular spin of the freely moving heads of microswimmers and the beating patterns of their flagella over a sample volume more than two orders-of-magnitude larger compared to existing optical modalities. Using this platform, we quantified the 3D locomotion of 2133 bovine sperms and determined the spin axis and the angular velocity of the sperm head, providing the perspective of an observer seated at the moving and spinning sperm head. In this constantly transforming perspective, flagellum-beating patterns are decoupled from both the 3D translation and spin of the head, which provides the opportunity to truly investigate the 3D spatio-temporal kinematics of the flagellum. In addition to providing unprecedented information on the 3D locomotion of microswimmers, this computational imaging technique could also be instrumental for micro-robotics and sensing research, enabling the high-throughput quantification of the impact of various stimuli and chemicals on the 3D swimming patterns of sperms, motile bacteria and other micro-organisms, generating new insights into taxis behaviors and the underlying biophysics. A label-free imaging system that can image the swimming behaviour of thousands of sperm cells has been developed by a team in the U.S. Conventional lens-based microscopy systems have been used to explore sperm motion in two dimensions, but they are not effective at imaging microswimmer motion in three dimensions. Aydogan Ozcan and co-workers from the University of California built a lens-free holographic microscope to capture the three-dimensional locomotion of freely moving sperm. The system captures images over a large volume of 1.8 μl and can be used to analyze the beating pattern of the sperms’ flagella and head spin. A sperm sample is placed onto an image sensor chip and illuminated with light from two light-emitting diodes to simultaneously generate holograms. This computational imaging system has been used to capture the trajectories of over 2100 bovine sperm cells in three dimensions.