Effective roles of the head and flagellum in sperm motility
In the low Reynolds number fluid environment that microswimmers encounter, back-and-forth motion cannot lead to net displacement. In mammalian sperm, the mechanical wave propagating along their single flagellum breaks the cancellation between back-and-forth motion and, therefore, is assumed to define the movement direction. Here, we show experimentally that the sperm head movement direction is not opposite to the wave propagation direction when sperm move in a viscoelastic fluid next to a solid substrate. The oscillation of the movement direction is out of phase with the oscillation of the wave direction, and the movement has a larger amplitude than the wave direction. When we tried to reconstruct the movement direction as a linear combination of the head orientation and the wave direction, we found that the contributions from these two changed in time. Further, the last bend of the flagellum does not move in the lab frame (as observed under the microscope) on a deformable polydimethylsiloxane (PDMS) substrate, which likely results from the flagellum indenting the surface, providing a new mechanism for locomotion. Overall, our results highlight the appearance of the sperm head in steering motility and provide quantitative tests on sperm chemotaxis and flagellar bending mechanism.