Resemblance between motile and magnetically actuated sperm cells

The active flagellum propels a motile sperm cell by traveling bending waves. Here, we demonstrate that non-motile cells have the capacity to be wirelessly actuated by external magnetic fields and reveal insights into their propulsion characteristics. Partial coating of the sperm head with nanoparticle aggregates is achieved by electrostatic-based self-assembly. The coating enables propagation of helical traveling waves along the passive flagellum under the action of a periodic magnetic field. We compare the waveforms of active and passive flagellated motion and show noticeable asymmetry in the case of magnetically actuated cells, leading to lower linearity (LIN = VSL / VCL) of the taken pathway. The average curvature of the flagellar beat cycle is 10.4 ± 8.1 rad mm^-1 (mean ± s. d.) for an active flagellum, whereas the curvature of a passive flagellum exhibits a linear increase (37.4 ± 18.1 rad mm^-1) and decreases toward the distal end. We also show that the maximum amplitude of the bending wave occurs at the distal end of the active flagellum and at the middle of the passive flagellum. Our experiments also show the ability of the actuating field to control the rate of progression of the bending waves along the passive flagellum to match that of motile cells.