We disclose a peculiar rotational propulsion mechanism of Ray sperms enabled by its unusual heterogeneous dual helixes with a rigid spiral head and a soft tail, named Heterogeneous Dual Helixes (HDH) model for short. Different from the conventional beating propulsion of sperm, the propulsion of Ray sperms is from both the rotational motion of the soft helical tail and the rigid spiral head. Such heterogeneous dual helical propulsion style provides the Ray sperm with high adaptability in viscous solutions along with advantages in linearity, straightness, and bidirectional motion. This HDH model is further corroborated by a miniature swimming robot actuated via a rigid spiral head and a soft tail, which demonstrates similar superiorities over conventional ones in terms of adaptability and efficiency under the same power input. Such findings expand our knowledge on microorganisms’ motion, motivate further studies on natural fertilization, and inspire engineering designs. [PNAS, 118.23 (2021): e2024329118]
Movie S1. The heter-stiffness helical morphology of the Ray sperm. The video shows the heter-stiffness helical morphology of Ray sperms under an optical microscope, scanning electron microscope, and confocal microscope.
Movie S2. The heter-stiffness helical propulsion of the Ray sperm. This video shows the 3D rotational motion of both the spiral head and helical tail. The head and tail rotates counter-clockwise to the forward direction, and both propels the body.
Movie S3. Bi-directional motions of Ray sperms. The sperm (sample 1) first moves backward with the head rotating. Then, it is in a state of transition, with the change of the rotational direction. Finally, it moves forward with both head and tail rotation. For sample 2, the sperm first moves backward to avoid the obstacle. After escaping from the obstacle, it moves forward.
Movie S4. Environmental adaptability of Ray sperms. This video shows the motility of Ray sperms in the normal solution, viscous solution i, viscous solution ii, and viscous solution iii. In the normal solution, the Ray sperms move forward with dual helical propulsion. In viscous solutions, although the tails twine as circles, the sperms still can move forward with the propulsion from the head.
Movie S5. Motions of the bio-inspired robot in various viscous solutions. First, the robot is driven by a single spiral head or helical tail with the same power input in solutions with increasing viscosities (Figs. S25-S26). In the diluted solution, the robot driven by a single tail moves faster. On the contrary, the robot with a single head is more efficient in highly viscous solutions. Then, the video displays the performance of robots with single or dual helical driven mode. The robot is driven by heter-stiffness helical morphology, single spiral head, or single helical tail with the same power input in different solutions. The robot with heter-stiffness helical sections moves fastest in all solutions. The robot with a single helical head or tail relatively adapts to the viscous and diluted solution, respectively.
