It is generally believed that sperm “swim” by beating or spinning their limp tails. However, a research team led by scientists from the City University of Hong Kong (CityU) found that ray sperm move by spinning both the tail and the head. The team further studied the motion model and demonstrated it with a robot. Their study expanded knowledge about the movement of microorganisms and inspired the design of robotic engineering.
The research is co-led by Dr Shen Yajing, Associate Professor in the Department of Biomedical Engineering (BME) at CityU, and Dr Shi Jiahai, Assistant Professor in the Department of Biomedical Sciences (BMS). Their findings were published in the scientific journal Proceedings of the National Academy of Sciences.
Their research revealed a new and peculiar mode of movement of ray sperm, which they call the “Heterogeneous Double Helix Model (HDH)”. “It was actually an accidental discovery,” said Dr Shi who focused on developing different biotherapies.
It all started with another team research to develop artificial insemination techniques for breeding cartilaginous fish, including sharks and rays, whose skeletons are made entirely or largely of cartilage. “Cartilaginous fish can be used as a ‘factory’ to produce antibodies against diseases, including COVID-19. So we wanted to develop artificial insemination techniques to cultivate them for high-value aquaculture,” he said. -he declares.
During this process, the team were greatly surprised when they first observed the unique structure and swimming motion of the sperm rays under a microscope. They found that the stingray sperm head is in a long helical structure rather than round, and that it rotates with the tail when swimming.
The team further investigated its propulsion mechanism, in particular the exact role of the head in motion. They discovered that ray sperm are made up of heterogeneous helical sections: a stiff spiral head and a soft tail, which are connected by an “intermediate piece” that provides energy for the rotational motion. The head of the ray sperm is not only a “container” of genetic material, but also facilitates propulsion with the soft tail.
High energy efficiency of HDH propulsion
To better understand the mode of movement, the team analyzed a large amount of swimming data and observed the internal structure of sperm at the nanoscale. Since the head and tail of the ray sperm spun in the same direction with different rotational speeds and amplitudes when swimming, the team named it heterogeneous double helix propulsion (HDH).
According to their statistical analysis, the head contributed about 31% of the total propulsive force, which is the first head propulsion recorded in all known sperm. Due to the contribution of the head, the efficiency of ray sperm movement is higher than that of other species like sterlet and bull, which are only driven by the tail.
“Such a non-traditional mode of propulsion not only provides ray sperm with great adaptability to a wide range of viscous environments, but also leads to superior movement ability and efficiency,” explained Dr Shen, whose he research objective is robotics as well as micro / nano manipulation and control.
High environmental adaptability
Environmental adaptability is crucial in natural selection. The head and tail of the sperm of the rays can adjust their movement and propulsion contribution according to the viscosity of the environment and swim at different speeds for the forward motion. Therefore, ray sperm can move in various environments with a wide range of viscosities, demonstrating great environmental adaptability.
The team also found that stingray sperm have a unique two-way swimming ability, which means they can swim not only forward but backward as well. Such an ability provides benefits to sperm in nature, especially when encountering obstacles. And other sperm with spherical or rod-shaped head cannot achieve bidirectional movement.
Thanks to the HDH model, the spiral head of ray sperm has an active rotational capacity. As the head and tail contribute to propulsion, the angle between them will produce a lateral force on the body, allowing the sperm to rotate, showing great flexibility in its movement.
Bio-inspired robot demonstrates HDH model
The particular HDH model showed extensive motility and efficiency characteristics and inspired the team to design microrobots. The bio-inspired robot, also featuring a rigid spiral head and flexible tail, demonstrated similar superiorities over conventional robots in terms of adaptability and efficiency under the same power input. He could move skillfully in an environment with liquid, even when the viscosity changed.
Such capabilities can provide information for the design of swimming robots for difficult engineering tasks and biomedical applications inside the human body with complex fluidic environments, such as inside blood vessels.
“We believed that understanding this unique propulsion would revolutionize the knowledge of the movement of microorganisms, which would facilitate the understanding of natural fertilization and provide inspiration for the design of bio-inspired robots in viscous conditions,” concluded Dr. Shen. .