As if the ray fishes weren't unusual enough already, it turns out that their sperm is also unique within the animal kingdom. Scientists have now created a robot inspired by those sperm, which may someday lead to smaller descendants that swim within the human body.
Most sperm essentially consist of a round tadpole-like head and a soft tail. That head remains in a fixed orientation while the tail either whips back and forth or rotates in a circle relative to it. Such is not the case with ray sperm, however.
Led by Drs. Shen Yajing and Shi Jiahai, researchers at City University of Hong Kong recently discovered that those sperm propel themselves by rotating their rigid head along with their soft tail. What's more, that head is long and helical in structure (corkscrew-shaped, in other words), allowing it to auger its way through liquid environments.
This means that not only can the sperm travel quicker and more efficiently than others, but they can also adapt to moving within liquids of varying viscosities by independently adjusting the propulsive contributions of the head and tail. When analyzed, it was determined that the head ordinarily contributes about 31 percent of the total propulsive force.
As is the case with other sperm, the genetic material that is passed onto the egg is contained within the head. And joining the head to the tail is a flexible midsection, which provides the energy for rotational movement. That midsection also allows for enhanced maneuverability, as it lets the sperm bend like an accordion bus, with both the head and tail rotating to pull it around turns.
Additionally, if the sperm encounters an insurmountable obstacle, it can reverse away from it simply by rotating its head in the opposite direction. By contrast, the sperm of other animals are incapable of moving backwards.
"Such an untraditional way of propulsion not only provides ray sperms with high adaptability to a wide range of viscous environments, but also leads to superior motion ability, and efficiency," says Dr. Yajing.
The scientists dubbed this system of movement "heterogeneous dual helixes" (HDH) propulsion, and incorporated it into a ray-sperm-inspired robot that is small enough to fit on the palm of a hand. It consists of rigid iron-wire spiral head, a soft cotton-wire tail, and two electric motors in between. And for a given power input, it is more adaptable and energy-efficient than other swimming robots, when it comes to moving through different liquids of various viscosities.
It is hoped that much smaller versions of the HDH robot may someday be able to swim within patients' bodies, perhaps making their way through blood vessels to deliver drugs to specific locations.
The research is described in a paper that was recently published in the journal PNAS. You can see the robot in action, in the video below.
Source: City University of Hong Kong
