Do human sperm defy Newton's third law? Scientists finally crack the mystery of their swim through thick fluids

Economic Times

2 days ago

Scientists at Kyoto University unravelled a biological mystery. They discovered how human sperm navigate thick fluids. Sperm tails use 'odd elasticity' to bypass Newton's laws. This allows efficient movement in viscous environments. The finding explains sperm's rapid swimming. It could inspire new designs for microscopic robots. This principle applies to other microscopic swimmers like algae.
Scientists decode the Physics trick that lets sperm speed through fluids that should stop them. (Representative Image)
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The physics problem in the micro world
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Odd elasticity: The tail's secret weapon
Biology and robotics
Rethinking newton in active systems
Scientists have been puzzled by a biological mystery: how human sperm, just 50 microns long, can glide so effortlessly through thick cervical mucus or viscous lab-made gels that should slow them down.Now, researchers from Kyoto University say they have decoded the secret: sperm tails exploit a phenomenon called 'odd elasticity,' enabling them to move in ways that sidestep one of Newton's basic laws of motion.The findings, published by researchers Kenta Ishimoto, Clément Moreau, and Kento Yasuda, help solve a decades-old puzzle: how sperm can travel swiftly through thick cervical mucus or gel-like fluids that should slow such tiny swimmers to a crawl.At the human scale, swimmers push water backward to propel forward, with inertia balancing the forces. But at the microscopic level, inertia disappears and is replaced by syrupy drag, a situation physicists call low Reynolds number flow.In such conditions, a simple back-and-forth flick of a tail wouldn't work. Instead, objects like sperm must perform a continuous, asymmetrical wiggle pattern that never repeats in reverse, allowing forward motion.Using high-speed video of human sperm and the green alga Chlamydomonas, the team mapped tail movements in 'shape space' and created an elastic matrix to calculate internal forces. They found that the tails are powered by molecular motors that constantly inject energy instead of acting like passive springs.This creates odd elasticity, an imbalance where a bend in one part of the tail sends tension through the entire structure without a mirrored counterforce. The result is a traveling wave that moves forward without an equal push in the opposite direction.The study suggests that as odd elasticity increases, so does propulsion speed. This explains how human sperm beat their tails around 20 times per second, even in thick fluids.The principle also applies to other microscopic swimmers like algae, and could inspire soft robots that navigate through viscous environments using similar mechanics, without rotating motors.The work doesn't overturn Newton's third law, which still governs passive systems. Instead, it shows that force symmetry can be bypassed in active systems, where energy is constantly absorbed and expended.In nature, this flexibility may help sperm adapt their swimming mechanics in response to chemical cues or changes in viscosity on their way to the egg.