Human sperm can navigate through thick fluids in a manner that challenges established principles of physics, specifically Newton’s third law of motion. A research team led by Kenta Ishimoto at Kyoto University conducted a study to understand how these microscopic swimmers manage to propel themselves through substances that should theoretically hinder their movement. The findings, published in October 2023 in the journal PRX Life, reveal new insights into the mechanics of sperm and similar organisms.
Newton’s third law states that for every action, there is an equal and opposite reaction. This principle describes interactions in many physical systems, such as colliding marbles. However, the chaotic nature of biological systems introduces complexities that challenge this symmetry. The study highlights non-reciprocal interactions, which occur in systems ranging from flocks of birds to swimming sperm. These interactions suggest that traditional physics does not fully capture the behavior of these motile agents.
The team focused on how sperm and single-celled algae such as Chlamydomonas swim using flexible structures known as flagella. These flagella change shape to generate movement. In highly viscous fluids, one would expect these flagella to lose significant energy, limiting movement. Surprisingly, the research shows that the elastic nature of these flagella allows sperm to move efficiently without significant resistance from the surrounding fluid.
The study identified a property termed “odd elasticity,” enabling the flagella to propel cells forward while minimizing energy loss. Yet, this characteristic alone did not account for the unique wave-like motion that drives propulsion. To delve deeper, the researchers introduced a new concept called the “odd elastic modulus,” which describes the internal mechanics of the flagella and their interactions with the surrounding environment.
The research team employed both experimental data and modeling techniques to uncover these dynamics. “From solvable simple models to biological flagellar waveforms for Chlamydomonas and sperm cells, we studied the odd-bending modulus to decipher the nonlocal, nonreciprocal inner interactions within the material,” the researchers stated.
The implications of these findings extend beyond biological understanding. The insights gained could inform the design of small, self-assembling robots that emulate living materials. Additionally, the modeling methods may enhance comprehension of collective behavior in various systems.
This research not only challenges long-standing physics principles but also opens new avenues for technological innovation. By examining the intricate mechanics of sperm and other microscopic swimmers, scientists are paving the way for applications that could mimic biological efficiency in engineering and robotics.
