CAREER: Establishing Swimming Sperm as a Model Active Matter System

Non-Technical Summary:

Swimming sperm actively consume energy and propel themselves. Meanwhile, they interact with neighboring sperm and form different phases, similar to a large number of water molecules forming either vapor or liquid phases. Under different conditions, sperm may swim individually (like molecules in a gas) or form dense, collective motion, which is called the flocking phase (think of a flock of birds but replacing birds with sperm). The research field that focuses on the behaviors of a large number of self-propelling objects is referred to as active matter. In this project, the research group aims to produce experimental data to rigorously test the existing theoretical models, and establish swimming sperm as an experimental model system for active matter. Fundamental questions answered in this project can be applied to help biologists understand how sperm pass through the mammalian female reproductive tract. Engineers can also use the information to utilize sperm as carriers or develop mechanical switches for the self-propelled objects. Success of a research project in an academic setting depends on the contribution of talented students, and the principal investigator utilizes tutoring activities to send undergraduate students to tutor students in the surrounding school systems. This education plan aims to boost both the performance of existing undergraduate students, as well as strengthening the pipeline for future students. Combined with the interdisciplinary research project that involves physics, biology, and information theory, the project aims to create an intellectually stimulating environment for students at North Carolina Agricultural and Technical State University, one of the Historically Black Universities and Colleges.

Technical Summary:

Theoretical models that consider momentum conservation between the microswimmers and the surrounding fluids suggest that the flocking phase should not be seen among low-Reynolds-number swimmers such as sperm. However, given the appropriate viscoelastic fluid and initial conditions, the principal investigator has observed flocking of sperm. These exhibit critical exponents similar to those predicted in the dry active matter theoretical model, which does not consider momentum conservation between swimmers and fluid. This project first addresses this by tracing the fluid flow generated by individual sperm swimming to test the momentum conservation proposition, which will also help us better understand sperm propulsion mechanism in viscoelastic fluid, which simulates the rheology of mucus that they need to swim through in the female tract. Sperm-sperm and sperm-sidewall interactions are mediated by the surrounding fluid and direct collisions. Thus, the research team next examines the interaction rules between sperm along with mechanical pressure generated on the sidewalls by swimming sperm colliding into them through the flow and cell tracing. These studies may lead to understanding how the female reproductive tract, particularly the uterotubal junction, regulates the movement of sperm, as well as the development of mechanical actuators for self-propelled objects in microfluidic systems. Finally, this project employs information theory approaches to analyze sperm behaviors. Specifically, it examines information transfer and the information/entropy of different phases, trying to identify organizational principles underlying living active matter systems.

This project is co-funded by the Condensed Matter Physics program in the Division of Material Research and the Historically Black Colleges and Universities – Undergraduate Program (HBCU-UP) in the Division of Human Resource Development.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.