Search and response mechanisms in active soft matter

Abstract

Efficient migration in complex environments is crucial for biological systems that search for a target across various length scales. Examples include T-cell locomotion, algae's viscotaxis, bacterial chemotaxis in complex environments, and mammalian sperm cell migration within the female reproductive tract. These microswimmers must search and respond to biophysical and/or biochemical cues from the environment to regulate their motion. This regulation of motion is essential, as the searcher needs to perform various tasks at different regions or carry out a multiphasic search to locate targets. Therefore, identifying the relevant signals present in the microenvironment, and their regulatory effect on the searcher's motion, is key to understanding the search mechanism. As a model system, we study bull sperm migration using microfluidics and apply concepts from soft matter and statistical physics to understand how sperm regulates its motion in a complex environment. I will present a quantitative yet overarching description of the regulation of sperm motility in response to the biophysical/biochemical cues within the female reproductive tract. We develop a holistic view that shows sperm motility can be categorized into multiphasic motions, including progressive, hyperactive, and circular. I will further discuss the outcomes of such interactions between sperm, biochemical stimulants, and biophysical cues, including the rheological properties of mucus and the architecture of the microenvironment. Leveraging this understanding, I will demonstrate new platforms that can detect and, with exquisite precision, isolate cells with high quality to improve assisted reproductive technologies. The approaches and tools we have developed can be further applied to decipher the search strategies utilized by other microorganisms, and study interactions between colloids and polymers under dynamic and non-equilibrium conditions.