Surface acoustic wave-driven pumpless flow for sperm rheotaxis analysis

Sperm rheotaxis, the phenomenon where sperm cells swim against the direction of fluid flow, is one of the major guiding mechanisms for long-distance sperm migration within the female reproductive tract. However, current approaches to study this pose challenges in dealing with rare samples by continuously introducing extra buffer. Here, we developed a device utilising acoustic streaming, the steady flow driven by an acoustic perturbation, to drive a tuneable, well-regulated continuous flow with velocities ranging from 40 μm s^-1 to 128 μm s^-1 (corresponding to maximum shear rates of 5.6 s^-1 to 24.1 s^-1) in channels of interest - a range suitable for probing sperm rheotaxis behaviour. Using this device, we studied sperm rheotaxis in microchannels of distinct geometries representing the geometrical characteristics of the inner-surfaces of fallopian tubes, identified sperm dynamics with the presence of flow in channels of various widths. We found a 28% higher lateral head displacement (ALH) in sufficiently motile rheotactic sperm in a 50 μm channel in the presence of acoustically-generated flow as well as a change in migration direction and a 52% increase in curvilinear velocity (VCL) of sufficiently motile sperm in a 225 μm channel by increasing the average flow velocity from 40 μm s^-1 to 130 μm s^-1. These results provided insights for understanding sperm navigation strategy in the female reproductive tract, where rheotactic sperm swim near the boundaries to overcome the flow in the female reproductive tract and reach the fertilization site. This surface acoustic wave device presents a simple, pumpless alternative for studying microswimmers within in vitro models, enabling the discovery of new insights into microswimmers' migration strategies, while potentially offering opportunities for rheotaxis-based sperm selection and other flow-essential applications.