Simulation of sperm kinetics in a bubble-actuated acoustofluidic trap for sperm motility screening

Microfluidics has recently emerged as a promising, non-invasive, and cost-effective approach for isolating high-quality motile sperm from semen samples. Passive microfluidic mechanisms primarily rely on channel geometries embedded with barriers or structures to trap or divert sperm, but they are less efficient in selectively isolating highly motile sperm. In contrast, active mechanisms utilize external fields to actively sort sperm based on their motility characteristics, allowing for more selective isolation as well as higher throughput without altering sperm's natural swimming behavior. In this study, we aimed to investigate sperm kinetics in an acoustic field within a novel acoustofluidic device. This device employs a piezoelectric transducer mounted next to the microfluidic device to generate bulk acoustic waves. The bulk acoustic wave induces oscillation in a trapped bubble inside the microfluidic channel, resulting in the formation of microvortices around the bubble. We performed numerical analyses of sperm trajectories and subsequently validated the findings through experimental characterizations. Our results show that less motile sperm are trapped in microvortices, while highly motile sperm escape toward the channel walls, following natural swimming behaviors. Sperm kinetics were analyzed across various sperm velocities and microvortex strengths, demonstrating how variations in acoustic excitation influence sperm–vortex interactions. Our results demonstrate that increasing applied voltage enhances the separation of faster swimming sperm, with the mean velocity increasing from 65.5 μm/s at 10 Vpp to 85.5 μm/s at 25 Vpp. This trend is consistent with our experimental observations, further validating the efficiency of the proposed acoustofluidic device.