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.

中文翻译：

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用于精子流变性分析的表面声波驱动无泵流

精子流变性是精子细胞逆着流体流动方向游动的现象，是精子在女性生殖道内长距离迁移的主要指导机制之一。然而，目前研究这种方法的方法是通过不断引入额外的缓冲液来处理稀有样本。在这里，我们开发了一种利用声流的设备，即由声扰动驱动的稳定流，以驱动速度范围为 40 μm s ^-1至 128 μm s ^-1（对应于最大剪切率）的可调谐、调节良好的连续流5.6 s ^-1至 24.1 s ^-1) 在感兴趣的通道中——适合探测精子流变行为的范围。使用此设备，我们研究了代表输卵管内表面几何特征的不同几何形状的微通道中的精子流变性，确定了精子动力学与不同宽度通道中存在的流动。我们发现在存在声学产生的流动的情况下，在 50 μm 通道中充分运动的流变精子的横向头部位移 (ALH) 增加了 28%，并且迁移方向发生了变化，曲线速度 (VCL) 增加了 52%通过将平均流速从 40 μm s ^-1增加到 130 μm s ^{-1 ，使精子在 225 μm 通道中充分运动}. 这些结果为理解女性生殖道中的精子导航策略提供了见解，其中流变精子在边界附近游动以克服女性生殖道中的流动并到达受精部位。这种表面声波装置提供了一种简单的无泵替代方案，用于在体外模型中研究微型游泳者，从而能够发现对微型游泳者迁移策略的新见解，同时可能为基于流变性的精子选择和其他流动必需的应用提供机会。