Single-cell encapsulation in droplet microfluidics is commonly hindered by the tradeoff between cell suspension density and on-chip focusing performance. In this study, we introduce a novel droplet microfluidic chip to overcome this challenge. The chip comprises a double spiral focusing unit, a flow resistance-based sample enrichment module with fine-tunable outlets, and a crossflow droplet generation unit. Utilizing a low-density cell/bead suspension (2 × 10⁶ objects/mL), cells/beads are focused into a near-equidistant linear arrangement within the double spiral microchannel. The excess water phase is diverted while cells/beads remain focused and sequentially encapsulated in individual droplets. Focusing performance was assessed through numerical simulations and experiments at three flow rates (40, 60, 80 μL/min), demonstrating successful focusing at 40 and 80 μL/min for beads and cells, respectively. In addition, both simulation and experimental results revealed that the flow resistance at the sample enrichment module is adjustable by punching different outlets, allowing over 50% of the aqueous phase to be removed. YOLOv8n-based droplet detection algorithms realized the counting of cells/beads in droplets, statistically demonstrating single-cell and bead encapsulation rates of 72.2% and 79.2%, respectively. All the results indicate that this on-chip sample enrichment approach can be further developed and employed as a critical component in single-cell encapsulation in water-in-oil droplets.

液滴微流控中的单细胞封装通常受到细胞悬浮液密度和片上聚焦性能之间的权衡的阻碍。在这项研究中，我们引入了一种新型液滴微流控芯片来克服这一挑战。该芯片包括双螺旋聚焦单元、带有微调出口的基于流阻的样品富集模块和横流液滴生成单元。利用低密度细胞/珠悬浮液（2 × 10 ^{6 个}物体/mL），细胞/珠在双螺旋微通道内聚焦成近等距的线性排列。多余的水相被转移，而细胞/珠子保持聚焦并顺序封装在单个液滴中。通过数值模拟和三种流速（40、60、80 μL/min）的实验评估聚焦性能，证明珠子和细胞分别在 40 和 80 μL/min 下成功聚焦。此外，模拟和实验结果都表明，样品富集模块的流阻可以通过冲制不同的出口来调节，从而可以去除50%以上的水相。基于YOLOv8n的液滴检测算法实现了液滴中细胞/珠子的计数，统计显示单细胞和珠子封装率分别为72.2%和79.2%。所有结果表明，这种片上样品富集方法可以进一步开发并用作油包水滴中单细胞封装的关键组成部分。