Microparticle separation using micropore membranes is crucial in various applications, including environmental and biomedical filtration. However, miniaturizing micropores to the 1µm level exacerbates the capillary pinning effect due to surface tension, hindering efficient filtration. Traditional methods like centrifugation overcome this, but introduce complexities, costs, and potential cell damage. Existing strategies to lower the gating threshold, such as increasing porosity or prefilling with low-surface-energy liquids, have limitations. This research introduces a novel tap-triggered self-wetting strategy, inspired by the teapot effect, which uses a 3D-printed microstructure to facilitate liquid leakage and spreading across the membrane, overcoming capillary pinning with only gravity. This method aims to improve filtration efficiency while avoiding the drawbacks of existing techniques. The paper details the design, characterization, and application of a 3D cone-shaped cell sieve employing this strategy for whole blood filtration.

The introduction extensively reviews existing microparticle separation methods using micropore membranes, highlighting the challenges posed by the capillary pinning effect at smaller pore sizes. It discusses the limitations of conventional methods such as high-speed centrifugation and the drawbacks of previously proposed solutions like increasing membrane porosity or employing prefilling with low-surface-energy liquids. These previous approaches are shown to be either impractical due to material and fabrication difficulties or potentially contaminating to the sample.

The study involved designing and 3D-printing a tap-trigger microstructure inspired by the teapot effect. This microstructure was strategically placed beneath an inclined micropore membrane to initiate self-wetting. The authors characterized the tap-trigger process and investigated different regions on the membrane to determine the optimal position for triggering liquid flow. Scanning electron microscopy (SEM) was used to characterize the micropore membranes, and contact angle measurements were performed to assess the hydrophilicity of the materials. The gating threshold, which is the pressure required to overcome capillary pinning, was measured for both plain horizontal membranes and those with the tap-trigger microstructures. A 3D cone-shaped cell sieve was fabricated and incorporated the tap-trigger microstructure. The throughput of the 3D sieve was evaluated using DPBS (Dulbecco's phosphate-buffered saline). Finally, the 3D sieve was used to filter leukocytes from whole blood samples, and the performance was evaluated in terms of leukocyte purity, platelet removal rate, and leukocyte simulation level.

The tap-triggered self-wetting strategy significantly lowered the gating threshold of micropore membranes. For 3µm and 5µm micropores, the gating threshold was reduced from over 3000 Pa to 80 Pa and 50 Pa, respectively, representing a 35-60 fold reduction. The 3D cone-shaped cell sieve, incorporating the tap-trigger microstructure, demonstrated a high throughput exceeding 20 mL/min of DPBS. When used to filter leukocytes from whole blood, the device exhibited comparable leukocyte purity to existing methods, but with a significantly higher platelet removal rate and lower leukocyte simulation, making it suitable for downstream single-cell analysis. SEM images and experimental results confirmed the efficacy of the tap-triggered self-wetting mechanism, showing that the liquid initially penetrates the membrane at the trigger point and then self-wets a larger membrane area, driven only by gravity.

The results demonstrate the success of the tap-triggered self-wetting strategy in overcoming the capillary pinning effect and enhancing passive microparticle filtration. The significantly reduced gating threshold, coupled with the high throughput achieved by the 3D sieve, makes this approach promising for various applications. The improved platelet removal and lower leukocyte simulation are particularly advantageous for biomedical applications requiring high-quality cell separation for single-cell analysis. The use of gravity as the sole driving force simplifies the device, reducing cost and complexity compared to existing methods that require external pumps or centrifuges. Future work could focus on optimizing the design of the tap-trigger microstructure, exploring different materials for the membrane and the microstructure, and further validating the performance of the device in diverse applications.

This study presents a novel tap-triggered self-wetting strategy for passive microparticle filtration, inspired by the teapot effect. The strategy successfully addresses the limitations of existing methods by utilizing a 3D-printed tap-trigger microstructure to significantly reduce the gating threshold of micropore membranes. The 3D cone-shaped sieve based on this strategy achieves high throughput and efficient whole blood filtration with improved performance compared to traditional techniques. Future research may focus on broader applications and optimizing device design for various filtration needs.

While the study demonstrates significant improvements in filtration performance, further research is necessary to comprehensively assess the long-term stability and durability of the 3D-printed microstructure and membrane. The study focused on whole blood filtration, and further testing with other complex fluid matrices is needed to confirm the generalizability of the findings. Additionally, a more detailed investigation into the effects of different fluid properties, such as viscosity and surface tension, on the self-wetting process could provide further insight into the method's applicability.