From the teapot effect to tap-triggered self-wetting: a 3D self-driving sieve for whole blood filtration

Micropore membranes are widely used for microparticle and cell separations, but when pore diameters approach ~1 µm, surface-tension-driven capillary pinning (Laplace pressure at pore mouths) drastically raises the gating threshold and limits passive flow. Lower porosity at small pore sizes further reduces connectivity between pores, making gravity-driven wetting insufficient. Conventional workarounds—centrifugation and pneumatic pumping—add complexity and can exert damaging hydromechanical forces on cells. To address this, the authors draw inspiration from the teapot effect: a localized “tap-trigger” initiates liquid permeation through an inclined membrane, after which self-wetting propagates across the membrane in a gravity-driven positive feedback loop. They integrate this strategy into a 3D cone-shaped sieve to enable passive, high-throughput filtration including whole blood leukocyte enrichment.

Prior strategies to overcome capillary pinning include: (1) increasing membrane porosity to reduce hydraulic resistance, which becomes challenging to fabricate reliably at ~1 µm pore sizes; and (2) liquid-based gating by prefilling pores with low-surface-energy liquids to tune the gating pressure, which risks contamination or damage from added liquids. In practice, external driving via centrifuges or pumps is common to surpass gating thresholds but introduces complexity, cost, and potential cell damage due to high accelerations and impacts. These limitations motivate a passive, surface-physics-based approach requiring only gravity.

Design and operating principle: An inclined micropore membrane is paired with a 3D-printed tap-trigger microstructure placed beneath it. A localized “tap” initiates fluid permeation through micropores and wets a small region on the underside; this self-wetting then spreads, merging menisci and lowering the effective barrier at pore mouths in a positive feedback loop driven solely by hydrostatic pressure. The membrane is conceptually divided into regions (R1–R5) based on local ΔP relative to the gating threshold P_th to identify positions suitable for triggering. Material characterization: SEM imaging was performed on membranes with nominal pore sizes 1, 3, 5, and 8 µm. Contact angles were measured: membrane 65.4°, 3D-printed device surface 84.3°. Red-ink droplet tests at different depths under the membrane verified distinct regions: droplets being drawn in (R2), held still (R3), or drawing more liquid to flow along the membrane (R4). Gating threshold measurements: Plain horizontal membranes were mounted at the bottom of transparent hollow polymer tubes (inner diameter 10 mm). Pre-filled water columns of specific heights (1 µm: 350 mm; 3 µm: 300 mm; 5 µm: 300 mm; 8 µm: 220 mm) remained pinned, indicating gating thresholds above the corresponding hydrostatic pressures. With the tap-trigger microstructure on inclined membranes, the trigger point was identified and liquid column depths (height difference between liquid level and first leakage point) were measured over repeated trials. Tap-trigger geometry optimization: Multiple trigger shapes/sizes were compared; a hemispherical trigger (radius 2 mm) provided the best performance, attributed to sufficient contact area and ability to collect drained liquid mass to overcome adhesion and promote runoff. 3D sieve fabrication and testing: A cone-shaped 3D-printed cover and a cone-shaped micropore membrane were assembled onto a holder equipped with tap-trigger microstructures contacting the membrane. For hydrodynamic tests, a precision balance recorded drained mass over time (converted to volume). DPBS drainage was characterized for membranes with 3 µm and 5 µm pores. Photomicrographs documented the self-wetting process: initial penetration at the trigger followed by progressive wetting and increased drainage area.

- Tap-trigger efficacy: For inclined membranes with tap-trigger structures, measured liquid column depths (mean ± SEM) were 24.4 ± 1.24 mm (1 µm), 7.6 ± 0.80 mm (3 µm), 5.0 ± 0.58 mm (5 µm), and 3.1 ± 0.75 mm (8 µm), indicating substantially reduced effective gating thresholds compared with horizontal plain membranes that remained pinned at columns of 350, 300, 300, and 220 mm, respectively. - Gating threshold reduction: The 3 µm membrane’s gating threshold was reduced from above 3000 Pa to approximately 80 Pa (~35-fold reduction). For 5 µm pores, it decreased from above 3000 Pa to approximately 50 Pa (~60-fold reduction). - Self-wetting dynamics: After triggering, the membrane exhibited progressive self-wetting that expanded the wetted area and increased flow via a positive feedback mechanism consistent with the teapot-effect-inspired design. - Throughput: The 3D sieve achieved high gravity-driven throughput. In DPBS tests, 20 mL drained within ~80 s for 3 µm pores and ~45 s for 5 µm pores. As reported, the device achieved throughput above 20 mL/min with 3 µm pores at 14.1% porosity in the cone-shaped sieve configuration. - Whole blood application: In leukocyte filtration from whole blood, the sieve maintained comparable leukocyte purity while achieving higher platelet removal and lower leukocyte stimulation levels, facilitating downstream single-cell analysis.

The tap-triggered self-wetting strategy directly addresses capillary pinning by initiating localized permeation and enabling meniscus merging on the underside of an inclined membrane. This reduces the effective gating threshold so that gravity alone suffices to sustain drainage, eliminating the need for centrifuges or pumps that can damage cells. Experiments across pore sizes (1–8 µm) confirmed large reductions in the pressure needed to open pores and demonstrated rapid, self-propagating wetting. Implemented in a 3D cone-shaped sieve, the approach delivered high passive throughput in buffer and effectively processed whole blood, yielding comparable leukocyte purity with improved platelet removal and reduced leukocyte stimulation—benefits for downstream single-cell workflows. These outcomes validate the teapot-effect-inspired, geometry- and wetting-controlled method as a practical solution for passive microfiltration.

The study introduces a teapot-effect-inspired, tap-triggered self-wetting strategy that markedly lowers micropore membrane gating thresholds and enables gravity-driven, high-throughput filtration. A 3D cone-shaped sieve integrating a hemispherical tap-trigger microstructure achieved substantial performance gains, including >20 mL/min DPBS throughput (with 3 µm pores at 14.1% porosity) and improved whole-blood leukocyte processing (higher platelet removal, lower leukocyte stimulation) without external pumping or centrifugation. These results highlight a simple, passive, and cell-friendly approach to microparticle and cell filtration with potential to facilitate downstream single-cell analysis.