A Bioinspired Flexible Film Fabricated by Surface-Tension-Assisted Replica Molding for Dynamic Control of Unidirectional Liquid Spreading

The unidirectional liquid spreading without external
energy input
has presently aroused widespread concern. Recently, on the peristome
of Nepenthes alata, a novel 2D unidirectional
liquid spreading has been reported. It has been revealed that its
exquisite superhydrophilic multistage microstructure, overlapping
microcavities with arc-shaped edges and wedge-shaped corners, is the
main reason for this phenomenon. To fabricate a peristome-inspired
surface, a replica molding method is highly efficient and provides
an ideal structure. However, the curved shape of the finally formed
surface cannot be adjusted, and a specific surface shows only one
type of liquid spreading state, greatly limiting its potential application.
Here, we aimed to develop a novel surface-tension-assisted replica
molding method to fabricate an artificial peristome film. The artificial
peristome film was fabricated by pouring styrenic block copolymers
(SBS) dissolved in organic solvents into a negative replica prepared
in polydimethyl­siloxane (PDMS), based on the natural peristome.
With volatilizing the organic solvent, the SBS agglomerates formed
an artificial peristome film via surface tension effects. More importantly,
the PDMS-negative replica swelled in the organic solvent and then
returned to the original size, which is conducive for replicating
microstructures. The liquid spreading speed could be dynamically controlled
by stretching the artificial peristome film. We demonstrated that
the microcavity wedge angle decreases with an increasing stretching
ratio. A smaller wedge angle can result in a much stronger unidirectional
liquid spreading ability. This study provides insight into the dynamic
control of unidirectional liquid spreading for novel pump-free medical
microfluidic devices.

无需外部能量输入的单向液体铺展，目前已引发学界广泛关注。近期，科研人员在翼状猪笼草（Nepenthes alata）的口缘区发现了一种新型二维单向液体铺展现象。研究表明，其精巧的超亲水多级微结构——带有弧形边缘与楔形拐角的重叠微腔，正是该现象产生的核心原因。为制备仿猪笼草口缘的表面，复制成型法高效且可实现理想的结构构型，但该方法制备的最终表面呈固定曲面，且单一表面仅能实现单一类型的液体铺展状态，极大限制了其应用潜力。为此，本研究旨在开发一种新型表面张力辅助复制成型方法，用于制备人工猪笼草口缘膜：以天然猪笼草口缘为模板，将溶于有机溶剂的苯乙烯类嵌段共聚物（styrenic block copolymers, SBS）浇注到聚二甲基硅氧烷（polydimethylsiloxane, PDMS）制备的负模中；随着有机溶剂挥发，SBS胶粒通过表面张力效应自组装形成人工猪笼草口缘膜。更关键的是，PDMS负模在有机溶剂中会发生溶胀，随后恢复至原始尺寸，这有利于微结构的精准复制。通过拉伸该人工口缘膜，可动态调控液体铺展速率。研究证实，微腔的楔形角随拉伸比增大而减小，而更小的楔形角可赋予表面更强的单向液体铺展能力。本研究为无泵新型医用微流控器件的单向液体铺展动态调控提供了重要的理论参考与技术支撑。