Sacrificial 3D Printing of Flexible Capillary Transistor for Unidirectional Liquid Transport

SHI Wenwan; WU Xinkun; SUN Xiaolu; ZHOU Yuning; GAO Xiaoxiang; SUN Jing; LIU Xiaojiang; GU Zhongze

doi:10.16490/j.cnki.issn.1001-3660.2025.21.006

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Surface Technology ›› 2025, Vol. 54 ›› Issue (21) : 87-100. DOI: 10.16490/j.cnki.issn.1001-3660.2025.21.006

Special Topic—Design and Applications of Hierarchical Surface Structure Exhibiting Superwettability

Sacrificial 3D Printing of Flexible Capillary Transistor for Unidirectional Liquid Transport

SHI Wenwan¹, WU Xinkun¹, SUN Xiaolu¹, ZHOU Yuning¹, GAO Xiaoxiang¹, SUN Jing², LIU Xiaojiang^1,*, GU Zhongze^1,*

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Abstract

To address the challenge of simultaneously achieving efficient unidirectional liquid transport and flexibility on capillary transistors, a method has been proposed in which the sacrificial template-assisted three-dimensional printing (sacrificial 3D printing) is utilized to fabricate asymmetric overhanging microstructures with interconnected channels on the surface of polydimethylsiloxane (PDMS). The method allows for precise control over the direction, height, width, and area of liquid capillary rise while imparting the ability for three-dimensional spatial deformation of the structure. In this work, a photopolymerization-based 3D printer was used to create a negative template with optimized exposure time and printing resolution for the capillary transistors. After PDMS casting, thermal curing, and alkaline solution hydrolysis of the template, flexible PDMS capillary transistors were obtained. The impact of different post-processing cycles (0, 1, 2, and 3 times) on the PDMS molding effect was evaluated. The morphology of the resulting structures was characterized by a microscope with extended depth of filed, and the dimensions were measured to calculate the dimensional deviations between the designed values and the measured values. A contact angle meter was used to assess the wettability of the structures with various liquids. The flexible capillary transistors with different dimensions were fabricated, and the self-driven capillary rise height in anhydrous ethanol was recorded to determine the optimal structural parameters. Liquid capillary rise and injection tests were performed to evaluate the unidirectional liquid transport behaviors and flexibility of the capillary transistors.
The results showed that the sacrificial template was successfully fabricated by 3D printing, with an optimal exposure time of 18 seconds and a minimum design feature size of 120 μm. The sacrificial resin was completely hydrolyzed in an alkaline aqueous solution within 140 minutes. After two post-processing cycles (including isopropyl alcohol cleaning, UV curing, and heating), high-precision flexible capillary transistor structures were successfully fabricated. Experimental results of liquid capillary rise demonstrated that the maximum capillary rise height of anhydrous ethanol on the flexible capillary transistors reached 25.3 mm. Furthermore, O₂ plasma treatment significantly enhanced the capillary rise of anhydrous ethanol, reaching a maximum height of 37.0 mm, which was attributed to the change in contact angle. By adjusting the direction and width of the asymmetric overhanging microstructure in the flexible capillary transistors, various functions such as liquid on/off valves, amplifiers, and attenuators were achieved. The fabricated PDMS flexible capillary transistors demonstrated remarkable structural flexibility. They were capable of bending to any angle, twisting, or even knotting, and could return to their original shape without any damage. This flexibility allowed the flexible capillary transistor to perform unidirectional liquid transport even on complex curved surfaces with angles greater than 360°. Additionally, by leveraging the hydrophobic and oleophilic properties of PDMS, the flexible capillary transistors enabled self-driven water-oil separation.
This study applies sacrificial 3D printing technology to the fabrication of microfluidic chips, enhancing the processing accuracy of PDMS three-dimensional microstructures. It successfully creates flexible capillary transistors that enable efficient unidirectional liquid transport and 3D deformability. Flexible capillary transistors hold significant potential for applications in microfluidics, liquid distribution, and oil-water separation technologies. This research provides important experimental insights for the design and application of flexible capillary transistors and broadens the potential applications of sacrificial 3D printing technology in the manufacturing of microfluidic devices.

Key words

asymmetric overhanging microstructure / capillary transistor / unidirectional liquid transport / PDMS / sacrificial 3D printing / oil-water separation

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SHI Wenwan, WU Xinkun, SUN Xiaolu, ZHOU Yuning, GAO Xiaoxiang, SUN Jing, LIU Xiaojiang, GU Zhongze. Sacrificial 3D Printing of Flexible Capillary Transistor for Unidirectional Liquid Transport[J]. Surface Technology. 2025, 54(21): 87-100 https://doi.org/10.16490/j.cnki.issn.1001-3660.2025.21.006

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Funding

National Natural Science Foundation of China (52033002, 82227808); Natural Science Foundation of Jiangsu Province (BK20241268); Open Research Fund of Southeast University and Jiangsu Province Hospital (2024-M02); Jiangsu Province Youth Science and Technology Talent Support Project (JSTJ-2024-096); Start-up Research Fund of Southeast University (RF028623292); Southeast University Interdisciplinary Research Program for Young Scholars (2024FGC1003)