Finger-triggered portable PDMS suction cup for equipment-free microfluidic pumping
© The Author(s) 2018
Received: 2 January 2018
Accepted: 1 March 2018
Published: 5 March 2018
This study presents a finger-triggered portable polydimethylsiloxane suction cup that enables equipment-free microfluidic pumping. The key feature of this method is that its operation only involves a “pressing-and-releasing” action for the cup placed at the outlet of a microfluidic device, which transports the fluid at the inlet toward the outlet through a microchannel. This method is simple, but effective and powerful. The cup is portable and can easily be fabricated from a three-dimensional printed mold, used without any pre-treatment, reversibly bonded to microfluidic devices without leakage, and applied to various material-based microfluidic devices. The effect of the suction cup geometry and fabrication conditions on the pumping performance was investigated. Furthermore, we demonstrated the practical applications of the suction cup by conducting an equipment-free pumping of thermoplastic-based microfluidic devices and water-in-oil droplet generation.
Over the past few decades, microfluidic devices have emerged as an effective platform for various bio and chemical analyses (e.g., point-of-care testing , dynamic microarray [2–4], single-cell analysis [5–7]) because of their numerous benefits, including reduced fluid volume, rapid analysis, high sensitivity, massive parallelization, and portability. In spite of these benefits, the broad and practical utilization of microfluidic devices is still limited because of the necessity of bulky, complex and expensive external power-consuming equipment for fluid pumping (e.g., peristaltic, syringe, and pneumatic pumps). Accordingly, researchers have attempted to realize an on-chip micropump, instead of using an external pumping equipment, to minimize the system to provide device portability [8, 9]. However, the integration of pumping components (e.g., microvalves and diaphragms) is required to perform on-chip pumping with a microfluidic device, which increases the device fabrication complexity. Moreover, power-consuming bulky and expensive equipment and complicated tube connections for the operation of integrated pumping components are still required.
In this regard, various methods were developed to enable equipment-free microfluidic pumping, such as capillary- [10–12], gas- , degas- [14, 15], pumping lid- , and evaporation-driven [17, 18] pumping. These approaches offer simplicity of execution by eliminating the need for an off-chip equipment. However, the broad implementation of these approaches is still limited by complicated fabrication, narrow range of applicable device materials (e.g., hydrophilic material for capillary), and pre-treatment requirement (e.g., pre-degassing). Accordingly, ‘finger-powered’ pumping strategies have been proposed for more practical and field-applicable fluid driving approaches [19–22]. Although these approaches provide a simple operation with a human finger, they require complex and expensive device fabrication such as multi-layer photolithography and computer numerical controlled (CNC) milling process. Moreover, an accurate assembly of the fluidic channel part and the pumping actuation part is necessary, making the device fabrication more complicated. This is because the microfluidic device for the pumping operation using a human finger in these studies should be integrated with many valve structures, which regulates the fluid flow (e.g., fluidic diode) under the pressure created by the human finger actuation. In addition, the finger actuation parts are irreversibly integrated with the microfluidic chip; hence, they cannot be used for other devices. With the aim of addressing the abovementioned limitations, this study presents a portable and power-free finger-triggered polydimethylsiloxane (PDMS) suction cup to generate the fluid flow in microfluidic devices. This method is very simple and easy, but effective and powerful. It offers the following benefits: (i) easy fabrication from a three-dimensional (3D) printed mold, (ii) easy-to-use and ready-to-use without any pre-treatment (e.g., pre-degassing), (iii) portable and facile assembly or disassembly, and (iv) broad application for various material-based microfluidic devices. To drive a fluid through the microfluidic channels, a user places the PDMS suction cup at the outlet of the device and dispenses the fluid at the inlet. The user then presses and releases the cup with a finger, thereby pulling the fluid toward the outlet. In addition, various pumping performances can be achieved by changing the cup geometry or fabrication conditions. An equipment-free pumping of thermoplastic-based microfluidic devices and a water-in-oil droplet generation using the PDMS suction cup were demonstrated herein for practical applications.
Materials and methods
Fabrication of a suction cup using 3D printed master mold
Preparation of the PDMS- and thermoplastic-based microfluidic device
For the master mold preparation, a photoresist (KMPR 1025, MicroChem, Inc.) was spin coated on a 4 in. silicon wafer with a height of 50 μm, then baked on a hot plate at 100 °C. The patterns on a photomask were transferred onto the silicon wafer by UV light exposure, followed by post-exposure baking and removal of the unexposed photoresist using a developer (SU-8 developer, MicroChem, Inc.).
The PDMS-based microfluidic device was fabricated by replica molding. The PDMS mixture (10:1 w/w ratio of the polymer base to the curing agent) was poured into the mold, cured, and peeled off from the mold. Inlet and outlet holes were then punched using a biopsy punch. The prepared PDMS replica and the glass substrate were bonded to each other by air plasma treatment (CUTE-MP, FemtoScience).
The thermoplastic-based microfluidic device was fabricated through PDMS-based hot embossing. A PDMS stamp was prepared by double-casting the PDMS replica from the master mold. Both polystyrene (PS) and polymethylmethacrylate (PMMA) substrates were prepared (thickness: 2 mm). Plates of a laboratory hot press machine (Qm900M_TD500, QMESYS) were heated to an embossing temperature of 145 °C. The materials for hot embossing were loaded on the heated plate in a stack consisting of a 4 in. silicon wafer, a thermoplastic substrate, and a PDMS stamp (from bottom to top). After 1 min, a force of 4.3 kN was applied to the stack at 145 °C for 3 min. The stack was then cooled down to 100 °C after 3 min and removed from the press. Subsequently, the embossed thermoplastic substrate was peeled off from the PDMS stamp. The embossed substrate was rinsed, dried, and bonded to a pressure-sensitive adhesive (3 M 9969, 3 M) using a roller to form a sealed microfluidic device.
Finger-triggered equipment-free fluid pumping
Results and discussion
Effect of cup diameter, thickness, and mixing ratio of PDMS on the pumping performance
Application for thermoplastic microfluidic devices and generation of water-in-oil droplets
This study presented a finger-triggered portable PDMS suction cup that facilitated equipment-free microfluidic pumping. A negative pressure-driven fluid flow can be generated by a simple finger-triggered operation. The effect of the diameter, thickness, and PDMS mixing ratio of the PDMS suction cup on the pumping performance was investigated. The pumping performance (i.e., pumping pressure and duration time) mainly depended on the restoration force of the deformed PDMS cup, and a trade-off existed between the pumping pressure and the duration time. Equipment-free pumping of thermoplastic-based devices and water-in-oil droplet generation were demonstrated for practical applications. We believe that this pumping method can be extensively utilized for point-of-care diagnostics or resource-limited applications.
LS performed the experiments, analyzed the data, and wrote the manuscript. KH supported the data analysis. LW performed the device fabrication. KJ supervised the research and reviewed the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
The datasets supporting the conclusions of this article are included within the article.
Ethics approval and consent to participate
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2017R1A2B4003328) and National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1C1B1008045).
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