AUTONOMOUS CAPILLARY MICROFLUIDIC DEVICES WITH CONSTANT
FLOW RATE AND TEMPERATURE-CONTROLLED VALVING
Lanhui Li
1,2, Eiko Westerbeek
1, Jeroen Vollenbroek
1,3,
Lingling Shui
2, Mathieu Odijk
1, Jan Eijkel
11
BIOS/Lab on a Chip Group, MESA+ Institute for Nanotechnology, University of Twente, Netherlands
2National Center for International Research on Green Optoelectronics & South China Academy of
Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China
3Dept. Nephrology & Hypertension, University Medical Center Utrecht, Netherlands
ABSTRACT
We report a capillary microfluidic device with temperature triggered stop valve function. The device consists of a Poly(N-isopropylacrylamide) (PNIPAm) grafted PDMS channel with thermosensitive surface wettability and an integrated platinum heating wire. By locally switching the channel temperature between 20 and 36 °C, a switchable stop valve is obtained. Interestingly, we found that PNIPAm grafted PDMS channel shows a capillary filling rate that is constant in time instead of the well-known Washburn behavior. We explain this by contact-line friction. The device can be applied for real-time flow control with steady velocity field.
KEYWORDS: capillary microfluidic, responsive wettability, stop valve, constant flow rate INTRODUCTION
Although both autonomous capillary pumping and flow control with hydrophobic patches as stop valves have been reported, characterized by cost-effective fabrication and facile operation, few works have combined capillary filling and switchable stop valves[1-3]. Challenges still remain for full control of single valve-switching and liquid flow for biologically relevant assays. In this work, we introduce a Poly(N-isopropylacrylamide) (PNIPAm) grafted PDMS (PNIPAm-g-PDMS) capillary microfluidic device. Due to the thermo-sensitive properties of PNIPAm, the device possesses a temperature-switchable surface wettability between 20 and 36 °C. By locally integrating a heating wire, a hydrophobic valving function can thus be obtained. The applied temperature range is suitable for most biomedical applications. Moreover, different from most capillary based microfluidic devices with liquid filling
behavior following Washburn equation, with the flow rate decreasing with √𝑡𝑡 [4], we found a capillary filling rate
that is constant in time, potentially providing more accurate diagnostic assay performance. EXPERIMENTAL
The device consists of a PDMS top plate with a fluid channel, and a glass bottom plate covered by a thin PDMS layer and with an integrated platinum heater fabricated by photolithography, wet etching, sputtering and lift-off. The PDMS channel walls were functionalized with PNIPAm by UV-initiated surface polymerization[5]. A Peltier element maintained at a constant temperature of 20 °C was placed under the glass to confine the heated area (Figure 1a). Valving was realized by locally switching heater temperature between 20 and 36 °C (Figure 1b). Devices were tested at different storage time after production (1 - 26 days).
Figure 1: Schematic drawing of liquid filling and temperature-controlled valving in a straight rectangular channel of constant width w and height h. (a) Capillary filling in a rectangular channel with walls at different wettability. θb,t,l,r are the contact
angles of the liquid on the bottom, top, left, right walls, respectively. (b) Valve operation: (i) autonomous liquid filling, (ii) closing the valve by local heating to 36 °C, (iii) opening the valve by switching local temperature to 20°C
978-1-7334190-1-7/µTAS 2020/$20©20CBMS-0001 380 24th International Conference on Miniaturized Systems for Chemistry and Life Sciences October 4 - 9, 2020 - Virtual
RESULTS AND DISCUSSION
PNIPAm-g-PDMS at 22 °C (Figure 2a) exhibits contact angle hysteresis and time-dependent wetting behavior, slightly varying with UV irradiation time (5-15 min) during preparation. PNIPAm-g-PDMS at 37 °C ((Figure 2b) shows similar advancing and receding angles but no obvious time-dependent wetting behavior. We interpret this as the result of the dissipative hydration process of hydrophilic PNIPAm below LCST (Lower Critical Solution Temperature) and the absence of hydration of the hydrophobic PNIPAm thin layer above LCST[6].
The dissipative hydration process causes strong contact-line friction during the capillary filling process, causing a constant capillary filling speed of the microchannel. Figure 3a shows double-logarithmic plots of filling length against time in channels based on experimental data. Slopes close to 1 are obtained for most filling processes, indicating that the contact-line friction dominates[7]. Only in a single case a slope close to 2 (Washburn-type)[4] is observed. Successful stop valve function is finally shown in Figure 3b.
Figure 2: Wetting property of PDMS and PNIPAm-g-PDMS surfaces at 22 °C (a) and 37 °C (b). PNIPAm-g-PDMS surfaces were prepared under UV irradiation for time periods of 5, 10, 15 min.
Figure 3: Liquid filling behavior and temperature controlled valving in PNIPAm-g-PDMS channel. (a) Liquid filling behavior in different channels, indicating predominant contact line friction. Double-logarithmic plots of filling length against time in channels of 100 (a), 300 (b), 500 (c) µm in width and for devices with different storage time periods. The channel height is 35 µm. (b) Temperature-controlled valving for liquid control performance.
CONCLUSION
In conclusion, we developed a PNIPAm-g-PDMS capillary microfluidic device with a constant capillary filling speed and individually controllable stop valve function operated between 20 and 36 °C.
ACKNOWLEDGEMENTS
This work was supported by China Scholarship Council (CSC) Grant # 201806750019. REFERENCES
[1] Londe, G., Chunder, A., Wesser, A., Zhai, L. & Cho, H. Sens. Actuators B Chem, 132, 431–438, 2008.
[2] Chunder, A., Etcheverry, K., Londe, G., Cho, H. J. & Zhai, L. Colloids Surfaces A Physicochem. Eng. Asp, 333, 187–193, 2009. [3] Tafti, E. Y. et al. J. Nanosci. Nanotechnol. 11, 1417–1420, 2011.
[4] Washburn, E. W., Phys. Rev. 17, 273 (1921). [5] Li, L. et al. ACS Appl. Mater. 2019, 11(18):16934-43. [6] Liu, Y. & Sakurai, K. ACS Omega. 4, 12194–12203, 2019.
[7] Joos, P., Van Remoortere, P. & Bracke, M. J. Colloid Interface Sci. 136, 189–197, 1990. CONTACT
* L.Li; phone: +31-682-146-510; l.li-3@utwente.nl 381
