ON-DEMAND CONTROL OF MICROFLUIDIC FLOW VIA SOLENOID MICROVALVE SUCTION

Q. Zhang, P.R. Zhang, Y.T. Su, M.L. Yang and B. Ma* Qingdao Institute of Biomass Energy and Bioprocess Technology, Chinese Academy of Sciences,

P. R. China ABSTRACT

A simple, low-cost and on-demand microfluidic controlling platform was developed based on a capillary-tuned solenoid microvalve suction effect without any outer pressure source. The suction effect was employed as a stable driving force for manipulating micro-fluid by connecting a piece of capillary tubing between the microvalve and the chip. The suction volume could be controlled from microliters to picoliters level by decreasing the energized duration or increasing the capillary length. Several microfluidic applications such as cell/droplet sorting and on-demand size-controllable droplet generation was demonstrated on the platform and both simulations and experiments confirmed this platform with good controllability and stability. KEYWORDS: Microfluidics, Single cells, Droplets

INTRODUCTION

Traditional active microfluidic controlling methods based on dielectrophoresis, optical tweezers, pulsed laser, surface acoustic waves and pneumatic valve suffered from either expensive instruments requirement or complicated chip fabrication. Recently, compressed air switched by an off-chip solenoid valve was utilized for controlling micro-fluid with the advantages of simple chip-fabrication and easy integration with chips[1][2]. However, high-speed reciprocating of the plunger in the valve would inherently cause undesired back and forth movements of fluid undermining the stability of the microsystem. Besides, the compressed air would delay the response time of the system and a finely adjustable pressure regulator was needed to precisely manipulate the fluids at picolitre level which also increased the complexity of the system. Here, we innovatively utilized the suction effect originated from the reciprocating of the valve plunger as a driving force instead of eliminating it through simply adding a short piece of capillary between the valve and the chip. In this way, a uniquely simple, low-cost and on-demand microfluidic controlling platform based on the capillary-tuned microvalve suction was developed without any other outer pressure source. RESULTS AND DISCUSSION

Figure 1: The mechanism of capillary-tuned solenoid valve suction. (a) Schematics of the operating principle of the solenoid valve in a full work cycle. (b) Back and forth movements (indicating “suction” and “push” effects) when the capillary inserted into “Comm” tubing. (c) Two sets (with v.s. without capillary) of zoomed pictures of “Comm” tubing.

978-0-9798064-7-6/µTAS 2014/$20©14CBMS-0001 213 18th International Conference on MiniaturizedSystems for Chemistry and Life Sciences

October 26-30, 2014, San Antonio, Texas, USA

The basic operating principle of a 3-Way Direct Lift solenoid valve (response time of 20 ms; Cole-Parmer, USA) is schematically illustrated in Figure 1a. In the energizing period, the “N.O.” orifice shows “push” and both the “Comm.” and “N.C.” orifices show “suction” (Figure 1a (ii)). Then, in the de-energizing period, both the “Comm.” and “N.C.” orifices show “push” and the “N.O.” orifice shows “suction” (Figure 1a (iv)). To utilize the suction effect as the only driving force for microfluidic controlling, we exploited the principle of asymmetric hydrodynamic resistances on different ports of the valve to eliminate the push effect by inserting a piece of capillary into one of the ports. Three PEEK tubings were separately connected with three ports of the solenoid valve. Then, a fused-silica capillary of 5 cm length (OD: 360 μm, ID: 150 μm) was inserted into one of the three PEEK tubings, which generated a significant high hydrodynamic resistance in the port with capillary. Very interesting result was achieved when the capillary was inserted in the “Comm.” tubing (Figure 1b and 1c). In this case, only a slight suction effect and no push presented in the “Comm.” tubing while significant both suction and push effect were still observed in the “NC.” and “NO.” tubings, which might result from the following two aspects. One is that the additional capillary increased the hydrodynamic resistance of “Comm.” port significantly, which greatly weakened both “suction” and “push” effects. The other may be attributed to the internal geometry structure of the valve which induced the suction volume much bigger than the pushing volume as presented in the experiments without capillary (Figure 1c). It was owing to these two reasons that the “push” effect was weakened to a negligible level.

Figure 2: Development and characterization of the microfluidic controlling platform based on the capillary-tuned solenoid microvalve suction. (a) The schematic (left) and real picture (right) of the platform. (b) Simulative and experimental characterization of the suction process.

A solenoid-microvalve-suction-based microfluidic controlling platform was developed (Figure 2a). The

controllability of the suction process in this developed platform was characterized by both simulations and experiments (Figure 2b). Computational fluid dynamics (CFD) was firstly used to simulate the suction process. The results showed that the negative pressure exerting on the fluid through the “Solenoid valve” port could actuate the splitting of the flow in the “Dye” channel into the “Outlet 2” channel to form a plug-like dye flow whose volume (i.e., suction volume) increased with the duration of the applied negative pressure from 10 ms to 40 ms (Figure 2b (ii)). The experiment results also indicated that the suction effect generated in the “Comm.” port of the valve could be well controlled by the energized duration which was consistent with the simulation results, suction volume increased with the energized duration from 25 ms to 55 ms (Figure 2b (iii)). Another key factor which can also be used to adjust the suction effect was the hydrodynamic resistance. The experiment results showed that the longer inserted capillary with higher hydrodynamic resistance resulted in the smaller suction volume in each suction cycle (no data shown here). Besides, the stability of the running process of the microfluidic platform during continuously operating of the solenoid valve was also investigated by experiments. The results indicated that the flow field in the chip could rapidly restore to its original state after each actuation (data not shown here).

214

Figure 3: A series of applications of solenoid-valve-suction-based on-demand microfluidic controlling platform. (a) Suction-based microfluidic cell isolation. (b) Suction-based droplet sorting. (c) Suction-based on-demand size-controllable droplet generation.

In proof of the concept, a series of important active microfluidic unit operations such as cell/droplet sorting, on-demand droplet generation were further demonstrated on this suction-based platform (Figure 3). The trajectory of isolating a target Yeast cell from the cell suspension into the collection chamber connected with a solenoid valve was showed in the time-lapse pictures (Figure 3a). The red droplet containing with amaranth dye could be sorted into the collection channel by the solenoid valve suction changing its flow direction (Figure 3b). Besides, the suction effect could also suck and split one drop of amaranth dye aqueous solution which flowed into the oil channel to form a W/O droplet (Figure 3c). And benefiting from the adjustable suction volume, different size of the droplets such as the length of 53, 99 and 123 μm could be produced with different valve energized duration of 30, 40 and 50 ms respectively, which indicated that the suction-based platform could more easily generate picolitre-sized droplets compared with the reported droplets generator based on external valves.

CONCLUSION

The suction force originated from actuating a capillary-connected external solenoid microvalve was innovatively and successfully employed as the only driving force for on-demand microfluidic controlling platform in this work. Compared with the existing approaches, the unique suction-based microfluidic con-trolling platform showed outstanding simplicity, controllability and stability. Furthermore, the capability and flexibility of the suction-based platform ensured a great deal of potential applications, for example, it could easily be coupled with other high speed detection/measurement technologies (such as Raman, fluo-rescence and imaging detectors), or integrated into much complicated microfluidic system to facilitate au-tomatic and high-throughput screening and single cell omics. ACKNOWLEDGEMENTS

This work was finically supported by Basic Research in Scientific Instrument Grant of National Natu-ral Science Foundation of China (No. 31327001), Scientific Instrument Development Grant and Key De-ployment Grant on Modern Agriculture of the Chinese Academy of Sciences, from the Chinese Academy of Sciences (No. YZ201236, No. KSZD-EW-Z-021-1-5).

REFERENCES [1] Z. Cao, F. Chen, N. Bao, et al. “Droplet sorting based on the number of encapsulated particles using a solenoid valve”. Lab

on a chip, 13, 171-178, 2013. [2] K. Churski, M. Nowacki, P.M. Korczyk, et al. “Simple modular systems for generation of droplets on demand”. Lab on a

Chip, 13, 3689-3697, 2013.

CONTACT * B. Ma; phone: +86-532-80662657; mabo@qibebt.ac.cn

215