FABRICATION METHOD OF BASED-PARAFFIN RECONFIGURABLE MICROFLUIDIC CHIPS USING LIQUID DIELECTROPHORESIS AND ELECTROWETTING TRANSDUCTIONS R. Renaudot1*, V. Agache1*, Y. Fouillet1, M. Kumemura2,3, L. Jalabert3, D. Collard3 and H. Fujita2. 1 CEA-LETI, Minatec Campus, Grenoble, FRANCE 2 IIS, The University of Tokyo, Tokyo, JAPAN 3 LIMMS/CNRS-IIS, The University of Tokyo, Tokyo, JAPAN
ABSTRACT A new and original concept aiming at rapid fabrication of continuous microfluidic chips with a channel-based reconfigurable and programmable geometry is reported in this paper. Interfaces between DI water and liquid paraffin-wax are precisely controlled using electromechanical forces onto an array of electrodes, similar to classical digital microfluidic platforms. As the liquid–liquid interfaces define the targeted geometry, the paraffin is solidified by decreasing the chip temperature using an integrated thermoelectric cooler. Then, the paraffin channel-based chip is used for in-flow continuous microfluidic experiments. First results describing the whole concept are shown with typical fluidic geometries.
KEYWORDS Digital microfluidics, continuous microfluidics, paraffin, phase transition, electrowetting on dielectrics, liquid dielectrophoresis.
INTRODUCTION Electrowetting on dielectric (EWOD) or liquid dielectrophoresis (LDEP) transductions are known to handle fluid-fluid interfaces onto surfaces. Complex and accurate fluidic functions (mixing, splitting, merging …) can be performed using these techniques by applying electrical potentials onto an electrode array. So far, these Digital MicroFluidics (DMF) techniques mainly address three application areas: Biochemical protocols at the micro-nanoliter scale [1]. Provide alternative miniaturized lenses/pixels for optics [2]. New mechanical actuators (pump, valves…) [3]. In parallel, continuous microfluidic (CMF) is considered to be a powerful tool for various applications (biology, medicine, cosmetic…). Nevertheless one major issue concerns the microfluidic chip fabrication with its more or less complex geometries (channels, pillars, chambers, inlets, outlets). Indeed, whatever the substrate materials, the chip fabrication process usually requires several steps (lithography, etching, bonding, moulding…), and thereof is costly and time consuming. Moreover the microfluidic component is often disposable after its first use [4]. The study aim is to present a new approach for CMF chip fabrication that overcomes such drawbacks. We propose hence to address a new DMF application area,
which is the fabrication of CMF based-paraffin chips by using EWOD/LDEP actuations. The basic idea is to take advantage of some chemical compounds phase transitions close to the temperature room, for instance paraffin-wax (see Fig. 1). The first step consists in monitoring precisely the DI water / liquid paraffin-wax interfaces with EWOD and/or LDEP actuations. Once the overall interface geometry is reached, represented by the overall DI water liquid finger inside the DMF platform, its temperature is cooled down, inducing the paraffin wax surrounding medium hardening. Hence a channel-based chip is created (see Fig. 2) and is ready-to-be used for CMF applications. One of the major advantages related to this concept is the possibility to program and choose a channel geometry, among a lot of variants, from a unique single DMF platform. Secondly, the chip geometry is reconfigurable, because of the reversibility of paraffin-wax phase transitions. Hence, various CMF chip geometries can be created with the same DMF platform that can be recycled. In this paper, the first promising results of this innovative concept are shown. Simple microfluidic basedchannel geometry have been created and in-flow CMF experiments have been performed to validate these new approach merits.
Figure 1: Schematics illustrating the principle of the programmable and reconfigurable geometry microfluidic chips fabrication using the paraffin-wax medium solidliquid transitions.
MATERIAL AND METHOD The two parallel-plates architecture chip, typically used for droplets operations in EWOD platforms [1], is made of a silicon substrate and a top glass cover, separated by thick Ordyl photosensitive resist walls (See Fig. 2). The silicon substrate is patterned with two electrode metal levels (Ti 10 nm + AlCu 200 nm + Ti 10 nm + TiN 40 nm). AlCu is mix of aluminum (wt > 90 %)
and copper. The EWOD/LDEP driving electrodes are then coated by a dielectric bi-layer. The dielectric bi-layer, made of 100 nm thick SiN (silicon nitride) and 100 nm thick SiOC (silicon oxycarbide), is well adapted for liquid actuations onto a surface, for both EWOD and LDEP techniques [1,5]. The glass cover is coated with a 140 nm ITO layer (Indium Tin Oxide), which plays the role of electrical ground for every substrate driving electrodes. Subsequently, the cover is coated by a thin 100 nm thick hydrophobic SiOC layer (θSiOC = 115° for a 1 µL DI water droplet).
Figure 2: Cross-section view of the technology used in this study. The electrodes width is w = 600 μm and the height between the silicon substrate and the glass cover is g = 110 μm. The electrodes are driven thanks to a signal generator (Agilent 33220A) coupled with a voltage amplifier (NF HSA 4101, gain x 10-20). The electrodes are individually switched on/off with a USB computer interface (National Instrument USB-6509) and three electromechanical relay boxes (National Instrument ER 8/16). The liquid finger displacements are captured with an optical digital microscope lens (Keyence VH-Z20R) coupled with classical camera (Keyence VHX-500). The chip temperature control is carried out by a thermoelectric cooler (TEC) implemented under the microfluidic chip. The TEC is monitored by a Keithley thermoelectric regulator (Model 2510 TEC SourceMeter). In this study, the paraffin-wax used is a mix of several n-alkanes, chemical formula CnH2n+2, (Sigma Aldrich, 327204), with an expected melting temperature around Tm = 55°C. The paraffin-wax electrical properties at liquid state are very close to mineral or silicon oil. The electrical conductivity is lower than 10-17 S.m-1 at 20°C and the dielectric constant is about 2. The overall concept protocol is described in Fig. 3.
Figure 3: Schematics illustrating the protocol steps for channel-based microfluidic chips fabrication using LDEP and EWOD actuations in a paraffin medium.
RESULTS AND DISCUSSION Liquid-liquid interfaces monitoring First results, illustrating the programmable and reconfigurable concept aspects, are shown in the following Fig. 4 and Fig. 5. The chip design features an array of 35 electrodes (see Fig. 4) which can be individually switched on/off. The electrodes shape mimics either simple channels shapes or more complex ones, which are representatives of a fluidic function (zigzag, chamber, micro flow-focusing device structure…). The overall design aims at building a given paraffin geometry based on combination of previous fluidic functions. Fig. 4 shows the DI water actuation in the liquid paraffin-wax medium by switching on, step by step (from a. to e.), seven electrodes required to create an inlet, a zigzag channel and an outlet. The actuation voltage is V = 100 Vpp, its frequency is f = 50 kHz, and the temperature chip is Tc = 60°C. This actuation voltage value is very close to the threshold actuation voltage value. The droplet mother reservoir needs to be regularly supplied with DI water, in order to ensure liquid finger motion all along the desired pathway. The liquid-liquid interfaces precisely follow the energized electrodes lateral edges, even atop of the zigzag angular geometries or right-angled shaped geometries. Virtually, a DI water channel, from a reservoir inlet to a reservoir outlet, is hence created inside a liquid paraffinwax medium.
Figure 4: Optical photographs illustrating the DI water displacement using electromechanical actuations in a liquid paraffin medium (V = 100 Vpp and f = 50 kHz).
Paraffin-wax phase transition step Paraffin-wax phase transition from liquid to solid phase occurs at Tm = 55 °C. In the Fig. 5 (snapshot top-
right), the geometry, previously established (see Fig. 4), is fixed by cooling down the chip at the room temperature (about 25°C). The paraffin-wax solidifies and a channelbased CMF chip is then produced. The phase transition is quasi-instantaneously obtained (in a few seconds). At the solid phase, the paraffin is opaque (white color), and its walls surface is hydrophobic (θpar > 90° with 1 µL DI water) The major drawback is the volume difference between liquid and solid paraffin-wax and therefore the controllability of the interface position while the phase transition is induced. As demonstrated in [6], the volume may vary as a function of the temperature. This effect is due to the thermal expansion coefficient which is quite high for the paraffin-wax compounds. The phenomenon is visible during the solidification process and may be an issue to control both channel walls roughness and its dimensions at the micrometer scale. Generally, during the solidification process, the paraffin retracts and the DI water expands, favored also by the pulling of DI water high volume quantity stored in the reservoirs. Hence, the created channel is usually wider than the liquid finger width (which corresponds to the substrate electrode width) before the solidification stage.
Programmable and reconfigurable aspect As explained in the introduction, this method provides a versatile and reconfigurable approach to build on demand CMF microfluidic chips. In other words, the initial DMF platform is generic and able to construct various based-channel geometries, chosen and programmed by the end-user. Moreover, the paraffin melting and hardening can be performed endlessly, on demand, inside the microfluidic chip. When a first geometry is fixed with paraffin solidification for a targeted CMF application, it is then possible to recycle the same device and perform another temperature cycle. When the paraffin comes back at its liquid phase, a new channel geometry can be created and fixed by paraffinwax solidification for another CMF application. In Fig. 5, from the same platform component used in Fig. 4, a second temperature cycle is carried out to establish a new geometry and create a different microfluidic overall channel network, by adding a Tjunction and an inlet. From that moment, the microfluidic chip may be used for mixing of two fluids (streamer function) [7], or for in-flow droplet generation [8].
Figure 5: Optical photographs illustrating two different geometry chip fabrications on a same platform component. The chips fabrication process is programmable and reconfigurable.
Continuous microfluidic experiments From the last cured geometry, shown in Fig. 5, in flow CMF experiments are performed to validate the overall concept. The first experiment consists in mixing two coloured liquids, the zigzag geometry being often considered as a characteristic mixer function in microfluidics. Microfluidic inlets and outlets with Tygon tubings are assembled at the glass cover holes locations, and connected to a pressure source controller (Fluigent MFCS-FLEX). Fig. 6 shows the flow of a green and a red colored fluid through the paraffin based chip. No leakage has been observed during the experiment, with pressures beyond 500 mbars for both inlets. Subsequently, the same platform component has been used for fluorescent DI water droplets generation in a soya oil continuous phase. No surface treatment, and addition of surfactant in both phases, has been performed. Fig. 7 shows the droplets formation as a function of time, while pressurizing DI water and oil phases at PDIwater = Poil = 200 mbars. Droplets generation process is not admittedly optimized because of insufficient shear stress at the T-junction intersection.
CONCLUSION Although some technical aspect still needs further improvements (in particular the paraffin walls shape regularity), this paper presents an innovative and versatile concept aiming at producing various microfluidic channels devices for CMF applications, from a single and reusable DMF platform component. This approach is intended to be a fast alternative technique for chip prototyping.
Figure 6: Photos illustrating the microfluidic chip after geometry fixing during a two coloured fluids mixing in a streamer channel. (R and G respectively stand for red and green).
[6]
[7]
[8]
[9]
Nanofluidics, 2013, DOI 10.1007/s10404-013-11562. P. R. Templin, “Coefficient of Volume Expansion for Petroleum Waxes and Pure n-Paraffins”, Ind. Eng. Chem., vol 48, pp 154–161, 1956. V. Mengeaud, J. Josserand, H. H. Girault, “Mixing Processes in a Zigzag Microchannel: Finite Element Simulations and Optical Study”, Anal. Chem., vol. 74, pp. 4279-4286, 2002 P. Garstecki, M. J. Fuerstmana, H. A. Stonec, G. M. Whitesides, “Formation of droplets and bubbles in a microfluidic T-junction-scaling and mechanism of break-up”, Lab Chip, 6, pp. 437-446, 2006. B. Hadwen, G. R. Broder, D. Morganti, A. Jacobs, C. Brown, J. R. Hector, Y. Kubota ,H. Morgan, “Programmable large area digital microfluidic array with integrated droplet sensing for bioassays”, Lab Chip, vol.12, pp. 3305-3313, 2012
CONTACT *R. Renaudot, tel: +33-438-781443;
[email protected]. *V. Agache, tel: +33-438-782653;
[email protected]. Figure 7: Video snapshots illustrating fluorescence DI water droplets formation in soya oil phase at a T-junction (PDI water = 200 mbars and Poil = 200 mbars). The results highlight the programmable and reconfigurable attributes of the chips geometry. As a perspective to that study, next investigations on high electrodes density DMF platforms will be carried out to fabricate on demand complex channel networks [9].
ACKNOWLEDGEMENT The authors want to thank M. Cochet, G. Castellan, and M. Allessio from CEA-Leti for respectively the silicon devices fabrication, the SiOC deposition and the silicon glue serigraphy process.
REFERENCES [1] Y. Fouillet, D. Jary, C. Chabrol, P. Claustre, C. Peponnet, “Digital microfluidic design and optimization of classic and new fluidic functions for lab on chip systems”, Microfluid Nanofluid, vol. 4, pp. 159–165, 2007. [2] B. Berge, J. Peseux, “Variable focal lens controlled by an external voltage: an application of electrowetting”, Eur. Phys. J. E, vol. 3, pp159-163, 2000. [3] K. S. Yun, I. J. Cho, J. U. Bu, C. J. Kim, E. Yoon, “A surface-tension driven micropump for low-voltage and low-power operations”, J. Microelectromech. Syst., vol. 11, pp. 454-461, 2002. [4] G.S. Fiorini, D.T. Chiu, “Disposable microfluidic devices: fabrication, function, and application” BioTechniques, vol. 38, pp. 429–446, 2005. [5] R. Renaudot, V. Agache, Y. Fouillet, M. Kumemura, L. Jalabert, D. Collard, H. Fujita, “Performances of a broad range of dielectric stacks for liquid dielectrophoresis transduction”, Microfluidics
