This work presents a miniaturized system combining, on the same microfluidic chip, capillarity and electrowetting-on-dielectric (EWOD) techniques for movement and control of fluids. The change in hydrophobicity occurring at the edge between a capillary channel and a hydrophobic layer is successfully exploited as a stop-and-go valve, whose operation is electronically controlled through the EWOD electrodes. Taking into account the variety of microfluidic operation resulting from the combination of the two handling techniques and their characteristic features, this work prompts the development of autonomous, compact and low-power consumption lab-on-chip systems

Bellini, Enrica

CAPRINI, DAVIDE

CAPUTO, Domenico

DE CESARE, Giampiero

Llorca, Pablo R.

LOVECCHIO, NICOLA

NARDECCHIA, MARCO

NASCETTI, Augusto

PETRUCCI, GIULIA

English

Archivio della ricerca- Università di Roma La Sapienza

 Procedia Engineering  168 ( 2016 )  1370 – 1373 1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conferencedoi: 10.1016/j.proeng.2016.11.380 ScienceDirectAvailable online at www.sciencedirect.com30th Eurosensors Conference, EUROSENSORS 2016 Integration of capillary and EWOD technologies for autonomous and low-power consumption micro-analytical systems Marco Nardecchiaa,b,*, Enrica Bellinib, Pablo R. Llorcac, Davide Caprinic, Nicola Lovecchiob, Giulia Petruccib, Domenico Caputob, Giampiero de Cesareb, Augusto Nascettia aS.I.A., University of Rome “La Sapienza”, Via Salaria 851/881, 00138 Rome, Italy  bD.I.E.T., University of Rome “La Sapienza”, Via Eudossiana 18, 00184 Rome, Italy  cD.I.M.A., University of Rome “La Sapienza”, Via Eudossiana 18, 00184 Rome, Italy  Abstract This work presents a miniaturized system combining, on the same microfluidic chip, capillarity and electrowetting-on-dielectric (EWOD) techniques for movement and control of fluids. The change in hydrophobicity occurring at the edge between a capillary channel and a hydrophobic layer is successfully exploited as a stop-and-go valve, whose operation is electronically controlled through the EWOD electrodes. Taking into account the variety of microfluidic operation resulting from the combination of the two handling techniques and their characteristic features, this work prompts the development of autonomous, compact and low-power consumption lab-on-chip systems. © 2016 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Keywords: Lab-on-chip, Microfluidics, ElectroWetting-On-Dielectrics, Capillarity, Dispensing, Hydrophobic and Hydrophilic Surfaces. 1. Introduction Lab-on-chip (LoC) devices are miniaturized systems able to perform biochemical analysis by integrating, in a small chip of few centimeters square, several modules that implement the functionalities of a standard laboratory for biomedical analysis and diagnostics [1, 2, 3]. The involved analytical and diagnostics aspects imply the movement of solutions along microfluidic channels. Actually, capillary microfluidic systems allow to obtain  an  autonomous  flow     * Corresponding author. Tel.: +39 0644585337. E-mail address: marco.nardecchia@uniroma1.it  2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommon .org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference1371 Marco Nardecchia et al. /  Procedia Engineering  168 ( 2016 )  1370 – 1373  without the need for external pumping system and bulky connection, but timing and synchronization of lab-on-chip operations are not easily controlled [4, 5]. In order to overcome these issues, our microfluidic system combines two different techniques to drive liquids through a microfluidic network: the capillarity, that takes advantage of the surface tension effect, and the electrowetting-on-dielectric (EWOD) approach, that changes from a hydrophobic to a hydrophilic status the wettability of a surface through an electric field. The main idea of this paper is to use the EWOD technique to control the dispensing of a fluid in a capillary-driven micro-channel only when an appropriate voltage is applied on the electrodes. In particular, in the implemented system, the electric fields applied over the hydrophobic regions control the starting time of the fluid injected in the dispenser. When it reaches the hydrophilic region, the capillary flow forces move the fluid through the microfluidic channels [6, 7].  These two technologies have been integrated on a single glass substrate. To this aim, the technological steps for the fabrication of regions characterized by different hydrophobicity have been designed and optimized taking into account the hydrophobic materials characterization. The performed tests prove the suitability of the proposed microfluidic chip as low-power consumption, autonomous and compact lab-on-chip system. 2. Device design The integration of EWOD and capillary technologies on the same glass substrate has been implemented through the basic device depicted in Fig. 1. The fluid is injected in a hydrophobic dispenser and it goes along the hydrophilic channel driven by capillaric forces only when a sequence of specified voltage is applied to the EWOD electrodes [8]. The complete system is reported in Fig. 2: a 5×5 cm2 glass substrate hosts the EWOD dispenser, the electrodes and their relative metal connections (red regions) and the capillary-driven micro-channels (blue regions).  Characterization of hydrophobic materials (Teflon AF2400 and Cytop) has been initially done through contact angle measurements with standard needle set-up in open configuration as a function of the applied voltage. The Cytop film allows to obtain the same contact angle variation applying lower voltage with the advantage to have lower baking temperature than Teflon. The variation of contact angle between the droplet and the Cytop surface as function of the applied voltage is reported in Fig. 3 that also shows how the wetting behavior of the liquid depends on the polarity of the applied voltage.  As shown in Fig. 3 a significant contact angle variation from 110° to about 71° is achieved applying a voltage around 15V in case of 2.6μm-thick dielectric layer. A good agreement between experimental results and the Lippmann-Young equation [9, 10] shown in (1) has been achieved. 2000 )(2coscos VVd lvEDL JHTT    (1) A contact angle saturation is observed applying positive voltages: according to literature this effect results from the trapped charges in the dielectric and/or hydrophobic layer upon voltage application [11, 12].   Fig. 1. Top view of the basic device. Fig. 2. Design of the device. 1372   Marco Nardecchia et al. /  Procedia Engineering  168 ( 2016 )  1370 – 1373  Fig. 3. Contact angle as a function applied voltage with Cytop hydrophobic layer.  Lines refer to the Lippmann-Young equation, symbols to the experimental measurements. 3. Device fabrication and testing The first technological step has been the fabrication on the same glass substrate of spatially selected hydrophilic and hydrophobic regions. This has been achieved through an optimization of the Cytop deposition parameters. Fig. 4 demonstrates the successful patterning: a glass substrate is covered with a hydrophobic surface having hydrophilic spots to capture the blue colored water droplets.  A cross section of the basic configuration, together with the used materials, is shown in Fig. 5. It includes a top and a bottom patterned glass substrates, which are coupled and bonded together through two mirrored SU-8 microfluidic networks. The device fabrication required the optimization of different technological steps and in particular of the  SU-8 thickness and bonding pressure.  The process steps for the fabrication of the bottom layer are:  x Deposition of a stack Cr/Al/Cr layer (300Å/1500Å/300Å) acting as electrode and its patterning by photolithography;  x Spin coating of a SU-8 3005 layer acting as dielectric layer and its lithography to cover the metal electrodes; x Spin coating of a Cytop as hydrophobic layer and its patterning through lift-off process to cover the electrodes; x Spin coating of a SU-8 3050 layer and its lithography to create microfluidic channels. The process steps for the fabrication of the top layer are:  x Cleaning of a glass already covered with Indium Tin Oxide (ITO); x ITO wet etching to define a conductive layer in correspondence of the bottom layers electrodes; x Spin coating of the Cytop hydrophobic layer with same process of bottom layer; x Spin coating of a SU-8 3050 layer and its lithography to create microfluidic channels, dispenser, capillary pumps and the space over the electrodes; x Drilling holes at the end of the capillary pumps and at the inlets. The last step is the alignment of the two layers and their bonding using 0.04 kg/cm2 pressure at 160°C for 40 minutes.   Fig. 4. Glass substrate with hydrophobic and hydrophilic area. Fig. 5. Cross section of the basic device. 1373 Marco Nardecchia et al. /  Procedia Engineering  168 ( 2016 )  1370 – 1373  Fig. 6. Device operation showing EWOD dispensing and capillary flow.  Sequence of voltages applied to the EWOD electrodes: a) Dispenser: ON; first, second and third electrode: OFF. b) Dispenser, first electrode: ON; second and third electrode: OFF. c) Dispenser, first electrode: OFF; second and third electrode: ON.  A detailed sequence of the basic structure operation is shown in Fig. 6. At first, the fluid is injected in the dispenser, which is a hydrophobic region. When a voltage equal to 30V is applied, it becomes hydrophilic and fluid flows at the border of the first electrode (Fig. 6a). Applying the voltage sequence reported in the caption of Fig. 6, the fluid covers progressively the EWOD electrodes (Fig. 6b) until reaches the hydrophilic microfluidic channel (Fig. 6c). 4. Conclusion This work demonstrates the successful combination of two techniques (EWOD and capillarity) and the possibility to develop autonomous and compact microfluidic networks, whose operations are electronically controlled. This is a very important result because the capillary action permits the development of miniaturized, low-consumptions systems while the EWOD technique allows the implementation of several microfluidic functions (triggering, mixing, dispensing, separation).  Acknowledgements Authors would like to thank the Italian Space Agency for its support through the project PLEIADES and the Italian Ministry of Education, University and Research (MIUR) through the University Research Project 2015 (prot. C26H15J3PX) for the financial supports. References [1]  R. B. Fair, Digital microfluidics: is a true lab-on-a-chip possible?, Microfluid Nanofluid, 3: 245-281, 2007. [2]  P. Abgrall, A. M. Gue, Lab-on-chip technologies: making a microfluidic network and cou-pling it into a complete microsystem - a review, J. Micromech. Microeng. 17(5), R15-R49, 2007. [3]  D. Caputo, M. Ceccarelli, G. de Cesare, A. Nascetti, R. Scipinotti. Lab-on-Glass system for DNA analysis using thin and thick film technologies,        Proceedings of Material Research Symposium, Vol. 1191, OO06-01, 2009. [4]  M. A. Miled, G. Massicotte, M. Sawan, Dielectrophoresis-Based Integrated Lab-on-Chip for Nano and Micro-Particles Manipulation and       Capacitive Detection, IEEE T. Bio-Med. Eng. 7(4), 557-557, 2013. [5]  D. Caputo, G. de Cesare, M. Nardini, A. Nascetti, R. Scipinotti, Monitoring of temperature distribution in a thin film heater by an array of  a-Si:H temperature sensors. IEEE Sensors Journal, 2012, 12 (5), 1209-1213. [6]  A. P. Washe, P. Lozano-Sanchez, D. Bejarano-Nosas, B. Teixeira-Dias, I. Katakis Elec-trochemically actuated passive stopgo microvalves for        flow control in microfluidic sys-tems, Microelectronic Engineering, 111, 2013, 416-420, 2013. [7] V. Kumar and N. Sharma, SU-8 as hydrophobic and dielectric film in electrowetting-on-dielectric based microfluidics device, Journal of        Nanotechnology, vol. 2012, 2012. [8]  F. Mugele and J.-C. Baret, Electrowetting: from basics to applications,Journal of Physics: Condensed Matter, vol. 17, no. 28, p. R705, 2005. [9]  G. Lippmann, Relations entre les phenomenes electriques et capillaires, Ph.D. dissertation, Gauthier-Villars, 1875. [10] Yan-You Lin, Erin R.F.Welch, and Richard B. Fair, Low voltage picoliter droplet manipu-lation utilizing electrowetting-on-dielectric  platforms, Sensors and Actuators B: Chemical, 173, 338-345, 2012. [11]  M. Nardecchia, N. Lovecchio, P. R. Llorca, D. Caputo, G. de Cesare, A. Nascetti. ”2-D digital microfluidic system for droplet handling using         Printed Circuit Board technology.” AISEM Annual Conference, 2015 XVIII. IEEE, 2015. [12] D. Caputo, G. de Cesare, N. Lovecchio, R. Scipinotti, A. 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Integration of capillary and EWOD technologies for autonomous and low-power consumption micro-analytical systems

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Integration of capillary and EWOD technologies for autonomous and low-power consumption micro-analytical systems

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