International Conference on Enabling Science and Nanotechnology 2012 (ESciNano 2012) 5-7 January 2012, Persada Johor International Convention Center, Johor Bahru, MALAYSIA

ESciNano 2012 – http://www.fke.utm.my/mine/escinano2012

Fabrication of PDMS multi-layer microstructure: The Electroosmosis mechanism in fluidics for life sciences

Tijjani Adama, U.Hashima, Pei Ling Leowb, Pei Song Cheeb and K.L. Fooa

a Institute Nano Electronic Engineering, Universiti Malaysia Perlis. (UniMAP), 01000 Kangar, Perlis Malaysia

b Faculty of Electrical Engineering, Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor Bahru, Malaysia

tijjaniadam@yahoo.comuda@unimap.edu.my

Abstract

The article contains report on fabrication of multi-layer microstructures that takes the advantage of electroosmosis to mix fluids. Microlaboratories for biochemical applications often require rapid mixing of different fluid streams. At the microscale, flow is usually highly ordered laminar flow and the lack of turbulence makes diffusion the primary mechanism for mixing. While diffusional mixing of small molecules can occur in a matter of seconds over distances of tens of micrometers, mixing of larger molecules such as peptides, proteins, and high molecular-weight nucleic acids can require equilibration times from minutes to hours over comparable distances. Such delays are impractically long for many chemical analyses. These problems have led to an intense search for more efficient mixers for microfluidic systems most microscale mixing devices are either passive mixers that use geometrical stirring or active mixers that use moving parts or external forces, such as pressure or electric field. In a passive mixer, one way of increasing the mixing is by “shredding” two or several fluids into very thin alternating layers, which decreases the average diffusion length for the molecules between the different fluids. Another way of improving mixing efficiency is to use active mixers with moving parts that stir the fluids. At the microscale level moving parts in an active mixer are very fragile. One alternative solution which we explored is to use electroosmotic effects to achieve a mixing effect that is perpendicular to the main direction of the flow.

Keywords: Multi-layer, Microstructure, Electroosmosis, Microlaboratories, Microfluidic, diffusion

1.0 Introduction Electroosmotic driven fluid mechanism is a widely used in microfluidic components, a common example is micropump. Electro statically driven fluid teclmique in micro/nano in life science application has tremendous advantages such as simple design; applicability to wide range of conductive fluids and favorable scaling of electrical forces[1-5] .Thus, the approach required Electrodes are necessary to establish the electrical forces and hence fluid flow inside the Micro channel[6-10].

2.0 Materials and Method

Microfluidic devices can be conveniently fabricated using polymers. The Polydimethylsiloxane (PDMS) gained a wide spread acceptance since it merges a number of advantageous features including chemical stability, biocompatibility and optical transparency. PDMS is the most

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International Conference on Enabling Science and Nanotechnology 2012 (ESciNano 2012) 5-7 January 2012, Persada Johor International Convention Center, Johor Bahru, MALAYSIA

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commonly used material in soft-lithography, a fabrication technique for the rapid prototyping of microfluidic chips. Large number of researchers have recently shown the feasibility of planar electroosmotic microchannels fabricated by soft-photoligraphy.the same technique being explored in this research. After the device design, the fabrication follows. The fabrication of three parts: two master templates fabrication, replica fabrication by low temperature curing process, electrodes and electric connections. The device fabrication started with mask design which was designed using AutoCAD software and printed with high resolution printer on a plastic transparent mask for alignment and exposure, the next is wafer cleaning, the heating the wafer surface for the removing moisture adsorbed on the wafer surface. A clean, dehydrated wafer is essential for adhesion of photoresist on the wafer surface. Poor adhesion can lead to failure of the photoresist patterning and can cause under cut during the subsequent etch processes. The next step is photoresist coating; the photoresist is coated using deposition process in which a thin layer of photoresist is applied on the wafer. The wafer is place on a spindle with a vacuum chuck that can hold the wafer during the high-speed rotation. Liquid photoresist is applied on the wafer surface, and centrifugal force from the wafer rotation spreads the liquids over the whole wafer. After the solvents in the Photoresist are evaporated, the wafer is coated with thin layer of photoresist. The photoresist thickness is determined by both photoresist viscosity and the wafer spin rate. The fabrication begins by creating two master template and each was done with separate photolithography process, the silicon was used as the substrate. The silicon substrate was first coated with Negative photoresist (SU-8) to 100µm, the coated substrate was exposed to UV light through the first mask ( for pattern transfer). And the second substrate was also exposed using similar procedure with first one. After exposed, both coated substrate were put into resist developer for 1minute, After the resist was completely developed, the dissolved part was washed with deionised water and process continue to realise the channel, after channel template was fabricated with standard soft-photolithographic techniques.. Then a 1:10 PDMS prepolymer: curing-agent (Sylgard 184 elastomers, Dow Corning) degassed mix was poured onto the master, cured, and pealed off. The obtained PDMS microchannel bears the negative SU-8 relief structure. The PDMS microchannels were finally aligned with the embedded ultra-thin aluminum sheet in it and irreversibly sealed to one another after O2 plasma surface activation (90 s at 250W and 60mTorr). An extended copper wire was soldered on the aluminum to save as conducting pad.

2.1 Fabrication process

Silicon wafer Silicon wafer

Coated with SU-8 Coated with SU-8

Soft bake at 65 oC Soft bake at 65 oC

Alignment & UV exposure Alignment & UV exposure

First Master template Second Master template (a) (b Figure2: Fabrication process flow (a) First master template (b) second Master template

International Conference on Enabling Science and Nanotechnology 2012 (ESciNano 2012) 5-7 January 2012, Persada Johor International Convention Center, Johor Bahru, MALAYSIA

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Figure3: (a) and (f) PDMS Preparation (b )and (g) PDMS was poured into master templates Figure3: (a) and (f) PDMS preparation (b) and (b) pouring PDMS on to the master templates (c) and (h) room temperature curing of the PDMS (d) and (i) detaches (e) and (k) fabricated devices (l) Plasma bonding (m) Bonded device.

3.0 Results 3.0 Results

Figure4:Assembly of the fabricated microchannel (a) Fabricated channels (b) fabricated micro chambers (c) Aligning for bonding (d) insertion of aluminium foil conductors (e) assembled devices (f) testing the device without electrical connection (g) testing the device with electrical connection,

4.0 Reference

[1]A. Brask, D. Snakenborg, J. P. Kutter , H. Bruus, (2006)"AC electro osmotic pump with bubble-free palladium electrodes and rectifying polymer membrane valves", Lab Chip, Vol. 6, pp.280-288,

[2]W. W. Y. Chou, K. F. Lei, G. Shi, W. J. Li, Q. Huang, ,(2006)"Microfluidic channel fabrication by PDMS-interface bonding", Smart Mater. Struct, VoLl5, pp.S112-S116.

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International Conference on Enabling Science and Nanotechnology 2012 (ESciNano 2012) 5-7 January 2012, Persada Johor International Convention Center, Johor Bahru, MALAYSIA

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[3]S. Doerner, S. Hirsch, R. Lucklum, B. Schmidt, P. R. Hauptmann, V. Ferrari, M. Ferrari, (2005) "MEMS ultrasonic sensor array with thick film PZT transducers", Ultrasonics Symposium IEEE 2005, VoLl, pp.487 - 490.

[4]K. A. D. Guzman, R. N. Kamik, J. S. Newman, A. Majumdar,(2006)"Spatially Controlled Microfluidics Using Low-Voltage Electrokinetics", JMEMS, Vo1.15, pp.237- 245.

[5] Nguyen N T et al 2004 A polymeric microgripper with integrated thermal actuators J.Micromech. Microeng. 14, 969–74

[6] Okkels F and Tabeling P 2004 Spatiotemporal resonances in mixing of open viscous fluidis Phys. Rev. Lett. 92 038301

[7]Park S J et al 2004 Rapid three-dimensional passive rotation micromixer using the breakup process J. Micromech.Microeng. 14 6–14

[8]Jang L-S, Kan W-H. (2007)Peristaltic piezoelectric micropump system for biomedical applications. Biomed Microdevices; 9: 619-626.

[9]Wu Z, Nguyen NT, Huang XY. Non-linear diffusive mixing in microchannels: theory and experiments. J Micromech Microeng 2004; 14: 604-611.

[10] N. Vourdas, A. Tserepi, A.G. Boudouvis and E. Gogolides, “Plasma processing for polymeric microfluidics fabrication and surface modification: Effect of super-hydrophobic walls on electroosmotic flow”, Microelectronic Engineering, vol. 85(5-6), pp. 1124-1127, 2007.