Procedia Engineering 47 ( 2012 ) 817 – 820

1877-7058 © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o.doi: 10.1016/j.proeng.2012.09.272

Proc. Eurosensors XXVI, September 9-12, 2012, Kraków, Poland

A platform for manufacturable stretchable Micro-Electrode Arrays

S. Khoshfetrat Pakazada,*, A. Savova, S. R. Braamb, R. Dekkerca

a ECTM / Delft University of Technology, Feldmannweg 17, 2628 CT Delft, The NETHERLANDS b Pluriomics BV Leiden, Einthovenweg 20, 2333 ZC Leiden, The NETHERLANDS

c Philips Research Eindhoven, High Tech Campus 4, 5656 AE Eindhoven, The NETHERLANDS

Abstract

A platform for the batch fabrication of pneumatically actuated Stretchable Micro-Electrode Arrays (SMEAs) by using state-of-the-art micro-fabrication techniques and materials is demonstrated. The proposed fabrication process avoids the problems normally associated with processing of thin film structures on polydimethylsiloxane (PDMS), by first fabricating the electrodes and electrical interconnects on the silicon wafer using fine-pitched stepper lithography, and afterwards transferring the structures to the elastomer. Stretchability is achieved by a novel spiral design for the interconnects. Experiments demonstrate the biocompatibility of the fabricated devices for in vitro cell culturing.

© 2012 Published by Elsevier Ltd.

Keywords: Stretchable electronics; MEA; Multi electrode arrays; Micro electrode arrays; Mechano-biology; Mechanotransduction.

1. Introduction

Micro-electrode arrays (MEAs) enable mapping of extra-cellular field potentials produced by excitable cells like cardiomyocytes, smooth muscle cells, skeletal muscle and neural cells seeded on the electrode array. Conventional MEAs are fabricated on rigid substrates which lack the ability of in situ mechanical stimulation while performing electrical measurements. Stretchable Micro-Electrode Arrays (SMEAs) are becoming increasingly important tools in biomedical research since they enable the study of the mechano-

* Corresponding author. Current address: High Tech Campus 4 M/S 02, 5656 AE Eindhoven, The Netherlands Tel.: +31-40-2748203; fax: +31-40-2743352.

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818 S. Khoshfetrat Pakazad et al. / Procedia Engineering 47 ( 2012 ) 817 – 820

biology of cells cultured on the SMEAs under electro-mechanical stimulation which can simulate the physiological/pathophysiological conditions for the cells [1].

(a) (b)

Fig. 1. (a) Device layout. Insets show close up views of an electrode (top), soft transition site (bottom) respectively; (b) optical micro-graphs of the SMEA as fabricated.

Currently, the fabrication of SMEAs involves either processing directly on PDMS, which poses serious fabrication challenges, or the use of stretchable conducting materials which are not compatible with state-of-the-art micro-fabrication techniques [2,3]. Here we demonstrate the feasibility of a manufacturable process which enables low-cost mass production of SMEAs for high-throughput clinical and pharmaceutical applications. In this method first the electrodes and electrical interconnects are fabricated on the silicon wafer enabling fine-pitch stepper lithography and afterwards the structures are transferred to the elastomer. Consequently, the problems normally associated with processing of thin film structures on PDMS are avoided.

2. Design of the SMEA

It is important that the interconnects and electrodes of the SMEA occupy a minimal surface area and do not alter the surface topography. Therefore, the use of coplanar and non-coplanar wavy and serpentine structures to achieve stretchability is not possible [4]. Consequently, a novel spiral design is used for the interconnects. The complete SMEA consists of pneumatically actuated circular PDMS membrane embedded with 16 symmetrically arranged circular electrodes 20 µm in diameter, spiral interconnects and micro-grooves to promote cell adhesion and alignment [5] (Fig 1&2).

(a) (b)

Fig. 2. (a) Schematic view of the cell stretching system; (b) optical micro-photograph of the actuated chip using 5 kPa back pressure.

819 S. Khoshfetrat Pakazad et al. / Procedia Engineering 47 ( 2012 ) 817 – 820

(a) (b)

Fig. 3. (a) FEM simulations results showing the magnitude and direction of principal strains in the PDMS. The spiral interconnect tracks are almost perpendicular to the first principal (radial) strain. The interconnects take up the second principal (tangential) stress and therefore confine the tangential strain in the membrane, as a result of which the second principal strain is approximately 4-times lower than the first principal strain allowing for directional stretching of the cells [5]; (b) Optical micro-graph of “soft transition sites” as fabricated (the membrane is actuated upwards).

The electrodes and interconnects are made of titanium nitride and insulated with parylene which is opened at the location of electrodes. The parylene is also used as structural material for the interconnects to give them mechanical rigidity. In order to make the interconnects robust with respect to strains in the membrane during inflation, the trajectory of the interconnects is designed to be perpendicular to first principle strain component in the membrane, and the interconnects are mechanically dimensioned to withstand the second principle strain component (Fig 3a).

The transition points where the interconnects traverse from the soft membrane to the rigid silicon substrate are critical regions where high degrees of bending happen in the interconnects shortening the fatigue life time. To increase the radius of bending curvature, recessions in the rigid substrate are incorporated which allow for a soft transition of interconnects from the soft to rigid material. Moreover, the “soft transition sites” prevent delamination of membrane from the interconnect as a result of high strain gradients which would otherwise develop in PDMS pulling at the interconnects to bend them at this low mechanical leverage regions (Fig 3b).

3. Fabrication

In order to avoid problems associated with processing on PDMS, the processing sequence is reversed by first fabricating the electrodes and interconnects using fine pitch lithography on silicon and finally applying PDMS and subsequently removing the silicon substrate underneath the membrane. The fabrication starts with a silicon wafer provided with 1 µm SiO2 on front side acting as etch-stop for the DRIE of silicon from the backside. Next, 3 µm SiO2 is deposited on the backside and patterned to define the membrane area. Afterwards, the interconnect stack (2 µm parylene - 100 nm TiN - 2 µm parylene) is fabricated by dry etching parylene and TiN using photo-resist masks in oxygen and chlorine plasmas respectively. Next the Al bondpads for wire-bonding the chips to PCB boards are fabricated. The processing continues by patterning a 10 µm thick AZ9260 resist mold to define the micro-grooves in the PDMS. Subsequently, 20 µm PDMS (Dow Corning, Sylgard 186) is spin coated and opened at the location of the Al bondpads in an ICP CF4, SF6 plasma using a 50 nm Al hard-etch mask. Finally, the underlying silicon is DRIE etched from the backside, and the oxide etch stop is wet-etched in BOE and the resist mold is dissolved in acetone (Fig 4a). A SEM micro-graph of the surface of the fabricated device is shown in Figure 3b top image.

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(a) (b)

Fig. 3. (a) Fabrication sequence; (b) (top) SEM micro-graphs of the surface of the SMEA; (bottom) bright field image of stem-cell derived cardiomyocytes cultured on the device. Inset shows immunofluorescent staining for the sarcomeric protein alpha-actinin (green), the ECM protein fibronectin (red) and nuclei (blue).

4. Results and conclusion

Mechanical tests confirm the absence of cracks in the interconnects for a 3.0 mm diameter membrane, cyclically inflated to 700 m height, corresponding to an average radial strain of 15 %. The change in the resistance of the tracks (~ 400 k ) is less than 1% at maximum inflation.

Biocompatibility testing was performed by coating the chips with a thin layer of Matrigel (BD biosciences 1:100) for 1-hour at room temperature. Pluriomics cardiomyocytes were dissociated at day 20 of differentiation using TrypLe (Invitrogen) and replated on the device. Within 3 days spontaneously contracting cells were observed, which formed synchronous beating structures after approximately 5 days. Culture medium was refreshed twice a week. The cells were fixed using 4% paraformaldehyde and stained immunofluorescent for the sarcomeric protein alpha-actinin, the ECM protein fibronectin and nuclei. The cardiomyocytes showed well-defined sarcomeric organization (Fig 3b bottom image).

In future works, in vitro measurements of field potential of the cells under mechanical stimulation as well as comprehensive electro-mechanical tests and fatigue analysis of the SMEA will be presented.

References

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[2] Adrega T, Lacour SP. Stretchable gold conductors embedded in PDMS and patterned by photolithography: fabrication and electromechanical characterization. Journal of Micromechanics and Microengineering 2010;20:055025.

[3] Wei P, Taylor R, Ding Z, Chung C, Abilez OJ, Higgs G, Pruitt BL, Ziaie B. Stretchable microelectrode array using room-temperature liquid alloy interconnects. Journal of Micromechanics and Microengineering 2011;21:054015.

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