Micro-Molded Glassy Carbon Electrodes for High Throughput Dielectrophoresis
Authors: Muhamed Sam Azhar , Natu Rucha , Keck Devin Monroe , Islam Monsur , Martinez-Duarte Rodrigo 
Published: February 2020

3D glassy carbon electrodes employed in dielectrophoresis (DEP) devices provide the advantages of simple and affordable fabrication, functionality at low voltage and the wider electrochemical stability of carbon in comparison to traditional metal electrodes [1]. Furthermore, the use of 3D electrodes offers high throughput of separation [2]. The state-of-the-art in the fabrication of glassy carbon micro- and nano-structures revolves around patterning and subsequent pyrolysis of organic precursors. In our previous work, we developed a computational model for high throughput sorting of stem cells using DEP [3]. Electrode dimensions in the range of hundreds of microns need to be fabricated to achieve practical throughput without compromising the cells by inducing increased shear stress. However, such dimensions are beyond the traditional operating range of SU-8 photolithography, making their fabrication challenging. On the other hand, precision machining techniques are still too bulky to enable easy and inexpensive fabrication in this scale. Here we propose the combination of 3D printed structures and Polydimethylsiloxane (PDMS) molds to shape an epoxy-based carbon precursor. The work presented here focuses on the development of reproducible carbon electrodes for application in high throughput dielectrophoretic separation.
A 3D printed “inverse” mold containing an array of 1200 microns diameter cylindrical posts is fabricated to suit PDMS molding, which, in turn, is used to mold the carbon posts. Commercial grade PDMS mixed in the ratio of 10:1 was used to create molds of cylindrical posts using the standard procedure. The resulting PDMS molds ensured sufficient flexibility to facilitate easy removal of hardened materials. Plant-based epoxy, chitin, cellulose and carrageenan were tested in different mixtures as carbon precursors. Powdered chitin, cellulose and carrageenan were separately mixed into the epoxy at various ratios and introduced into the PDMS mold. Upon setting, they were removed from mold and pyrolyzed at 900oC in an inert atmosphere to yield conductive carbon.
Initial results showed how bio-epoxy, which consists of a 2:1 ratio of resin to hardener, fails to retain the shape of the mold through the pyrolysis process. On the other hand, chitin, cellulose and carrageenan when formed into gel and pyrolyzed retain shape, but yield fragile structures. Hence, we combined the binding capacity of bio-epoxy with the structural integrity provided by biopolymers. A 40-80% weight by weight mixture of chitin in bio-epoxy ensured adequate concentration for a trade-off between shape retention and molding ability. This range also pyrolyzed to give cylindrical glassy carbon posts with an average electrical resistance of 7 ohms across the 4mm distance between the farthest electrodes. Pyrolysis also brought about an average diametric shrinkage of 40% in the electrodes.
Ongoing work involves using this molding process for SU-8 photoresist. We hypothesize that the production of carbon micro-electrodes is a forerunner to the adoption of the same process in the manufacture of other conductive carbon components in the same size range. By characterizing further complex geometric features and identifying their suitability to this process, we aim to develop a standard to manufacture a broader range of glassy carbon micro-components.
REFERENCES:
[1] Martinez‐Duarte, R., Renaud, P., & Madou, M. J. (2011). A novel approach to dielectrophoresis using carbon electrodes. Electrophoresis, 32(17), 2385-2392.
[2] Islam, M., Natu, R., Larraga-Martinez, M. F., & Martinez-Duarte, R. (2016). Enrichment of diluted cell populations from large sample volumes using 3D carbon-electrode dielectrophoresis. Biomicrofluidics, 10(3), 033107.
[3] Natu, R., & Martinez-Duarte, R. (2016). Numerical Model of Streaming DEP for Stem Cell Sorting. Micromachines, 7(12), 217.

Figure 1