Liquid Metal Key to Simpler Creation of Electrodes for Microfluidic Devices

Researchers from North Carolina State University have developed a faster, easier way to create microelectrodes, for use in microfluidic devices, by using liquid metal. Microfluidic devices manipulate small amounts of fluid and have a wide variety of applications, from testing minute blood samples to performing advanced chemical research.

“By making it easier to incorporate electrodes into microfluidic devices, we hope to facilitate research and development into new technologies that utilize those devices, such as biomedical tools,” says Dr. Michael Dickey, an assistant professor of chemical and biomolecular engineering at NC State and co-author of a paper describing the research.

Traditionally, microfluidic devices have incorporated solid metal electrodes that serve as sensors, pumps, antennas or other functions. However, these solid electrodes can be problematic, because they need to be physically aligned to a channel that runs through the device. The channel serves as the entry point for whatever fluid the device is designed to manipulate. Aligning the electrodes is tricky because the electrodes are only tens to hundreds of microns in diameter, as is the channel itself. It is difficult to manipulate objects of that size – a micron is one-millionth of a meter, and a human hair is approximately 100 microns in diameter.

The NC State team has addressed the problem by designing microfluidic devices that incorporate three channels, with the central channel separated from the other two by a series of closely set posts. The researchers inject the two outer channels with a liquid metal alloy composed of gallium and indium. The alloy fills the outer channels completely, but forms an oxidized “skin” that spans the space between the posts – leaving the central channel free to receive other fluids.

“This approach allows you to create perfectly aligned electrodes in a single step,” Dickey says. “The channels are built into the device, so the electrodes are inherently aligned – we get the metal to go exactly where we want it. This means creating these devices is easier and faster.”

In addition, this approach allows for the creation of electrodes in useful configurations that were previously difficult or impossible to achieve. This can be done by changing the shape of the channels that will be injected with the liquid metal. These configurations would create more uniform electric fields, for use in manipulating fluids and particles.

The paper, “Inherently aligned microfluidic electrodes composed of liquid metal,” was co-authored by Dickey and NC State Ph.D. student Ju-Hee So. The paper is forthcoming from the Royal Society of Chemistry’s journal Lab on a Chip. The research was supported, in part, by the National Science Foundation.

NC State’s Department of Chemical and Biomolecular Engineering is part of the university’s College of Engineering.

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Note to Editors: The study abstract follows.

“Inherently aligned microfluidic electrodes composed of liquid metal”

Authors: Ju-Hee So and Michael D. Dickey, North Carolina State University

Published: Forthcoming, Lab on a Chip

Abstract: This paper describes the fabrication and characterization of microelectrodes that are inherently aligned with microfluidic channels and in direct contact with the fluid in the channels. Injecting low melting point alloys, such as eutectic gallium indium (EGaIn), into microchannels at room temperature (or just above room temperature) offers a simple way to fabricate microelectrodes. The channels that define the shape and position of the microelectrodes are fabricated simultaneously with other microfluidic channels (i.e., those used to manipulate fluids) in a single step; consequently, all of the components are inherently aligned. In contrast, conventional techniques require multiple fabrication steps and registration (i.e., alignment of the electrodes with the microfluidic channels), which are technically challenging. The distinguishing characteristic of this work is that the electrodes are in direct contact with the fluid in the microfluidic channel, which is useful for a number of applications such as electrophoresis. Periodic posts between the microelectrodes and the microfluidic channel prevent the liquid metal from entering the microfluidic channel during injection. A thin oxide skin that forms rapidly and spontaneously on the surface of the metal stabilizes mechanically the otherwise low viscosity, high surface tension fluid within the channel. Moreover, the injected electrodes vertically span the sidewalls of the channel, which allows for the application of uniform electric field lines throughout the height of the channel and perpendicular to the direction of flow. The electrodes are mechanically stable over operating conditions commonly used in microfluidic applications; the mechanical stability depends on the magnitude of the applied bias, the nature of the bias (DC vs. AC), and the conductivity of the solutions in the microfluidic channel. Electrodes formed using alloys with melting points above room temperature ensure mechanical stability over all of the conditions explored. As a demonstration of their utility, the fluidic electrodes are used for electrohydrodynamic mixing, which requires extremely high electric fields.