An Origami Microfluidic Battery: A Low-cost Activity

Paper microfluidic technologies are emerging as a promising disruptive technology for low-cost sensing and detection. Researchers have developed a number of sensing and actuating devices that allow the design and creation of microfluidic devices using standard office software and equipment. These devices can be easily designed and produced in a firstor second-year engineering laboratory. This paper will discuss a novel design of a folded, paper microfluidic battery based on the work of N. Thom et al. that can power a surface-mounted light-emitting diode. This origami design, named for the Japanese art of folding paper called origami, allows one to print and assemble postage-stamp sized paper batteries for an initial equipment investment of under $1000 (a wax printer and micropipettes). Although the start-up cost of supplies is a few hundred dollars, the approximate cost per postage-stamp sized battery is on the order of $0.10. The design presented here has a folded footprint of 1 cm and outputs an open circuit voltage of 2.5 V for over 15 minutes. Once printed, the dosing of electrolytes and the salt bridge, assembly and testing can be done in about 2 hours. Like standard batteries, the voltage output reflects the chemical potential difference of the electrode metals and the flow of current happens through ion transport in the electrolytes and salt bridge. This origami paper microfluidic battery is a low-cost activity that deepens the understanding of capillary action, chemical potential, and charge transport in batteries. It also represents a hands-on way to introduce students to the emerging technology of paper microfluidics. Introduction to Paper Microfluidic Technology Paper-based analytical devices are an emerging, ultra-low cost, open-source, scalable, portable, solution for biological and chemical sensing assays pioneered by Carrilho, Martinez, & Whitesides (2009). These devices are made by printing hydrophobic channels on paper using the Xerox ColorQube 8580N solid-wax ink printer onto chromatography paper. Upon heating, the wax penetrates the entire thickness of the chromatography paper via capillary action. The hydrophilic channel is therefore defined by the wax that has infused the paper from the top surface, where it is originally printed, to the bottom side. An example of a figure of a husky dog printed in wax is shown in Figure 1a. The wax is heated so that it wicks through the paper thickness (Figure 1b) to form a hydrophobic barrier that constrains the liquid analyte (Figure 1c). The fibered, cellulose chromatography paper substrates functions as a “pump” that pulls small volumes, typically microliters (μL), of aqueous analyte through the hydrophilic (non-wax) channels through capillary action (Figure 1d). The analyte can be thus “pumped” toward regions with pre-deposited reagents, providing a chemical sensing platform that can be customized for the analyte. The analyte can be qualitatively or quantitatively characterized through color of the reaction product or other means, such as sensing current from an electrochemical reaction (Li, Ballerini & Shen, 2012). This technology platform has the potential to serve as an ultra-low cost sensor for disease vectors or toxins; upon completion of the test, the paper device, typically on the order of cm area, can be burned to eliminate hazardous waste. Figure 1. a. Printed wax image on chromatography paper. b. Heated image where wax has spread and wicked through the paper to form hydrophobic regions through the thickness of the paper. c. Water dyed with blue food coloring is contained in the hydrophobic regions. d. The water is “pumped” into the regions by capillary action of the chromatography paper fibers. This paper describes a simple, low cost design of one such paper microfluidic device that can work in conjunction with a paper microfluidic sensor--a folded paper microfluidic battery. We developed this design, which we describe as an “origami” design. Origami is the word for the Japanese art of paper folding; since the bulk of the battery is folded paper, we call it an “origami” design. The folding circumvents the alignment challenges of the pioneering microfluidic battery design by Thom et al. (2012, 2013). This origami battery provides sufficient power to light a small, surface-mounted light emitting diode. This activity can be completed in under three hours from an existing origami layout file. The learning objectives of such an activity are to: 1. Explore the connection between molecular structure of paraffin and cellulose paper and their properties of hydrophobicity and capillarity; 2. Use the difference in the chemical potential of metals in galvanic cells to produce electric power; 3. Learn how ionic conduction through electrolytes and a salt bridge can produce an electric current; 4. Discover how wax printing can be used to make a paper microfluidic device; 5. Apply the Nernst equation to the open circuit voltage of a battery.