Paper‐based capillary action

Abstract

Researchers in the US present integration of paper-based microfluidics with liquid metal as a method for liquid metal transport through capillary action. Capillary action transport moves liquid by using the intermolecular forces between the liquid and a solid. If you take a material, such as a polymer, that contains different sized microfluidic channels, a specific channel can be selected for fluid transport if that channel is actuated. Actuation of fluidic channels is used in a variety of applications where signals need to be blocked and activated at different times. These include circuit switches, filters and various applications within antennas. Typically, liquid metal is moved by mechanical (e.g. syringes or pumps), or electrochemical (e.g. applied voltage) methods. In this issue of Electronics Letters, Matthew Moorefield and colleagues at the University of Hawaíi at Mānoa, report the first use of paper actuation to transport liquid metal using hydrophobic filter paper. Compared to mechanical and electrochemical methods, the method removes the need for an external power supply, providing a sustainable and unique alternative for liquid metal transport. “A potential application for this transportation method is a planar circuit that would be hampered by an electrical actuation network, or that doesn't have enough power or room in the device for a mechanical pump,” said Moorefield. Moorefield and colleagues have been working with the liquid metal Galinstan - a dense eutectic alloy of gallium, tin and indium - since 2011. The idea to use paper-based microfluidics to reconfigure a circuit grew out of a problem that the authors had when using syringes to remove a carrier fluid from around liquid metal. They had to use specialised geometries in microfluidic channels to position the liquid metal, and relied on its high surface tension to keep it in place. However, using the syringe was creating more force than was needed. A paper towel was then used to remove the carrier fluid as an attempt to reduce the required actuation force of the liquid metal. These experiments revealed that paper-based actuation was possible and became a natural development step for the authors. Moorefield and colleagues tested the initial idea with various grades of filter paper. “The change to filter paper in place of commercial paper towel was the most challenging part of the process” explains Moorefield. After optimising the experimental conditions required for paper-based liquid metal transport, the authors began designing a functional device. In their Letter the team present a simple switch device, designed to have a wide operating bandwidth. The switch is based on a 2.4 mm-wide copper microstrip, which has a 20 mm gap cut into its structure to create the non-operational OFF state. The size of the gap in the microstrip was calculated to give the switch a wide operating frequency range of between 50 MHz and 10 GHz. Liquid metal is transported between a reservoir and main channel, once the channel is actuated by placing the paper at the outlet of the main channel. Capillary forces then pull the liquid metal through a polymer placed within the gap of the microstrip into the main channel, and the device is switched onto the ON state with measurable microwave frequency transmission. Two members of the liquid-metal research team conducting research at the University of Hawaíi at Mānoa. Microstrip series switch used to demonstrate paper-based capillary actuation. The switch is fabricated by creating a physical break in the copper of a microstrip transmission line and placing a microfluidic channel over the break. When filled with Galinstan liquid metal and an electrolyte, paper-based capillary action is used to reposition the liquid metal turning the switch on. The choice of liquid metal being transported is limited by the operational environment. Galistan is a popular choice for room temperature liquid metal transport owing to its high viscosity and low toxicity properties. Eutectic-gallium-indium is also used in the research environment, and mercury is still used in some applications. The work reported in this Letter has the potential to expand the use of liquid metal for electronic circuits and other applications, by providing a simple and robust method for liquid metal actuation. Moorefield's research group is focused on adapting the controlled motion of liquid metal in microwave devices and are continuing their work on reconfigurable adaptive antennas. Moorefield explained that they “expect the use of liquid metals in reconfigurable devices to become more common in the next decade, and these devices may become commercialised products as the actuation and integration techniques mature”.