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Collin Ladd, Ju-Hee So, John Muth, and Michael D. Dickey* This paper describes a method to direct-write liquid metal microcomponents at room temperature. Three-dimensional (3D) printing is gaining popularity for rapid prototyping and patterning. Many 3D printers extrude molten polymer that quickly cools and solidifies. The ability to pattern liquids into arbitrary shapes both in and out of plane is usually limited by interfacial tension. A classic example is the break-up of cylinders of liquid into droplets when the aspect ratio of the cylinder exceeds the Rayleigh stability limit of π.[1] Here, we show it is possible to direct-write a low viscosity liquid metal at room temperature into a variety of stable free-standing 3D microstructures (cylinders with aspect ratios significantly beyond the Rayleigh stability limit, 3D arrays of droplets, out of plane arches, wires). A thin (∼1 nm thick), passivating oxide skin forms rapidly on the surface of the liquid metal and stabilizes the microstructures despite the low viscosity and large surface energy of the liquid.[2] The ability to directly print metals with liquid-like properties is important for soft, stretchable, and shape reconfigurable analogs to wires, electrical interconnects, electrodes, antennas, meta-materials, and optical materials. In the absence of external fields, interfacial tension dictates the shape and the behavior of liquids at the sub-mm length scale. It can cause smaller droplets to coalesce, jets of liquid to break up, and thin films of liquid to dewet.[3,4] Several techniques have been developed to stabilize the shape of fluids into desirable shapes despite the destabilizing influence of surface tension. The most prevalent approach is to convert a liquid into a solid to trap it in a non-equilibrium shape (e.g., polymer melt processing). Liquid droplets can be stabilized against coalescence by the inclusion of solid particles, macromolecules, or surfactants on their surface. However, these structures (e.g., liquid marbles, colloidosomes, and emulsions) typically adopt spherical shapes.[5,6] The ability to pattern materials into arbitrary 3D microstructures is important for electronics,[7] microfluidic networks,[8,9] tissue engineering scaffolds,[10] photonic band gap structures,[11] and chemical synthesis.[12] There are several strategies to pattern 3D microstructures including lithography, laser writing, colloidal assembly and direct-write techniques.[11,13–17] Direct-write techniques are appealing because they are additive processes, although most work has focused on the patterning Mr. C. Ladd, Dr. J.-H. So, Prof. M. D. Dickey Department of Chemical and Biomolecular Engineering North Carolina State University Raleigh, NC 27695, USA E-mail:
[email protected] Prof. J. Muth Department of Electrical and Computer Engineering North Carolina State University Raleigh, NC 27695, USA
DOI: 10.1002/adma.201301400
Adv. Mater. 2013, 25, 5081–5085
of polymers. Recently, there has been great interest in new methods to direct write 3D conductive microstructures for antennas,[18] flexible displays[19,20] and wire bonds.[21,22] One approach is to pattern composites of conductive additives in insulating materials (e.g. polymers), but these composites often have limited conductivity and mechanical properties. Recently, two new methods have been demonstrated for direct writing solid metal wires including extrusion of metal particles from a nozzle[21] or by electrodeposition from a conductive tip.[22] Here, we demonstrate that it is possible to direct write structures composed of a low-viscosity liquid with metallic conductivity at room temperature. The liquid metal is useful for soft, stretchable, or shape reconfigurable electronics. We focus on the binary eutectic alloy of gallium and indium (EGaIn, 75% Ga 25% In by weight), but any alloy of gallium will also work. EGaIn is liquid at room temperature (m.p. ∼15.7 °C) with metallic conductivity.[23] The liquid metal exhibits a negligible vapor pressure and low toxicity. Upon exposure to air, the metal forms a thin (∼1 nm) passivating “skin” composed of gallium oxide.[2] Passivation occurs nearly instantaneously under atmospheric oxygen levels and electrical resistance remains largely unaffected because the skin is thin.[24] In addition, the liquid metal adheres to most surfaces and alloys with many metals to form ohmic contacts. Injection of liquid metal into microchannels is an established method to shape the metal for reconfigurable wires and antennas,[25,26] interconnects,[27,28] electrical components for microfluidics,[29] and “soft” electrodes for electrical characterizations of thin films.[30] The use of microfluidics limits the ability to shape the metal because the channels are in a single plane, whereas many applications (e.g., interconnects, meta-materials, optical materials) require out of plane or 3D architectures. In addition, the necessity of inlet and outlet ports makes it difficult to pattern arrays of small, discrete structures of the metal via microfluidic injection. A number of other methods for patterning liquid metal in 2D are beginning to emerge.[31–34] Here, we present free standing liquid metal structures such as wires, fibers, interconnects, and stacks and arrays of droplets using four separate yet related direct-write patterning methods. We elucidate the mechanism by which these structures form and present a working proof-of-concept flexible circuit using liquid metal wire bonds. Fabrication of Free Standing Wires: Figure 1 depicts the general approach for printing liquid metal structures. Modest gauge pressure (
