Freeze‐Printing of Liquid Metal Alloys for Manufacturing of 3D, Conductive, and Flexible Networks

Abstract

liquid metal alloys including injection of liquid metals into elastomeric channel networks, [10][11][12][13][14] freeze-casting, [15] surface patterning using lithographic techniques, [16][17][18][19] laser engraving, [20] or additive approaches including flow-based direct-writing, [21,22] droplet-by-droplet deposition, [23] heated roller-pen printing, [24,25] ink-jetting, [26] deposition at microscale in the nanoparticle ink form, followed by sintering, [27] or combination of direct-writing and transfer printing. [28] These methods have been utilized to realize various flexible electronic components including interconnects, [10,29] passive circuitry, [11] diodes and memristors, [30,31] antennas for wireless communication, [21,32,33] and wearable force and pressure sensors. [12,34] Despite their demonstrated success, the 3D geometric capability of these methods is still limited. The geometric capability of the injection and lithography is determined by that of the 2D methods used to fabricate the elastomeric channel networks, stencils or stamps. The additive manufacturing methods provide a higher level of geometric capability, however, still are mostly limited to 2D planar designs since the liquid metal cannot maintain its mechanical stability as complex 3D structures. Using these methods, obtaining a limited range of 3D designs still require a number of rigorous process steps including molding, metal injection and vacuuming, as well as layer bonding. In a recent study, Ladd et al. showed that the moldability of liquid metal alloys can be utilized to form vertically extended structures in mm-level sizes, [22] demonstrating the promise for true 3D-printing using liquid metals. However, as also demonstrated in this work, the structural stability of the oxide skin is not sufficient to form complex 3D networks of liquid metal alloys in practical size scales. A number of recent studies combined liquid metal and elastomer printing to build multilayer planar networks of liquid metals where the printed elastomer essentially used as the support material. [35][36][37] Even though these studies improved the geometric capability of additive manufacturing of EGaIn networks, printing of EGaIn along truly 3D paths in a continuous fashion has not been realized. Realization of such a printing method will enable high-density 3D interconnect networks, antennae, and passive circuitry for flexible electronics with substantially increased design complexity.In this paper, we present the freeze-printing method to address this need. This method is schematically described in Figure 1a-d. Here, the liquid metal alloy, EGaIn is dispensed from a nozzle on a substrate, the temperature of which is kept below the melting point of the alloy. As the nozzle is translated in 3D, more liquid is added to the printed structure along the translation path and maintains its cylindrical shape owing to the spontaneous formation and deformation of the oxide skin. In the meantime, the printed structure starts freezing from the substrate generating a moving f...

Liquid Metal–Based Soft Microfluidics

“…The frozen liquid metal exhibits substantially higher structural stability than the oxide skin alone. Finally, the 3D free‐standing liquid metal patterns could be wrapped in silicone rubber, thus obtaining a flexible 3D circuit …”

“…Finally, the 3D free-standing liquid metal patterns could be wrapped in silicone rubber, thus obtaining a flexible 3D circuit. [80]…”

Wiring up Liquid Metal: Stable and Robust Electrical Contacts Enabled by Printable Graphene Inks

“…[15,16] Moreover, the fluid nature of GaLMAs such as eutectic gallium-indium (eGaIn) enables broad process compatibility with additive printing methods such as direct write, inkjet, transfer, and 3D printing. [17][18][19][20][21][22][23] As such, research toward the control and integration of GaLMAs for printed, stretchable, and reconfigurable electronics has attracted broad scientific and practical interest.Despite their promise, the development of liquid metal electronics must overcome several challenges for widespread application. In particular, stable electrical contacts have been identified as a critical challenge for the integration of GaLMAs in electronic circuits and systems.…”

Shear‐Driven Direct‐Write Printing of Room‐Temperature Gallium‐Based Liquid Metal Alloys

“…[20] When confining mercury to microchannels, Plateau-Rayleigh instabilities can disconnect electrical connections. [32,33] These approaches offer an alternative to injecting the metal into a prefabricated channel network. Although GaLMAs exhibit high surface tension (624 mN m À1 for eutectic gallium indium (EGaIn)), [21] the spontaneous formation of a passivating solid-state surface oxide skin dominates the surface properties and enables patterning of the metal into shapes that would normally not be stable.…”