This review contains a comparative study of reported fabrication techniques of gallium based liquid metal alloys embedded in elastomers such as polydimethylsiloxane or other rubbers as well as the primary challenges associated with their use. The eutectic gallium-indium binary alloy (EGaIn) and gallium-indium-tin ternary alloy (galinstan) are the most common non-toxic liquid metals in use today. Due to their deformability, non-toxicity and superior electrical conductivity, these alloys have become very popular among researchers for flexible and reconfigurable electronics applications. All the available manufacturing techniques have been grouped into four major classes. Among them, casting by needle injection is the most widely used technique as it is capable of producing features as small as 150 nm width by high-pressure infiltration. One particular fabrication challenge with gallium based liquid metals is that an oxide skin is rapidly formed on the entire exposed surface. This oxide skin increases wettability on many surfaces, which is excellent for keeping patterned metal in position, but is a drawback in applications like reconfigurable circuits, where the position of liquid metal needs to be altered and controlled accurately. The major challenges involved in many applications of liquid metal alloys have also been discussed thoroughly in this article.

Liquid‐metal alloys are now extensively used for stretchable electronic applications due to their superior electrical conductivity, non‐toxicity, and mechanical stability in micro‐channels. Needle‐injection and direct‐writing are the most popular techniques for patterning micro‐structured liquid metal alloys. However, embedded channels often require a very high pressure to inject liquid‐metal, and direct writing by dispensing is relatively complex due to the low viscosities and high surface tension of the metal which cause liquid to normally dispense in droplets rather than a stream. In this work, a technique to co‐axially extrude liquid‐metal alloy within an encapsulating cover fluid has been presented to obtain a continuous stable stream of liquid‐metal. Fused deposition modeling (FDM) 3D printing has been adapted to co‐extrude a liquid‐metal core with a shell made from a thermoplastic elastomer. A custom extruder system is used to directly produce conductive micro‐wires (diameter: ≈25 μm) of liquid‐metal having an insulating shell of styrene‐ethylene‐butylene‐styrene which can be stretched up to four times the original length without any noticeable mechanical and electrical loss. The system is capable of printing in‐plane conductive pathways as well as out‐of‐plane functional devices with direct‐stable encapsulation of liquid‐metal wires. This technology has been successfully used to print 2D‐pressure and 3D‐strain sensors.

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Purpose This paper aims to target to print functionally gradient materials (FGM) devices made of immiscible polymers in multi-material fused deposition modelling (FDM) systems. The design is intended to improve adhesion of dissimilar thermoplastics without the need for chemical compatibilization so that filaments from many different sources can be used effectively. Therefore, there is a need to invent an alternative solution for printing multiple immiscible polymers in an FDM system with the desired adhesion. Design/methodology/approach In this study, the authors have developed a bi-extruder for FDM systems which can print two thermoplastics through a single nozzle with a static intermixer to enhance bonding between input materials. The system can also change the composition of extrudates continuously. Findings The uniqueness of this extruder is in its easy access to the internal channel so that a static intermixer can be inserted, enabling deposition of mechanically interlocked extrudates composed of two immiscible polymers. Without this intermixer, the bi-extruder extrudes with simple side-by-side co-extrusion having no mechanical interlocking. The bi-extruder was characterized by printing objects using pairs of materials including polylactic acid, acrylonitrile butadiene styrene and high impact polystyrene. Microscope images of the cross-sections of the extrudates confirm the ability of this bi-extruder to control the composition as desired. It was also found that the mechanically interlocked extrudates composed of two immiscible polymers substantially reduces adhesion failures within and between filaments. Originality/value In this study, the first-ever FDM extruder with a mechanical blending feature next to the nozzle has been designed and used to successfully print FGM objects with improved mechanical properties.

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