Direct 3D Printing of Stretchable Circuits via Liquid Metal Co‐Extrusion Within Thermoplastic Filaments

Abstract

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.

Innervated, Self‐Sensing Liquid Crystal Elastomer Actuators with Closed Loop Control

“…To fabricate iLCEs, we co‐extrude pure LM and a main‐chain LCE ink developed previously [ 23–25 ] through a core–shell nozzle mounted on a custom‐built, direct ink writing platform. Because alignment of the LCE director to a prescribed print path requires sufficient shear and extension during extrusion, the nozzle shell is retracted relative to the core [ 43,44 ] and the nozzle is tilted 20° from vertical to create a coaxial LCE (shell)‐LM (core) fiber (Figure 1a and Figure S1: Supporting Information). These iLCEs are printed within the nematic phase at 25 °C and subjected to UV curing immediately upon exiting the core–shell nozzle to preserve the prescribed director alignment and the uniformity of LM deposition.…”

Smart Textiles for Visible and IR Camouflage Application: State-of-the-Art and Microfabrication Path Forward

“…The stress–strain curves were successfully described using continuum bidimensional models. Other deformable metamaterial complex structures produced by 3D printing include stretchable circuits manufactured by coextrusion of liquid metal within thermoplastic filaments [ 72 ] and a robotic gripper combining soft and hard thermoplastics [ 73 ]. Recent progress in 3D printing includes the use of a polycarbonate (PC)/acrylonitrile butadiene styrene (ABS) core/sheet filament as feedstock for fused filament fabrication 3D printing, yielding a ductile and tough composite ABS/PC meso-structured part after annealing at a temperature between the glass transition temperatures of ABS and PC [ 74 ].…”

Liquid Metal‐Based Soft Electronics for Wearable Healthcare

“…e) Reproduced with permission. [ 81 ] Copyright 2019, Wiley‐VCH GmbH. f) Reproduced with permission.…”

“…proposed the coaxial extrusion of liquid metals and thermoplastic elastomer (styrene‐ethylene‐butylene‐styrene copolymer (SEBS)) (Figure 5e). [ 81 ] To make continuous fiber without the disconnection of liquid metal core, viscous drag force should overcome the surface tension. The diameter of the extruded fiber was dependent on the ratio of extrusion speed to drawing speed.…”