Metallic gels for conductive 3D and 4D printing

•Metallic gels consisting of pendular suspension Cu, EGaIn, and water•The are 3D-printable at room temperature over a wide compositional window•Shear alignment by dispensing enables 4D printing conductive metal composites•Printed parts have metallic conductivity 1.05 × 105 S/m without sintering step Four-dimensional (4D) refers to 3D-printed structures that change shape with respect time in response an external stimulus. To date, techniques focused primarily on electrically insulating materials such as polymers. Adding fillers polymers could increase the functionality printed parts, but high loadings necessary achieve represent trade-off printability. Here, we connect copper (Cu) particles soft eutectic gallium indium alloy (EGaIn) bridges form ink (Cu-EGaIn) gel-like properties well suited for printing. The final total content 97.5 wt %, remainder is methylcellulose, rheological modifier. extremely electrical (1.05 S/m) requiring step. Cu-EGaIn addresses contradiction between printability should open up new opportunities electronic, thermal, composite devices. This paper reports printable (pendular suspensions) aqueous connected liquid (EGaIn). Pendular suspensions rely capillary forces networks solid composition-dependent rheology, prior studies suspensions. rheology solid-liquid-liquid tuned 3D varying composition pH; latter promotes wetting. dry (1.05×105 Drying elevated temperatures can accelerate removal water while creating stress drives (i.e., printing). As demonstration, print spider lifts assembles its own body from initially flat shape. Such inks promising under ambient conditions. conductive, printable, heat-responsive wetted “bridges” solution. In general, wetting play important role stabilizing multiphase systems. For example, liquids strengthen wet granular materials,1Mitarai N. Nori F. Wet materials.Adv. Phys. 2006; 55: 1-45Crossref Scopus (345) Google Scholar stabilize foams2Murakami R. Bismarck A. 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Printing ferromagnetic domains untethered fast-transforming materials.Nature. 558: 274-279Crossref (1089) features program subsequent stress. filamentary geometries respond anisotropically heat post-processing transform them shapes, flower petals “Spiderbot.” We created mixing (diameter 10 25 μm) volumes shown Figure 1A. facilitate surface setting 1.0 HCl. After printing, HCl evaporate characterize “wet” state. Following convention “saturation” S characterizes major phase water) each sample V volume:S=VwaterVEGaIn+Vwater.(Equation First, = 1. When volume fraction (φCu) exceeds 0.45 (Figure 1B), mixture becomes granular. However, φCu falls below 0.40, extra increases separation distance forming liquidous dispersion easily S1A). second (here, metal) significantly interactions particles.5Koos arises fluid bridging addition makes dispersed (which principle, this study). 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Graphene-based encapsulation particles.Nanoscale. 12: 23995-24005Crossref range, macroscopically settled (apparent photographs 1C); consistent having insufficient prepared 13 elastic behavior, apparent 1C. 1C taken appear Samples scanning electron microscopy (SEM) imaging distributed bridged no obvious coalescence (Figures 1D 1E). typically coalesces absence crowded environment. observed stirring (from ∼3), evaporation consumption H+. dissolved acidic conditions, leaving behind corresponding chloride salts. since limited fresh powder, residual expected low. evidenced photoelectron spectroscopy results S4), overall Cl drying 0.1%. reasonable assume salts negligible effect materials. alkaline solutions, NaOH cannot therefore greatly corrosion parts. focus paper. further explore Cu/EGaIn/water report ternary diagram 1B. Compared oil-water network (PNS), used (green area 1A), presumably strong upper bottom boundary roughly defined φEGaIn/φCu 1.09 φWater/φCu 1.22, maximum concentration only (without excess minimum continuous phase, respectively. Beyond boundaries (yellow area), aggregate S1B). expected, excessive (φWater > 0.7, blue area) macroscopic region distinguished 1B, S1C, S1D). With moderate (0.4 < φWater purple area)—though still form—the engulfs entire significant decrease strength S1E). At 0.4 solid-like, so intense will induce S1F). green-colored depicts stable "metallic gels"). rheometer estimate identify might 1F). orders magnitude 0.08 then gradually reaches plateau. lowers stress: 0 0.79, stress; however, 2.0. Moreover, (beyond 0.79) sensitive fraction, attributed richer packing density. proceeded 0.48 (see experimental procedures details Table S1 formulation). reported large (s) separating radius (rp) unconventional normal PNS decreases s critical value (sc) pinch off. 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