Printing solid-state elastic conductors into self-supporting three-dimensional (3D) geometries promises the design diversity of soft electronics, enabling complex, multifunctional, and tailored human-machine interfaces. However, the di culties in manipulating their rheological characteristics have only allowed for layerwise deposition. Here, we report omnidirectional printing of elastic conductors enabled by emulsifying elastomer composites with immiscible, nonvolatile solvents. The strategy simultaneously achieves superior viscoelastic properties that provide the structural integrity of printed features, and pseudoplastic and lubrication behaviours that allow great printing stability. Freestanding, lamentary, and out-of-plane 3D geometries of intrinsically stretchable conductors are directly written, achieving a minimum feature size <100 μm and excellent stretchability >150%. Particularly, the evaporation of the continuous phase in the emulsion results in microstructured, surface-localized conductive networks, signi cantly improving their electrical conductivity. To illustrate the feasibility of our approach, we demonstrate skin-mountable electronics that visualize temperature on a matrix-type stretchable display based on omnidirectionally printed elastic interconnects. Full TextSkin electronics augment the capability of shareable signals from personal and metabolic activities over communication networks by blurring the physical discontinuity between electronic devices and human skin [1][2][3][4] . With their unique mechanical characteristics, such as lightweight design, softness, and stretchability, skin electronics can be functionalized on various body parts 5,6 and even brains 7 and hearts 8 in the forms of biosensors, processors, and displays. For high-delity operation under these challenging circumstances, the design of skin electronics needs to be tailored elaborately to individuals 9,10 . However, traditional mask-based lithography primarily optimized for the mass production of standardized, uniform electronics cannot effectively deal with the morphological diversity of the human bodies. Moreover, existing manufacturing processes still lack strategies to implement threedimensional (3D) structures with soft functional materials such as vertical interconnect accesses (VIAs) and multilayer circuitries that are crucial to the realization of high-performance, multifunctional applications.Printing electrical wirings into 3D structures could be a promising solution for maximizing the customizability of skin electronics and achieving circuit complexity. However, most conventional 3D printing processes still deposit one layer at a time, which is unsuitable for complex, lamentary, and omnidirectional wirings (including a z-directional component). Alternatively, viscoelastic inks that simultaneously exhibit high quasi-static stiffness and strong shear-thinning behaviour can immediately solidify after extrusion from a nozzle-based printhead, allowing direct writing of self-supporting 3D structures [11][12][13][14][15...
Intrinsically stretchable solid-state conductors can shed light on the realization of further biocompatible and reliable wearable electronics. However, their material composition should be optimized considering the compatibility of target stretchable platforms. In this paper, we report directly printable conductive elastomeric composites for intrinsically stretchable conductors. Pneumatic direct ink writing (DIW) system is employed to deposit well-defined patterns. Polydimethylsiloxane (PDMS), Ag particles, and multi-walled carbon nanotubes (MWCNTs) were used as elastomeric matrix, conductive filers, and auxiliary fillers, respectively. Because there is a critical trade-off between the conductivity and stretchability depending on the concentration of conductive fillers, we optimize the Ag concentration to 77.5 wt% to fulfill these requirements. Especially, we introduce multi-solvent Ag composite inks to deliver excellent printability and enhanced conductivity simultaneously. We further investigate the electromechanical reliability of the encapsulated conductors undergoing cyclic strains, and they exhibited stable R/RO values over 50% strain.
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