Water-Transferred, Inkjet-Printed Supercapacitors toward Conformal and Epidermal Energy Storage

Abstract

Rapid growth of the internet of things and health monitoring systems have stimulated the development of flexible, wearable, and conformal embedded electronics with the unprecedented need for energy storage systems fully adaptable to diverse form factors. Conventional fabrication methods, such as photolithography for electronics and electrode winding/stacking for energy storage systems, struggle as fabrication strategies to produce devices with three-dimensional, stretchable, and conformal form factors. In this study, we demonstrate the fabrication of supercapacitors on 3D objects through inkjet and water-transfer printing. The devices are initially printed on a water-soluble substrate, which is then placed on the surface of water. Once the substrate is dissolved, the level of water is lowered until the devices are transferred on to the submerged 3D object. As a proof of concept, planar supercapacitors constituted of a silver nanoparticle-based current collector, nickel­(II) oxide (NiO) nanoparticle-based active electrodes, and ultraviolet-cured triacrylate polymer-based solid-state electrolyte were used as model materials. The conformal supercapacitors showed a maximum areal capacitance of 87.2 mF·cm–2 at a voltage window of 0–1.5 V. Moreover, the concept of water transfer was further explored with a particular focus on wearable applications by transferring the supercapacitors onto the skin of a human subject to realize epidermal energy storage. This new class of conformal electrochemical energy storage offers a new alternative approach toward monolithically integrated/object-tailored energy storage systems that are essential for complex-shaped devices for internet of things and flexible/on-skin electronic applications.

Developing the Nondevelopable: Creating Curved‐Surface Electronics from Nonstretchable Devices

“…Since surfaces with 2D curvature (also known as nondevelopable surfaces, e.g., spheres, bowls, hyperboloids) can only be formed from a flat sheet through applying membrane strain (i.e., stretch or compression), [ 1 ] much of the rigid, planar infrastructure used to fabricate traditional consumer devices is unsuitable for curved electronics. As a result, many innovative techniques, including 3D printing, [ 2–4 ] water‐transfer printing, [ 5–7 ] stamp patterning, [ 8–10 ] spray‐coating, [ 11,12 ] and stretching, [ 13–16 ] have been implemented to create electronics conformal to curved surfaces. Recent progress in this field has brought remarkable advances, leading to a variety of curved devices with unique applications in human–computer interaction, imaging, and environmental sensing.…”

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infrastructure used to fabricate traditional consumer devices is unsuitable for curved electronics. As a result, many innovative techniques, including 3D printing, [2][3][4] water-transfer printing, [5][6][7] stamp patterning, [8][9][10] spray-coating, [11,12] and stretching, [13][14][15][16] have been implemented to create electronics conformal to curved surfaces. Recent progress in this field has brought remarkable advances, leading to a variety of curved devices with unique applications in human-computer interaction, imaging, and environmental sensing.

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Hydroprinting Technology to Transfer Ultrathin, Transparent, and Double‐Sided Conductive Nanomembranes for Multiscale 3D Conformal Electronics

“…[ 18 ] However, methods for direct printing of undeformable electronics, including laser direct writing, [ 19 ] curved lithography, [ 20 ] and inkjet printing, [ 21 ] need expensive equipment and have limited substrates. Therefore, studies focused on the development of fabrication technologies for deformable electronics: techniques such as lithography, [ 22 ] deformable plastic stamping, [ 23 ] and hydroprinting [ 6,24–27 ] have gained considerable attention. However, flexible electronic devices fabricated with lithography and plastic stamps are inherently incompatible with complex conformal surfaces (e.g., microstructure surfaces, spherical, and re‐entrant shapes), because of the strain determined by curvature.…”

“…Hydroprinting (or water transfer/hydrographic printing) is considered a promising technology for transferring conductive/patterned films onto a target 3D object with excellent conformability characteristics, such as bendability, stretchability, and wrapping capability. [ 6,24–27 ] However, although hydroprinting with water‐soluble polyvinyl alcohol (PVA) films as a sacrificial layer has reportedly been used to directly transfer conductive/patterned films to conformal structures, [ 24–26 ] studies focusing on transferring transparent electrodes using such films have not been conducted. For transparent electrodes to successfully form conformal contact with the macro‐ and microstructures, it is necessary to use flexible and transparent electrodes (FTEs) (e.g., carbon nanotubes (CNTs), [ 28 ] graphene, [ 9 ] and silver nanowires (AgNWs) [ 29 ] ) with a smaller scale than that of the structure itself.…”

Skin-conformable printed supercapacitors and their performance in wear

“…They can be printed onto flexible substrates and easily integrated with other printed components [9][10][11][12][13][14] . Several efforts to produce wearable and stretchable supercapacitors have been reported [15][16][17][18] , but so far the reports concerning the durability of the device when worn on the skin are very limited 19,20 , and the existing reports present very short term trials 21,22 . With these types of devices, the required wear times can range from minutes to hours to even a full day of continuous monitoring.…”