The water transfer printing method is used to transfer patterned films on random three-dimensional objects. This industrially viable technology has been demonstrated to intimately wrap metallic and polymeric films around different materials. This method avoids the use of rigid substrate during the transfer step. Patterns can be transferred to objects without folds even when holed, addressing a challenging issue in the field of conformal electronics. This technique allows high film bending properties to be reached. This promising method enables us to integrate large-area films onto daily life objects. A bent capacitive touchpad is fabricated showing the potential applications of this technology.

flexible electronics have demonstrated extreme bendability. [21,22] Even so, wrinkles and folds are inevitable when wrapping flexible sheets on 3D surfaces with two nonzero principal curvatures. A recently developed curved image array demonstrated that selectively cutting the flexible sheet can effectively reduce folding and wrinkling when conforming it to a 3D surface. [23] On the other hand, stretchable electronics enabled by stretchable serpentine interconnects, [24,25] liquid metal, [26] nanomeshes [27] and nanoscrolls, [28] or intrinsically stretchable conductive and semiconducting polymers [29][30][31][32] are able to not only bend but also expand inplane, hence capable of conforming to 3D surfaces such as spherical domes. But so far, there are limited studies on the conforming process, especially when the target object has a complex 3D surface. Except for conforming planar electronics to 3D surfaces, direct printing of electrical components on 3D surfaces has been demonstrated. [33] However, only limited materials such as conductive inks and devices such as 3D antenna can be printed so far. As a result, simple but versatile transferring techniques are in need for conformable electronics.Water transfer printing (WTP) technology, also known as hydrographic printing, is commonly used to transfer planar graphics to 3D surfaces. [34] The process begins by printing graphics on a water-soluble substrate, which is then placed on water surface. As the substrate gets dissolved by water, the ultrathin graphics layer stays floating on water. A solid Perfectly wrapping planar electronics to complex 3D surfaces represents a major challenge in the manufacture of conformable electronics. Intuitively, thinner electronics are easier to conform to curved surfaces but they usually require a supporting substrate for handling. The water transfer printing (WTP) technology utilizes water surface tension to keep ultrathin electronics floating flat without supporting substrate, enabling their conformal transfer on 3D surfaces through a dipping process. In many cases, however, the size of the microfabricated electronics is much smaller than the target 3D surface. This work proposes that such mismatch in size can be overcome by leveraging stretchable electronics in WTP. Stretchable electronics are compliant to inplane stretch induced by water surface tension, hence can first self-expand in water and then be transferred onto 3D objects. Uniaxial and biaxial expansion ranging from 41% to 166% has been achieved without any externally applied tension. The results demonstrate that expansion-enhanced WTP is a promising fabrication process for conformable electronics on large 3D surfaces.

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.

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