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.

International audienceThis paper presents strain sensor arrays on flexible substrates able to measure local deformation induced by radii of curvature of few millimeters. Sensors use n-type doped microcrystalline silicon (μc-Si) as piezoresistive material, directly deposited on polyimide sheets at 165 °C. Sensitivity of individual sensors was investigated under tensile and compressive bending at various radii of curvature, down to 5 mm. A Transmission Line Method was used to extract the resistivity for each radius. The devices exhibited longitudinal gauge factors of −31 and longitudinal piezoresistive coefficients of −4.10−10 Pa−1. Reliability was demonstrated with almost unchanged resistances after cycles of bending (standard deviation of 1.7%). Strain gauge arrays, composed of 800 resistors on a 2 cm2 area, were fabricated with a spatial resolution of 500 × 500 μm2. Strain mapping showed the possibility to detect local deformation on a single resistor or to detect larger objects. These strain sensor arrays can find applications when high sensitivity and high spatial resolution is required. This paper also showed that μc-Si can be a relevant semi-conductor candidate for flexible electronic