Human skin is capable of transducing pressures in the range of 100 Pa (light touch) to 1 MPa (full body weight bearing); common tasks such as object manipulation develop contact pressures on the order of 10 kPa. [ 21,22 ] Moreover, sensitivity of human skin to applied pressures is complex and varies widely by type of mechanoreceptor and type of stimulation (normal pressure, shear pressure, frequency, magnitude). [ 23 ] Although distributed sensing using arrays of thin-fi lm transistors on ultrathin plastic foils combined with soft mechanical sensors has also been demonstrated, [ 11,[24][25][26] most reported skin-like sensors are discrete elements. An unmet demand for truly wearable e-skin is mechanical compliance. Natural skin is soft and elastic. Electronic skins should therefore wrap over the external surface of the body and accompany movement, in particular over joints and articulations. To date, pressure sensing data gloves and tactile skins are mainly prepared with fl exible polymers [27][28][29] and conductive textiles. [ 30,31 ] These constructs conform well to developable surfaces (e.g., the arm and fi nger phalanges) but wrinkle and often fail when placed over articulations (e.g., the elbow and fi nger joints). [ 32 ] E-skins prepared entirely with stretchable materials appear as a necessary starting point. Over the last decade, multiple designs of stretchable tactile sensors using elastomers, thin fi lms, composites, [ 19,33 ] and conductive liquids [34][35][36] have been reported, but their systematic characterization in real-life conditions is often incomplete. Stretchable strain sensors are often demonstrated in complex real-life scenarios, [ 37,38 ] but in the literature related to stretchable tactile sensors, demonstrations involving dynamic states where bending and stretching of the sensors occur simultaneously are not common, likely due to the challenges of removing cross-sensitivities to strain and noise received from the body. [ 19,20,39 ] In this paper, we report on a stretchable e-skin designed to be worn over the hand, monitor live fi nger movement, and register distributed pressure along the entire length of the fi nger. The sensory skin is thin and made entirely of elastic materials, thereby can be mounted on a glove and worn without impeding hand movement ( Figure 1 ). The read-out electronics are integrated in a small printed circuit board (PCB) located immediately at the base of each fi nger. Capacitive pressure sensors combine stretchable gold thin-fi lm electrodes with porous silicone foam (Figure 1 a) and display high sensitivity across much of the large dynamic pressure range of human skin. Six adjacent pressure sensors cover the entire length of the fi nger; two soft metallic shielding layers eliminate noise and cross-sensitivity over the skin and enable multi-touch with This report demonstrates a wearable elastomer-based electronic skin including resistive sensors for monitoring fi nger articulation and capacitive tactile pressure sensors that register distributed pressure along ...

Liquid metals, encapsulated in soft materials, have therefore attracted much attention in recent years [2a,8] to manufacture soft conductors with metallic conductivity, high stretchability and reconfigurability. [9] Gallium-based alloys, rather than toxic mercury, are widely used. The high surface tension and the passivating oxide skin that spontaneously forms on the surface of these liquids hinder their patterning using conventional techniques. Alternative methods focus on injection into channels, molding and printing for rapid manufacturing of highly conductive and stretchable metal networks but none of these patterning techniques offer high-resolution batch processing over large (wafer-scale) surface areas. [10] Based on these observations, we developed a new class of stretchable electronic conductors formed of biphasic solidliquid thin metal films. A bilayer metallization sequence starting with the sputtering of an alloying gold film followed by the thermal evaporation of liquid gallium (that displays a melting point of 29.8 °C [11] ) results in a heterogeneous film composed of clusters of the solid intermetallic alloy AuGa 2 and supercool liquid gallium forming a continuous network and dispersed bulges [11b,12] (Figure 1a-c). We designed and engineered the biphasic metallic films to be compatible with large-area and standard microfabrication. Figure 1d,e shows examples of fine patterns produced at wafer scale on elastomeric substrates. Multilayered stretchable circuits can be readily integrated by covalently bonding membranes hosting patterned biphasic conductors connected through soft vias. Figure 1e displays a 4 × 4 wafer-sized hybrid array of surface mounted light emitting diodes interconnected with a two-level network of biphasic solid-liquid conductors. The array withstood demanding multiaxial inflation cycles, constantly delivering power to the optoelectronic devices (Movie S1, Supporting Information).To prepare the stretchable biphasic solid-liquid thin metal films, a two-step process was developed in which liquid gallium was evaporated on a substrate preliminarily coated with a wetting and alloying thin film. We selected poly(dimethylsiloxane) (PDMS), a silicone, as the soft carrier substrate and a gold film sputtered on the PDMS as the alloying layer. However, our process is not limited to those materials ( Figure S1 and S2, Supporting Information). Non-noble metals may be used, provided the alloying thin film is not oxidized.The high surface tension of the liquid metal prevented the formation of an evaporated continuous liquid metal film on bare silicone substrates. Instead, the surface of the elastomer was covered with a nonconducting arrangement of liquid gallium microdroplets ( Figure S3, Supporting Information). In contrast, evaporating gallium on an alloying metal film, first deposited on the silicone surface, overcame the cohesive forces Stretchable conductors are the foundation of soft electronic circuits. [1] Manufacturing elastic wiring networks to distribute and carry electrical pote...

Emerging applications of the Internet of Things in healthcare, wellness, and gaming require continuous monitoring of the body and its environment, fueling the need for wearable devices able to maintain intimate, reliable, and unobtrusive contact with the human body. This translates in the necessity to develop soft and deformable electronics that match the body's mechanics and dynamics. In recent years, various strategies have been proposed to form stretchable circuits and more specifically elastic electrical conductors embedded in elastomeric substrate using either geometrical structuring of solid conductors or intrinsically stretchable materials. Gallium (Ga)-based liquid metals (LMs) are an emerging class of materials offering a particularly interesting set of properties for the design of intrinsically deformable conductors. They concomitantly offer the high electrical conductivity of metals with the ability of liquids to flow and reconfigure. The specific chemical and physical properties of Ga-based LMs differ fundamentally from those of solid conductors and need to be considered to successfully process and implement them into stretchable electronic devices. In this Account, we report on how the key physical and chemical properties of Ga-based LMs can be leveraged to enable repeatable manufacturing and precise patterning of stretchable LM conductors. A comprehensive understanding of the interplay between the LM, its receiving substrate chemistry and topography, and the environmental conditions is necessary to meet the reproducibility and reliability standards for large scale deployment in next-generation wearable systems. In oxidative environments, a solid oxide skin forms at the surface of the LM and provides enough stiffness to counterbalance surface tension, and prevent the LM from beading up to a spherical shape. We review techniques that advantageously harness the oxide skin to form metastable structures such as spraying, 3D printing, or channel injection. Next, we explore how controlling the environmental condition prevents the formation or removes the oxide skin, thereby allowing for selective wetting of Ga lyophilic surfaces. Representative examples include selective plating and physical vapor deposition. The wettability of LMs can be further tuned by engineering the surface chemistry and topology of the receiving substrate to form superlyophobic or superlyophilic surfaces. In particular, our group developed Ga-superlyophilic substrates by engineering the surface of silicone rubber with microstructures and a gold coating layer. Thermal evaporation of Ga on such engineered substrates allows for the formation of smooth LM films with micrometric thickness control and design freedom. The versatility of the available deposition techniques facilitates the implementation of LM conductors in a wide variety of wearable devices. We review various epidermal electronic systems using LM conductors as interconnects to carry power and information, transducers and sensors, antennas, and complex hybrid (soft-rigid) ele...

Compliant thin film model for stretchable rectangular gold thin films on elastic soft substrateWhen the gauge axis is aligned with the uniaxial strain direction, the change in resistance of the gold track of length L, width W and constant thickness t under tensile strain ε is given by:where ρ is the constant resistivity of the metal layer and υ the Poisson's ratio of PDMS. Hence :When the gauge is oriented longitudinally relative to the uniaxial strain direction, the change in resistance of the gold track of length L, width W and constant thickness t under tensile strain ε is given by:2,ax.

Liquid metals have recently gained interest as a material of choice for soft and stretchable electronic circuits, thanks to their virtually infinite mechanical failure strain and high electrical conductivity. Gallium-based thin films are obtained by depositing gallium in the vapor phase to form a class of liquid metal conductors. The films, with an average thickness below 1 µm, withstand mechanical strain in excess of 400%. However, modes of failure other than mechanical ones have not yet been thoroughly investigated. In particular, electromigration, a well-known cause of failure in solid thin film traces for integrated circuits, also occurs in bulk liquid metals. In this work, microscopic observation of the thin conductive traces reveals that gallium is displaced from the anode terminal toward the cathode terminal after direct current stressing. This results in a catastrophic increase in the trace resistance and electrical failure. The mean time to failure decreases with increasing current density, following Black’s equation, an empirical mathematical model originally developed to describe failure in solid metal thin-film tracks due to electromigration. We show that using alternating current, e.g., symmetric square wave, rather than direct current can extend the lifetime of the thin liquid metal film conductor by several orders of magnitude. These results may help stretchable circuit designers who select liquid metal thin-film conductors as the stretchable interconnect technology to predict devices’ lifetime and implement mitigation strategies at the system level or at the material level.