Skin is the largest organ of the human body, and it offers a diagnostic interface rich with vital biological signals from the inner organs, blood vessels, muscles, and dermis/epidermis. Soft, flexible, and stretchable electronic devices provide a novel platform to interface with soft tissues for robotic feedback and control, regenerative medicine, and continuous health monitoring. Here, we introduce the term "lab-on-skin" to describe a set of electronic devices that have physical properties, such as thickness, thermal mass, elastic modulus, and water-vapor permeability, which resemble those of the skin. These devices can conformally laminate on the epidermis to mitigate motion artifacts and mismatches in mechanical properties created by conventional, rigid electronics while simultaneously providing accurate, non-invasive, long-term, and continuous health monitoring. Recent progress in the design and fabrication of soft sensors with more advanced capabilities and enhanced reliability suggest an impending translation of these devices from the research lab to clinical environments. Regarding these advances, the first part of this manuscript reviews materials, design strategies, and powering systems used in soft electronics. Next, the paper provides an overview of applications of these devices in cardiology, dermatology, electrophysiology, and sweat diagnostics, with an emphasis on how these systems may replace conventional clinical tools. The review concludes with an outlook on current challenges and opportunities for future research directions in wearable health monitoring.

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In an electrochemical cell, crystalline silicon and lithium react at room temperature, forming an amorphous phase of lithiated silicon. The reaction front-the phase boundary between the crystalline silicon and the lithiated silicon-is atomically sharp. Evidence has accumulated recently that the velocity of the reaction front is limited by the rate of the reaction at the front, rather than by the diffusion of lithium through the amorphous phase. This paper presents a model of concurrent reaction and plasticity. We identify the driving force for the movement of the reaction front, and accommodate the reaction-induced volumetric expansion by plastic deformation of the lithiated silicon. The model is illustrated by an analytical solution of the co-evolving reaction and plasticity in a spherical particle. We derive the conditions under which the lithiation-induced stress stalls the reaction. We show that fracture is averted if the particle is small and the yield strength of lithiated silicon is low. Furthermore, we show that the model accounts for recently observed lithiated silicon of anisotropic morphologies.Lithium-ion batteries dominate the market of power sources for wireless electronics, and are being implemented in electric vehicles. 1, 2 Intense efforts are dedicated to developing next-generation lithiumion batteries with high energy density, long cycle life, and safe operation. 3, 4 Silicon can host a large amount of lithium, making it one of the most promising materials for anodes. 5 Lithiation of silicon, however, causes large volumetric expansion and mechanical stress, often leading to the shedding of active materials and rapid decay of capacity. 6 This mode of failure can be mitigated in nanostructured silicon anodes. Examples include nanowires, 7 thin films, 8 nanoporous structures, 9 hollow nano-particles, 10 and carbon-silicon composites. 11 Specifically, recent experiments and theories show that nanostructured silicon may avert fracture when the lithiation-induced expansion is accommodated by plasticity. 12 -18 To develop such nanostructured anodes, it is urgent to understand lithiation-induced stress, deformation, and fracture.Nanostructured electrodes of silicon are often fabricated with crystalline silicon. In an electrochemical cell, crystalline silicon and lithium react at room temperature, forming an amorphous phase of lithiated silicon [ Fig. 1]. 7, 19-23 The reaction front is atomically sharpthe phase boundary between the crystalline silicon and the lithiated silicon has a thickness of ∼1 nm. 24 Evidence has accumulated recently that, in the nanostructured electrodes, the velocity of the reaction front is not limited by the diffusion of lithium through the amorphous phase, but by the reaction of lithium and silicon at the front. For example, it has been observed that under a constant voltage the displacement of the reaction front is linear in time. 25 This observation indicates that the rate of lithiation is limited by short-range processes at the reaction front, such as breaking and forming ...

During charging or discharging of a lithium-ion battery, lithium is extracted from one electrode and inserted into the other. This extraction-insertion reaction causes the electrodes to deform. An electrode is often composed of small active particles in a matrix. If the battery is charged at a rate faster than lithium can homogenize in an active particle by diffusion, the inhomogeneous distribution of lithium results in stresses that may cause the particle to fracture. The distributions of lithium and stress in a LiCoO 2 particle are calculated. The energy release rates are then calculated for the particle containing preexisting cracks. These calculations predict the critical rate of charging and size of the particle, below which fracture is averted.

Silicon can host a large amount of lithium, making it a promising electrode for high-capacity lithium-ion batteries. Recent experiments indicate that silicon experiences large plastic deformation upon Li absorption, which can significantly decrease the stresses induced by lithiation and thus mitigate fracture failure of electrodes. These issues become especially relevant in nanostructured electrodes with confined geometries. On the basis of first-principles calculations, we present a study of the microscopic deformation mechanism of lithiated silicon at relatively low Li concentration, which captures the onset of plasticity induced by lithiation. We find that lithium insertion leads to breaking of Si-Si bonds and formation of weaker bonds between neighboring Si and Li atoms, which results in a decrease in Young's modulus, a reduction in strength, and a brittle-to-ductile transition with increasing Li concentration. The microscopic mechanism of large plastic deformation is attributed to continuous lithium-assisted breaking and re-forming of Si-Si bonds and the creation of nanopores.