Wearable devices provide an alternative pathway to clinical diagnostics by exploiting various physical, chemical and biological sensors to mine physiological (biophysical and/or biochemical) information in real time (preferably, continuously) and in a non-invasive or minimally invasive manner. These sensors can be worn in the form of glasses, jewellery, face masks, wristwatches, fitness bands, tattoo-like devices, bandages or other patches, and textiles. Wearables such as smartwatches have already proved their capability for the early detection and monitoring of the progression and treatment of various diseases, such as COVID-19 and Parkinson disease, through biophysical signals. Next-generation wearable sensors that enable the multimodal and/or multiplexed measurement of physical parameters and biochemical markers in real time and continuously could be a transformative technology for diagnostics, allowing for high-resolution and time-resolved historical recording of the health status of an individual. In this Review, we examine the building blocks of such wearable sensors, including the substrate materials, sensing mechanisms, power modules and decision-making units, by reflecting on the recent developments in the materials, engineering and data science of these components. Finally, we synthesize current trends in the field to provide predictions for the future trajectory of wearable sensors. 

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End-to-end design of wearable sensors

We investigate the residual stress in diamond films grown on (001) silicon substrates as a function of film thickness. The diamond films were deposited at 1070 K by the conventional hot filament technique using a gas mixture of methane (1.0% vol) and hydrogen (99.0% vol). The film thickness, obtained from cross section scanning electron micrographs, varied from 3.0 to 42 μm as the growth time increased from 1 to 10 h. These images evidenced that the columnar growth is already established for films thicker than 10 μm. Top view micrographs revealed predominantly faceted pyramidal grains for the films at all growth stages. The grain size, obtained from these images, was found to vary linearly with film thickness. Using a high resolution x-ray diffractometer, the residual stress was determined by measuring, for each sample, the (331) diamond Bragg diffraction peak for Ψ values ranging from −60° to +60°, and applying the sin2 ψ method. For the micro-Raman spectroscopy, we used the summation method, which consists in recording and adding a large number of spectra in different places of a selected area of the sample. All Raman spectra were fitted with Lorentzian lines to separate the contribution of the pure diamond and the other nondiamond (graphite) phases. This spectral analysis performed in each sample allowed the determination of the residual stress, from the diamond Raman peak shifts, and also the diamond purity, which increases from 70% to 90% as the thickness goes from 3 to 42 μm. The type and magnitude of the residual stress obtained from x-ray and micro-Raman measurements agreed well for films thicker than 10 μm. For films thinner than this value, an opposite behavior between both results was observed. We attributed this discrepancy to the domain size characteristic of each technique.

/pdf/analysis-of-residual-stress-in-diamond-films-by-x-ray-pmvlit80se.pdf

Analysis of residual stress in diamond films by x-ray diffraction and micro- Raman spectroscopy

Hard physical vapour deposition (PVD) coatings as CrN, TiN and NbN have been successfully exploited for wear and corrosion protection. One limit to their performances under service is caused by the presence of large residual stresses, which adversely affect their properties as fatigue, fracture, corrosion, friction and wear resistance. X-ray diffraction (XRD) has been used for measuring residual stress on CrN/Cr/CrN multilayer coatings, produced by cathodic arc evaporation on substrates of three different steels (AISI H 13, AISI 1040 and K340). The thickness of the external and internal CrN layer was 1.6 mu m in all the samples, while three thicknesses of the intermediate Cr layer, respectively, 0.5, 1.0 and 1.6 mu m were used. The stress along two perpendicular directions (phi = 0 degrees, 90 degrees) on the coating surface have been measured by recording the peak positions of CrN {111} reflection for each 0 at different tilt angles psi. Regardless of the type of steel employed as substrate, residual compressive stresses are present in the external CrN layers. Furthermore, the results reveal stress anisotropy (sigma(phi=0 degrees)not equal sigma(phi=90 degrees)) in the coating plane. In. the coatings deposited on AISI 1040 steel stress is more isotropic independently on the Cr interlayer thickness. (c) 2005 Elsevier B.V. All rights reserved.

X-ray residual stress analysis on CrN/Cr/CrN multilayer PVD coatings deposited on different steel substrates

Interfacial structure, residual stress and adhesion of diamond coatings deposited on titanium

A CrN monolithic 5 μm thick was deposited on a substrate of H11 tool steel and was characterised by XRD analysis and nanoindentation measurements. Hardness, elastic modulus, chemical composition, texture and lattice constants were determined along with the through-thickness residual stress distribution. A high level of compressive stress was obtained both at the surface and inside the coating; data always higher than 2 GPa were measured. Low values of compressive stresses were measured in the steel side near the coating/substrate interface. Those results, coupled with a good adhesion of the interface, allow, when cycling loads are applied to the coated component, a nucleation of the fatigue crack inside the substrate material. Therefore, the critical microstructural parameter that strongly affects the fatigue resistance of the CrN coated samples investigated becomes the size of nonmetallic inclusions working as discontinuity inside the steel substrate, close to the location of the coating/substrate interface. Indeed, in such a location the compressive residual stresses induced by the deposition technique are low and, therefore, insufficient to oppose the tensile stress induced by the applied loading condition during four-point bending fatigue tests.

Relationship between through-thickness residual stress of CrN-PVD coatings and fatigue nucleation sites

Continuous detection of raised intraocular pressure (IOP) could benefit the monitoring of patients with glaucoma. Current contact lenses with embedded sensors for measuring IOP are rigid, bulky, partially block vision or are insufficiently sensitive. Here, we report the design and testing in volunteers of a soft and transparent contact lens for the quantitative monitoring of IOP in real time using a smartphone. The contact lens incorporates a strain sensor, a wireless antenna, capacitors, resistors, stretchable metal interconnects and an integrated circuit for wireless communication. In rabbits, the lens provided measurements that match those of a commercial tonometer. In ten human participants, the lens proved to be safe, and reliably provided accurate quantitative measurements of IOP without inducing inflammation.

A soft and transparent contact lens for the wireless quantitative monitoring of intraocular pressure.

Comparative study of residual stresses measurement methods on CVD diamond films

A clothing-type wearable display can be utilized in fashion, bio-healthcare, and safety industries as well as smart textiles for the internet of things (IoTs) and wearable devices. In response to this trend, we demonstrate a textile display that can endure the active movements of a human body. It can be applied to any kind of textile, and is durable against conditions such as rain, sweat, and washing. As a key technology for realizing the multi-directional wrinkle-able textile display, we fabricated a stress-lowering textile platform with an ultrathin planarization layer replicated from the flat surface of glass. An elastomeric strain buffer for reducing mechanical stress is also inserted into the textile platform. Here, organic light-emitting diodes (OLEDs) with red, green and blue color, thin film transistors (TFTs) fabricated at a low temperature below 150 °C, and a washable encapsulation layer blocking both gas and liquid were demonstrated on the textile platform.

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Multi-directionally wrinkle-able textile OLEDs for clothing-type displays

Evaluating mechanical properties of 100nm-thick atomic layer deposited Al2O3 as a free-standing film

Fan-Out Packages (FOPs) technology is actively researched because it enables high Input / Output (I/O) pads density, thin package, and cost reduction. Epoxy Molding Compound (EMC) materials play a major role in implementing FOPs. EMC acts as a substrate and also protects the chips from the outside. Recently Epoxy Molding Films (EMFs) have been introduced because EMFs are advantageous for large-area molding and can solve planarization problem. However, there are many problems before using at FOPs such as voids, delamination, warpage, and die shift. Voids and delamination occur when moldability is bad. Coefficient of Thermal Expansion (CTE) mismatch between the EMFs and the chips induces thermos-mechanical stress resulting in a warpage during the molding process. Furthermore, die shift from their original position can be also found by warpage, resin flow, and curing shrinkage. Therefore, EMFs materials properties and their effects on FOPs characteristics should be investigated. In this study, thermo-mechanical properties of EMFs were optimized by epoxy formulation to improve FOPs characteristics. Epoxy, silica contents, curing agent, and additives (coupling agent, carbon black) were investigated to produce low CTE and high Glass Transition Temperature (Tg) properties. Then, multiple 10 mm × 10 mm × 200 x03BC;m chips were molded with the size of 100 mm × 100 mm using 400 x03BC;m thick EMFs. The chip to chip distance could be reduced from 12 mm to 5.7 mm without voids and delamination. The warpage of EMFs were measured using 3 Dimensional - Digital Image Correlation (3D - DIC) method, and the effects of process conditions on the warpage of EMFs were also evaluated. And the die shift after molding was quantitatively measured assessed by optical microscopy. Finally, thermal cycle and 85 °C / 85 % RH tests were performed to evaluate the FOPs using optimized EMFs

Junmo Kim

Papers

A soft and transparent contact lens for the wireless quantitative monitoring of intraocular pressure.

Comparative study of residual stresses measurement methods on CVD diamond films

Multi-directionally wrinkle-able textile OLEDs for clothing-type displays

Evaluating mechanical properties of 100nm-thick atomic layer deposited Al2O3 as a free-standing film

Effects of the Materials Properties of Epoxy Molding Films (EMFs) on Fan-Out Packages (FOPs) Characteristics