Though the widely available, low-cost, and disposable papers have been explored in flexible paper-based pressure sensors, it is still difficult for them to simultaneously achieve ultrahigh sensitivity, low limit and broad range of detection, and high-pressure resolution. Herein, we demonstrate a novel flexible paper-based pressure sensing platform that features the MXene-coated tissue paper (MTP) sandwiched between a polyimide encapsulation layer and a printing paper with interdigital electrodes. After replacing the polyimide with weighing paper in the MTP pressure sensor, the silver interdigital electrodes can be recycled through incineration. The resulting pressure sensor with polyimide or paper encapsulation exhibits a high sensitivity of 509.5 or 344.0 kPa–1, a low limit (∼1 Pa) and a broad range (100 kPa) of detection, and outstanding stability over 10 000 loading/unloading cycles. With ultrahigh sensitivity over a wide pressure range, the flexible pressure sensor can monitor various physiological signals and human movements. Configuring the pressure sensors into an array layout results in a smart artificial electronic skin to recognize the spatial pressure distribution. The flexible pressure sensor can also be integrated with signal processing and wireless communication modules on a face mask as a remote respiration monitoring system to wirelessly detect various respiration conditions and respiratory abnormalities for early self-identification of opioid overdose, pulmonary fibrosis, and other cardiopulmonary diseases.

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We consider a liquid meniscus inside a wedge of included angle 2β that wets the solid walls with a contact angle θ. Under an imposed axial temperature gradient, the Marangoni stress moves fluid toward colder regions whereas the capillary pressure gradient drives a reverse flow, leading to a steady state. The fluxes driven by these two mechanisms are found by numerical integration of the parallel flow equations. Perturbation theory is applied to derive an expression for the capillary pressure, which is typically dominated by the transverse curvature of the circular arc inside the cross section perpendicular to the flow axis, and corrected by a higher order axial curvature resulting from the axial variation of the interface. Lubrication theory is then used to derive a thin film equation for the shape of the interface. Solutions are determined by two primary parameters: D, a geometric parameter giving the relative importance of the two curvatures and M, a modified Marangoni number. Numerical solutions indicate that for sufficiently large M, the Marangoni stress creates a virtual dry region. The value of M at dryout is found to depend linearly on D. A simplified analytical model is developed which agrees very well with the numerical solution for large values of D. It is found that dryout occurs more easily for larger wedge and/or contact angles except for a special case of β+θ=π∕2. In that case the axial curvature dominates and the dependence of the dryout condition on β and θ is nonmonotonic, but only weakly so.

The surge in air pollution and respiratory diseases across the globe has spurred significant interest in the development of flexible gas sensors prepared by low-cost and scalable fabrication methods. However, the limited breathability in the commonly used substrate materials reduces the exchange of air and moisture to result in irritation and a low level of comfort. This study presents the design and demonstration of a breathable, flexible, and highly sensitive NO2 gas sensor based on the silver (Ag) decorated laser-induced graphene (LIG) foam. The scalable laser direct writing transforms the self-assembled block copolymer and resin mixture with different mass ratios into highly porous LIG with varying pore sizes. Decoration of Ag nanoparticles on the porous LIG further increases the specific surface area and conductivity to result in a highly sensitive and selective composite to detect nitrogen oxides. The as-fabricated Ag/LIG gas sensor on a flexible polyethylene substrate exhibits a large response of -12‰, fast response/recovery of 40/291 s, a low detection limit of a few ppb at room temperature. Integrating the Ag/LIG composite on diver fabric substrates further results in breathable gas sensors and intelligent clothing, which allows permeation of air and moisture to provide long-term practical use with an improved level of comfort.

We consider a liquid meniscus inside a wedge of included angle 2␤ that wets the solid walls with a contact angle . The meniscus has a convex interface that satisfies /2Ͻ + ␤ Ͻ . The capillary pressure gradient due to a small disturbance of the location of the contact line moves fluid from a neck region to a bulge region, causing instabilities. A dynamic contact-line condition is considered in which the contact angle varies linearly with the slipping speed of the contact line with a slope of G : G = 0 representing perfect slip and a fixed contact angle. A nonlinear thin film equation is derived and numerically solved for the shape of the contact line as a function of parameters. The result for G = 0 shows that the evolution process consists of a successive formation of bulges and necks in decreasing length and time scales, eventually resulting in a cascade structure of primary, secondary, and tertiary droplets. When G Ͼ 0, there is a similar but slower nonlinear evolution process. The numerical results agree qualitatively with very recent experimental results.