Wearable sensors are increasingly used in a wide range of applications such as tactile sensors and artificial skins for soft robotics, monitoring human motions for wellbeing and sports performance, and pressure control of compression garments for wound healing. In this work, we present an ultrasensitive resistive pressure sensor based on conductive polydimethylsiloxane (PDMS) thin films with different microstructures. These microscopic features include micro-pyramids, micro-semi-spheres, and micro-semi-cylinders which are created by soft lithography replication of 3D printing templates. To enable piezoresistivity, a thin layer of carbon nanofibers (CNFs) is spray-coated on the textured PDMS film. Pressure sensors are created by pairing the resultant conductive PDMS thin film with a conductive electrode made of coating a poly(ethyleneterephthalate) film with indium tin oxide. The resistance changes of the three microstructure designs under compression loading show that the micro-semi-cylinder-based sensor has the highest sensitivity of -3.6 kPa -1 . Finite element modelling reveals that among the three designs, the micro-semi-cylinders show the largest change in contact area under the same pressure, consistent with the experimental results that the largest resistance change under the same pressure. This sensor is capable of detecting pressure as low as 1.0 Pa. All of the three designs show good reproducibility and excellent cyclic stability. This 3D printing technology is a promising fabrication technique to design microstructured piezoresistive layers, paving the way to tailor sensor performance by engineering their microstructures and to produce ultrasensitive pressure sensors at low cost.

Wearable temperature sensors with high sensitivity, linearity, and flexibility are required to meet the increasing demands for unobtrusive monitoring of temperature changes indicative of the onset of infections and diseases. Herein, we present a new method for engineering highly sensitive and flexible temperature sensors made by sandwiching a poly(3,4-ethylenedioxythiophene):polystyrene (PEDOT:PSS) sensing film between two poly(dimethylsiloxane) (PDMS) substrates. Pre-stretching the sensor to a certain strain can create stable microcracks in the sensing layer that bestow high senstivity and linearity. The average length and density of the microcracks, which dictate the sensor's temperature sensitivity, can be engineered by controlling three key processing parameters, incuding (a) pre-stretching strain, (b) sulfuric acid treatment time, and (c) surface roughness of the substrate. For a given acid treatment time and surface roughness condition, the density and average length of the microcracks increase pre-stretching strain. A smooth PDMS substrate tends to yield long and straight cracks in the PEDOT:PSS film, compared to shorter microcracks with higher density on rough surfaces. Crack density can be further increased via sulfuric acid treatment with an optimum duration of approximately 3 h. Prolonged treatment would result in weak adhesion between the PEDOT:PSS film and the PDMS substrate, which in turn reduces the microcrack density but increases the crack length. By optimizing the three design parameters we have designed a high performance PEDOT:PSS−PDMS sensor that provides a combined high temperature sensitivity of 0.042 °C−1 with an excellent linearity of 0.998 (from 30 to 55 °C), better than the highest temperature sensitivity of PEDOT:PSS based sensors reported in the literature. With a good optical transparency, high temperature sensitivity, excellent linearity, and high flexibility, this microcrackbased sensor is a very promising wearable temperature-sensing solution.

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Microcracking mechanism is an effective technique for creating highly sensitive piezoresistive strain sensors, but such sensors tend possess limited sensing range. Herein we present a new method of widening the...

Cite this article as: P. Blanloeuil, A. Meziane, C. Bacon, 2D Finite Element modeling of the non-collinear mixing method for detection and characterization of closed cracks, NDT&E International, http://dx.doi.org/10.1016/j. ndteint. 2015.08.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. AbstractThe non-collinear mixing technique is applied for detection and characterization of closed cracks.The method is based on the nonlinear interaction of two shear waves generated with an oblique incidence, which leads to the scattering of a longitudinal wave. A Finite Element model is used to demonstrate its application to a closed crack. Contact acoustic nonlinearity is modeled using unilateral contact law with Coulomb's friction. The method is shown to be effective and promising when applied to a closed crack. Scattering of the longitudinal wave also enables us to image the crack, giving its position and size.

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The nonlinear interaction of shear waves with a frictional interface are presented and modeled using simple Coulomb friction. Analytical and finite difference implementations are proposed with both in agreement and showing a unique trend in terms of the generated nonlinearity. A dimensionless parameter ξ is proposed to uniquely quantify the nonlinearity produced. The trends produced in the numerical study are then validated with good agreement experimentally. This is carried out loading an interface between two steel blocks and exciting this interface with different amplitude normal incidence shear waves. The experimental results are in good agreement with the numerical results, suggesting the simple friction model does a reasonable job of capturing the fundamental physics. The resulting approach offers a potential way to characterize a contacting interface; however, the difficulty in activating that interface may ultimately limit its applicability.

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