An ultrahigh sensitive capacitive pressure sensor based on a porous pyramid dielectric layer (PPDL) is reported. Compared to that of the conventional pyramid dielectric layer, the sensitivity was drastically increased to 44.5 kPa −1 in the pressure range <100 Pa, an unprecedented sensitivity for capacitive pressure sensors. The enhanced sensitivity is attributed to a lower compressive modulus and larger change in an effective dielectric constant under pressure. By placing the pressure sensors on islands of hard elastomer embedded in a soft elastomer substrate, the sensors exhibited insensitivity to strain. The pressure sensors were also nonresponsive to temperature. Finally, a contact resistance-based pressure sensor is also demonstrated by chemically grafting PPDL with a conductive polymer, which also showed drastically enhanced sensitivity.

Tactile sensors that can mechanically decouple, and therefore differentiate, various tactile inputs are highly important to properly mimic the sensing capabilities of human skin. Herein, we present an all-solution processable pressure insensitive strain sensor that utilizes the difference in structural change upon the application of pressure and tensile strain. Under the application of strain, microcracks occur within the multiwalled carbon nanotube (MWCNT) network, inducing a large change in resistance with gauge factor of ∼56 at 70% strain. On the other hand, under the application of pressure to as high as 140 kPa, negligible change in resistance is observed, which can be attributed to the pressure working primarily to close the pores, and hence minimally changing the MWCNT network conformation. Our sensor can easily be coated onto irregularly shaped three-dimensional objects (e.g., robotic hand) via spray coating, or be attached to human joints, to detect bending motion. Furthermore, our sensor can differentiate between shear stress and normal pressure, and the local strain can be spatially mapped without the use of patterned electrode array using electrical impedance tomography. These demonstrations make our sensor highly useful and important for the future development of high performance tactile sensors.

Electronic skin are devices that mimic the functionalities of human skin, which require high sensitivity, large dynamic range, high spatial uniformity, lowcost and large-area processability, and the capacity to differentiate various external inputs. We herein introduce a versatile droplet-based microfluidic-assisted emulsion self-assembly process to generate three-dimensional microstructure-based highperformance capacitive and piezoresistive pressure sensors for electronic skin applications. Our technique can generate uniformly sized micropores that are selfassembled in an orderly close-packed manner over a large area, which results in high spatial uniformity. The size of the micropores can easily be tuned from 100 to 500 μm, through which sensitivity and dynamic range were controlled as high as 0.86 kPa −1 and up to 100 kPa. Our device can be printed on curvilinear surfaces and be molded into various shapes. We furthermore demonstrate that by simultaneously utilizing capacitive and piezoresistive pressure sensors, we can distinguish between pressure and temperature, or between pressure and proximity. These demonstrations make our process and sensors highly useful for a wide variety of electronic skin applications.

Going with the grain Changes in surface structure can make metal nanoparticles supported on oxides more active for certain catalytic reactions. Huang et al . show that steam pretreatment of palladium nanoparticles on alumina led to a high density of twin boundaries, unlike other oxidation and reduction pretreatments. The density of these stable grain boundaries at the surface correlated with higher methane oxidation rates and lower temperatures for the initiation of the reaction. The introduction of additional defect sites through laser ablation created even more active catalysts. —PDS

Piezoresistive pressure sensors based on elastomer-conductive material composite is particularly promising due to their many advantages such as simple readout circuit, low crosstalk, low susceptibility to electromagnetic pick-up, and low-cost and simple fabrication process. [5a,6] Various works have been reported to improve the performance of piezoresistive pressure sensors, most of which have been focused on increasing the sensitivity. [3a,7] For instance, microstructuring of the piezoresistive element into porous structure, [4b,8] pyramids, [7a,9] microdomes [3d,10] have been demonstrated to improve the sensitivity, which has been attributed to the decrease in the compressive modulus. [7a,11] Porous structures, in particular, was utilized in various pressure sensors due to their facile fabrication process and scalability. Porous structure can be fabricated either by filling a 3D template such as sugar, [12] nickel foam [13] with an elastomer and subsequently etching away the template, or by mixing aqueous and oil solutions to form an emulsion and removing the solvents. [4b,14] Despite its significance, maximizing sensitivity in composite-based piezoresistive pressure sensors is not necessary for many applications (i.e., often moderate levels are sufficient). On the other hand, sensor-to-sensor uniformity and hysteresis are two properties that are of critical importance to realize any application. In fact, without assuring high uniformity and low hysteresis, using the sensor in a practical setting is unrealistic. However, there is currently a lack of reported work that specifically addresses these issues. As far as it is known, no quantitative assessment of sensor-to-sensor uniformity (error bars are sometimes included in the sensor performance plots but are not specifically addressed) or hysteresis was reported in composite-based piezoresistive pressure sensors. The importance of sensor-to-sensor uniformity is obvious. If sensors with largely varying characteristics are used together as an array, each sensor has to be individually calibrated, making accurate measurement impractical with increasing number of sensors. Hysteresis, which is the difference in the output signal under loading and unloading of pressure, also causes inaccuracy in measurement. Hysteresis is especially problematic in piezoresistive sensors, which originates from weak interactions Sensor-to-sensor variability and high hysteresis of composite-based piezoresistive pressure sensors are two critical issues that need to be solved to enable their practical applicability. In this work, a piezoresistive pressure sensor composed of an elastomer template with uniformly sized and arranged pores, and a chemically grafted conductive polymer film on the surface of the pores is presented. Compared to sensors composed of randomly sized pores, which had a coefficient of variation (CV) in relative resistance change of 69.65%, our sensors exhibit much higher uniformity with a CV of 2.43%. This result is corroborated with finite element simulation, w...

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The demand for display technology is expected to increase with the continuous spread of portable electronics and with the expected emergence of flexible, wearable, and transparent display devices. A touch screen is a critical component in display technology that enables user interface operations, and the future generation of touch screens, the so-called 3D touch screens, is expected to be able to detect multiple levels of pressure. To enable 3D touch screens, transparent pressure sensors with high linearity over a working range that encompasses the pressure range of human touch (10–100 kPa) are required. In this work, a transparent and linear capacitive pressure sensor is reported with a transmittance over 85% and high linearity (R 2 = 0.995) over 5–100 kPa of pressure. To render the sensor transparent, a microstructured “hard” elastomer layer was filled in with a refractive index matching a “soft” elastomer layer, through which light scattering was minimized. High linearity was attained from the sensor’s unique architecture that increases the effective area of the capacitor with applied pressure. These attributes render our sensor highly suitable for future 3D touch screen applications.

A highly sensitive bending sensor composed of patterned Pt lines, integrated with energy harvesting capability, is reported. The sensitivity of the bending sensor increases as the width of the Pt lines decreases, owing to the increase in crack density with decreasing line width. Furthermore, sensitivity increases with increasing bending cycles, but saturates at around 1000 cycles. Such a behavior corresponds to the increase and eventual saturation of crack density with increasing bending cycles. A microstructured polydimethylsiloxane layer is placed on top of the Pt lines to serve as a triboelectric energy harvesting layer, where human skin and the Pt lines are utilized as electrodes. Voltage and current of 18.6 V and 209 nA are generated, respectively, from gentle finger tapping. These demonstrations make the device highly useful for a wide variety of portable and wearable flexible electronic applications.