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.
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...
Unlike creatures that walk, flying animals need to control their horizontal motion as well as their height above the ground. Research on insects, the first animals to evolve flight, has revealed several visual reflexes that are used to govern horizontal course. For example, insects orient toward prominent vertical features in their environment [1-5] and generate compensatory reactions to both rotations [6, 7] and translations [1, 8-11] of the visual world. Insects also avoid impending collisions by veering away from visual expansion [9, 12-14]. In contrast to this extensive understanding of the visual reflexes that regulate horizontal course, the sensory-motor mechanisms that animals use to control altitude are poorly understood. Using a 3D virtual reality environment, we found that Drosophila utilize three reflexes--edge tracking, wide-field stabilization, and expansion avoidance--to control altitude. By implementing a dynamic visual clamp, we found that flies do not regulate altitude by maintaining a fixed value of optic flow beneath them, as suggested by a recent model [15]. The results identify a means by which insects determine their absolute height above the ground and uncover a remarkable correspondence between the sensory-motor algorithms used to regulate motion in the horizontal and vertical domains.
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