Highly Uniform and Low Hysteresis Piezoresistive Pressure Sensors Based on Chemical Grafting of Polypyrrole on Elastomer Template with Uniform Pore Size

Abstract

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...

Micro‐Nano Processing of Active Layers in Flexible Tactile Sensors via Template Methods: A Review

“…With the rise of the template method in the field of flexible tactile sensors, apart from the above four common template methods with obvious advantages, there are other template methods including microsphere array template method and prestretched template method. The microsphere array template method is a kind of method to obtain ordered anti-microsphere array by using various microsphere (PS microsphere, [149,150,28,151] SiO 2 microsphere, [33] and droplet microsphere [152] ) as ordered templates and sacrificing the microsphere array after polymer coating. This template method is simple, economical, and can effectively adjust the size of the micro-nano structure in the template by controlling the size of the microspheres, thereby obtaining an optimal sensing performance.…”

“…With respect to the developed intelligent robot gripper, the first function that needs to achieve is enabling to repeatedly grasp and release objects to ensure reliability in practical applications. Park et al [152] proposed a method for preparing a resistive pressure sensor based on porous PPy/PDMS composite elastomers by means of the microfluidic emulsion droplet template. Because of the strong covalent bond (as opposed to weak physical adsorption) between PPy and PDMS, and the porous nature of PDMS (minimum viscoelasticity), the sensor exhibited negligible hysteresis.…”

“…Besides that, an anti-pyramidal Si template was prepared by lithography and anisotropic etching, and an Au/PDMS thin film with pyramidal structure array was replicated, thereby providing excellent sensitivity (6.7 × 10 7 kPa −1 , 1-5 kPa; 3.8 × 10 5 kPa −1 , 5-50 kPa). Meanwhile the memristor could realize basic synaptic function under low energy consumption (10-200 pJ), and remain stable status in [152] Copyright 2019, Wiley-VCH. b) Schematic of the synthesis process of the GO-AgNF-PI sponge, and its variable stiffness touchsensing demonstration.…”

“…a) Schematic depiction of the sensor based on porousPPy/PDMS composite elastomers fabrication process, and the optical photograph of robotic hand gripper grasping an egg and a balloon. Reproduced with permission [152]. Copyright 2019, Wiley-VCH.…”

Recent Progress in Essential Functions of Soft Electronic Skin

“…Traditional conductive elastomer composite consisting of stretchable elastomer and conductive fillers is able to sense the external stress because of the piezoresistive behavior, which can be designed as pressure sensor for the detection of pressure stimuli. [127][128][129][130][131][132][133][134][135][136][137][138][139][140][141][142] However, these pressure sensors normally possess poor sensitivities, obvious hysteresis, and delayed response time, which hinder their practical applications in the e-skins. To overcome the above limitations of traditional pressure sensors, novel geometrical designs of the sensing structures are desired.…”