Ultrastretchable Strain Sensors Using Carbon Black-Filled Elastomer Composites and Comparison of Capacitive Versus Resistive Sensors

DOI: 10.1002/admt.201700284 system.[11] Researchers have developed highly stretchable strain sensors made of compliant elastomers and various conductive materials, such as silver nanowire,[12] carbon nanotube (CNT),[13–18] carbon grease,[19] graphene,[20] graphite,[21] laser-carbonized polyimide,[22] conductive acrylic elastomer,[10] liquid metal,[23,24] ionic liquid,[25–27] and conductive fabric.[28] However, not all of these technologies can be manufactured in large scale at low cost. Here, we propose the use of carbon black (CB)-filled elastomer composites for highly stretchable strain sensors (up to 500%) that can be batch manufactured at low cost. CBs are a type of low-cost conductive nanoparticle, which, when used as a filler in an elastomeric matrix, enhances the mechanical strength, abrasion resistance, UV resistance, and light absorbency of the composite.[29–31] The CB-filled elastomer can be printed in large areas by means of a layer-by-layer process,[32] with good wettability and high adhesion to silicone surfaces. Mixing various types of CBs and elastomers[33] gives material designers flexibility to achieve high compliance and stretchability. Our layer-by-layer CB-filled elastomer fabrication process can be used to create resistive or capacitive sensors.[11] Resistive sensing relies on the piezoresistive effect and geometrical changes of electrodes, where mechanical strain causes a change in electrical resistivity. Capacitive sensing exploits changes of the capacitance between a pair of electrodes sandwiching a dielectric layer. Strain expands the area of the electrodes and reduces the thickness of the dielectric layer, leading to an increase of the capacitance. A recent review on strain sensors has pointed that resistive type strain sensors have high sensitivity but hysteresis and nonlinear response, while capacitive type strain sensors display excellent linearity and hysteresis performance but low sensitivity.[11] On the other hand, according to other literature, both resistive and capacitive type strain sensors show good linearity, low hysteresis, and repeatability.[10,13,15,28] Therefore, there is a lack of comprehensive knowledge of highly stretchable strain sensors that clarifies advantages and disadvantages of the two sensing methods. In addition, other characteristics, such as responses to different strain speed and temperature, have not yet been compared. This would result in difficulty when it is required to select an appropriate sensor The advent of soft robotics has led to the development of devices that harness the compliance and natural deformability of media with nonlinear elasticity. This has led to a need of batch-manufacturable soft sensors that can sustain large strains and maintain kinematic compatibility with the systems they track. In this article, an approach to address this challenge is presented with highly stretchable strain sensors that can operate at strains up to 500%. The sensors consist of a carbon black-filled elastomer composite that is batch manufactured using film-casting techniques and CO2 laser ablation. This process facilitates the rapid multilayered fabrication of both capacitive and resistive sensing elements. When measuring capacitance, these sensors exhibit high linearity (R2 = 0.9995), low hysteresis under cyclic loading with varying strain amplitude (50–500%), and high repeatability (≥104 cycles). The sensors possess gauge factors of 0.83–0.98 in capacitive mode and 1.62–3.37 in resistive mode.