However, shape changes in terms of tensile strain will lead to a change in conductivity of the electrodes in addition to the change in electrode separation, making the material combination complicated to characterize as a simple strain sensor. For example, if a parallel capacitive sensor is designed using a flexible electrode, the electrode area increases as the sensor is compressed. Then the sensor signal changes due to the decrease in gap distance between the electrodes, as well as due to the change in resistance of the electrode geometry changing [14,16]. With dielectric elastomers one primary research goal is the development of compliant electrodes, where a change in mechanical dimension does not lead to a large change in resistivity of the material [17].

This will be very useful for flexible circuit applications, but is not useful at this time for tailoring a high-stretch strain sensor, where a change in resistivity of the material with respect to applied mechanical deformation is required [9,18�C20].1.3. Piezoresistive Polymer SensorsDifferent types of piezoresistive sensors have been developed over the past decade with a focus on strain or force-sensing applications. Historically flexible fiber sensors have followed different directions including the development of yarns with conductive fibers [5,21] or nanotube yarns [22], electroactive polymers with carbon fillers [9,19], carbon or silver-coated polymer fibers [10] and the integration of carbon nanotubes into a polymer matrix [23]. The yarn design includes a sensor fiber element woven or twisted together with structural but electrically insulating fibers.

The conductive fiber could be metallic or carbon-coated [5,24]. This combination allows the integration of sensing properties into a structural Brefeldin_A package, where the sensor material can support mechanical loads and provide a signal (while traditional strain gauges only provide passive sensing of substrate deformation). The basic design of a piezoresistive sensor monofilament entails combining conductive particles together with a flexible polymer matrix, creating a CPC material. The material can then be formed into various forms from monofilaments to plates for specific applications. Forms of carbon are natural choices for the conductive filler. The basic system includes combining carbon black and a non-conductive mechanically flexible matrix to produce a composite. As the content of carbon black increases, the mechanical flexibility of the monofilament decreases, while the electrical conductivity increases. This can result in a monofilament which conducts current well, but will fail mechanically if stressed too much. Conversely,
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