Compared with traditional conductive fillers, carbon nanotubes (CNTs) have unique advantages, i.e., excellent mechanical properties, high electrical conductivity and thermal stability. Nanocomposites as piezoresistive films provide an interesting approach for the realization of large area strain sensors with high sensitivity and low manufacturing costs. A polymer-based nanocomposite with carbon nanomaterials as conductive filler can be deposited on a flexible substrate of choice and this leads to mechanically flexible layers. Such sensors allow the strain measurement for both integral measurement on a certain surface and local measurement at a certain position depending on the sensor geometry. Strain sensors based on carbon nanostructures can overcome several limitations of conventional strain sensors, e.g., sensitivity, adjustable measurement range and integral measurement on big surfaces. The novel technology allows realizing strain sensors which can be easily integrated even as buried layers in material systems. In this review paper, we discuss the dependence of strain sensitivity on different experimental parameters such as composition of the carbon nanomaterial/polymer layer, type of polymer, fabrication process and processing parameters. The insights about the relationship between film parameters and electromechanical properties can be used to improve the design and fabrication of CNT strain sensors.

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Carbon nanotubes-based nanocomposites have gained a great amount of attraction and play a key role in the realization of strain sensors owing to their remarkable physical properties. In this study, the piezoresistivity of multi-walled carbon nanotubes (MWCNTs)/epoxy-based nanocomposite-based strain sensor under static tensile load is examined using electrochemical impedance spectroscopy. Morphological examinations show that MWCNTs are randomly and homogeneously distributed in the epoxy polymer matrix. A simplified resistance constant phase element model is proposed and validated by impedance spectrum to fit the impedance spectra and the equivalent circuit parameters are extracted under uniaxial static load. Impedance results suggest that depending on the frequency regions, the sensor exhibits different responses under loading. Moreover, the proposed sensor gives high sensitivity, linearity and low hysteresis under cyclic quasi-static loading and unloading that makes the sensor a promising candidate for practical strain sensor applications.

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