The use of carbon nanotubes for piezoresistive strain sensors has acquired significant attention due to its unique electromechanical properties. In this comprehensive review paper, we discussed some important aspects of carbon nanotubes for strain sensing at both the nanoscale and macroscale. Carbon nanotubes undergo changes in their band structures when subjected to mechanical deformations. This phenomenon makes them applicable for strain sensing applications. This paper signifies the type of carbon nanotubes best suitable for piezoresistive strain sensors. The electrical resistivities of carbon nanotube thin film increase linearly with strain, making it an ideal material for a piezoresistive strain sensor. Carbon nanotube composite films, which are usually fabricated by mixing small amounts of single-walled or multiwalled carbon nanotubes with selected polymers, have shown promising characteristics of piezoresistive strain sensors. Studies also show that carbon nanotubes display a stable and predictable voltage response as a function of temperature. 1. Introduction Carbon nanotubes have drawn much attention since their discovery in 1991 [1] because of their unique electronic and mechanical properties. Electronically, single-walled carbon nanotubes can be metallic, semiconducting, or small-gap semiconducting, depending on the graphene lattice orientation, with respect to the axis of the tube. They also have very interesting electromechanical properties [2] and could be useful in applications for piezoresistive strain sensors such as strain gauges. The applications of strain sensors are mainly used in engineering fields for damage detection and characterization of structures. Strain measurements by the traditional sensors (metal and semiconductor strain gauges) have shown high sensitivities and are less expensive. But despite these advantages, there are drawbacks. They are fixed directional sensors, meaning strain can only be measured in a specific direction; they have low resolution at nanoscale and cannot be embedded in structural materials. The fiber Bragg grating (FBG) sensors also known as optical fiber sensors mostly used as trunk lines for the transmission of information have been attracting special interest recently due to their strain sensing characteristics [18]. In a manner similar to traditional strain gauges, calibration of displacement optical fiber is related to the strain resistivities of the fiber by appropriate gauge factors. FBG strain sensors match quite well with other composites materials like glass and carbon fiber composites,
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