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.

In this paper, we propose strain-sensitive thin films based on chemically reduced graphene oxide (GO) and multi-walled carbon nanotubes (MWCNTs) without adding any further surfactants. In spite of the insulating properties of the thin-film-based GO due to the presence functional groups such as hydroxyl, epoxy, and carbonyl groups in its atomic structure, a significant enhancement of the film conductivity was reached by chemical reduction with hydro-iodic acid. By optimizing the MWCNT content, a significant improvement of electrical and mechanical thin film sensitivity is realized. The optical properties and the morphology of the prepared thin films were studied using ultraviolet-visible spectroscopy (UV-Vis) and scanning electron microscope (SEM). The UV-Vis spectra showed the ability to tune the band gap of the GO by changing the MWCNT content, whereas the SEM indicated that the MWCNTs were well dissolved and coated by the GO. Investigations of the piezoresistive properties of the hybrid nanocomposite material under mechanical load show a linear trend between the electrical resistance and the applied strain. A relatively high gauge factor of 8.5 is reached compared to the commercial metallic strain gauges. The self-assembled hybrid films exhibit outstanding properties in electric conductivity, mechanical strength, and strain sensitivity, which provide a high potential for use in strain-sensing applications.

Polymer/CNT nanocomposites as piezoresistive films provide an innovative approach for the realization of scalable strain sensors with high sensitivity and low manufacturing costs. These nanocomposites can be realized by thin film deposition or printing techniques on flexible substrates, e.g. foils, mats and textiles, and lead to flexible functional layers. Depending on the sensor geometry the sensing layers allow the strain measurement of integral measurements and local measurement at a certain position. Strain sensors based on carbon nanostructures can overcome several limitations of conventional strain sensors and provide interesting advantages, e.g. high sensitivity, adjustable measurement range and integral measurement on large surfaces. Some of these aspects will be exemplarily discussed for CNT/epoxy and CNT/PEDOT: PSS nanocomposites. Owing to their aforementioned features, CNT based flexible strain sensors are highly attractive for applications in the fields of structural health monitoring (SHM) and wearable devices.

The piezoresistive properties of thin films based on carbon nanotubes dispersed in conductive polymer were investigated. Conductive multiwalled carbon nanotubes / Poly (3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS) films were prepared and deposited on flexible polymer substrates (Kapton HN, 250 μm) by drop casting and the aqueous dispersions with different amounts of MWCNTs in PEDOT:PSS were prepared at different sonication times. The influences of the MWCNT load on the film properties such as electrical conductivity; strain sensitivity and hysteresis were investigated. Negative resistance coefficient was observed when the films are subjected to a force from 10 N to 110 N. Furthermore, it was observed that at low amounts of MWCNTs dispersed in PEDOT:PSS with sonication time of 15 min have better strain sensitivity then pure PEDOT:PSS and other prepared thin film composites. Moreover, the strain sensitivity was found to be strongly dependent on the MWCNT concentration. In general, MWCNT/PEDOT:PSS films have potential as a highly linear strain sensitive material.