Carbon nanotubes (CNTs) are known to exhibit outstanding mechanical, electrical, thermal, and coupled electromechanical properties. CNTs can be employed towards the design of an innovative strain sensor with enhanced multifunctionality due to their load carrying capability, sensing properties, high thermal stability, and outstanding electrical conductivity. All these features indicate the prospect to use CNTs in a very wide range of applications, for instance, highly sensitive resistance-type strain/force sensors, wearable electronics, flexible microelectronic devices, robotic skins, and in-situ structural health monitoring. CNT-based strain sensors can be divided into two different types, the individual CNT- based strain sensors and the ensemble CNT-based strain sensors e.g. CNT/polymer nanocomposites and CNT thin films. In contrast, to individual CNT-based strain sensors with very high gauge factor (GF) e.g. ~3000, the ensemble CNT-based strain sensors exhibit very low GFs e.g. for a SWCNT thin film strain sensor, GF is ~1. This research discusses the mechanisms and the optimizing principles of a SWCNT thin film piezoresistive sensor, and provide an experimental validation of the numerical/analytical investigations. The dependence of the piezoresistivity on key parameters like alignment, network density, bundle diameter (effective tunneling area), and SWCNT length is studied. The tunneling effect is significant in SWCNT thin films showing higher degrees of alignment, due to greater inter-tube distances between the SWCNTs as compared to random oriented SWCNT thin films. It can be concluded that SWCNT thin films featuring higher alignment would have a higher GF. On the other hand, the use of sparse network density which comprises of aligned SWCNTs can as well intensify the tunneling effect which can result to a further increase in the GF. In addition, it is well-known that percolation is greatly influenced by the geometry of the nanotubes e.g. bundle diameter and length. A study on the influence of bundle diameter of SWCNTs on the piezoresistivity behavior of mechanically drawn SWCNT thin films showed the best performance with an improved GF of ~10 when compared to the randomly oriented SWCNT thin films with GF of ~1. The non-linear piezoresistivity of the mechanically drawn SWCNT thin films is considered to be the main mechanism behind the high strain sensitivity. Furthermore, information about the average length and length distribution is very essential when examining the influence of individual nanotube length on the strain sensitivity. With that in mind, we use our previously developed preparative ultracentrifuge method (PUM), and our newly developed gel electrophoresis and simultaneous Raman and photolumiscence spectroscopy (GEP-SRSPL) to characterize the average length and length distribution of SWCNTs respectively.
