# Review of Flexible Piezoresistive Strain Sensors in Civil Structural Health Monitoring
## 5 Applications and Prospects in Civil SHM
### Challenges and Prospects of Large-Scale Application  
Although carbon-type composite materials have shown many advantages for sensing damage in SHM, there are still challenges to realize large-scale application of carbon-based strain sensors. For instance, the high-purity CNTs and their corresponding dispersion have high cost, and the blast risk of CNTs brings safety concerns [67, 68]. In addition, a tedious fabrication process hinders the application of graphene-based strain sensors [160]. In fact, most current fabrication techniques of carbon-type strain sensors are still in their prototype stage. Moreover, practical application of the proposed piezoresistive strain sensors requires an overall integrated system, which includes sensors, power supplement, data analysis and processing, wireless data transmission and a remote operation system [93]. Besides, the integrated sensing system should have robustness and high reliability against the performance degradation in long-term monitoring [128]. In terms of the self-healing flexible sensor, there are still challenges for simple, economical and scale-up fabrication strategies. Another important challenge is to keep the strain sensors maintaining good performance even after multiple healings [22].  
Furthermore, researchers explored novel ideas of carbon-type sensors using pure macro-graphene, which showed potentials for field application and scale-up production [161]. [PERSON] et al. reported a pure graphene-assembled film (GAF), which can be directly used as a strain sensor without flexible substrates [160]. The GAF significantly simplifies the fabrication process in comparison with most reported strain sensors, and mass production of this material has already been realized. Besides, the resistance of the film remains unchanged with increasing temperatures from 20 \({}^{\circ}\)C to 100 \({}^{\circ}\)C, indicating a good thermal stability (Figure 13a).  
On the other hand, the GAF, which was originally used as a wireless antenna with a flexible radio frequency, can also be used as a wireless strain sensor via resolving the  
Figure 13: Flexible strain sensor based on pure macro-graphene: (**a**) fabrication of the GAF [160] (Copyright 2016, Elsevier Ltd.); (**b**) GAF antenna-based wireless strain sensor for measuring compression and tension [162] (Copyright 2020, American Chemical Society).  
signal variation resulting from the deformation of the GAF antenna [162]. In their work, the high-conductive GAF has advantages of high flexibility and excellent mechanical stability. As a result, a flexible GAF-antenna-based wireless strain sensor was achieved, which has high GF of 35 in measuring tensile bending and compressive bending (Figure 13b). Moreover, a two-dimensional wireless displacement sensor was developed based on a bilayer graphene-based frequency selective surface [163]. A two-dimensional displacement measurement with high accuracy was carried out based on the variation trend of the resonant frequencies of the top and bottom layers of the sensor. One more benefit is that the bilayer graphene material shows outstanding mechanical and chemical stability in complicated environments, indicating long-term-use prospects.