# Review of Flexible Piezoresistive Strain Sensors in Civil Structural Health Monitoring
## 6 Conclusions  
This study presented a literature review of flexible piezoresistive strain sensors in several aspects, including typical piezoresistive mechanisms, elaboration of sensor materials, optimization of sensor performances, applications and prospects in civil SHM. The conclusions are summarized as follows:  
The four common piezoresistive mechanisms successively predominate the electrical conductivity of the strain sensor within their respective working strain levels. Each mechanism corresponds with a different sensing parameter. The proportion of tunneling resistance in total resistance determines the sensitivity, while the proportion of interface contact resistance governs the linearity. Thus, the sensing performances of piezoresistive sensors are highly related to the morphology of the conductive network.  
Selections of conductive materials and substrate materials are crucial to electrical conductivity, sensing properties and stretchable range in flexible piezoresistive sensors. Hybrid conductive materials usually perform better than single-type materials because the conductive networks and the mechanical properties are improved by the complementation of different materials.  
The sensing performances of flexible sensors can be further enhanced through strategies such as the proper combination of different types of conductive fillers, strong bonding and cooperative deformation between the fillers and the substrates, and assistances of microcracks or microstructure. Nevertheless, the incompatibility of high sensitivity, high stretchability, satisfactory linearity and durability is still a great challenge in developing high-performance strain sensors.  
Applications of piezoresistive strain sensors in civil SHM mainly focused on tracking concrete cracks and detecting damage in FRP structures. Feasible strategies for selecting suitable piezoresistive strain sensors for civil SHM are provided. Although carbon-based flexible piezoresistive sensors have been well-developed in recent years, challenges such as complex fabrication, performance degradation during long-term use and overall integrated systems still exist for further field application. More attempts are required to develop advanced piezoresistive sensors targeted for civil SHM.  
Conceptualization, S.W. and H.Z.; Data curation, Z.Z.; Investigation, H.Y.; Methodology, K.G.; Resources, S.W.; Supervision, H.Z.; Visualization, T.P.; Writing--original draft, K.G. and Z.Z.; Writing--review & editing, K.G. All authors have read and agreed to the published version of the manuscript.  
The authors are grateful for the financial support from the National Key R&D Program of China (Grant No.: 2021 YFF0501001), the National Natural Science Foundation of China (Grant Nos.: 51922046 and 51838006), Natural Science Foundation of Hubei Province: 2020 CFA047, and the Research Fund of China Railway Siyuan Survey and Design Group Co., Ltd. (contract numbers: 2021K085 and 2020D006). The findings and opinions expressed here, however, are those of the authors alone; they do not necessarily reflect the views of the sponsors.  
Not applicable.  
The authors declare no conflict of interest.