# Review of Flexible Piezoresistive Strain Sensors in Civil Structural Health Monitoring
## 3 Elaboration of Sensor Materials
### Flexible Substrates  
Flexible substrates provide a large stretchable range for piezoresistive sensors. In the application of SHM, the substrates are pasted to the structural surface by using epoxy or another adhesive. The generally used flexible substrates in SHM include PDMS [74; 78; 79], Ecoflex [80] and thermoplastic polyurethane (TPU) [81; 82].  
PDMS has the advantages of low elastic modulus, high elasticity, good formability, transparency, good thermal stability and corrosion resistance; therefore, it is the most widely used substrate for fabricating flexible sensors. [PERSON] et al. prepared a strain sensor based on CNT arrays encapsulated by PDMS [74]. In their study, the flexibility of PDMS was consistent with that of CNTs; hence, the sensor still maintained reliability and stability after cyclic bending it 8000 times. [PERSON] et al. produced a metallic CNTs/PDMS composite film, which had 92% high transparency in the visible light range and all-round outstanding performance in sensitivity, hysteresis, linearity and drift [79]. Nevertheless, PDMS is sensitive to temperature change and is prone to ageing in the long term.  
Ecoflex has excellent mechanical flexibility, water resistance and long-term stability. As an example, [PERSON] et al. employed Ecoflex as a substrate and CNTs as conductive filler to fabricate an ultrasoft strain sensor. Given the strong interface between the CNTs and the Ecoflex, the composite sensor has high reliability, high linearity and an ultra-large strain range [80].  
TPU has higher toughness, higher tear strength and stronger resistance to mechanical damage than PDMS. Moreover, TPU-based substrates still maintain the original function after more than 1000 large-tensile cycles. As a result, TPU has gradually become utilized as a common matrix material for flexible sensors in recent years [81; 82].  
High-performance polymers, such as polyimide (PI) [83], polyurethane [84], polyvinyl alcohol [85] and polyethylene terephthalate [86; 87], have also been employed as substrates in different application scenarios.  
Table 2 shows the materials of the above piezoresistive sensors and their corresponding sensing properties. Specifically, the maximum detectable strain range, the GF within the  
Figure 7: Hybrid conductive materials.  
working range, the linearity and the cyclic times for long-term durability are illustrated. It is found that the properties of different piezoresistive sensors are quite different from each other, even though they have the same conductive filler. Another phenomenon is that a flexible sensor can hardly achieve co-existence of high GF, large detectable range and good linearity. A sensor with a relatively large range and high GF always has a nonlinear strain-resistance relation, while a sensor that possesses a large range and good linearity regularly meets a low GF. The main reason for this phenomenon is probably the mutual conflict between the concentration of conductive filler and the ratio of resistance variation. Therefore, the sensing properties should be improved to meet the demands of practical application, as presented in the following Section 4.