# Review of Flexible Piezoresistive Strain Sensors in Civil Structural Health Monitoring
## 4 Optimization of Sensor Performance
### Improvement of Hysteresis and Durability
Self-healing material will significantly extend the service life of a flexible strain sensor and reduce the maintenance cost of civil SHM [136]. Table 3 shows some typical flexible strain sensors with improved sensing properties. Compared with the property values in Table 2, the most remarkable improvement of the flexible sensors shown in Table 3 is the more accurate linearity. Most of the sensors showed linear or multi-stage linear characters. For instance, the flexible sensor proposed in [91] showed multi-stage linearity within the strain ranges of \(\varepsilon<2\%\), \(2\%<\varepsilon<15\%\) and \(\varepsilon>15\%\). Owing to the improved linearity, the mathematical sensing model became more precise, namely with fixed GF values and clear working ranges. The improved linearity and precise GF are mainly attributed to the implementation of braid micro-structures or micro-cracks, the reinforced bonding between the conductive filler and substrate, and the integration of hybrid materials. In addition, a few sensors showed the compatibility of multiple properties. Noting that the sensor described in [105] achieved all-round performances of large range (200% strain), relatively good sensitivity (GF = 10.5), good linearity and outstanding durability (180,000 cycles under 30% strain), this all-round sensor is very promising not only in the civil SHM but also other fields. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & **Materials** & **Maximum** & **Gauge Factor** & **Linearity** & **Cyclic Times for** \\  & **Filler + Substrate** & **Detectable Range** & **(Sensing Range)** & **(Strain Range)** & **Durability** \\ \hline \multirow{3}{*}{[91]} & \multirow{3}{*}{SWCNT+ PDMS} & \multirow{3}{*}{150\%} & 161 (\(\varepsilon<2\%\)), & \multirow{3}{*}{Multi-stage linear} & 100,000 \\  & & & 0.58 (\(\varepsilon>15\%\)) & & (\(\varepsilon<60\%\)) \\ \cline{3-5}  & & & & (\(\varepsilon<0.8\%\)) \\ \cline{3-5}  & & & & Linear & 1800 \\ \cline{3-5}  & & & 27.8 (\(\varepsilon<40\%\)), & & \\ \cline{3-5}  & & & 1084 (40\% \(<\varepsilon<90\%\)) & & \\ \cline{3-5}  & & & 0.75 (\(\varepsilon<4\%\)) & & \\ \cline{3-5}  & & & 3.4 (\(\varepsilon<2.5\%\)), & & \\ \cline{3-5}  & & & 100 (2.5\% \(<\varepsilon<5\%\)), & & \\ \cline{3-5}  & & & 400 (5\% \(<\varepsilon<7.5\%\)) & & \\ \cline{3-5}  & & & 223 (\(\varepsilon<3\%\)) & & \\ \cline{3-5}  & & & 200\% & 10.5 & Linear (\(\varepsilon<100\%\)) & \\ \cline{3-5}  & & & 1 (\(\varepsilon<30\%\)), & & \\ \cline{3-5}  & & & 2 (30\% \(<\varepsilon<50\%\)) & & \\ \cline{3-5}  & & & 457 (\(\varepsilon<35\%\)), & & \\ \cline{3-5}  & & & 268 (35\% \(<\varepsilon<100\%\)) & & \\ \cline{3-5}  & & & & (\(\varepsilon<0.5\%\)) & \\ \cline{3-5}  & & & 2.2 \(\times 10^{3}\) (\(\varepsilon<15\%\)), & & \\ \cline{3-5}  & & & \(4.2\times 10^{4}\) (22\% \(<\varepsilon<150\%\)) & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Flexible strain sensor with improved sensing performance (\(\varepsilon\)’ is strain). Above all, the sensing performances of the flexible sensors were effectively improved through different methods, which inspired researchers to develop more advanced flexible strain sensors. Nevertheless, the compatibility of high sensitivity, high stretchability, low hysteresis and long-term stability is still a common challenge, especially for civil SHM, in which not only subtle strain but also large strain on the structural surface need to be captured.