# Review of Flexible Piezoresistive Strain Sensors in Civil Structural Health Monitoring
## 4 Optimization of Sensor Performance
### Improvement of Sensitivity  
The sensitivity of a sensor is also called the gauge factor (GF), which reflects the amplification factor of the conductive materials for resistivity response to external deformation. It is regarded as the most important parameter of sensor performance, especially for subtle strains or cracks. In terms of a piezoresistive strain sensor, the GF is defined as the ratio of the relative resistance variation to external strain [92]. Note that the GF is usually a linear regression constant within a linear strain range of the sensor [93]. The GF value is influenced by many factors, such as the type, content and dispersion of the conductive fillers, and the interaction between fillers and substrates. In general, fillers with a content near the electrical percolation threshold show the largest GF value, while this GF hardly reappears because of the unstable conductive network. In this case, the sensitivity is mainly determined by the ratio of tunnel resistance to total resistance [94].  
Therefore, the key point of sensitivity improvement is to regulate the morphology of the conductive network. An effective way to tune the conductive network is to combine different types of fillers. For example, the sensitivity of a CNTs-CB hybrid filler is regulated by changing the mass ratio of CNTs to CB. The CNTs act as conductive pathways in the conductive network, and the CB particles are located between the adjacent CNTs. When the CNT has a low content, the connections between CB particles and between CB particles and CNTs were weakened as the sensor stretched. In this case, the conductive structure became more susceptible to tensile strain; therefore, the lower mass ratio led to a higher GF value at a low strain level. Additionally, strong interfacial binding between conductive fillers and elastic substrates is also helpful to realize higher GF. For example, [PERSON] et al. developed tunable sensitivity composites using CNT and thermoplastic elastomer [74], and [PERSON] et al. combined reduced graphene oxide with polyvinyl fibers [95].  
On the other hand, high sensitivity can also be achieved through crack-assisted conductive structures and micro- or nanostructures [96; 97]. Inspired by the crack-shaped organs of spiders, a microcrack-assisted piezoresistive strain sensor using AgNWs and patterned PDMS substrates was fabricated, with many microcracks inside the composite. Attributed to its ultrasensitive microstructure, the sensor showed a remarkable GF value of 1.5 \(\times\) 10\({}^{5}\) within a 60% strain range [49]. Besides, a crack-assisted strain sensor composed of AgNWs and graphene was developed, which showed a GF = 20 within a 0.3% strain, GF = 10\({}^{3}\) within a 0.3-0.5% strain, and GF = 4 \(\times\) 10\({}^{3}\) between a 0.5-1% strain [43]. According to recently studies, microcracks are generally generated by the pre-stretching or bending, thermal solvent evaporation, and cooling process [43; 98]. Moreover, other microstructures, such as interlocked structure, whisker structures and cotton fabric molded structures, have also been applied for higher sensitivity [99]. In addition to high sensitivity, the microstructure-assisted strain sensors also have the benefits of fast response and high stability [100; 101].