# Review of Flexible Piezoresistive Strain Sensors in Civil Structural Health Monitoring
## 4 Optimization of Sensor Performance
### Improvement of Hysteresis and Durability  
Hysteresis and dynamic durability are key parameters to indicate sensing performances under applied dynamic forces. Hysteresis is defined as the inconsistent or non-monotonic resistivity variations during a loading-unloading cycle [128]. Dynamic durability reveals the endurance of strain sensors to long-term cyclic loadings, including stable and reproducible resistive responses and mechanical integrity. In general, hysteretic behavior and response delay exist in all polymer-based strain sensors. They are regularly attributed to reasons that include the intrinsic viscoelastic nature of polymers, the viscoelastic characteristics of conductive fillers, poor interfacial adhesion, and the imperfect recovery of conduction paths because of friction between the sensing element and the elastomeric matrix [92; 118]. First, a large interfacial area of conductive fillers and polymer raises the time delay [129]; thus, strong binding between the elastomeric fillers and the polymer matrix is helpful to eliminate the hysteresis effect [42; 80]. Second, ultra-soft substrate provides a lower force for quick formation of the conductive network [97]. Third, polymers with smaller viscoelastic properties are recommended to ensure accurate strain measurement [92]. Besides, woven-based piezoresistive fiber/yarn/textiles also provide solutions to lower hysteresis because of the high stretchability and fast response of the woven structure for overcoming the viscoelastic delay of polymer substrates [108; 109]. Generally, the piezoresistive fibers/yarns/textiles are fabricated by integrating conductive materials on the surface of the fiber/yarns through coating, ink writing and wrapping processes [130; 131]. In fact, the hysteretic behavior depends on the integrity and robustness of the conductive network at different strain levels [93; 118]. For instance, the \"shoulder peak\" hysteretic behavior for CNT/TPU composites at a 30% strain is much more obvious than that at a 5% strain [77]. This is because the weak destruction of the CNT network is easy to recover under small strain, while the destruction and establishment of the conductive network under large strain is more complicated. Similarly, [PERSON] et al. presented that the evenly distributed and densely packed CNT conductive network in polymer suffered almost no evident changes, resulting in stable dynamic durability under a wide strain range within 10% to 50% [68]. Additionally, the response time of the crack-based strain sensors is faster because of their quick and easy reconnection-disconnection transformation [129; 132]. In terms of long-term application, there is usually a declining trend of the sensing performance with the increasing number of cyclic stretching/releasing [133; 134], owing to the unequal deconstruction and reconstruction of the conductive pathways during cyclic loadings. Another reason for the degradation is the irreversible plastic deformation of substrates under large strain, and the fracture of conductive fillers. Additionally, long-term exposure to a harsh working environment will also lead to the performance decline and service life reduction in a flexible sensor [42; 118]. In this case, growing efforts toward self-healing technologies have been made to improve the sensor's durability. At present, the self-healing of a polymer structure is achieved via passive extrinsic methodologies and active intrinsic methodologies [135]. Extrinsic self-healing is being performed via the leakage of a pre-embedded healing agent or catalyst from cracked microcapsules or microvascular systems. Extrinsic self-healing is usually restricted to only happening once and is incapable of multiple healings. Conversely, intrinsic self-healing is offered by reversible covalent bonding and supramolecular interactions, which are capable of multiple healings. Once the healing cycle proceeds, the sensor material will recover partial mechanical properties with or without external trigger intervention.