# Review of Flexible Piezoresistive Strain Sensors in Civil Structural Health Monitoring
## 1 Introduction  
With the worldwide development of infrastructure construction, enormous numbers of buildings, bridges, tunnels, railways and other infrastructure have been constructed during the past half century. Given that much infrastructure has been operated for decades, degradation and damage of structures are inevitable because of environmental erosion, fatigue load and natural disasters [1, 2]. For instance, in 1995, the collapse of the Mitutoyo Department Store in Seoul, Korea, caused by functional modifications and design flaws, killed 502 people. In 2021, the collapse of a 12-story residential building in Miami, USA, killed 97 people [3]. Consequently, demands for structural health monitoring (SHM) and safety assessment of civil structures have been growing in the past decade [4, 5]. Structural strain is a direct reflection of structural deformation under external force or environmental change. Thus, accurate strain measurement is one of the most important things in SHM to analyze structural performance and detect potential damage [6, 7]. In practice, structural strain is usually measured using metallic wire resistance sensors, which depend on the variation in the sensor resistance under deformation. For instance, the relative change in resistance \(\Delta R/R\) of a metal wire is given by \((1+2v)A/(\rho l)\), where \(v\), \(\rho,l\) and \(A\) are the Poisson's ratio, electrical resistivity, wire length and cross-sectional area of the metal wire, respectively [8]. The gauge factor (GF) of a metal foil strain sensor is around 2 because the Poisson's ratio is generally within \(-1\)-\(0.5\). This sensitivity level is insufficient for a low-strain measurement, making the signal vulnerable to external noises. Meanwhile, metal wire sensors usually sustain a maximum effective strain range lower than 2% strain,which is incapable of addressing large structural deformation. Aside from the low sensitivity and small working range, conventional metal sensors also encounter short fatigue life and environmental problems [9]. In the past few years, as novel alternatives, nanomaterial-based flexible strain sensors have displayed much higher sensitivity and larger range because of their ultrasensitive conductive material and elastic substrate. One benefit of the flexible sensor is the capability to measure both small and large strains [10]. Therefore, the flexible strain sensors show promising applicability in various fields, including SHM [11], human health care [12,13] and human-machine interfaces [14,15]. The construction industry is one of the industries with the largest carbon dioxide (CO\({}_{2}\)) emissions in the world. According to the global status report for buildings and construction publicized by the United Nations Environment Program, in 2021, the construction and operation of buildings were responsible for 37% of global energy-related CO\({}_{2}\) emissions and 36% of global final energy consumption [16]. Under this circumstance, the development of smart SHM based on flexible strain sensors will assist managers to make maintenance decisions, which is effective for ensuring the safety of civil structures and extending the service life of structures [17]. Thus, SHM is helpful in reducing CO\({}_{2}\) emissions indirectly. Moreover, SHM is of great significance to ensure the safety of people's lives and property and to achieve a sustainable society (e.g., requirements of energy consumption) and economy (e.g., investment in building energy efficiency) [18]. In accordance with the sensing principle of different conductive materials, flexible strain sensors are mainly divided into three categories, namely, piezoelectric-, capacitive- and piezoresistive-based strain sensors. Flexible piezoelectric sensors measure strain on the basis of the relative potential difference between the top and bottom electrodes of the piezoelectric material under deformation.