Traditional spinning techniques, such as wet spinning, dry spinning and melt spinning, are the most common methods to prepare a 1D stretchable conductive composite materials; they mix the conductive filler and the elastic matrix directly and uniformly, and then extrude it through the spinneret hole to a coagulating bath to form the composite fiber. Li et al. uniformly mixed Gr into SBS and prepared SBS/Gr composite fiber flexible strain sensors by a simple wet-spinning method, and the Gr content had a significant impact on the morphology, mechanical properties and electromechanical properties of the composite fiber (Figure 1) [70]. The fiber with the 5 wt% graphene content has a wide working strain, which reaches 100%. However, its sensitivity increases with the increase in strain, and the sensitivity within 50% strain is changeable at different stretching speeds. He et al. proposed multiwalled carbon nanotube/thermoplastic polyurethane (MWCNT/TPU) fibers by wet spinning [71]. The gauge factors (GF) of the MWCNT/TPU fiber are about 550 and 2800 in the strain ranges of 1 to 4% and 5 to 100%, respectively. The strain of the MWCNT/TPU fibers decreases significantly under large hysteresis after multiple stretching–releasing cycles, indicating poor sensing repeat stability. At the same time, the influence of different weight ratios of MWCNTs to TPU on the mechanical and electrical properties of composite fibers has been studied. It was found that the concentration and arrangement of MWCNT would change the working strain range and GF of the sensor [72]. Wang et al. manufactured a fiber strain sensor with a wide response range (320%) and a fast response time (<200 ms) based on MWCNTs and TPU by a simple wet-spinning method [39]. However, the electrical response of the MWCNT/TPU strain sensor decreased slightly in the initial stage when multiple stretching–releasing cycles were carried out at 100% strain, and it exhibited unstable sensitivity at the same time. To improve the conductivity and the stability of the conductive network, hybrid conductive fillers have been used to achieve a composite synergistic effect to prepare strain-sensing fibers. For instance, Zhang et al. demonstrated a highly conductive AgNW/MWCNT/TPU composite fiber by wet spinning, in which MWCNTs were regarded as the sensitive materials and silver nanowires were used to improve electrical conductivity [73]. When the contents of AgNWs reached the optimal amount (3%), the working strain range was 254%, and the conductivity was 0.0803 S/cm (Figure 2). Compared with single-filler composite fibers, the increase in AgNWs improves the conductivity and working strain range of the composite fiber, but its sensitivity decreases. In the case of a strain range of 50–150%, the relative resistance change of the sensor continues to decrease in stretching–releasing tests within 1000 s, showing poor stability.

The conductive network was also designed by controlling the distribution of the fillers, such as selective positioning in multiple phases to form a co-continuous structure or a sea-island structure. In this case, the conductivity of the composite is improved by forming a double or triple permeation structure in the polymer matrix. The selective positioning of the fillers at the interface of the co-continuous polymer structure can further reduce the filler content, which is required to form the continuous conductive network. Zhou et al. used the coaxial wet-spinning method and post-treatment process to prepare the thermoplastic elastomer/single-walled carbon nanotube (TPE/SWCNT) ribbon coaxial fiber with good stretchability and high sensitivity (Figure 3a) [38]. The strain sensor composed of this fiber has a GF of 48 at 0–5% strain and a GF of 425 at 20–100% strain; a linear change cannot occur in in the full strain range. Tang et al. designed a stretchable core sheath fiber using a one-step coaxial wet-spinning assembly method, in which a high-stretch polymer elastomer Ecoflex wrapped CNT/Ecoflex composite material [74]. Similar to traditional cables, the outer insulating sheath effectively avoids short circuits and the falling off of conductive fillers. At the same time, it can have good conductivity under a low permeability threshold (0.74 vol%). Strain sensors made of this fiber achieve a high sensitivity of 1378 under 300% strain and show high durability under 100% strain, but they exhibit low sensitivity in a small strain range, non-linear resistance change and obvious overshoot behavior. Yue et al. demonstrated a highly stretchable TPU-CB@TPU fiber strain sensor with a porous core–sheath structure through the coaxial wet-spinning method (Figure 3b,c) [37]. Due to the countercurrent diffusion and coagulation of the solvent, this fiber has a porous structure with a wide strain range. The highest GF is 28,084 when the strain is 204%. However, its sensitivity is not large enough in a small strain range, and the resistance change gradually declines over multiple cycles of stretching. A coaxial fiber with an outer layer of MXene/PU composite and an inner layer of PU was prepared by Seyedin et al. [75]. Compared with the non-coaxial composite fiber, the coaxial fiber shows a larger strain range, a smaller data drift, and an improvement in the cyclic stability of the sensor response. Gao et al. fabricated a coaxial stretchable composite fiber with a double-layer hollow structure (Figure 3d), in which the conductive outer layer has a CNT/TPU composite as the sensitive area, and the insulating inner layer is made of pure TPU with a hollow core to serve as a flexible support [76]. The prepared composite fiber (TPU-8CNT@TPU) has an ultralow percolation threshold (0.17 wt%), good durability, and small compression deformation that can be detected. With an increase in the stretching speed, the relative resistance differently changes under the same strain. Additionally, there is an obvious shoulder phenomenon, which may disturb signal identification in an accurate strain monitoring. However, the reason for this shoulder phenomenon is still not clear. The mainstream is attributed to the competition between the destruction and reconstruction of CNT conductive networks in the fiber, which needs further verification.

The characteristics of various fiber-based strain sensors prepared by spinning technology are summarized in Table 4. In general, the preparation of stretchable conductive composite fibers as strain sensors by mixing conductive materials and spinning is a process technology that can be produced on a large scale and is widely used in industry. However, the addition of conductive filler will enhance the rigidity of the elastic matrix, and shrink the tensile strain range of the fiber, which leads to the narrow working strain range of the fiber sensor. On the contrary, if the amount of conductive material is too low, the conductivity of the composite fiber will also limit its working strain range. Therefore, there is a paradox between the conductivity and the working strain range of the stretchable conductive fiber, which needs to be balanced. According to the percolation theory [60,77,78,79,80,81], the content of conductive materials in stretchable conductive composites has a percolation threshold. When the percolation threshold is exceeded, the polymer elastomer changes from an insulator to a conductor, and the conductivity increases with the increase in the content of conductive materials. When the content is near the percolation threshold, the sensitivity of the material is at its greatest [82]. Therefore, it is still a huge challenge to achieve a high strain range and high sensitivity at the same time for conductive composite fibers. In addition, there is a limit on the production costs of practical commercial applications with the increase in conductive fillers. To reduce the permeation threshold while achieving high conductivity, different strategies have been studied [50,83,84], such as functionalizing conductive fillers’ surfaces, increasing the aspect ratio of fillers, controlling the arrangement of fillers, and using different mixture of fillers. However, such permeation-based composite strain sensors rarely exhibit good linearity. When the composite fiber is stretched, its resistance is mainly caused by changes in geometry and tunnel theory [37,39,85]. With an increase in tunneling distance and the destruction of the conductive path, the resistance of composites increases significantly during the tensile process. The maximum GF usually occurs when the conductive material content is close to the permeation threshold. Other shortcomings of strain sensors made of composite fibers include hysteresis, fatigue and so on, which are mostly due to the viscoelasticity and elastic recovery rate of composite fibers.

In order to improve the workable strain range, a conductive coating with a wrinkle structure was added to the fiber surface to design a flexible strain sensor. Wang et al. designed a highly stretchable NTSm@rubber@fiber strain sensor with a dual-sheath buckling structure by the pre-stretching method, in which NTS is the carbon nanotube sheets and m represents the number of NTS layers (Figure 6a) [89]. The elastic support fiber coaxially coated a curved rubber intermediate layer and a curved NTS conductive layer. At the same time, the GF of the sensor was controlled by changing the manufacturing parameters to adjust the buckling structure, but its overall sensitivity values were very low, only 0.5 (0–200%) and 0.14 (200–600%). The design of a wrinkle structure makes the strain sensor bear high tensile deformation without destroying the conductivity of the material, thereby increasing its sensing range. However, it also causes a small resistance change in the stretched state, showing lower sensitivity. As shown in Figure 6b, CNT ink/PU yarns with a wrinkle-assisted crack microstructure were created by Sun et al. [90]. The yarn sensors have an ultralow detection limit and excellent repeat stability. As mentioned above, the complex and multistep manufacturing process poses challenges to realize a large-scale production of strain sensors.

Building a multilayered structure at the fiber scale is another way to design flexible strain sensors. Cao et al. introduced a AgNW/PU fiber with a composite multilayer structure by using an adhesive layer with adjustable adhesion to adjust the interface adhesion and fiber microstructure (Figure 7a) [91]. The GF and stretchability of the strain sensor were adjusted by changing the interface layer combination (Figure 7b). However, its sensitivity was weak and the resistance response was nonlinear. After 100 and 1000 cycles of stretching at 10% strain, the drift rate values of the relative resistance of the sensor were 29.4 and 53.1%, respectively, showing poor repeatability. Liu et al. reported the silver plating polyurethane filaments (SPPF) with good electrical resistivity (4.5 ± 0.1 Ω/cm) [92]. These AgNPs are bound to the surface of the filament by polydopamine, which remarkably improves the bonding between the conductive material and the fiber interface, but the nonlinear error and hysteresis of the SPPF strain sensor are up to 29.3 and 34.3%, respectively.

The characteristics of fiber strain sensors prepared by coating technology are summarized in Table 5. In general, the coating method is easy to implement and the 1D strain sensors produce via this method show good sensing performance. As the mechanical properties of conductive coatings and elastic substrates are inconsistent, conductive coatings propagate small and dense microcracks, which destroys conductive networks and causes the changes of resistance. However, the poor adhesion and the mechanical mismatch between the elastic substrate and the conductive coating often leads to degradation of the sensor response. Therefore, it is still a big challenge to achieve high linearity and cycle stability by a simple coating. Due to the irreversible fracture and shedding of the conductive layer, it is very necessary to explore the stress distribution and interface strength between the conductive coating and the substrate. Although the added adhesive (like the LBL method) has improved adhesion, the fatigue durability of the strain sensor is still a challenge. Additionally, there is a lack of systematic studies on how to control the crack propagation and stability, and on how it affects the sensing performance of strain sensors.

Another design is to use braided yarns to fabricate yarn-based strain sensors. Shi et al. reported a sensor (BWY-AgNWs) composed of stretchable yarns with a braided structure and silver nanowires by dip coating (Figure 9) [97]. The fiber sensor can not only detect various deformations such as stretching, torsion, and bending, but also has a high stable sensitivity (GF = 65) in a larger sensing range (strain can reach 100%). However, due to the insufficient recovery of the microstructure and the brittleness of the AgNWs film, the microcracks cannot be completely merged after release, resulting in the poor repeatability of the strain sensor during multiple cycles of stretching. Furthermore, the high hysteresis of the strain sensor makes its strain response slow, which limits its wearable application. Yang et al. proposed a PET/AgNW/PDMS yarn sensor with braid yarns as the substrate, AgNW as the active material and PDMS as the protective layer by dip coating [98]. The yarn sensor has high conductivity and a wide range of stretchable strain. However, the resistance change does not increase monotonously with the increase in strain, instead of a downward trend after 40% strain. In addition, the relative resistance changes in PET/AgNW/PDMS yarns with an upward trend show relatively instability during multiple stretching and bending cycles. Pan et al. designed a yarn sensor with a core–sheath yarn structure, in which a braided composite yarn coated with CNTs is used as the core (BYs-CNT) and electrospun polyurethane nanofibers are used as the sheath [35]. This kind of combination of the yarn has extremely high sensing sensitivity (maximum GF up to 980) and long-term stability, but poor linearity. Additionally, the yarn preparation process is complicated and cannot be easily produced en masse. Similarly, the relative resistance change shows a downward trend after the strain exceeds 40%, due to the changes in the braid angle and contact area of braided yarns PET during the stretching process.

In addition to fancy yarn, unconventional yarn sensors have been formed by twisting and winding conductive coated films, which remarkedly enhance the tensile strain range of one-dimensional sensors. Compared with the conventional planar wave structure, the coil structure has greater stretchability because the local stress is suppressed during the stretching process and the local maximum strain is reduced due to the non-planar motion of the coil [99]. Ultrahigh stretchable conductive helical yarn with CNT/PU nanocomposite fiber helical yarn was prepared by simple electrospinning, spraying and twisting processes (Figure 10a) [99]. With the help of the synergistic effect of the flexible polymer chain and the nanofiber spiral coil structure, the CNT/PU helical yarn will break the limitation of material stretchability due to its rigidity and excellent stretchability. Its recovery is within 900% strain, and the maximum of tensile elongation can reach 1700%, while its sensitivity is very low. Xie et al. designed a SWCNT-RGO/TPU spiral layered composite yarn by spraying and winding technology (Figure 10b,c) [100]. Due to the special spiral layered structure of the composite yarn, the conductive layer is wrapped and protected by the elastic polymer layer, and there is no obvious interruption or crack on the surface of the yarn. Compared with the SWCNT-RGO/TPU thin-film sensor, the yarn sensor has a wider working strain range and has five linear regions. In the 50% tensile strain cycles, the relative resistance of the sensor continued to increase during the initial 100 cycles and then began to stabilize.

The performance of yarn-based strain sensors with different structures are summarized in Table 6. On the basis of the coating, improving the linearity and stability of the strain sensor by changing the yarn structure is an excellent method because the resistance change mainly depends on the structure of the composite yarn. For example, in terms of wrapped yarn-based sensors, the decrease in the contact of the spiral winding leads to an increase in resistance, but this also reduces the sensitivity and working strain range to a certain extent. Therefore, it is necessary to discuss the influence of structural changes on the sensing performance so that the yarn strain sensor has balanced performance indicators.