Metals, as the most common conductive materials in life, show high mechanical strength, thermal conductivity, and electrical conductivity of about 5 × 10⁵ S cm⁻¹ [49]. For example, EGaIn, silver, aluminum, copper, nickel, and others are very suitable for making conductive layers of metal-based conductive fibers, and they have been successfully prepared as various conductive fibers with superior electrical conductivity [50,51,52]. Among them, liquid metal (EGaIn) has become a popular material to prepare metal-based conductive fibers due to its low melting point (29.8 °C), low viscosity, high surface tension, high electrical conductivity (3.4 × 10⁴ S cm⁻¹), and good thermal conductivity [25,53,54]. It can be well combined with various elastomers, such as PDMS and PU, which can be manufactured as a flexible resistive strain sensor, followed by capacitive pressure sensors and piezoelectric pressure sensors [55,56,57,58]. Though the cost of EGaIn is very high, it is still highly favored by researchers. In a study, Liu et al. [59] injected EGaIn into hollow PDMS fibers to prepare a liquid-metal-based conductive fiber with high conductivity and excellent tensile properties. This conductive fiber was used to develop a resistive strain sensor. Silver is also often prepared as a metal-based conductive fiber by depositing silver nanowires or nanoparticles onto elastic fibers (e.g., nylon, PU fibers), which is mainly in the form of silver nanowires or silver nanoparticles [60]. Yan et al. [61] integrated AgNWs on top of PU fibers to form a conductive fiber with high conductivity and good stability. The conductive fiber was successfully prepared as a resistive strain sensor with high sensitivity. During the preparation process, the most important thing is to ensure that the coating is uniform, continuous, and thin, maintaining good electrical conductivity. The silver-based conductive fibers are generally prepared as a resistive strain sensor. Plus, there are also metals such as aluminum, copper, and nickel that have been applied as raw materials for the metal-based conductive fiber [62,63,64]. Those metals are mainly employed as plating layers combined with various elastomers, whose cost is relatively low compared to EGaIn and silver. However, copper and nickel would reduce the conductivity of the obtained metal-based conductive fibers due to oxidation, affecting the performance of the corresponding wearable sensors. In general, metal-based conductive fibers exhibit good electrical conductivity, durability, thermal conductivity, and high mechanical strength, which are suitable for manufacturing into functional wearable sensors, such as resistive strain sensors, resistive pressure sensors, piezoelectric pressure sensors, capacitive pressure sensors, etc. [65,66,67,68].

The development of carbon-based materials has attracted much attention in recent years due to their potential applications in a wide range of fields, such as energy storage devices, fuel cells, sensors, and electromagnetic shielding. Carbon and its derivatives are used in the manufacture of conductive fibers because of their remarkable characteristics, such as low cost, high electrical conductivity, large specific surface area, excellent chemical stability, and good mechanical durability [69]. The most commonly used carbon and its derivatives are graphene (G), graphene oxide (GO), reduced graphene oxide (rGO), CNTs, activated carbon (AC), and CB [70]. Among them, CB is in the form of black powder, with particle diameters ranging from 10 to 100 nm [71]. Compared with other types of carbon materials, CB is often employed for preparing carbon-based conductive fibers due to its wide sources. In the preparation process of carbon-based conductive fibers, CB is mainly dispersed uniformly in a solution and then coated on the surface of elastomers or it is mixed with the elastomer to form a solution or melt which is then cured into carbon-based conductive fibers. For example, Souri et al. [72] prepared a conductive ink using CB as the main raw material and applied it to elastic cotton fabric to produce conductive fibers. Such conductive fibers are usually made as resistive strain sensors and resistive pressure sensors [73]. CNTs are allotropes of carbon, which are also often used to fabricate carbon-based conductive fibers. However, CNTs have certain toxicity, which is super harmful to humans. Thus, safety precautions need to be taken when using CNTs to manufacture carbon-based conductive fibers [74]. Usually, CNTs are connected with the surface of the fiber to obtain conductive fibers [11]. Sometimes, CNTs as conductive fillers are put into hollow elastic fibers to prepare carbon-based conductive fibers [75]. In a study, Zhou and his colleagues prepared a highly stretchable conductive fiber by uniformly filling CNTs inside thermoplastic elastomer (TPE) tubes [62]. Similarly, G, GO, rGO, AC, etc., are used in a comparable way to prepare carbon-based conductive fibers [76,77,78,79]. For example, Souri et al. [80] fabricated a type of carbon-based conductive fiber by mixing G nanosheets into conductive inks and then coating the mixer onto cotton fabrics. In terms of safety, G nanomaterials are safer compared with CNTs [75]. In addition, it possesses better mechanical flexibility than metal nanomaterials [10]. In general, carbon-based conductive fibers possess superior advantages of electrical conductivity, chemical and mechanical durability, and low cost, which can be applied to make resistive strain sensors, resistive pressure sensors, humidity sensors, etc. They play a key role in the conductive fibers, resulting in significant parts in the field of functional wearable sensors.

Currently, there are many types of polymers, some of which have become the material choices for manufacturing multifunctional fibers or films owing to their high electrical conductivity, good processability, lightweight, high elasticity, and strong corrosion resistance [65,66]. Typical polymers used for conductive fibers are PPy, PANI, polythiophene (PTh), PEDOT: PSS, etc. [81]. Their conductivity is usually between 10⁻⁸ and 10² S cm⁻¹ at room temperature [82]. Commonly, the polymer-based conductive fibers are prepared by solidifying a co-blended solution of polymers and elastomers. Sometimes, the monomers of the conductive polymer are polymerized directly on the surface of the elastomer to form the conductive fiber. A few polymer-based conductive fibers are made via directly coating the conductive polymer on the elastomer surface, as with some metal-based conductive fibers. For example, Tadesse and his colleagues prepared the conductive paste PEDOT: PSS as the main materials. The conductive paste was then added to the appropriate amount of rheology modifier to increase the viscosity of the conductive paste. Finally, the conductive paste is coated on the fiber surface to form a polymer-based conductive fiber [29]. Polymer-based conductive fibers are often fabricated as resistive strain sensors, resistive pressure sensors, capacitive pressure sensors, piezoelectric sensors, etc. In addition, polymers including PPy, PAIN, PTh, etc., show different degrees of sensitivity to various gases (NH₃, N₂, CO, etc.), resulting in being developed as gas sensors. For instance, a polymer-based conductive fiber can be made into a wearable gas detection sensor, which uses PAIN and PAN nanofibers as the main raw materials. It possesses excellent sensitivity, fast response and recovery time, good reproducibility, and stability for NH₃ at 10–2000 ppm at room temperature [83]. In summary, polymer-based conductive fibers are composed of a variety of polymers, which are not only prepared as resistive strain sensors, resistive pressure sensors, and capacitive pressure sensors, but also as diverse gas sensors [84,85]. This is an advantage that metal-based conductive fibers and carbon-based conductive fibers do not have.