Fig. 1. Thermoelectric effect. (a) Seebeck effect for power generation; (b) Peltier effect for refrigeration; (c) Thomson effect for cooling or heating

Fig. 2. Preparation method of stretchable thermoelectric fibers. (a) Schematic of the chemical structure of PEDOT∶PSS and WPU, as well as the wet spinning device for producing PEDOT∶PSS/WPU composite fiber^[69]; (b)(c) optical micrographs of PEDOT: PSS/WPU composite fiber in relaxed and stretched states^[69]; (d) schematic of an experimental device for manufacturing CNT composite TE fiber^[70]; (e) schematic of the preparation process of stretchable thermoelectric fiber yarn^[71]; (f) stress-strain curves of thermoelectric fibers and pure PU fiber^[71]; (g) resistance change of thermoelectric fibers under different strains^[71]

Fig. 3. Preparation method of stretchable TE fibers. (a) Schematic of continuous alternating extrusion process^[73]; (b) schematic of automatic extrusion section assembly line^[73]; (c) Seebeck coefficient and conductivity of thermoelectric fibers with different PEI contents^[73]; (d) schematic of the fabrication of TE fibers^[74]

Fig. 4. Structural design of stretchable thermoelectric spiral fibers. (a) Schematic of hydrogels with different macromolecular conformations^[75]; (b) process of Janus fibers continuously prepared by parallel dual channel spinning, and spiral fibers prepared by strain programming^[75]; (c) formation of helical Janus fiber in response to 900% prestrain^[75]; (d) spiral fibers formed under prestrain of 400% and 900%^[75]; (e) relationship between diameter and number of coils of spiral fibers and prestrain^[75]; (f) structural schematic of 3D thermoelectric spiral coil^[76]; (g) schematic of preparing spiral thermoelectric structures based on screw templates^[77]

Fig. 5. Thermoelectric woven fabric. (a)(b) Schematic of TET in 3D and 2D modes^[78]; (c) relationship between the output voltage and temperature difference of the two modes^[78]; (d) structural schematic of TET^[79]; (e) images of twisting, knotting, bending, and stretching TETs^[79]; (f) schematic of energy collection and heat conduction in woven fabrics^[80]; (g) schematic of structural changes during fabric stretching ^[80]

Fig. 6. Application of stretchable thermoelectric fibers in self-powered sensors. (a) Voltage signal of fibers under periodic heating^[69]; (b) voltage signal of stretched fibers under different temperature differences^[69]; (c)(d) voltage signal of fibers under finger touch and non-contact cold source stimulation conditions under repeated stretching ^[69]; (e) intelligent gloves based on thermoelectric fibers^[71]; (f) thermal response time of the output voltage and voltage signal of smart gloves when they come into contact with hot and cold water respectively^[71]; (g) change in resistance of fibers under different strains^[83]; (h) change of fiber current at~10% strain^[83]; (i) change of fiber open circuit voltage under different temperature differences and strains^[83]

Fig. 7. TEG for thermal energy harvesting. (a) Photo of Janus hydrogel spring composed of >100 TE coils connected in series^[75]; (b) photo of wearable TE bracelets^[75]; (c) voltage and current of TE bracelets and coils^[75]; (d) infrared image of TE coil wrapped around hot water pipes^[75]; (e) cross section infrared and optical images of the TE device^[69]; (f) open circuit voltage change before and after placement of the TE device on the forearm^[69]; (g) TEG pictures^[78]; (h) output voltage of TEG (15 units)^[78]

Fig. 8. Cooling TET. (a)(b) Schematic of the cooling device TET for thermal regulation of the human body^[79]; (c) cooling effect of TET over time^[79]

Table 1. Common TE materials and properties

Table 2. Materials and properties of stretchable fiber-based thermoelectric devices