Fig. 1. (a) Stretched area created by pressing AgNW-PUU-PDMS (silver nanowires-polyurethane-urea poly(dimethylsiloxane)) film with the end of a glass rod; (b) photographs of sample mounted on a stretching tester and sample after 50% increase in film length^[14]; (c) schematic illustration of the fabrication process of AgNW-polymer composites; (d) scanning electron microscope (SEM) image of the conductive cross-section surface of a AgNW-poly(TBA-co-AA) (poly(tert-butylacrylate-co-acrylic acid)) composite; (e) photographs of deformed circular active area of actuator in response to electric fields with different amplitude^[34].

Fig. 2. (a) Optical photographs and schematic illustration of the formation process of Ag-PDMS/Ecoflex; (b) effect of skin deformation on electrocardiogram signals collected by electrically conductive composites electrode. The insets show the conformal contact at the skin-electrode interface during compression and stretch^[38]; (c) stencil printing process to fabricate a stretchable antenna^[37].

Fig. 3. (a) Schematic illustration of the formation process of MXene hydrogel; (b), (c) optical photographs of the 3D MXene foam and 3D MXene xerogel; (d) rate performance of the the Mxene hydrogel, Mxene/ reduced graphene oxide film, and MXene film electrodes at current densities ranging from 0.2 to 1000 A/g; (e) cyclic voltammetry profiles collected at 1000 mV/s^[43].

Fig. 4. (a) A liquid metal (LM)-elastomer composite being stretched and twisted with an intricate design of electrically conductive traces. The lower left inset shows the undeformed sample and lower right inset is an optical micrograph showing the LM microdroplets in the elastomer at ϕ = 50%; (b) a soft quadruped with autonomously self-healing soft-matter electronics; (c) movie frame sequence from the top-down view of the soft robot traversing smooth terrain^[46].

Fig. 5. (a) SEM images of the liquid metal-filled magnetorheological elastomer (LMMRE). Scale bars are 10 µm; (b) resistance-strain curve of the LMMRE; (c) resistance-strain curve of the LMMRE as a sensor. Inset are finger with different gestures; (d) exploded schematics and thermal images of the hand-held heating column. Scale bars are 1 cm^[47].

Fig. 6. (a) Photograph of graphene-AgNW hybrid film on a polyethylene terephthalate (PET) substrate. The scale bar indicates 2 cm. The inset shows a SEM image of this hybrid (scale bar: 5 μm); (b), (c) relative difference in resistance as a function of radius of curvature and tensile strain; (d) schematic illustration of the device layout; (e) photograph of the contact lens device (scale bar: 5 mm). (Inset: optical microscopic image of slightly sunken inorganic light-emitting diode (ILED) on the surface of hybrid electrode and contact lens. Scale bar is300 μm); (f) a photograph of the ILED-hybrid electrode-contact lens device on an eye of a mannequin. Scale bar is 5 mm^[27].

Fig. 7. (a) Printed elastic conductors on a PDMS sheet. The insets show single walled carbon nanotubes (SWNTs) dispersed in paste and a micrograph of printed elastic conductors with a line width of 100 µm; (b) SEM image of the elastic conductor^[19]; (c) surface plot of transparency for various AgNW and carbon nanotubes (CNT) concentration; (d) stretchability comparison of AgNW only percolation network and hierarchical multiscale AgNW/CNT hybrid nanocomposite^[57].

Fig. 8. (a) SEM images of CNT ribbons directly drawn from CNT forest (arrow shows the drawing direction), and magnified CNT ribbons; (b) resistance of a CNT/PDMS film as a function of tensile strains in the 1 st stretching, 1 st releasing and 2 nd stretching cycles^[58]; (c) illustration of strips of cross-stacked films with three typical directions; (d) SEM image of a 2-layer cross-stacked CNT film; (e) change of resistance of a (45°, 45°) SACNT/PDMS film during three sequential stretch processes after the first stretch process^[59]; (f) photographs of a fully fabricated e-skin device under stretching conditions^[60].

Fig. 9. (a) Chemical structures of poly(3, 4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and representative ionic additives–assisted stretchability and electrical conductivity (STEC) enhancers; (b) schematic diagram representing the morphology of a stretchable PEDOT film with STEC enhancers; (c) stress/strain of freestanding PEDOT/STEC films; (d) conductivity under various strains for PEDOT with different STEC enhancers. Inset: photograph showing a freestanding PEDOT/STEC film being stretched^[67].

Fig. 10. (a) Dependency of resistance of the polyvinyl alcohol (PVA)-coated planar and wrinkled graphene sheets with different chemical vapor deposition (CVD) deposition times on the tensile strain ^[68]; (b) SEM image of transferred Cu nanoparticle random network on a PET film; (c) resistance change against bending radius. Bottom inset pictures show the digital image at maximum and minimum bending radius. Right inset pictures show permanently damaged (folded) transparent conductor at maximum bending radius, yet electrically conductive^[69].

Fig. 11. (a) Fabrication of the serpentine transparent conductors; (b) the relationship between the transmittance and sheet resistance of the composites; (c) the resistance of composites plotted as a function of the applied strain percentage^[74].

Fig. 12. (a) Steps in the fabrication of an NTS_m@fiber; (b) SEM images showing long- and short-period buckles for an NTS₁₈₀@fiber at 100% applied strain ^[70]; (c) schematic illustration of the melt-draw method used for the preparation of micrometer-size rubber fibers; (d) fiber electrical conductivity versus applied strain (with an inset for small applied strains)^[75].

Fig. 13. (a) SEM image showing the lateral buckling of CNTs; (b) resistance change of a typical CNT-PDMS film as a function of applied strain^[77]; (c) schematic for the preparation of AgNW networks with wavy configurations; (d), (e) optical microscope images of AgNW networks floated on water (d) before and (e) after compression^[78].

Fig. 14. (a) Schematic illustration of the electrospinning with different collectors to obtain various PVA NFs with controlled fiber orientation; (b) technological flow chart of the patterned electrode, including metallization, nanofibers (NFs) transfer, patterning, and packaging; (c) SEM images of different oriented PVA NFs. Scale bar is10 μm; (d) optical photographs of a fabricated stretchable transparent AgNWs electrode^[86].

Fig. 15. (a) Schematic illustration of the fabrication of the CNT/rGO/CNF (carbon nanotubes-reduced grapheme oxide-cellulose fibers) aerogel^[90]; (b) variation of the resistance of the copper nanowires-polyvinyl alcohol (CuNW-PVA) as a function of tensile strain up to 60% in the first two stretch-release cycles; (c) variation of the resistance of the CuNW-PVA as a function of stretching cycles at a strain of 60%. The inset showed the resistance changes for the 5th, 100th, and 1000th stretching cycles, respectively^[91].

Fig. 16. (a) Photograph of transparent flower-shaped, knotting and stretching Agar-PAM (polyacrylamide) hydrogel; (b) resistance change and gauge factor variations of the Agar-PAM hydrogel as sensors on applied tension^[92]; (c) demonstration of hydrogels as sensor on a wooden mannequin and pressure-dependent conductivity by LED bulb after the hydrogels self-adhered on a wooden mannequin model; (d) the application of (poly-N-isopropylacrylamide) PNIPAM/L/CNT hydrogel as a strain sensor, showing repeatable regular resistance changes in different parts^[93].

Fig. 17. (a) Schematic illustration of the CNT/rGO-PDMS preparation; (b) conductivity of the composite as a function of tensile strain, inset curve shows the electrical conductivity of the CNT/rGO-PDMS with 2 wt.% graphene/CNT loading under stretching ^[13]; (c) the brightness of LED lamps depending on the strains and bends^[98].

Fig. 18. (a) Fabrication procedure for PUS-AgNW-PDMS stretchable conductors^[101]; (b) SEM images for cross-sections of 35% biaxial pre-strain SWNT/PP (polypropylene foam) at applied strains; (c) resistance change as a function of applied strain for SWNT/PP with different biaxial fabrication strains^[25].

Fig. 19. (a) Schematic drawing of a fully stretchable OLED using GO-AgNW/PUA composite electrode; (b) optical photographs of an OLED stretched to specified strains^[25]; (c) schematic drawing of OLED using PAM-LiCl-ZnS composite electrode; (d) optical photographs of an OLED stretched to specified strains^[104].

Fig. 20. (a) Schematic illustration of the configuration of a fully printed FET based on screen-printed AgNW source/drain electrodes; (b) optical image of a fully printed stretchable 10 × 6 FET array^[105]; (c) FET adhered to a textile and stretched to 30%^[106].

Fig. 21. (a) The relative capacitance change of the Ag-Au core-shell NW-based supercapacitor during the repeated stretching cycles^[107]; (b) fabrication step of Ag/Au/PPy core-shell NW network mesh film; (c) CV curves of the supercapacitor based on Ag/Au/PPy coreshell NW mesh at a scan rate of 50 mV/s at indicated strain rates. Insets are schematic illustration of transparent and stretchable supercapacitor on strain condition^[108].

Fig. 22. (a) Schematic illustration of a twist-inserted rectangular sandwich fiber, which comprises an Ecoflex rubber core and two symmetric, buckled CNT electrodes; (b) capacitance and fractional change in fiber thickness versus tensile strain. Inset: photograph showing a side view of the sandwich fiber^[109]; (c) Illustration of the crack-based sensor^[115]; (d) response of the sensor to motions of arm muscle with different gestures^[112].

Fig. 23. (a) Schematic illustration of the stretchable and transparent heater composed of Ag NW-PDMS composites; (b) transient temperature evolution of heater under stepwise application of 0−30% strain at a constant voltage. Insets are temperature field at each strain^[22].