Self-healing actuatable electroluminescent fibres

Abstract

Alternating-current electroluminescent fibres are promising candidates as light sources for smart textiles and soft machines. However, physical damage from daily use causes device deterioration or failure, making self-healable electroluminescent fibres attractive. In addition, soft robots could benefit from light-emitting combined with magnetically actuated functions. Here, we present a self-healing and actuatable Scalable Hydrogel-clad Ionotronic Nickel-core Electroluminescent (SHINE) fibre which achieves a record luminance of 1068 cd × m^-2 at 5.7 V × μm^-1. The SHINE fibre can self-heal across all constituent layers after being severed, recovering 98.6% of pristine luminance and maintaining for over 10 months. SHINE fibre is also magnetically actuatable due to the ferromagnetic nickel electrode core, enabling a soft robotic fibre with omnidirectional actuation and electro-luminescence. Our approach to this multifunctional fibre broadens the design of fibre electronics and fibre robots, with applications in interactive displays and damage-resilient navigation.

Fig. 1. Design of SHINE fibre with self-healing and magnetic actuation capabilities.

a Schematic and composition of SHINE fibre. Inset at the top right is a cross-sectional optical microscope image of a SHINE fibre. Scale bar, 200 μm. b A SHINE fibre of 5.5-meter length lit up under indoor light (V_ac = 400 V, f_ac = 100 Hz, T_EL = 167 μm, and $\overrightarrow{\mathrm{E}}$ = 2.4 V × μm⁻¹). Scale bar, 2 cm. c Healed SHINE fibres with green and orange colors under bending. Scale bar, 1 mm. d Magnetic responsiveness of an interactive display consisting of SHINE fibres with different colors. The display changes between “sad” and “smiley” expressions under an external magnetic field (B) provided by a permanent magnet underneath. The working distance of the magnet is 5 mm. Scale bar, 3 cm.

Fig. 2. Scalable fabrication and luminance performance.

a Schematic of the scalable fabrication process combining liquid-solid phase separation in coaxial wet-spinning and ion-induced gelation in hydrogel coating. b SEM images and corresponding energy-dispersive X-ray spectroscopy (EDX) mapping of fluorine (F) and chloride (Cl) elements of bilayer coaxial fibres coated by hydrogel with and without pre-coating of ion source solution. Scale bar, 1 mm. c Water content and ionic conductivity of PVA-B/SA-C/LiCl/Gly hydrogel measured for over 10 months. d Transmittance in the visible spectrum measured for PVA-B/SA-C/LiCl/Gly hydrogel (~370 μm thickness). Inset is an optical microscope image of a SHINE fibre surface, showing the good transparency of the hydrogel. Scale bar, 1 mm. e The fibre can be fabricated to a few meters and lit up (V_ac = 400 V, f_ac = 50 Hz, T_EL = 167 μm, and $\overrightarrow{\mathrm{E}}$ = 2.4 V×μm⁻¹). Scale bar, 10 cm. f The luminance performance of SHINE fibre at different $\overrightarrow{\mathrm{E}}$ and f_ac. T_EL = 141 μm. g Comparison of the luminance versus $\overrightarrow{\mathrm{E}}$ of our SHINE fibre with previously reported EL fibres. h Ratio between luminance at a point in time (L) and pristine luminance (L₀) of SHINE fibre for over 10 months. Samples for long-term characterizations were stored under ambient conditions with a temperature of (20.8 ± 0.5) °C and relative humidity of (74.3 ± 2.7) %. Error bars are standard deviations of results from three samples.

Fig. 3. Self-healing.

a Schematic of self-healing process. The hydrogel electrode self-heals under ambient conditions while the EL interlayer and Ni electrode self-heals at 50 °C. Water loss occurs in the hydrogel during heating, leading to a decrease in hydrogel conductivity; however, both water content and conductivity recover when the hydrogel regains water by absorbing moisture from air at RT (rehydration). b Photos showing a lit pristine fibre, the cut fibre, and the healed fibre which light up entirely again and is bent to a bending radius of ~5 mm. Scale bars, 1 cm. c SEM images showing self-healing of the cut interface on hydrogel electrode, EL interlayer, and Ni electrode, respectively. The black dashed boxes indicate the cut and healed area of the hydrogel electrode, which started to heal under ambient conditions during preparation for SEM and remained intact after heating. All fibres were heated at 50 °C for 24 h. Scale bar, 20 μm. d and e The decrease in water content (d) and ionic conductivity (e) of different hydrogels after heating at 50 °C for 24 h, followed by rehydration for 24 h. f The luminance at increasing f_ac for SHINE fibre before and after heating on the hotplate with 50 °C for 9 days followed by rehydration. g Representative tensile stress-strain curves of SHINE fibre before and after self-healing at different temperatures and the average self-healing efficiencies. Healing time for RT samples is 24 h while it is 48 h for 50  °C samples (heating for 24 h followed by rehydration for 24 h). The tensile rate is 20 mm·min⁻¹. h Luminance of pristine SHINE fibre compared to luminance after the fibre was cut and healed. V_ac = 400 V, f_ac = 300 Hz, T_EL = 138 μm, and $\overrightarrow{\mathrm{E}}$ = 2.9 V × μm⁻¹. i Ratio between luminance at a point in time (L) and pristine luminance (L₀) of healed SHINE fibre for over 10 months at different f_ac under ambient conditions. The Ni and ZnS loadings in samples for Fig. 3 were 60 wt% and 50 wt%, respectively. Error bars are standard deviations of results from three samples.

Fig. 4. Magnetic actuation.

a SHINE fibre maintained its luminance after bending for 2000 cycles at a bending radius of 8 mm (κ = 125 m⁻¹). The insets reveal that the fibre maintained its compact coaxial structure before and after bending; no interface failure was observed (white dashed areas). L and L₀ refer to the luminance at a bending cycle and the initial luminance without bending, respectively. Scale bar, 60 μm. b A SHINE fibre with a length of ~65 cm actuated by an external magnetic field (B) to perform curling, with potential applications as light-emitting ferromagnetic soft robotic fibres. The direction of B is labeled at the bottom left in blue color. The working distance of the magnet was 5 mm. Scale bar, 10 cm. c Illustration of a millimeter-scale 3D pipe of two inner diameters (d₁ = 5.2 mm; d₂ = 4.2 mm) and three bend shapes (L shape, κ = 107.0 m⁻¹; U shape, κ = 63.7 m⁻¹; V shape, κ = 228.1 m⁻¹). A SHINE fibre (~35 cm long) navigated this 3D pipe successfully. d Self-healing of SHINE fibre during navigation in a U-bend pipe (inner diameter = 5.2 mm, κ = 63.7 m⁻¹). The fibre restored its pristine electro-luminescence and actuation capabilities via self-healing after being severed. The dashed white boxes highlight the cut location. All arrows indicate the directions of motion. Scale bars in (c and d) 10 mm.