Extremely stretchable and self-healing conductor based on thermoplastic elastomer for all-three-dimensional printed triboelectric nanogenerator

Advances in next-generation soft electronic devices rely on the development of highly deformable, healable, and printable energy generators to power these electronics. Development of deformable or wearable energy generators that can simultaneously attain extreme stretchability with superior healability remains a daunting challenge. We address this issue by developing a highly conductive, extremely stretchable, and healable composite based on thermoplastic elastomer with liquid metal and silver flakes as the stretchable conductor for triboelectric nanogenerators. The elastomer is used both as the matrix for the conductor and as the triboelectric layer. The nanogenerator showed a stretchability of 2500% and it recovered its energy-harvesting performance after extreme mechanical damage, due to the supramolecular hydrogen bonding of the thermoplastic elastomer. The composite of the thermoplastic elastomer, liquid metal particles, and silver flakes exhibited an initial conductivity of 6250 S cm−1 and recovered 96.0% of its conductivity after healing.

film. e and f Enlarged Raman Spectra of the pristine undamaged PUA film and the damaged PUA film.
g Complete Raman spectra of the pristine undamaged PUA film and the healed PUA film. h and i Enlarged Raman spectra of the pristine undamaged PUA film and the healed PUA film.

Supplementary Note 3.
To confirm the role of multiple H-bonds in the healability of the PUA, Raman spectroscopy was carried out to evaluate the molecular structure of PUA networks. For the damaged PUA film, Raman spectra were obtained from the site of damage (by scratching, Supplementary Figure 7b Table 1). This is primarily due to the decomposition of the urethane bonds. The 50% weight loss in the degradation processes of the PUA films was observed in the temperature range of 354to 365 o C, and the Tmax2 is 353 to 374 o C. This can be attributed to the 12 degradation of soft segments from the polyether. When the OH number of the HEMA increases from 10 to 30 wt%, significant thermal stability can be observed, probably because of the competition between the increase in the crosslink density and the higher content of labile hard segments. Figure 11. Storage modulus and loss tangent delta of polyurethane acrylate (PUA) with different amount of 2-hydroxyethyl methacrylate (HEMA) as a function of temperature measured using dynamic mechanical analysis. (Storage modulus and tan delta concluded that when the temperature is lower than Tg, all the PUA show higher E' values with a small decrease. At a higher temperature it displays a sharp decrease due to molecular mobility which could allow stored energy as a mechanical restoring force that brings the materials to the original state.

Supplementary Note 7.
Dynamic mechanical analysis (DMA) was used to investigate the thermal transition and viscoelastic behaviour of the PUA films. As shown in Supplementary Figure

Supplementary Note 8.
To demonstrate the change in the resistance of the conductor during the self-healing process, the conductor was bifurcated into two parts; then manually attached and heated for 24 h at 100 °C and subsequently allowed to heal for 24 h at room temperature (30 o C). The electrical self-healing efficiency of the healable conductor is calculated by measuring the resistance of the conductor before and after healing, using Supplementary equation 1. Supplementary Figure 16a represents the resistance of the conductor before and after the healing process. PUA matrix is infused with silver flakes, which encompasses the liquid metals, thus preventing the aggregation. After the conductor is damaged, the liquid metals can be seen well distributed in the matrix without being 'leak out' (Supplementary Figure 17d). If a higher amount of liquid metal is used, aggregation will occur due to high surface tension thus the liquid metal will drop out of the device when damaged or cut.

Supplementary Note 14.
In addition to the single electrode mode, lateral sliding mode TENG is demonstrated. As shown in Supplementary Figure 24a, the PUA layer is fixed on the left end and a lateral stretch sliding action is used to exert the frictional triboelectric effect. The pulling tensile stress is equivalent to 0.3 MPa (determined from the stress-strain curve (Fig. 1d).
However, the voltage of the device in the sliding mode is low compared to the single electrode mode.
In single electrode mode, compressive force is exerted which creates macroscopic deformations at the interface, thus resulting in high surface charge density, and thus high voltage output. However, in the later sliding mode (achieved by stretching the PUA layer), the contact and separation of the two triboelectric layers are not effective, thus generating low surface charge, resulting in low voltage output.
Thus, high output performance can be generated from the single electrode mode compared to the lateral sliding mode TENG. (ii) When the triboelectric layer is stretched, the thickness of the layer decreases due to Poisson's effect.
The decrease in the thickness increases the capacitance, which will increase the surface charge density of the triboelectric layer, resulting in improved triboelectric performance.
(iii) When the triboelectric layer is stretched, the microstructures on the surface of the triboelectric layers will deform, which will decrease the effective surface area of the triboelectric layer, this will decrease the net surface charge, resulting in lower triboelectric performance.
(iv) As the area of contact between the two triboelectric layer increases the total surface charge increases, thus the performance will improve.
Thus, the net output performance of the TENG when subjected to lateral deformation depends on the most dominating factor, which will vary according to the device design, configuration and electrode.
For the first case in our manuscript, Fig. 4b, when the area of the impacting force (area of the top latex layer, 3 × 3 cm 2 ) was kept constant, and the measurements were carried out by straining the PUA layer of the SH-TENG device, the output performance (the output voltage and current density (upon application of a mechanical force of 40 N at a frequency of 5 Hz)) decreased with an increase in the 28 uniaxial strain. When the PUA is stretched the microstructures on the PUA surface deforms, thus decreasing the effective contact area of PUA exposed to the exerted mechanical force. This reduces the net surface charge and the energy-harvesting performance. In the second case (Fig. 4c), when the area of the impacting force (area of the top latex layer) is increased so that it was similar to the uniaxial strain of the device, the output voltage increases. This increase can be attributed to the increment in the surface area of contact between the two triboelectric layer which dominates the overall performance.

Supplementary Tables
Supplementary Table 1. Summary of the glass transition temperature of polyurethane acrylate (PUA) with different amount of 2-hydroxyethyl methacrylate (HEMA) from differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and thermogravimetric analyser (TGA) studies.