# Shape Memory-Enhanced Electrical Self-Healing of Stretchable Electrodes
## 3 Results and Discussion
### Shape Memory-Enhanced Self-Healing with Pre-Stretching  
In order to evaluate the reversibility and stretch-ability, the electromechanical measurements for the crack-free samples were performed firstly. The conduction-strain relationship was disclosed under constant voltage of 0.1 V and strain speed of 10 mm/min. Figure 2a shows the current variation which was monitored in the overall shape memory programming. After a slight drop as heated at 80 \({}^{\circ}\)C, the current dramatically decreased and stabilized as the samples were extended to the temporary shapes. After cooling down to fix the temporary shapes, heating again allowed the shape recovery along with the recovery of the current. The conduction could partially recover under the heat stimulations, depending on the extension percentages. The recovery ratio was calculated according to the ratios of the recovered and pristine currents, which was determined to be 99%, 70% and 40% as the pre-stretching percentage was 20%, 40% and 60%, respectively. Too large extension may result into the irreversibility associating with plastic deformation of the polymer matrix, permanent break of the conductive Ag network and local movement of the AgNPs [11]. Thus, the pre-stretching was limited to within 20% in the next electromechanical measurements, ensuring that it was in the reversible region for the study on the shape memory-enhanced self-healable conductions. Furthermore, as for the samples exposed upon the scratch in the temporary shapes (shown in Figure 2b), pre-stretching with four different percentages ranging from 0 to 20% led to dramatic decrease in the current. The cracks were made in the temporary shapes, enabling the currents to approach zero. Afterwards, the self-healing was triggered at 80 \({}^{\circ}\)C for several seconds, which was above the \(T_{m}\) of the PCL. The healing efficiency (\(\eta\)) was calculated by the following equation:  
\[\eta=(I_{o}-I_{c})/(I_{h}-I_{c}) \tag{1}\]  
where \(I_{o}\), \(I_{c}\), \(I_{h}\) were the currents as the sample was in the states of original, cracked and healed states. Generally speaking, the healing efficiency could also be determined according to the variation in mechanics of the samples in the pristine, cracked and healed states. We once compared the heal-ability of the conductive shape memory composites with respect to the conduction and the mechanical properties. It was found that the healing efficiency of the mechanics was less than 50% which was lower than that of other healable polymeric systems based on reversible chemical bonding [9]. Considering that the emphasis of this study was put on the healable conductivity of the polymeric electrodes, herein the healing efficiency was chosen to calculate by the currents rather than the peak stress in the tensile tests. The dependence of the healing efficiency against the pre-stretching percentages is demonstrated in Figure 2c. The efficiency was calculated to be 13.4%, 18.1%, 47%, 63% and 60% as the pre-stretching percentage was 0%, 5%, 10%, 15% and 20%, respectively. The results suggested obvious enhancing effect of the pre-stretching on the healing behaviors. According to the previous report on the self-healing improved by the shape memory pre-stretching, the enhancement may attribute to the release of the strain energy driving the local shape recovery and favoring the closure of the scratch and its complete healing [12; 13]. The SEM images, shown in Figure 2d-f, clearly disclosed that the thermal stimulation made the scratch gap closer and reconstructed the conductive pathways of the AgNPs. Comparatively, the sample with 20% of the pre-stretching had the best morphology in the healed state. The SEM investigations were consistent with the above current monitoring results, providing with the evidence that the shape memory pre-stretching significantly enhanced the healing performance in both the structure and the conduction recovery.