Supporting Information

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1 Supporting Information Self-Healing, Highly Sensitive Electronic Sensors Enabled by Metal-Ligand Coordination and Hierarchical Structure Design Yangyang Han, Xiaodong Wu, Xinxing Zhang, * and Canhui Lu* State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, No.24 South Section 1 of First Ring Road, Cheng Du , China Corresponding author: Xinxing Zhang & Canhui Lu. address: xxzwwh@scu.edu.cn & canhuilu@scu.edu.cn. Tel: Fax: S-1

2 Figure S1. FTIR spectra of ENR, PDA and (a). Inset photos in (a) show ENR and suspensions. XPS spectra of ENR and (b). FTIR and XPS analysis were employed to characterize (Figure S1). As shown in Figure S1a, the color of pristine ENR suspension is white. After modifying ENR with PDA, the color of suspension turns to black. In FTIR spectra, compared with ENR and PDA, shows a new peak around cm -1 arising from the N-H bending vibration of the amine group of the PDA, 1 indicating the successful modification of ENR with PDA. Additionally, the XPS spectrum of PDA@ENR shows peak components of C 1s, N 1s, and O 1s, ascribing to C, O, and N elements (Figure S1b). The extra peak of N, compared to that of ENR, proves that the ENR was modified by PDA. S-2

3 Figure S2. XPS spectrum of Fe 3+ (a) and UV-Vis spectra of and Fe 3+ suspensions (b). As shown in Figure S2a, the XPS spectrum of Fe 3+ elastomer shows peak components of C 1s, N 1s, O 1s, and Fe 2p, respectively. The atomic ratio of N:Fe is calculated to be 2.89:1 (nearly 3:1). Because each dopamine unit has only one N atom, we can figure out that the ratio of catechol:fe is approximately 3:1. Besides, as shown in UV-Vis spectra (Figure S2b), the band around 280 nm is attributed to the π π* transition of the aromatic moieties of PDA. This band of Fe 3+ /PDA@ENR suspension is red-shifted to 292 nm, indicating the coordination bonding between PDA and Fe 3+. S-3

4 Figure S3. SEM image of the healing line in the self-healed Fe 3+ elastomer. S-4

5 Figure S4. Pictures showing the cutting/healing process of sulfur cross-linked ENR (a). Typical stress-strain curves of ENR, and Fe 3+ elastomers after healing (b). Hysteresis loops of and Fe 3+ elastomers (c). For comparison, we prepared ENR cross-linked by conventional sulfur system (based on our previous studies). 2 To evaluate the self-healing ability, the sample was cut into two pieces with a razor blade. Then, the two parts of the cut sample were reconnected and allowed to rest at room temperature without any external stimuli. As given in Figure S4a, the sulfur cross-linked ENR after cutting could not heal without external stimuli because of the irreversibility of sulfur-based covalent bonds. In contrast, when reversible catechol-fe 3+ coordination was incorporated into the Fe 3+ /PDA@ENR elastomer, desirable self-healing behavior could be attained as expected (Figure 2). The self-healing ability of PDA@ENR elastomer was also evaluated. As shown in Figure S4b, the mechanical properties of PDA@ENR can be partly restored due to the fact that the S-5

6 glass-transition temperature (T g ) value of ENR is below room temperature and inter-diffusion of ENR across the interfaces may occur. However, the healed strength is very limited (only 0.9 MPa), which might hinder its applications as robust materials or devices. In contrast, when Fe 3+ was introduced into PDA@ENR system, Fe 3+ /PDA@ENR system showed much higher self-healing efficiency and mechanical properties than that of PDA@ENR. This result confirms that the interactions between Fe 3+ and catechol play a key role for the desirable self-healing efficiency. Besides, Fe 3+ /PDA@ENR sample has higher deformation recovery rate than PDA@ENR (Figure S4c), benefiting from the metal-ligand coordination interaction. Figure S5. Schematic illustration for the self-healing behavior of Fe 3+ /PDA@ENR elastomer (a). Typical stress-strain curves of pristine and self-healed samples under different harsh conditions: at different temperatures (b), in damp air for 12 h before self-healing (c), and in water (d). Pictures showing the self-healing capability of the elastomer in water (e). S-6

7 Figure S6. Typical stress-strain curves of CNT filled elastomer and layered composite (a) and hysteresis loops of Fe 3+ elastomer, CNT filled Fe 3+ elastomer and layered composite. CNT was incorporated into the self-healing Fe 3+ matrix to prepare CNT filled elastomer (5 wt%). As shown in Figure S6a, elongation at break of CNT filled elastomer is 120.4±14.2% and tensile strength is 3.4±0.2 MPa, while elongation at break of layered composite is 115.5±2.3% and tensile strength is 2.2±0.08 MPa. It can be noticed that the elongation of Fe 3+ elastomer does not show a decline after filled with CNT or coated by CNT composite. However, tensile strength of CNT filled elastomer is much higher than that of Fe 3+ /PDA@ENR elastomer, while the tensile strength of layered composite does not change obviously. Notably, the elongation of Fe 3+ /PDA@ENR elastomer filled with CNT or coated by CNT composite could still maintain at above 100%, capable of monitoring normal human physiological activities. Besides, the layered composite shows similar hysteresis compared with Fe 3+ /PDA@ENR elastomer (Figure S6b). S-7

8 Figure S7. Electrical resistance change of the sensors with increasing conductive layer numbers during drop-casting process. Resistance of samples with various numbers of conductive layers was measured. As depicted in Figure S7, the electrical resistance shows a decline when numbers of conductive layers are below 11 because of the gradual construction of conductive structure. However, with further increasing conductive layers, no obvious decrease in resistance could be observed. This is due to the fact that conductive structure has been fabricated completely and conductivity of the conductive layer would be stable (approaching its bulk conductivity) with more numbers of conductive layers. S-8

9 Figure S8. Pictures (a) and zeta potential (b) of ENR, and aqueous suspensions (from left to right). To intuitively illustrate the suspension stability of ENR, and photographs of their aqueous suspensions before and after standing for one month are given in Figure S8a. All suspensions remained stable after one month standing. Zeta potential of these suspensions was further measured to evaluate their colloidal stability. As depicted in Figure S8b, the zeta potential value of ENR is mv while that of and is and mv, respectively. Notably, zeta potential value of suspension does not change obviously after dispersing and stabilizing CNT, indicating good stability of suspension. These results demonstrate that CNT could be well dispersed by suspension. S-9

10 Figure S9. SEM images of latex microspheres (a) and mixture (b, c). Scale bar: 1 µm for a, c and 5 µm for b. To investigate the template effect of PDA@ENR, the morphology of PDA@ENR and CNT-PDA@ENR was observed using SEM. As shown in Figure S9a, microspheres of PDA@ENR latex could be seen clearly. In Figure S9b-c, CNT-PDA@ENR microspheres connect with each other, forming a continuous segregated conductive network. S-10

11 Figure S10. Schematics for CNT/elastomer composites with randomly dispersed fillers prepared via directly blending (a), conductive composites based on surface coating strategy (b), and hierarchically structured composites with thin and segregated conductive network (c). Relative resistance changes of CNT/elastomer composites with randomly dispersed CNT (red line) and hierarchically structured CNT/elastomer composites with segregated CNT conductive network (black line) under increasing strain (d). Relative resistance change of conductive composites via surface coating and hierarchically structured CNT/elastomer composites during various harsh conditions (e). Here, elastomer refers to the as-prepared Fe 3+ elastomer. For comparison, CNT/elastomer composites with randomly dispersed CNT (30 wt%) were prepared by two-roll milling. Although large amount of CNT was added into the composites, their resistance was still much higher (62.2 kω) than that of hierarchically structured CNT/elastomer S-11

12 composites (6.7 kω). Moreover, the prepared composites with randomly dispersed CNT show much lower sensitivity to strain (GF) than that of the hierarchically structured sensors, indicating the effectiveness of the hierarchical structure design in enhancing both conductivity and sensitivity of the composites. On the other hand, for comparison, we prepared conductive composites based on surface coating strategy. To evaluate the robustness, we exposed the hierarchically structured CNT/elastomer and composites to various harsh conditions, such as washing in detergent solution, ultrasonic treatment in ethanol, peeling off using scotch tape and bending to 180 o. As shown in Figure S10e, the conductive stability of hierarchically structured CNT/elastomer composites is much better than that of composites during these harsh treatments. This is because that the conductive CNT network in hierarchically structured composites was embedded in the elastomer matrix, protecting it from injury. S-12

13 Figure S11. SEM image showing the healing-line of self-healed CNT-Fe 3+ conductive layer (scale bar= 50 µm). In order to further evaluate the self-healing ability of the Fe 3+ elastomer after filled with CNT, SEM was conducted to observe the interface of the self-healed CNT-Fe 3+ conductive layer. As shown in Figure S11, a scar could be observed and a well-bonded interface between the two broken parts of CNT-Fe 3+ /PDA@ENR layer could also be identified. The desirable self-healing performance of the CNT-Fe 3+ /PDA@ENR conductive layer provides the prerequisite for the self-healing capability of the entire sensors. S-13

14 Figure S12. Normalized current changes of pristine sensors (a) and self-healed sensors after bending times (bending radius: 3 mm) (d) under different tiny strains (0.05%, 0.1% and 0.2%). Illustration for the bending process of the self-healed sensor (c). Normalized current changes of pristine sensors after exposure to water and simulative sweat (b). Figure S13. Representative current signals of pristine sensors during monitoring coughing (a) and saliva swallowing (b). S-14

15 Figure S14. Representative current signals of pristine sensors during monitoring pronouncing different words: hi (a), hello (b) and how are you (c), respectively. Figure S15. Representative current signals of self-healed sensors after bending times during pronouncing different words (hi and hello) and a phrase (How are you) respectively (a-c). S-15

16 Figure S16. Representative current signals of self-healed sensors after washing in detergent solution, acid condition, and alkaline condition during pronouncing word hi. As shown in Figure S16, after washed in commercially available detergent solution (0.05 wt%), acid condition (1 mol/l H 2 SO 4 ), and alkaline condition (1 mol/l NaOH) with magnetic stirring for 24 h, the self-healed sensors could still detect tiny motions. S-16

17 Figure S17. Representative current signals of pristine sensors during finger bending (a), finger touching (b), neck bending (c) and neck shaking (d). References: (1) Liu, W.; Li, Y.; Meng, X.; Liu, G.; Hu, S.; Pan, F.; Wu, H.; Jiang, Z.; Wang, B.; Li, Z.; Cao, X. Embedding Dopamine Nanoaggregates into a Poly(dimethylsiloxane) Membrane to Confer Controlled Interactions and Free Volume for Enhanced Separation Performance. J. Mater. Chem. A 2013, 1, (2) Wang, S.; Zhang, X.; Wu, X.; Lu, C. Tailoring Percolating Conductive Networks of Natural Rubber Composites for Flexible Strain Sensors via a Cellulose Nanocrystal Templated Assembly. Soft Matter 2016, 12, S-17