Supporting Information. Highly Stretchable and Biocompatible Strain Sensors Based on. Mussel-Inspired Super-Adhesive Self-Healing Hydrogels for Human

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1 Supporting Information Highly Stretchable and Biocompatible Strain Sensors Based on Mussel-Inspired Super-Adhesive Self-Healing Hydrogels for Human Motion Monitoring Xin Jing 1,2,3, Hao-Yang Mi 1,2,3*, Yu-Jyun Lin 2, Eduardo Enriquez 2, Xiang-Fang Peng 3, and Lih-Sheng Turng 1,2* 1 Wisconsin Institute for Discovery, University of Wisconsin Madison, 53715, WI, USA 2 Department of Mechanical Engineering, University of Wisconsin Madison, 53706, WI, USA 3 Department of Industrial Equipment and Control Engineering, South China University of Technology, Guangzhou, China * Corresponding authors: Lih-Sheng Turng, turng@engr.wisc.edu Hao-Yang Mi, mehymi@scut.edu.cn S-1

2 1 H NMR spectroscopy analysis The 1 H NMR spectra of DA, AM, and DA AM were analyzed to investigate the interactions between DA and PAM. The 1 H NMR spectra were recorded in deuterium oxide (D 2 O) with a Bruker AM 400 spectrometer (600 MHz). *Website of Biological Magnetic Resonance Data Bank database: X-ray Diffraction Measurement XRD patterns of talc and PDA-talc were analyzed using an X-ray diffractometer (XRD, Bruker D8 Discover, Bruker Company, USA) operating at a voltage of 50 kv and a current of 1000 µa and employing Cu Kα filter radiation (λ = 1.54 nm). Rheological Experiments The rheological properties of the hydrogels were examined with a TA rheometer using different methods. (1) A strain amplitude sweep (0.1% 500%) test was first performed under a fixed angular frequency (1 rad/s) to determine the linear viscoelastic region. (2) A dynamic frequency sweep was performed from 0.1 to 100 rad/s. The preset strain was 1% based on the strain sweep test result. (3) The alternate step strain sweep test was also carried out to observe the healing ability of the hydrogels at a fixed angular frequency (10 rad/s) using the same rheometer. The amplitude strain was switched from small strain (γ = 1%) to subsequent large strain (300%) for 100 s for every strain interval. Characterization of the Surface Energies of the Substrates and the Hydrogels S-2

3 The water contact angles of the substrates and DTPAM hydrogel were measured using the sessile water drop method at room temperature with a video contact angle instrument (Dataphysics OCA 15). The droplet size was set at 4 µl. Three samples were tested for each surface type. The surface energies were calculated based on the Owens Wendt (OW) method. The surface energy of a solid is related to the contact angle (θ) as follows 1 : (1+ cos θ ) γ = 2( γ d γ d + γ p γ p ) L s L s L where γ L is the liquid surface tension and γ d L and γ p L are its dispersion and polar components, respectively. Parameters γ d s and γ p s are the dispersion and polar components of the solid surface tension, respectively. The final surface energy of the solid (γ s ) is the sum of its dispersion γ d s and polar γ p s components. In this study, the two testing liquids were deionized water (γ = 72.8 mj/m 2, γ d = 21.8 mj/m 2, γ p = 51.0 mj/m 2 ) and glycerol (γ = 63.4 mj/m 2, γ d = 37.0 mj/m 2, γ p = 26.4 mj/m 2 ) 2-3. S-3

4 Figure S1. (a) XPS C1s core level scan of dopamine powder. The curve was Gauss-fitted to analyze the components of the various bonds. (b) The surface properties of the DTPAM (0.75%D, 0.75%T) hydrogel surface and middle part. The spectrum of DA in D 2 O is the same as that listed in the Biological Magnetic Resonance Data Band database. For the spectrum of DA, the characteristic resonance signals located at 6.79 and 6.74 ppm were phenyl protons (Figure S2). The methylene protons appeared around 3.12 and 2.77 ppm. The spectrum of AM showed characteristic resonance signals of methyne and methylene protons at 6.73 and 5.72 ppm. The peaks at 7.7 and 6.93 ppm might have been the chemical shift of amide groups in the PAM structure. After AM was mixed with DA to form the DA AM solution, the active hydrogen in DA shifted from 6.74 to 6.7 ppm, thus indicating that there were interactions between the oligomers of PDA and PAM. S-4

5 Figure S2. The 1 H NMR spectra of DA, AM, and DA AM in D 2 O. Figure S3. SEM images show the morphology of a freeze-dried hydrogel: (a) PAM, (b) DTPAM (0.75%D, 0.75%T), (c) DTPAM (1%D, 1%T), (d) DTPAM (0%D, 1%T), and (e) DTPAM (1%D, 5%T). The scale bar is 5 µm. S-5

6 Figure S4. The XRD patterns of talc and PDA-talc. Figure S5. The images of the prepared hydrogels. S-6

7 Figure S6. (a) The strain sweep test result of DTPAM (0.75%D, 0.75%T) and (b) the frequency sweep test results of pure PAM and DTPAM (0.75%D, 0.75%T). Figure S7. Recovery rate of the DTPAM (0.75%D, 0.75%T) hydrogel in the (a) cyclic tensile test and (b) cyclic compression test. S-7

8 Figure S8. (a) The dissipated energy, (b) the Young s modulus, and (c) the tensile stress of the DTPAM hydrogel according to the cyclic tensile test. (d) The dissipated energy, (e) the Young s modulus, and (f) the compression stress of the DTPAM hydrogel according to the cyclic compression test. S-8

9 Figure S9. (a) The DTPAM (0.75%D, 0.75%T) hydrogel stuck to the back of the author s hand and did not cause any irritation when peeled off. (b) The DTPAM (0.75%D, 0.75%T) hydrogel easily adhered to an iphone and a monitor. S-9

10 Figure S10. Demonstration of the reversible adhesive behavior of the DTPAM (0.75%D, 0.75%T) hydrogel. S-10

11 Figure S11. The reversible adhesive strength of the DTPAM (0.75%D, 0.75%T) hydrogel to different substrates including PET, wood, and steel. Figure S12. The alternate strain sweep test with a small strain to a large strain at 100 s intervals on DTPAM (0.75%D, 0.75%T). S-11

12 Figure S13. The resistance change of the sensor attached to a mechanical length gauge: (a) the resistance change of the sensor stretched by 50%, and (b) the resistance change of the sensor stretched by different strains. S-12

13 Figure S14. Fluorescence images of live/dead assay results of HEF1 fibroblasts cultured on PAM and DTPAM hydrogels, and on the tissue culture plate control. The control group represents cells cultured on non-treated tissue culture polystyrene plates. S-13

14 Table S1. Compositions of the prepared hydrogels. Hydrogels DA/AM (wt.%) Talc/AM (wt.%) AM (g) APS/AM (wt.%) MBAA/AM (wt.%) TEMED (µl) Water (ml) KCl (mol) Talc-AM 0 DTPAM 0.25 DTPAM 0.5 DTPAM 0.75 DTPAM S-14

15 Table S2. Contact angle measurements. Substrate Water Glycerol Steel 34.5 ± ± 4.7 Fabric 38.9 ± ± 1.9 Wood 43.3 ± ± 3.4 Glass 34.9 ± ± 4.6 PET 72.6 ± ± 2.3 Pig Skin 9.8 ± ± 1.6 Hydrogel 15.3 ± ± 3.9 Table S3. Surface energies of substrates and the representative hydrogel. Substrate γ d s (mj/m 2 ) γ p s (mj/m 2 ) γ s (mj/m 2 ) Steel Fabric Wood Glass PET Pig Skin Hydrogel To further investigate the effect of talc on the oxidization of DA, we investigated the oxidization behavior of DA under the protection of N 2. We found that the ph of the DA solution changed from 5.1 to 7.8 after adding talc to the solution for 8 h. The color of the solution became light pink, which indicated that dissolved ions from the talc can provide a slight alkaline S-15

16 environment for the oxidization of DA (shown in Figure S15 (a)). In addition, two commonly used oxidative agents iron chloride (FeCl 3 ) and sodium periodate (NaIO 4 ) were also used to oxidize DA. However, the hydrogel could not form and only a very viscous solution was obtained 4 (shown in Figure S15 (b)). Moreover, Tris buffer was used to adjust the ph of the talc/da to 8.5, and the DA was left to oxidize for 8 h to prepare the composite hydrogel, which displayed some precipitate on the bottom due to the fast oxidation of DA. The obtained hydrogel was named DTPAM (8.5) for comparison. We also used an optical microscope to observe the self-healing behavior of the fully oxidized DTPAM hydrogel. The self-healing process is shown in Figure S16 (e). We found that, after the hydrogel healed for 30 min, the cutting area was still clearly visible. Even 4 h later, the cutting area could still be seen. S-16

17 Figure S15. (a) An image of the DA solution (clear), the DA-talc solution (white), and the DAtalc solution (light pink) after being magnetically stirred for 8 h under the protection of N 2 at room temperature. The ph of the DA solution was 5.1, while the ph of the DA-talc solution was 7.8. (b) Bottle 1: Pure PAM hydrogel, bottle 2: DTPAM hydrogel (DA/AM=0.75%, talc/am=0.75%), and bottle 3: DTPAM hydrogel ((DA/AM=0.75%, talc/am=0.75%). The ph of DA-talc was adjusted to 8.5. Bottles 4 and 5 showed that the hydrogel could not form when the talc was replaced by an equal amount of FeCl 3 and NaIO 4. (c) Photos of the inversed bottles in (b). (d) An image of the DTPAM (8.5) hydrogel ((DA/AM=0.75%, talc/am=0.75%). (e) Optical images of DTPAM (8.5) healed for 0 min, 30 min, and 4 h. References 1. Lu, X.; Zhao, Z. F.; Leng, Y., Biomimetic calcium phosphate coatings on nitric-acid-treated titanium surfaces. Mat Sci Eng C-Bio S 2007, 27 (4), Zhong, Z. Y.; Yin, S.; Liu, C.; Zhong, Y. X.; Zhang, W. X.; Shi, D. F.; Wang, C., Surface energy for electroluminescent polymers and indium-fin-oxide. Appl Surf Sci 2003, 207 (1-4), Feng, B.; Chen, J. Y.; Qi, S. K.; He, L.; Zhao, J. Z.; Zhang, X. D., Characterization of surface oxide films on titanium and bioactivity. J Mater Sci-Mater M 2002, 13 (5), Han, L.; Lu, X.; Liu, K. Z.; Wang, K. F.; Fang, L. M.; Weng, L. T.; Zhang, H. P.; Tang, Y. H.; Ren, F. Z.; Zhao, C. C.; Sun, G. X.; Liang, R.; Li, Z. J., Mussel-Inspired Adhesive and S-17

18 Tough Hydrogel Based on Nanoclay Confined Dopamine Polymerization. Acs Nano 2017, 11 (3), S-18