Supporting Information Highly Stretchable and Transparent Thermistor Based on Self-Healing Double Network Hydrogel Jin Wu a, Songjia Han a, Tengzhou Yang a, Zhong Li c, Zixuan Wu a, Xuchun Gui a, Kai Tao b, *, Jianmin Miao c, Leslie K. Norford d, Chuan Liu a, * and Fengwei Huo e a State Key Laboratory of Optoelectronic Materials and Technologies and the Guangdong Province Key Laboratory of Display Material and Technology, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China b The Ministry of Education Key Laboratory of Micro and Nano Systems for Aerospace, Northwestern Polytechnical University, Xi'an, 710072, China. c School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore d Department of Architecture, Massachusetts Institute of Technology, Cambridge, MA 02139, USA e Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P. R. China Corresponding Author *E-mail: taokai@nwpu.edu.cn or liuchuan5@mail.sysu.edu.cn S-1
Figure S1. Fourier transform infrared spectroscopy (FTIR) spectra of the DN hydrogel. Figure S2. Photographs show that the double network (DN) thermistor can be stretched to different strain from 0% to 330%. A home-made stretching stage was utilized. S-2
Figure S3. Tensile stress/strain curves obtained in the loading-unloading process for the original (black, two cycles) and self-healed (red) DN hydrogels. The tensile tests were performed on an Instron machine (S6566) with the speed of 5 mm/min. Figure S4. Plot of normalized resistance change Δ R/R 0 (%) versus number of stretching cycles, where R 0 is the initial resistance at the first cycle. The thermistor was stretched to 150% strain at 25 C in the 41 repeated cycles. The resistance was recorded after each stretching cycle. S-3
Figure S5. (a) Plots of the resistivity of the DN hydrogel versus temperature at different strains. (b) Plots of the resistivity as a function of strain at different temperatures. The resistivity of the DN hydrogel slice was calculated according to the equation: R = ρ*l/a (S1) in which R, ρ, L and A are the resistance, resistivity, length and cross-sectional area of the hydrogel slice, respectively. 1 Assuming the hydrogel slice was isotropic and incompressible, the sizes of the deformed sample can be expressed as: L = λ*l 0 (S2) W = 1/λ ½ *W 0 (S3) T = 1/λ ½ *T 0 (S4) λ = 1 + S (S5) where L 0, W 0 and T 0 are the length, width and thickness of the sample in relaxed state, and L, W and T are the length, width and thickness of the sample at the strain of S. 2 The relationship between resistivity S-4
and temperature was calculated according to the above equations S1-5 using the experimental data in Figure 2b. The results in Figure S5 indicate the resistivity of the DN hydrogel decreases with both temperature and strain. Figure S6. a) Plots of relative resistance change of the DN thermistor versus time when it was stretched from 33% to 233% strain. b) Relative resistance variation of the thermistor with the strain. The relative resistance changes versus strain can be fitted into a parabolic equation y=0.004x 2 +0.58x-6.8, where y is the relative resistance change and x is the tensile strain. S-5
Table S1 Comparison of various flexible thermistors based on different materials Thermal sensing Stretchability Sensitivity Self-healing Transparent Response Recovery materials (Maximal %/ C time (s) time (s) strain) DN hydrogel [this work] 330% 2.6% Yes Yes 13 120 Graphene 3 50% 0.86% No No ~ 6 ~ 20 Reduced Graphene 70% 1.34% No Yes ~ 17 ~ 280 Oxide /PU 4 Oligomers/SWCNTs 5 73% 0.78% Yes No ~ 260 ~ 260 Ultrathin Au film 6 10% 0.12% No - ~ 0.005 ~ 2.5 Si nanoribbons 7 20% 0.28% No No ~ 100 ~ 1700 PAM/PPI 8-1.3% No No 26 26 PANI nanofiber 9 20% 1.0% No No 1.8 3.0 In this table, PU, SWCNTs, PAM, PPI and PANI are the abbreviations of polyurethane, single-wall carbon nanotubes, polyacrylamide, polyiodides, and polyaniline, respectively. S-6
Figure S7. Plot of resistance versus bending radius of curvature in the bending test. S-7
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