Parallel Microcracks-based Ultrasensitive and. Highly Stretchable Strain Sensors

Transcription

1 Supporting Information for Parallel Microcracks-based Ultrasensitive and Highly Stretchable Strain Sensors Morteza Amjadi 1,*, Mehmet Turan 1, Cameron P. Clementson 1,2, and Metin Sitti 1,* 1 Physical Intelligence Department, Max-Planck Institute for Intelligent Systems, Heisenberstr. 3, 70569, Stuttgart, Germany 2 Department of Biomedical Engineering, University of Arizona, AZ 85721, Tucson, USA Corresponding sitti@is.mpg.de and amjadi@is.mpg.de This PDF file includes: Figures S1-S15 Table S1 Captions for Movies S1 and S2 S-1

2 Stiffness gradient design: We added dental polymer into the Ecoflex solution to make the electrode areas stiffer than that of the strain responsive region which was made of bare Ecoflex. The mixed polymer in the electrode parts was strongly bonded to the bare Ecoflex elastomer film in the sensing region after bringing the cured mixed polymer films in contact with the liquid Ecoflex layer between them (Figure S1). The stiffness gradient design minimized the harmful stress in the electrode areas while induced maximum strain in the sensing film. In addition, it was easier to connect rigid wires in the stiffer electrode sides with noise free signals. Figure S1: a) Stiffness gradient of the mixed polymer (Ecoflex and dental polymer) and bare Ecoflex elastomer. b) Highly flexible strain responsive film made of the low-density graphite thin film coated on elastomer films bonded with the high-density embedded graphite thin films in the stiffer electrode areas. S-2

3 Figure S2: Control of the parallel microcrack formation in the graphite thin films coated on the plasma-exposed elastomer films. Morphology change of the graphite thin films coated on the non-exposed and exposed (50, 100, and 150 s) elastomer films under different stretching conditions from 0% to 150%. Scale bar: 100 μm. S-3

4 Figure S3: a) Increase of the crack width by the exposure time (at 100% of stretching). b) Increase of the crack width by tensile strain for the graphite thin film coated on a 100 s exposed elastomer film. c) Morphology tracking of the graphite thin film coated on a 50 s exposed elastomer film under different strains; increase of the crack length, width, and density by further stretching. Scale bar: 100 μm. S-4

5 Localized parallel microcrack generation: To investigate the local generation of microcracks, we prevented exposure of the entire elastomer films by a paper mask. Moreover, elastomer films were only plasma-exposed in the patterned areas of the paper mask. The exposed elastomer films were then coated by graphite thin films. Figure S4 depicts the microcrack generation in the graphite thin film coated on a 40 s exposed elastomer film when it was stretched to 150% and released to 50%. As shown in the figure, parallel microcracks were only generated in the graphite thin film in the exposed area while there is no crack formation in the graphite thin film in the non-exposed regions. Figure S4: Local microcrack generation in the graphite thin film with a paper mask; microcrack formation in the graphite thin film in the exposed areas only. Scale bar: 100 μm. S-5

6 Figure S5: a) Surface profile measurement of a 50 s exposed Ecoflex layer under 150% of strain. b) Depth information of a microgroove, showing valley-like structure formation on the top surface of the exposed elastomer (for example, depth of microgrooves in a 50 s exposed elastomer under 150% of strain was measured to be around 1 μm). c) Surface profile measurement of the graphite thin film coated on a 50 s exposed Ecoflex under 150% of strain. d) Depth information of the microcracks, showing that microcracks were generated in the microgroove edges. S-6

7 Figure S6: a and b) Response of a non-exposed strain sensors under 100% and 150% cyclic loading, respectively. Figure S7: a) Piezoresistivity of a 60 s exposed sensor, as compared with a non-exposed sensor. b) Resistance recovery of the 60 s exposed sensor under cyclic loading/unloading from 0% to 50%, showing ultrasensitivity and high stretchability of the 60 s exposed sensor. For more than 50% of tensile strain, the sensor was not conductive and the resistance shifted to infinite value. S-7

8 Figure S8: Small-strain measurement of a 40 s exposed strain sensor. a) Response of the sensor to stepwise cyclic loadings (0.1, 0.3, and 0.5%). b) Output signal of the sensor under stepwise cyclic stretching (0.2, 0.4, and 0.6%). S-8

9 Strain sensing characterization: The strain sensing performance of strain sensors was further investigated. Figure S9a shows the piezoresistivity of the 5 non-exposed and 5 exposed sensors fabricated with the same processes. The strain sensors possessed same piezoresistive response under the tensile strain. It should be noted that it was difficult to carefully control the density of graphite thin films by hand spray and bar coatings. Automated systems would give much better thin film quality and sensory performance, as a future work. As shown in the figure, non-exposed strain sensors possessed high linearity (R ) while exposed sensors showed exponential increase of the resistance. Figure S9, b and c illustrate the durability test of the non-exposed and 40 s exposed sensors. After critical straining, the strain sensors were stretched to 25% and released to 0% under 2,000 cycles. The initial resistance of both sensors slightly decreased after the first 30 cycles and then stabilized and remained constant for more than 2,000 cycles. The GFs of the non-exposed (44.06±2.96) and 40 s exposed (95.54±5.39) sensors were almost constant under cyclic loading/unloading. Moreover, the electromechanical behavior of both types of sensors was highly reversible during durability test (Figure S9d). For large stretching/releasing cyclic tests (e.g. 50%), we observed sliding of the strain sensors in our moving stage due to the ultra-softness of the Ecoflex elastomer and were not able to characterize the durability of strain sensors under very large strain conditions. S-9

10 Figure S9: a) Piezoresistive response of the non-exposed and 40 s exposed sensors (5 samples per each). b and c) Durability tests of the non-exposed and 40 s exposed sensors under 2,000 cycles from 0% to 25%, respectively. d) Electromechanical performance of the strain sensors at the last 10 cycles (1990 to 2,000). S-10

11 Simulation of flake movements: Since it was impossible to track the movement of each flake under stretching/releasing condition, we conducted a simulation to visually demonstrate the thin film morphology under stretching. Graphite flakes, as rigid circles with different diameters, were randomly assigned on the top of elastomer films. The position of each flake was calculated according to the center of mass coordinate of flakes with respect to a reference point. Next, the overlapped areas between connected flakes were identified and visually illustrated. Then, the repositioning of flakes and reorientation of the conductive network were recalculated according to Poisson s effect and the connectivity between flakes was analyzed again. Figure S10 shows the effect of the flake size on the topology changes of the conductive networks. Thin films with different flake sizes (i.e. 1, 1.5, and 2 µm) with same density were stretched to 50% and the topology changes were visually tracked. As shown in the figure, there was more disconnection between conductive chains for thin films made of the small size flakes. Moreover, the overlapped flakes lost their connection under smaller strains due to the shorter overlapped areas. Therefore, thin films made of the small size flakes are expected to have higher piezoresistivity, but with limited stretchability. S-11

12 Figure S10: Topology changes of the graphite thin films made of different flake sizes. S-12

13 Figure S11: a) Optical microscope top-view images of the graphite thin film coated on a 20 s exposed elastomer film at original length and 50% and 100% strains; wrinkling of the graphite thin film in perpendicular direction of stretching at critical straining (100%). b and c) Bottleneck locations between parallel microcracks in the graphite thin film coated on a 60 s exposed elastomer film. S-13

14 Contribution of microcracks & adhesion improvement: To explore the contribution of the microcrack generation on the piezoresistivity, we measured the resistance of the non-exposed and 40 s exposed sensors before and after microcrack formation in the exposed sensor. Both samples were fist stretched to 50% and released, meaning that there was no wrinkling or cracking in the graphite thin film coated on the 40 s exposed elastomer film. As illustrated in Figure S12, both samples possessed the same piezoresistivity since there was no wrinkling or microcrack generation in the exposed sensor and thin film structural deformations were similar for both samples. Then, we measured the resistance of both samples after stretching them to the critical strain (100%) and releasing, pointing that there were microcracks in the graphite thin film coated on the 40 s exposed film. As shown in the figure, the non-exposed sensor exhibited piezoresistive response similar to that before critical straining. On the other hand, there was a significant improvement in the piezoresistive response of the 40 s exposed sensor after critical straining, showing significant contribution of the microcrack formation in the piezoresistivity of the strain sensors. In addition, similar piezoresistivity of the non-exposed and exposed crack-free thin films revealed that the effect of the possible interfacial adhesion enhancement between graphite flakes and elastomer films by the plasma-exposure is negligible, due to the strong adhesion strength between graphite flakes and non-exposed Ecoflex films. Figure S12: Comparison of the piezoresistivity for the non-exposed and 40 s exposed strain sensors before and after microcrack formation in the exposed sensor. S-14

15 Sound visualization: We explored the ability of the strain sensors for the acoustic wave visualization. A two edge-fixed freestanding 40 s exposed sensor was attached to a speaker and saw-tooth sound waves with different frequencies were played with a computer. As shown in Figure S13a, the sensor was able to response to the acoustic waves for up to 14 Hz. Next, we measured signal of the sensor when frequency of the sound wave was linearly increased from 0 to 150 Hz. Figure S13b illustrates the output signal of the strain sensor. Several resistance peaks were appeared in the signal while frequency was increased, showing intrinsic frequency modes of the Ecoflex elastomer film (i.e. 32, 50, and 99 Hz). Figure S13: a) Response of a 40 s exposed sensor to acoustic waves. Response of the sensor to saw-tooth acoustic waves with different frequencies. b) Detection of the intrinsic frequency modes of the Ecoflex film. S-15

16 Figure S14: a) Detection of the sound waves when the sensor was attached to a mobile phone and a music track was played by phone. b) Detection of the vibration patterns when a phone is calling (a 40 s exposed sensor is attached to the phone). S-16

17 Pressure sensing: The schematic representation and photograph (inset images) of a sample pressure sensor design are shown in Figure S15a. A strain responsive film was two edge-fixed suspended on the top of the polydimethylsiloxane (PDMS) cavity and electrical signals were measured through two side electrodes. Suspended responsive film was highly sensitive and could deform under extremely low pressures. Figure S15a shows the relative change of the resistance versus applied pressure from 0 to 120 Pa. The sensing film could detect subtle pressures in Pa ranges with high sensitivity ( S R R0 P ~6.6 kpa -1 ). Figure S15b indicates the response of the sensing film to dropping a piece of paper (1.5 mg, only 0.59 Pa) on the top of the film. The responsive film could detect the pressure with high sensitivity and fast bandwidth (~ 80 ms). Figure S15: a) Relative resistance change of the responsive film under extremely low pressure ranges; insets, schematic and photograph of the pressure sensor made of the suspended strain responsive film. b) Output signal of the sensing film to the paper drop (0.59 Pa). S-17

18 Table S1: Performance comparison between our sensors and recently reported strain sensors. Reference Stretchability (%) Maximum Gauge Factor This work Non-exposed: S exposed: s exposed: s exposed: 11,344 1 Nature, ,000 2 Nat. Nanotechnol., Adv. Mater., ACS Nano, Nat. Commun., Adv. Mater., ACS Nano, ACS Nano, ACS Nano, ACS Appl. Mater. Interfaces, Adv. Electron. Mater Adv. Mater., S-18

19 Movie Captions: Movie S1 This movie illustrates the real-time topography of the graphite thin films coated on the nonexposed, 40 s, and 60 s exposed elastomer films under stretching from 0% to 100% and releasing from 100% to 0%. There is no microcrack formation in the graphite thin film coated on the nonexposed elastomer film while parallel microcracks are generated in the graphite thin films coated on the exposed elastomer films. The length, width, and density of the microcracks increase by the longer exposure time. Movie S2 The movie depicts the real-time surface morphology change of the non-exposed and exposed Ecoflex elastomer films, without graphite thin film coating. Microgrooves are generated in the case of the exposed elastomer film. References: 1. Kang, D.; Pikhitsa, P. V.; Choi, Y. W.; Lee, C.; Shin, S. S.; Piao, L.; Park, B.; Suh, K.-Y.; Kim, T.-i.; Choi, M. Ultrasensitive Mechanical Crack-based Sensor Inspired by the Spider Sensory System, Nature 2014, 516, Yamada, T.; Hayamizu, Y.; Yamamoto, Y.; Yomogida, Y.; Izadi-Najafabadi, A.; Futaba, D. N.; Hata, K. A Stretchable Carbon Nanotube Strain Sensor for Human-Motion Detection, Nat. Nanotechnol. 2011, 6, Yan, C.; Wang, J.; Kang, W.; Cui, M.; Wang, X.; Foo, C. Y.; Chee, K. J.; Lee, P. S. Highly Stretchable Piezoresistive Graphene Nanocellulose Nanopaper for Strain Sensors, Adv. Mater. 2014, 26, Amjadi, M.; Pichitpajongkit, A.; Lee, S.; Ryu, S.; Park, I. Highly Stretchable and Sensitive Strain Sensor Based on Silver Nanowire-Elastomer Nanocomposite, ACS Nano 2014, 8, Gong, S.; Schwalb, W.; Wang, Y.; Chen, Y.; Tang, Y.; Si, J.; Shirinzadeh, B.; Cheng, W. A Wearable and Highly Sensitive Pressure Sensor with Ultrathin Gold Nanowires, Nat. Commun. 2014, 5, Article number: Xiao, X.; Yuan, L.; Zhong, J.; Ding, T.; Liu, Y.; Cai, Z.; Rong, Y.; Han, H.; Zhou, J.; Wang, Z. L. High Strain Sensors Based on ZnO Nanowire/Polystyrene Hybridized Flexible Films, Adv. Mater. 2011, 23, Boland, C. S.; Khan, U.; Backes, C.; O Neill, A.; McCauley, J.; Duane, S.; Shanker, R.; Liu, Y.; Jurewicz, I.; Dalton, A. B. Sensitive, High-Strain, High-Rate Bodily Motion Sensors based on Graphene Rubber Composites, ACS Nano 2014, 8, Hwang, B.-U.; Lee, J.-H.; Trung, T. Q.; Roh, E.; Kim, D.-I.; Kim, S.-W.; Lee, N.-E. Transparent Stretchable Self-Powered Patchable Sensor Platform with Ultrasensitive Recognition of Human Activities, ACS Nano 2015, 9, Roh, E.; Hwang, B.-U.; Kim, D.; Kim, B.-Y.; Lee, N.-E. Stretchable, Transparent, Ultrasensitive, and Patchable Strain Sensor for Human Machine Interfaces Comprising a Nanohybrid of Carbon Nanotubes and Conductive Elastomers, ACS Nano 2015, 9, Gong, S.; Lai, D.; Wang, Y.; Yap, L. W.; Si, K. J.; Shi, Q.; Jason, N. N.; Sridhar, T.; Uddin, H.; Cheng, W. Tatoo-Like Polyaniline Microparticle-Doped Gold Nanowire Patches as Highly Durable Wearable Sensors, ACS Appl. Mater. Interfaces 2015, 7, Gong, S.; Lai, D. T.; Su, B.; Si, K. J.; Ma, Z.; Yap, L. W.; Guo, P.; Cheng, W. Highly Stretchy Black Gold E Skin Nanopatches as Highly Sensitive Wearable Biomedical Sensors, Adv. Electron. Mater. 2015, 1, Liu, Z.; Qi, D.; Guo, P.; Liu, Y.; Zhu, B.; Yang, H.; Liu, Y.; Li, B.; Zhang, C.; Yu, J. Thickness Gradient Films for High Gauge Factor Stretchable Strain Sensors, Adv. Mater. 2015, 27, S-19