Highly Sensitive Wearable Pressure Sensors Based on Three-Scale

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Highly Sensitive Wearable Pressure Sensors Based on ThreeScale Nested Wrinkling Microstructures of Polypyrrole Films Chengfeng Yang, Lele Li, Jingxin Zhao, Juanjuan Wang, Jixun Xie, Yanping Cao, Mianqi Xue, and Conghua Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08666 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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ACS Applied Materials & Interfaces

Highly Sensitive Wearable Pressure Sensors Based on Three-Scale Nested Wrinkling Microstructures of Polypyrrole Films

Chengfeng Yang,† Lele Li,† Jingxin Zhao,† Juanjuan Wang,† Jixun Xie,† Yanping Cao,‡ Mianqi Xue,§and Conghua Lu†*

† School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, P. R. China ‡ AML, Department of Engineering Mechanics, Tsinghua University, Beijing, 100084, P. R. China § Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China

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ABSTRACT Pressure sensors have a variety of applications including wearable devices and electronic skins. In order to satisfy the practical applications, pressure sensors with a high sensitivity, a low detection limit, and a low-cost preparation are extremely needed. Herein, we fabricate highly sensitive pressure sensors based on hierarchically patterned polypyrrole (PPy) films, which are composed of three-scale nested surface wrinkling microstructures through a simple process. Namely, double-scale nested wrinkles are generated via in-situ self-wrinkling during oxidative polymerization growth of PPy film on an elastic polydimethylsiloxane (PDMS) substrate in the mixed acidic solution. Subsequent heating/cooling processing induces the third surface wrinkling and thus the controlled formation of three-scale nested wrinkling microstructures.

The

multiscale

nested

microstructures

combined

with

stimulus-responsive characteristic and self-adaptive ability of wrinkling morphologies in PPy films offer the as-fabricated piezoresistive pressure sensors with a high sensitivity (19.32 kPa-1), a low detection limit (1 Pa), an ultrafast response (20 ms), and excellent durability and stability (more than 1000 circles), with these comprehensive sensing properties higher than the reported results in literature. Moreover, the pressure sensors have been successfully applied in the wearable electronic fields (e.g., pulse detection and voice recognition) and microcircuit controlling, as demonstrated here.

KEYWORDS: pressure sensor, polypyrrole film, two-scale wrinkle, three-scale wrinkle, human motion detection.

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1. Introduction In the recent years, artificial electronic skins (E-skins)1-6 have attracted abundant attention due to their potential applications in motion detection,7,8 medical diagnostics, 9,10

and robotic skins, 11 etc. As a fundamental and crucial part of the E-skin, pressure

sensors can convert the change of external force to an electrical signal or other recognized

signals.

Basically,

different

sensing

mechanisms

including

piezocapacitive,8,12-15 triboelectric,16,17 piezoelectric,18-21 and piezoresistive4,6,22-24 behaviors are involved in the as-reported pressure sensors, in which piezoresistive pressure sensors (PPS) have many intriguing advantages such as simple mechanism, fast response, low energy consumption, and easy signal detection (e.g., by probing a change in current or resistance during varying the pressure).2-6 The typical PPS is generally composed of a flexible conformable substrate (e.g., polydimethylsiloxane (PDMS) and polyurethane) and a compliant conductive film (e.g., Au metal nanowires,7

ZnO

nanowires

arrays,25

carbon

nanotubes,26 and

conductive

polymers8,27). In order to satisfy the practical applications, it is of great importance that pressure sensors possess an excellent durability, an ultrafast response time, a low detection limit, and a high sensitivity, especially in the low pressure region (e.g., < 0.5 kPa)5. It is widely accepted that introducing nonplanar microstructures into conductive films4,6,12 and/or flexible substrates5,10,13 could greatly improve the sensitivity of the pressure sensors, comparing to the corresponding planar counterparts. The presence of surface topographical microstructures in the conductive layers and/or substrates can provide more contact interface/area and current pathways along with the pressure change, which will have a positive effect on the significant enhancement of the sensing performances.28 Hereinto, the microstructured compliant substrate can be generated by the template replication (e.g., the templates from leaf,5 silk scarf,10 PDMS stamp,29 and silicon master30). The fabrication of microstructured active conductive layers can be realized by the direct deposition of conductive layer on microstructured supports such as electrospun fibers,4 tissue papers,3,6,7and fabrics.9 On the other hand, as a non-lithographic surface patterning technique, mechanical instability-driven surface wrinkling has attracted increasing interest owing to its 3



simplicity, versatility, large-scale production, and so on.31,32 Furthermore, owing to the surface stress-relaxation characteristics, smart stimulus-responsive systems based on surface wrinkling have been well developed, and reversible switching between the surface wrinkling and de-wrinkling states is easily achieved by the external stimuli such as mechanical straining,33,34 solvent treatment,35-37 and light processing.38,39 Recently, the reversible wrinkling morphology evolution induced by the mechanical force has been well utilized for the fabrication of PPS.40-42 It is concluded that the combined hierarchical microstructures from the as-wrinkled conductive films and the microstructured compliant substrate can greatly enhance the sensitivity of the pressure sensors, in comparison with the case of patterned microstructures only existing in the active films or substrates.40-42 In this work, we report highly sensitive pressure sensors based on three-scale nested wrinkling polypyrrole (PPy) films for the first time. The hierarchical microstructures fully come from the simple facile surface wrinkling without using any lithographic techniques and any patterned substrates. Combined with the stress-relief surface characteristics and self-adaptive ability of surface wrinkling morphologies, the three-scale nested wrinkling microstructures in the PPy films offer the as-fabricated pressure sensors with outstanding comprehensive sensing performances such as a high sensitivity (19.32 kPa-1), an ultrafast response and relaxation (20 ms and 30 ms, respectively), a low detect pressure limit (1 Pa), and a good stability and durability (more than 1000 cycles). These excellent sensing properties endow pressure sensors with great potentials in the applications for in-situ monitoring human body’s signals (e.g., pulse detection and voice recognition) and microcircuit controlling.

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2. Results and Discussion

Figure 1. a) Schematic illustration of the procedure to prepare three-scale nested wrinkling PPy films: in-situ double-scale (λ1/λ2) wrinkling microstructures formed during the growth of PPy films on a compliant PDMS substrate (I); heating-induced the third wrinkling to generate the three-scale (λ1/λ2/λ3) nested wrinkling microstructures (II). b) Model of the pressure sensors based on the face-to-face assembly of three-scale nested wrinkling PPy films.

2.1 Preparation of Three-Scale Nested Wrinkling PPy Films and Pressure Sensors Figure 1 schematically illustrates the two-step preparation of the conductive PPy films with three-scale nested wrinkling microstructures, followed by their face-to-face assembly into a pressure sensor. Firstly, the PPy film is deposited on a thick PDMS substrate (~ 700 µm) via a solution-phase oxidation polymerization of pyrrole monomer, accompanied by in-situ self-wrinkling and the generation of two-scale nested surface wrinkles43 with the wrinkle wavelength of λ1 and λ2, respectively (Figure 1a(I)). Subsequent heating/cooling processing leads to the third surface wrinkling with a larger wrinkle wavelength (i.e., λ3) and thus the formation of three-scale nested wrinkling microstructures (i.e., λ1/λ2/λ3) (Figure 1a (II)). Finally, the hierarchically wrinkled PPy films on the PDMS substrate were placed face to face to construct the ﬂexible pressure sensor (Figure 1b). Figure 2a-c show the as-formed double-scale nested wrinkles on the PPy films using the polymerization time tp of 7 h. The 1st wrinkling wavelength λ1 is about 0.54 5



µm, which is roughly unchanged with the increase of tp (inset of Figure 2b and Supporting Information: Figures S1 and S2). As for the 2nd wrinkling patterns, λ2 is dependent of tp (i.e., the deposited PPy film thickness h) (Figure S2, Supporting Information), which is easy to distinguish from the recorded optical microscopy images and 3D optical microscopy images shown in Figures S1 (Supporting Information). Namely λ2 increases with tp at the beginning state and then reaches a saturated value of ~ 4.24 µm after tp ≥ 7 h (Figure S2, Supporting Information). Meanwhile, the roughly similar relations of tp with the wrinkling amplitude (A) and the surface roughness (Rq) derived from the 3D optical images occur (Insets of Figure S1a1-a3 and Figures S2, Supporting Information). The dependences of λ and A on tp are basically consistent with our previous results.43 The formation of the double-scale nested wrinkles as well as the dependences of λ and A on tp (or film thickness h) is intimately related to the in-situ self-wrinkling process during polymerization growth of PPy films on the PDMS substrate.43 After the double-scale wrinkling PPy films are subjected to the 140 °C heating and subsequent air cooling, the 3rd surface wrinkling with a larger wavelength λ3 is induced (e.g., λ3= 14.77 µm for tp = 7 h) (Figure 2d-f). From the combined information from the recorded optical/SEM/AFM images shown in Figure 2d-f, it is interesting to see that the initial double-scale wrinkles are still reserved on the heating-processed PPy films. Namely, the in-situ self-wrinkling λ1/λ2 nested wrinkles can stand the external heating/cooling processing. The enhanced stability for the self-wrinkling λ1/λ2 nested patterns is attributed to the in-situ self-reinforcement effect existing in the in-situ self-wrinkling, which has been well investigated in our previous paper.43 As a result, the heating-induced λ3 wrinkles coupled with the retained double-scale λ1/λ2 wrinkles form the newly three-scale

λ1/λ2/λ3 nested wrinkling microstructures (Figure 2d-e). Compared with the initial double-scale λ1/λ2 nested wrinkles, the corresponding three-scale λ1/λ2/λ3 nested patterns have a larger A and Rq, which is clearly seen from the recorded AFM and 3D optical images shown in Figure 2c, f and Figures S3-S5 (Supporting Information). Furthermore, the recorded A and Rq of the three-scale nested wrinkles can be tuned by tp and the heating duration (Figures S3-S5, Supporting Information). 6


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Figure 2. Optical (a,d), SEM (b,e), and AFM (c,f) images of the microstructured PPy films with the two-scale (a-c) and three-scale (d-f) nested surface wrinkles using tp = 7 h. Insets in panels b and e show the corresponding magnified SEM images. Insets in panels c and f show the corresponding AFM cross-section profiles.

2.2 Performance and Application of Microstructured PPy Film-Based Pressure Sensors 2.2.1. Sensitivity of the Microstructured PPy Film-Based Pressure Sensors and Analysis of the Structure–Performance Dependence Given that the as-prepared nested wrinkling PPy films are grown on the thick PDMS elastic substrate, we can construct the well-defined pressure sensors by direct face-to-face assembly of two microstructured PPy films (Figure 1b). Namely no additional active layers and compliant substrates are needed. Figure 3a，b illustrate the piezoresistive sensing mechanism of the assembled pressure sensor based on the hierarchical microstructures of PPy films. In the pristine state, the top and bottom microstructured PPy films partially contact each other with a relatively small contact area to form the conductive paths. With an external minute pressure/force applied, the contact area between the two active microstructured PPy films will increase significantly owing to the synergetic effect from the hierarchically nested microstructures and the stimulus highly-responsive wrinkling morphologies with a 7



Figure 3. a,b) Schematic illustration of the as-proposed pressure sensing based on the microstructured PPy films with the two-scale (a) and three-scale (b) nested surface wrinkles, respectively. c) Sensitivity of the pressure sensor based on the two-scale wrinkling PPy films from different tp. d) Sensitivity of the pressure sensor based on the three-scale nested wrinkling PPy films from tp = 3 and 7 h, respectively. e) Change in the current of the pressure sensor of the three-scale nested wrinkling PPy film from tp of 7 h with the applied pressure ranging from 27 to 2036 Pa. f) Response signal under the pressure of 1 Pa.

good elastic deformation capability (Figure 3a,b). Simultaneously, a rapid huge change in current/resistance occurs, endowing the pressure sensor with an excellent sensitivity, especially in the low pressure region. Furthermore, compared to that of the

λ1/λ2 nested microstructures, the sensitivity can be enhanced remarkably for the pressure sensor based on the λ1/λ2/λ3 nested PPy film with more hierarchical 8


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microstructures (Figure 3a,b). This is supported by the comparison results shown in Figure 3c,d. Firstly, we investigate the pressure sensing based on the double-scale λ1/λ2 nested wrinkling PPy films (Figure 3c). As one of the most important parameters of pressure sensors, the sensitivity (S) is defined as S = (∆I/I0)/∆P. ∆I is the current change when a certain pressure P is applied on the sensor. I0 is the initial current without pressure applied. ∆P is the change of the applied pressure. As shown in Figure 3c, the plot of ∆I/I0 of the pressure sensor as the function of the pressure P is composed of more than one regions, which is similar to most of the reported pressure sensors.4-6,10,13 Furthermore, ∆I/I0 increases rapidly at first and then slowly as the applied P increases gradually. Namely, the sensitivity S of the pressure sensor from the double-scale λ1/λ2 nested wrinkling PPy films is higher at a lower P applied than at a larger P applied, which is also in accordance with most of the reported pressure sensors.10,28,44 Furthermore, the sensitivity in the low P region (denoted as S1) increases with tp at tp < 7 h and then reaches a saturated value of ~ 3.25 kPa-1 at tp ≥ 7 h (Figure 3c). This dependence of S1 on tp is basically similar to the dependence of λ2, A and Rq of the

λ1/λ2 nested PPy films on tp as shown in Figures S1,S2 (Supporting Information). Combined with the independence of the resistivity of the double-scale wrinkling PPy films on tP (Figure S6, supporting Information), it is believed that the pressure sensing property can be improved by the increase of the surface roughness of the active layers because of more active sites and more contact areas accessible during the pressure-induced deformation. Secondly, we investigate the case of the three-scale λ1/λ2/λ3 nested PPy films. As expected, the sensitivity S1 in the low pressure region is enhanced by 5 ~ 6 times after the three-scale λ1/λ2/λ3 nested wrinkling PPy films are employed (Figure 3d). For example, S1 = 1.5 and 7.27 kPa-1, when the double-scale λ1/λ2 and three-scale λ1/λ2/λ3 nested PPy films using tp of 3 h were applied, respectively. In the same case, S1 = 3.25 and 19.32 kPa-1, when the double-scale and three-scale nested PPy films from tp of 7 h were used, respectively. It is noted that during the heating/cooling processing, more than a twenty-fold increase in the resistivity of the as-prepared three-scale λ1/λ2/λ3 9



nested PPy films occurs, in comparison with that of the double-scale λ1/λ2 nested PPy films before the processing (Figures S5 and S6, Supporting Information). On the other hand, although the resistivity (i.e., ~23.8 Ω·m) of the three-scale nested PPy films from tp of 7 h is a bit larger than that (i.e., ~19.3 Ω·m) of the three-scale nested films from tp of 3 h, the sensitivity S1 from tp of 7 h is about 2.7 times bigger than that of using tp of 3 h (Figure 3d). Additionally, we also study the influence of the heating duration on the S1 of the as-formed λ1/λ2/λ3 nested PPy films. It is seen that S1 = 4.6, 19.32, and 21.8 kPa-1 when the heating duration at 140 °C is 0.5, 1, and 2 h (Figure 3d and S7, Supporting Information), respectively. Correspondingly, the resistivity of the as-formed λ1/λ2/λ3 nested PPy films is 2.3, 23.8, and 33 Ω·m, while the surface roughness Rq is 0.363, 0.429, and 0.432 µm (Figure S5, Supporting Information), respectively. Based on the above comparisons, we conclude that the significant enhancement in the sensitivity of pressure sensors should be mainly attributed to the heating-induced multiscale microstructures in the PPy films, rather than the heating-induced resistivity change of the PPy films. This conclusion is further validated by the control experiment, in which the PPy film is directly deposited on a microstructured PDMS stamp. The PDMS stamp is replicated from a wrinkled template with the wavelength λs3 of ~ 9.24 µm. Thus we obtain the three-scale

λ1/λ2/λs3 nested PPy films on the PDMS stamp, in which λs3 comes from the replicated substrate and λ1/λ2 come from the in-situ self-wrinkling (Figure S8a, Supporting Information). In this case, the two kinds of nested PPy films have the similar resistivity (Figure S9, Supporting Information). As expected, the sensitivity of the pressure sensor from the three-scale λ1/λ2/λs3 nested PPy films is higher than that of the double scale λ1/λ2 nested PPy films (Figure S8b, Supporting Information). In a word, the hierarchically λ1/λ2/λ3 nested microstructures on the active PPy films combined with the self-adaptive ability of the stimulus-responsive wrinkling patterns endow the as-formed pressure sensor with superior sensitivity, which are much higher than the reported results in literature (Table S1, Supporting Information). If not specified, the following pressure sensor is based on the PPy films with the three-scale

λ1/λ2/λ3 nested wrinkles from tp of 7 h. Figure 3e illustrates the relation of the current 10


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response of the pressure sensor with various P applied. It is seen that no matter the magnitude of the applied P and 5 cycles’ uploading/loading, our pressure sensors possess super sensing ability and good stability.

2.2.2. Detection of an Ultralow Pressure Our pressure sensors from the three-scale λ1/λ2/λ3 nested wrinkling PPy films can be used to detect a minute pressure due to their ultrahigh sensitivity. As shown in Figure 3f, when cycling loading/unloading a little foam on the pressure sensor (inset of Figure 3f), the elicited current response signal is obvious and can be repeatable with the cycling. It is noted that the pressure induced by the little foam is only 1 Pa. Furthermore, comparing to most of the reported pressure sensors (Table S1, supporting Information), the detection limit of our pressure sensors is quiet low, indicating our pressure sensors own an ultralow detection limit.

2.2.3. Reliability, Stability, and Fast Response of the λ1/λ2/λ3 Nested Wrinkling PPy Film-Based Pressure Sensors To characterize the reliability and stability of our λ1/λ2/λ3 nested wrinkling PPy film-based pressure sensors, repeatable loading/unloading of the pressure (P = 673 Pa) for 1000 cycles was applied on the devices (Figure 4a). When the pressure is loaded, the relative current variation of ∆I/I0 increases from 0 to 7.67, and then recovers to 0 after unloading the pressure. During the 1000 cycles of loading/unloading of the pressure, the response value of the pressure sensors is almost identical (Figure 4a). From the magnified waveforms extracted from Figure 4a at the beginning and ending stages, respectively, we further see that the repeatable and reliable response with no evident performance degradation occurring on the pressure sensors during the 1000 cycles (Figure 4b and c). Furthermore, when applying a minute pressure (e.g., 45 Pa), the response and relaxation time of the pressure sensor can reach to be ~ 20 and ~ 30 ms, respectively, indicating that the pressure sensors own the superfast sensing and no obvious hysteresis. 11



Figure 4. a) Performance of the pressure sensor with cycling loading-unloading of a 673 Pa pressure up to 1000 cycles. b,c) Magnified waveforms extracted from panel a at the starting and ending stages, respectively. d) Step response of the pressure sensor with the applied pressure of 45 Pa. e,f) Magnified sensor responses extracted from panel d to show the response and relaxation times.

2.2.4. Applications of the λ1/λ2/λ3 Nested Wrinkling PPy Film-Based Pressure Sensor Considering the high sensitivity, excellent response, and repeatability properties of the

λ1/λ2/λ3 nested wrinkling PPy film-based pressure sensors, a variety of applications are explored. For example, to detect the weak stress/pressure from the wrist pulse, the hierarchically microstructured PPy film-based pressure sensor is simply attached to the tester’s wrist (24 years old, 170 cm height, 72 kg weight) with a transparent semi-permeable dressing (Figure 5a). It is seen that the pressure sensor has a good response to the pulse beat. Before the exercise, the pulse rate is about 72 times/min (bottom line, Figure 5b), which is within the normal range (60 ~ 100 times/min). After the tester excises for 10 min, the pulse rate increases to approximately 106 times/min (top line, Figure 5b). This tested pulse rate conforms to the practical level. From the point of view of medicine, the pulse rate is one of the most important indicators of human body, which can reflect the health level, especially for some persons suffering 12


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from asthma, heart attack, or other diseases.4,6,7,10,17,21 Additionally, the typical 2/3-peak signal of wrist pulse is not observed in the current pressure sensor. It is believed that the shape of the wrist pulse signal is intimately related to the tester’ age, height, weight, and other factors,4,6,7,10,17,21 which maybe lead to the above difference between our case and the reported case in the references. Thus the real-time detection of health status and monitoring of some heart diseases are of great significance in the wearable medical health field, which can be achieved by our multiscale wrinkling PPy film-based pressure sensor.

Figure 5. a) Digital image of the pressure sensor attached to a wrist. b) Response curves of the pressure sensor for the wrist pulse before and after the exercise. c-f) Response curves of the pressure sensor attached to the wearer’s throat when saying the words of “PPy”, “Pressure”， “Sensor”，and“Tianjin University”, respectively. 13



The pressure sensor is also quite effective for detecting the vibration signal of vocal cords when the tester is speaking. As shown in the inset of Figure 5c, the pressure sensor is adhered to the tester’s neck by a transparent semi-permeable dressing. When the tester says different words like “PPy”, “Pressure”, “Sensor”, and “Tianjin University”, respectively, the PPy-based pressure sensor exhibits distinct signal patterns with a remarkable sensitivity (Figure 5c-f). It can be seen that different pronunciations and accents of the words correspond to different waveforms of signal curves. These results indicate that the pressure sensor based on the three-scale nested wrinkling PPy films can provide a facile and efficient technique for the voice recognition. Additionally, the pressure sensor can be connected into a microcircuit (Figure S10, Supporting Information). When a pressure is applied on the pressure sensor, the brightness of the bulb will increase enormously owing to the evident increase of the contact area and the current (Video 1, Supporting Information), implying that our pressure sensors hold a great promise in the field of the microcircuit controlling.

3. Conclusion In conclusion, we have developed a simple and efficient method to prepare PPy films with three-scale nested wrinkling microstructures and further fabricated highly sensitive pressure sensors. The three-scale nested microstructures combined with the stimulus-responsive characteristics and self-adaptive ability of wrinkling patterns in the patterned PPy films offer the pressure sensor with an ultrahigh sensitivity (19.32 kPa-1), a fast response time (20 ms) and relaxation time (30 ms), a great durability over 1000 cycles, and an ultralow detection limit (1 Pa). Owing to the above prominent comprehensive sensing properties, the three-scale nested wrinkling PPy film-based pressure sensors have been successfully applied in the fields of the pulse detection, voice recognition, and microcircuit controlling. More applications related to the smart wearable devices need to be explored in the near future. Furthermore, the proposed strategy by the introduction of multiscale wrinkling microstructures, which is simple, versatile, low-cost, and very suitable for the large-scale production, is 14


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highly expected to design/fabricate other smart wearable devices with unprecedented performances and applications.

4. Experimental section Preparation of the PDMS Substrate: A polydimethylsiloxane substrate was prepared by mixing the base/curing agent (Sylgard 184, Dow Corning) at a designed weight ratio of 10:1. After being degassed for 30 min, the mixed base/curing agent was poured into a culture dish and baked at 70 °C for 4 h to obtain the elastic PDMS substrate. Preparation of Two-Scale Wrinkled and Three-Scale Nested Wrinkling PPy Films: To fabricate PPy films with two-scale wrinkled structures, a chemical oxidation polymerization of pyrrole on the PDMS substrate was employed in the mixed solution. Typically, under continuous stirring at room temperature, the pyrrole monomer was added dropwise into HCl solution to obtain a mixed solution composed of 0.1 M pyrrole and 1 M HCl (simply referred to A). Similarly, the mixture solution composed of 0.2 M FeCl3 and 1 M HCl was also prepared (simply referred to B). Subsequently, the PDMS substrate was immersed into solution A. Then solutions A and B were kept at 2-5 °C for 10 min. Finally, solution B was poured into solution A to obtain a newly thoroughly mixed solution. After a designed polymerization duration (tp) at 2-5 °C, the PPy film-grown PDMS substrate was taken out, followed by water washing and air drying. To prepare the three-scale wrinkling patterns, the above obtained PPy/PDMS system was heated at 140 °C for 1 h, followed by air cooling to room temperature. Fabrication of PPy Pressure Sensor: First, copper wires were fixed on the nested wrinkling PPy films with a silver paste. It was worth noting that the silver paste in the each PPy film should not touch the conductive part of the other film. Then, two layers of the microstructured PPy films on the compliant PDMS substrates were placed face to face each other with the face area of 1.5 × 1.5 cm2. At last, the pressure sensors were encapsulated with a Scotch 15



tape. Characterization and Performance Testing: The morphologies and microstructures of PPy films were characterized by an inverted Observer AI microscope (Zeiss, Germany), atomic force microscope (Agilent 5500 AFM/SPM) in tapping mode, a three-dimensional (3D) microscope (LEXT OLS4100, Olympus), a scanning electron microscope (Hitachi S-4800) equipped with different detectors and imaging models. The resistivity of PPy films was tested by a 4-Point Probes Resistivity Measurement System (RTS-9, Guangzhou Four-Probe Technology). The I-t characteristics of pressure sensors were collected by an electrochemical workstation (LK2005A) which was connected to a computer at an operation voltage of 1 V. To evaluate the response and relaxation time of the sensor, the pressure of 45 Pa was applied on the pressure sensor. The response to the applied pressure was performed by gently loading different weights of objects onto the pressure sensor, corresponding to different pressures. A digital source meter (Keithley 2400) was used to characterize the electrical properties and to record the electrical response of the pressure sensor.

ASSOCIATED CONTENT Supporting Information Optical, 3D optical, and AFM images of double-scale nested wrinkling films from different polymerization time tp (Figure S1); Dependence of the first/second wrinkling wavelength and the Rq of the double-scale wrinkling films on tp (Figure S2); Optical and 3D optical images of the three-scale nested wrinkling films of tp = 7 h after 140 °C heating for different duration (Figure S3); Optical and 3D optical images of three-scale nested wrinkling films from tp = 3 h after 140 °C heating for 1 h (Figure S4); Dependences of Rq and resistivity of the three-scale nested wrinkling films on the heating t (Figure S5); Dependence of the resistivity of the double-scale nested wrinkling films on tp (Figure S6); Sensitivity of the pressure sensor based on the three-scale nested films using the heating duration of 0.5 and 2 h (Figure S7); 16


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Optical and sensitivity of the pressure sensor from the three-scale λ1/λ2/λs3 nested films (Figure S8); Resistivity of the λ1/λ2 nested wrinkling films deposited on the smooth PDMS substrate and the λ1/λ2/λs3 nested films wrinkling films deposited on the replicated PDMS stamp (Figure S9); Circuits based on the pressure sensor composed of the three-scale nested wrinkling films (Figure S10); Summary of the performances of pressure sensors reported in the literature (Table S1); In-situ recording the change in the light brightness of the LED bulb in the circuit during loading/unloading applied on the pressure sensor (Video 1). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C.L.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest

Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 21374076, 21574099, 11572179, 21622407).

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Highly Sensitive Wearable Pressure Sensors Based on Three-Scale

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