Hierarchical Reduced Graphene Oxide Ridges for ... - ACS Publications

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Hierarchical Reduced Graphene Oxide Ridges for Stretchable, Wearable, and Washable Strain Sensors Jia Song, Yinlong Tan, Zengyong Chu,* Min Xiao, Gongyi Li, Zhenhua Jiang, Jing Wang, and Tianjiao Hu College of Liberal Arts and Sciences, National University of Defense Technology, Changsha 410073, P. R. China

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S Supporting Information *

ABSTRACT: Recently, flexible and wearable devices are increasingly in demand and graphene has been widely used due to its exceptional chemical, mechanical and electrical properties. Building complex buckling patterns of graphene is an essential strategy to increase its flexible and stretchable properties. Herein, a facile dimensionally controlled four-dimensional (4D) shrinking method was proposed to generate hierarchical reduced graphene oxide (rGO) buckling patterns on curved substrates mimicking different parts of the uniforms. The reduced graphene oxide ridges (rGORs) generated on the spherical substrate seem isotropic, while those generated on the cylindrical substrate are obviously more hierarchical or oriented, especially when the cylindrical substrate are shrinking via two steps. The oriented rGORs are superhydrophobic and strain sensitive but obviously anisotropic along the axial and circumferential directions. The sensitivity of rGORs along the axial direction is much higher than those along the circumferential direction. In addition, the intrinsic solvent barrier property of graphene enables the crack-free rGORs an excellent chemical protective performance, withstanding DCM immersion for more than 2.5 h. The flexible rGORs-based strain sensors can be used to detect both large and subtle human motions and activities by achieving high sensitivity (maximum gauge factor up to 48), high unidirectional stretchability (300−530%), and ultrahigh areal stretchability (up to 2690%). Excellent durability was also demonstrated for human motion monitoring with resistance to hand rubbing, ultrasonic cleaning, machine washing, and chemical immersion. KEYWORDS: reduced graphene oxide, ridges, 4D-shrinking, strain sensors, chemical protection

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buckling patterns of graphene so as to further increase its flexible and stretchable properties.26 Wang et al.27 deposited GO solution on a prestretched elastomeric substrate to fabricate wrinkled multilayer graphene surfaces, which can be used in cell and tissue engineering. To obtain hierarchical architectures, Chen et al.28 used a multiple sequential approach to generate GO crumples, which enables the design of feature sizes and orientations across multiple length scales. This texturing method can be extended to other 2D materials and has potential applications in stretchable electronics, actuators and energy storage.1−5,14−16,29−31 The highly textured graphene films can be used as templates as well to prepare textured metal oxides by ion intercalating.32 However, the method suffers from the multistep operations or multitransfers of the films. Recently, Tan et al.33 in our group demonstrated a one-step shrinking method to fabricate brain cortex-like buckling patterns with high gyrification index on a spherical balloon substrate. The highly folded hydrophobic GO surface can be used as a bilayer actuator with excellent reversible, bidirectional, large curling properties. More recently, Chen et al.34 showed that the textured graphene-based coatings can serve

INTRODUCTION

Flexible and wearable devices are becoming increasingly important for us these days, such as the pressure/strain sensors,1−5 artificial electronic skins,6,7 biomedical monitors,8,9 and motion detections.10,11 Imagine that if the leak of toxic chemicals happens, the firemen need to wear chemicalprotective uniforms. It will be much better if the uniform is incorporated with strain sensors and health monitors, so that we can timely track the health state of the firemen from a distance. The uniform also needs to be wearable and reusable. Up to now, however, there are still many challenges in fabricating highperformance flexible and wearable devices. Graphene, a two-dimensional (2D) material, exhibits exceptional chemical, mechanical and electrical properties suitable for wearable electronics.12−16 The ideal monolayer graphene is known to be impermeable to all gases and liquids.17,18̅Graphene oxide (GO) and reduced graphene oxide (rGO) are also promising for chemical barriers.19,20 In most cases, graphene is free-standing in a naturally wrinkling state.21 In fact, wrinkling or buckling is very common in nature,22,23 for example, crumples on the tree epidermis are induced from the growth strain of trees. The skin crumples at the joints enable the bones move freely, and the unique fingerprint is often used in identification.24,25 Recently, researchers have tried many ways to build complex © XXXX American Chemical Society

Received: October 17, 2018 Accepted: December 10, 2018 Published: December 10, 2018 A

DOI: 10.1021/acsami.8b18143 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustrations (A, B) and optical recording images (C−E) of the 4D-shrinking fabrication processes for rGORs-S, rGORs-N, and rGORs-C.

as ultrastretchable molecular barriers for chemical protection and detection. Strain sensors are the materials or devices that can transfer mechanical deformation into electrical signals, which have been widely applied in wearable electronics and health care devices.1−11,35,36 Compared with other kinds of strain sensors, resistive strain sensors has been investigated intensively because of the simple design, low driving power and excellent mechanical tunability.37,38 Most previous researches have focused on how to improve the stretchability and sensitivity of the resistive strain sensors and the most common approach is the proper distribution of conductive fillers (silver nanowires, carbon nanotubes, graphene, etc.) within an elastic substrate.1−5,14−16,37−41 When some external mechanical stimulation is applied, the internal conductive network becomes disconnected and results in the resistance change. Additional functionalities, such as transparent, tunable, multidimensional, and multisensing, are also encouraged for practical applications.7−11,16 However, in many cases these added functionalities have made large stretchability, high sensitivity and wide detection range difficult to achieve. In this work, we prepared hierarchical rGO ridges (rGORs) with well reversible sensitivities and stretchable protections through a facile dimensionally controlled four-dimensional (4D) shrinkage method. GO suspension was coated on the surface of an inflated latex balloon, and a highly buckling rGO film with multiscale ridges were obtained after a controlled deflation and reduction. The morphology of the hierarchical rGORs can be tuned by alteration of the shapes and the shrinking steps of the latex substrate. The ridges contact with each other and these contact points act as numerous switches which can be switched on/off reversibly with external force. The hierarchical rGORs exhibit high sensitivity (maximum gauge factor up to 48), high unidirectional stretchability (300−530%)

and ultrahigh areal stretchability (up to 2690%). Excellent durability was confirmed for human motion monitoring with hand rubbing, ultrasonic cleaning, machine washing and chemical immersion. It is promising for multifunctional uniforms with high sensitive, self-cleaning, chemical protective and wear resistive properties.

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RESULTS AND DISCUSSION As shown in Figure 1, suitable latex balloons with different shapes were selected and prestretched through air inflation. The detailed fabrication procedure is illustrated in Figure S1. The prestretched balloon was coated with GO suspension by a rotation-dip method, dried at room temperature, and released through deflation. The mismatched strain between the substrate and the GO coating induced hierarchical GO ridges (GORs). The GORs was in situ reduced to rGORs in the vapor of hydrazine. The thickness of the rGO film is mainly influenced by the GO concentration. As shown in Figure S2, when the GO concentration changes from 1.0 to 4.0 mg/mL, the thickness of rGO increases from ∼60 to ∼600 nm. In this work, we just rinsed the latex glove with ethanol to make the surface clean. No additional surface chemistry is needed between GO films and the latex glove. The success is based on the roughness of the latex substrate which provides strong interactions with GO and also makes contributions to the formation of the first-generation wrinkles (Figure S3). The rate of deflation also plays an important role. Rapid, explosive, and instantaneous shrinkage will lead to the irregular stacking of rGO sheets (Figure S4). So the prestretched balloon is deflated slowly so as to form the target buckling patterns, typical in a rate of ∼30 mL/s controlled using a flow meter. Figure 1A exhibits the isotropic, one-step shrinkage approach of a spherical balloon, and Figure 1B shows the anisotropic shrinkage approaches of a cylindrical balloon. Both balloons B


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Figure 2. SEM images of various rGORs with the variation of coating thickness: (A) rGORs-S, (B) rGORs-N, and (C) rGORs-C.

through the controlled shrinkage (rGORs-C) is much stronger than those through the natural shrinkage (rGORs-N). The hierarchical and oriented ridges based on the cylindrical balloon are mainly due to the anisotropic shrinking of the anisotropicshaped cylindrical substrate. For ease of description, we define the ridges with smaller feature sizes as the first generation ridges (GR1) and the ridges with larger feature sizes as the second generation ridges (GR2). GR1 and GR2 of rGORs-C are oriented along the axial and circumferential direction respectively, while the orientation of rGORs-N is much weaker than that of rGORs-C. The morphologies of rGORs can be tuned by varying the GO concentration, that is, the thickness of rGO. As shown in Figure 2, the widths of GR1 and GR2 increase with the increase of GO concentration. For example, the widths of GR1 and GR2 of rGORs-C increase from 279 nm and 6.4 μm to 651 nm and 21.9 μm, respectively when the GO concentration increases from 1.0 to 3.0 mg/mL. It is obvious that the widths of GR2 are much larger than that of GR1. This is because GR1 was formed at the initial stage, and GR2 was further induced on GR1 when the mismatched strain increased beyond a critical value. To explore the formation mechanism of hierarchical and oriented rGORs, the deflating and shrinking process of the cylindrical balloon was digitally recorded, as shown in Figure

were used to mimic different parts of the gloves. In addition, two more submethods were employed in Figure 1B, namely, the natural shrinkage in one step and the controlled shrinkage in two steps. When the rGORs was prepared through the controlled shrinkage method, the substrate shrinks along the circumferential direction first, then along the axial direction second. The rGORs based on the spherical substrate was denoted as rGORs-S, and those based on the cylindrical substrate were denoted as rGORs-N and rGORs-C, for the natural and controlled methods, respectively. Figure 1C−E is the optical digital images of the fabrication procedures of rGORs-S, rGORsN and rGORs-C respectively. Here, radius length, axial length, and time are all involved in controlling the inflation and deflation processes, so we name it the 4D-shrinking fabrication process. The ridges formed using different substrate shapes and different shrinking steps will have different topographies, as typically shown in Figure 2. Figure 2A−C exhibits the morphologies of rGORs-S, rGORs-N, and rGORs-C respectively. All the topographies consist of hierarchical microstructures. rGORs-S seems isotropic as a whole, while for the rGORs based on the cylindrical substrate, the morphologies are obviously more hierarchical and oriented. rGORs-N exhibits an oriented mesh morphology, while rGORs-C exhibits a quasiparallel microstructure. The orientation of the ridges fabricated C


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Figure 3. Contact angles and solvent barrier properties of GORs and rGORs. (A) The contact angles of GO paper, rGO paper, GORs-S, and rGORs-S with different GO concentrations. (B) The contact angles of GORs-N, GORs-C, rGORs-N, and rGORs-C with different GO concentrations. (C) The contact angles of rGORs-S, rGORs-N and rGORs-C along axial and circumferential directions. (D) The contact time with DCM at the failure of chemical protection.

1D−E, Movies S1 and S2, and Figure S5. When naturally deflated, the cylindrical balloon shrinks gradually from one side to the other side, which is much different from the spherical balloon. The tail of the cylindrical substrate first shrinks into a hemisphere and then continues to shrink into a smaller cylinder. Different from the simultaneous, isotropic shrinkage of the spherical substrate, the shrinkage of the cylindrical substrate is nonsimultaneous, and anisotropic. We define the axial displacement as Δl and the circumferential displacement as Δr. Δl increases linearly with time, while Δr increases sharply at a certain strain. The sharp increase in Δr is mainly induced by the sharp decrease of the volume of the balloon. This is because the volume of an inflated balloon is related to the inside air pressure, which equals to the outside air pressure plus the pressure exerted by the latex itself.42 At some critical state, the latex exerted pressure may have a sharp change, leading to sharp change of the balloon volume, which plays an important role in the formation of larger ridges, GR2. Other than the above-mentioned nonsimultaneous shrinkage, different fabrication strains in the axial and circumferential directions (i.e., nonproportional shrinking) also play a great role in the formation of hierarchical and oriented microstructures. Typically, as shown in Tables S1 and S2, the diameter and length of the original cylindrical balloon are 0.8 and 7.0 cm respectively, and increase to 5.0 and 30.1 cm respectively when the cylindrical balloon are fully inflated. Thus, the fabrication strains are about 530% in the circumferential direction and 330% in the axial direction (the areal stretchability is up to 2690%). Higher fabrication strain in the circumferential direction makes the width of ridges smaller and the buckling denser. That is the

smaller ridges, GR1. For rGORs-N, GR1 and GR2 are generated interlaced and almost synchronized, while for rGORs-C, the formation of GR1 and GR2 are completely separated. GR2 begin to be generated when the formation of GR1 is finished. So that is why both rGORs-N and rGORs-C are hierarchical but rGORsC is much more oriented. In this work, GORs was in situ reduced to rGORs in the vapor of hydrazine at ∼100 °C. Figure S6A and B shows the IR spectra and Raman spectra of rGORs with different reduction times. The reduction degree and the electrical conductance (Figure S6C) increases with the increase of reduction time, and when the reduction time increases up to 15 h, GORs can be mostly reduced. The reduction degree and the wrinkling degree will greatly influence the surface chemistry of GORs. So the contact angles of GORs and rGORs were performed and are shown in Figure 3A−C. The contact angles of both GO and rGO paper with zero fabrication strain are also provided in Figure 3A as references. The contact angles GO paper are in the range of 30−60°, while those of the rGO paper increase to 110−120°, changing from hydrophilic to hydrophobic due to the elimination of oxygencontaining groups, such as −COOH and −OH. When ridges are generated, the contact angles of GORs-N and GORs-C are slightly higher than those of the GO paper, but the contact angles of isotropic GORs-S (90−120°) are much higher. Moreover, all the contact angles of rGORs are much higher than that of GORs. All the contact angles of rGORs are larger than 120° (hydrophobic), some is larger than 150° (superhydrophobic). In addition, the contact angles also increase with increase of the GO concentrations. That is, the higher the GO D


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Figure 4. Electrical properties of rGORs. (A) The resistances of rGORs-N and rGORs-C with different GO concentrations. (B) The relative resistance change as a function of strain for rGORs-Ca. (C) The length-normalized resistance as a function of strain for rGORs-N and rGORs-C. The relative resistance changes of (D) rGORs-Sa, (E) rGORs-Na, (F) rGORs-Ca, (G) rGORs-Sc, (H) rGORs-Nc and (I) rGORs-Cc. (J) The gauge factors of rGORs-Sa and rGORs-Sc with different GO concentrations. (K) The gauge factors of rGORs-Na, rGORs-Nc, rGORs-Ca, and rGORs-Cc with different GO concentrations. (L) The comparison of gauge factors and strain ranges between our work and other reports.3,4,13,43−50

Since monolayer graphene is impermeable to gases and liquids, the film of piled small sized rGO is also a good candidate for molecular or solvent barrier.47 The solvent barrier properties of rGORs are shown in Figure 3D, and Supporting Information, Figure S8. A common industrial extraction solvent, dichloromethane (DCM), was selected as the target solvent and the contact time before the balloon exploded or deflated was recorded to characterize the chemical protection property. The longer the contact time, the better the barrier property. The bare balloon deflated gradually because of swelling and exploded after ∼1.5 min, but after coated with GO and GORs, the balloon can withstand DCM for ∼9 and ∼40 min, respectively. Surprisingly, if coated with rGORs, it can withstand DCM for more than 150 min. These results indicate that the flexible film of rGORs have excellent barrier properties, competitive to the commercial firemen uniforms or chemical protective clothing.

concentration, the higher the thickness and the widths of the ridges, which will lead to an increase in the surface roughness and the contact angle. However, since some rGORs are generated on the anisotropic shrinking substrates, it triggers us to observe if the contact angles are anisotropic or not. As shown in Figure S7, the axial direction and circumferential direction are denoted accordingly on the spherical and cylindrical substrates, so as to distinguish the anisotropic property of the patterns, as well as the poststretching directions (“a” for axial and “c” for circumferential). As shown in Figure 3C, the contact angles are different on the cylindrical substrate along different directions, higher along the axial direction, indicating that the hydrophobicity of rGORs-N or rGORs-C is anisotropic. This is because the ridges are denser along the axial direction. While for rGORs-S, the contact angles along the two directions remain almost the same, so its hydrophobicity is isotropic. E


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Figure 5. Cyclic repeatability and hysteresis loops of rGORs-Na sensor. 1000-times cyclic resistance changes in the strain ranges of (A) 0−50% and (B) 0−300%. Hysteresis loops of stretching/releasing in the strain ranges of (C) 0−50%, (D) 0−100%, (E) 0−200%, and (F) 0−300%.

electrical properties of rGORs on the cylindrical substrate are also anisotropic. Figure 4B exhibits strain dependence of the relative resistance change (ΔR/R0) of rGORs-Ca. Here, R0 is the resistance of rGORs-Ca at zero strain. R is the resistance under different tensile strains. The curve can be mainly divided into three intervals. The resistance experienced a rapid increase at first (interval I), followed by a gradual increase (interval II), and finally a sharp increase again (interval III). At interval I, the contact points between rGORs depart with each other gradually and the resistance increase rapidly; at interval II, the resistance changes gently because the separation of contact points has reached a certain threshold; at Interval III, the sharp increase of resistance is probably related to the deep organization or partial fracture of the rGO film. Figure 4C shows the length-normalized resistance as a function of strain for rGORs-N and rGORs-C along the axial

The electrical resistances of the rGORs are shown in Figure 4, which were performed according to the device shown in Figure S9. The sample was cut into a rectangle of 1 cm × 2 cm and fixed on two pieces of glasses with glue. Two copper electrodes were connected on the sample with conductive silver paste. The electrical resistances of rGORs-N and rGORs-C prepared using different GO concentrations were studied and are shown in Figure 4A. The resistances of all rGORs decrease with the increase of GO concentration. However, there are also some differences among the rGORs. The resistance of rGORs-N is smaller than rGORs-C due to its higher irregularity. Compared with the ordered structure of rGORs-C, the mesh microstructure of rGORs-N provides more contact points between the ridges and so results in a relatively smaller resistance. In addition, the resistances along the circumferential direction (rGORs-Cc, rGORs-Nc) are significantly larger than the resistances along the axial direction (rGORs-Ca, rGORs-Na). That is to say, the F


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Figure 6. SEM images of various rGORs at the applied strains of 0%, 50%, 100% and 300% corresponding to three different intervals. (A) rGORs-Sa, (B) rGORs-Sc, (C) rGORs-Na, (D) rGORs-Nc, (E) rGORs-Ca, and (F) rGORs-Cc.

rGORs-N and rGORs-C. In addition, if we compare the gauge factors and the strain ranges of our work with other reports, as shown in Figure 4L, our strain sensor located at a much competitive position, with the maximum gauge factor up to 48 and the maximum sensing range up to 300%. Figure 5A shows the stability of this kind of device up to 50% strain and the resistance remains steady even after 1000 cycles of stretching and releasing processes. Figure 5B exhibits the stability of the strain sensor up to 300% strain. The resistance of the strain sensor at zero strain only have a slight increase after 1000 times cycling. Figure 5C−F illustrate the hysteresis hoops of rGORs-Na strain sensor. As shown in Figure 5C, there is no hysteresis in the stretching/releasing cycle for the maximum strain εmax = 50%. With increase of the maximum applied strain (Figure 5D−F), the hysteresis is becoming more obvious due to the considerable stress relaxation of the latex substrate. However, even in this case, the initial resistance of the sensor is fully recovered after releasing to zero strain. To some extent, the fabrication strain will influence the stretching behaviors as well, as shown in the Supporting Information, Figure S10. The fabrication strain generally affects the length of the second stage, the higher the fabrication strain, the longer of the second stage. When the applied strain is larger than the fabrication strain, the resistance will increase sharply because of the partial fractures of rGORs. To deeply understand the organization of the ridges under extensions, SEM images of various rGORs under different tensile strains are shown in Figure 6. The morphologies of stretched rGORs-S are similar (Figure 6A and B), while the morphologies of the stretched rGORs-N and rGORs-C are

and circumferential directions. The length-normalized resistance (R/Lmax = ρ/S) is an indicator showing the stability of resistivity (ρ) under per unit cross-sectional area (S). It has a slight change with the variation of strain, similar to the curve in Figure 4B. They increase relatively higher along the circumferential direction (rGORs-Cc, rGORs-Nc) than those along the axial direction (rGORs-Ca, rGORs-Na). Figure 4D−I exhibits the relative resistance change (ΔR/R0) curves of rGORs-Sa, rGORs-Na, rGORs-Ca, rGORs-Sc, rGORs-Nc, and rGORs-Cc, respectively. The relative resistance changes of rGORs-N and rGORs-C along the axial direction (Figure 4E, 4H) are much higher than those along the circumferential direction (Figure 4F and I), while those of rGORs-S are similar along the two directions (Figure 4D and G), further evidencing their anisotropic and isotropic properties. Since the relative resistance changes of rGORs increase with the applied tensile strain, they can be used as strain gauges. Figure 4J−K shows the gauge factors of rGORs-Sa, rGORs-Sc, rGORs-Na, rGORs-Nc, rGORs-Ca, and rGORs-Cc with the variation of different GO concentrations. The gauge factor (GF) of a strain sensor is the ratio of relative change in electrical resistance, ΔR/R0, to the mechanical strain, ε. As the coating thickness increases, the gauge factors of rGORs increase as well. The reason is that the thicker the coating, the wider the ridges. Therefore, when the samples are stretched to the same length, the sensitivity of the thicker ridges will be larger due to less contact points. At the same time, due to the smaller number of ridges, thicker rGORs will be faster to reach the flat period ( interval II). The gauge factor is also much higher along the axial direction than that along the circumferential direction, for G


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Figure 7. Electromechanical properties of the strain sensors and wearable applications integrated with the strain sensor. (A) The frequency tests versus applied strain from 5% to 15%. (B) The multiple-cycle tests of the relative resistance change with different applied strain. (C, D) The relative resistance change of the strain sensor at different static strain states. (E, F) The corresponding signals of the bending fingers with different bending angles. Insets: Photographs of bending fingers. (G) The relative resistance change of five connected strain sensors on five fingers used to monitor the gesture. Insets: photographs of gestures. (H) The relative resistance change caused by swallowing or speaking. Inset: optical image showing the strain sensor attached on the laryngeal prominence. (I) The relative resistance change caused by the blood pulse. Inset: Optical image showing the strain sensor attached on the finger.

stretched in the intervals I and II, the ridges deform into oriented patterns but few cracks can be observed, which enables the excellent chemical defensive properties of the rGORs. However, when the applied strain goes into interval III, as indicated from the SEM images under the strain of 300%, the ridges are so extended that cracks might occur. A simple resistor model proposed for the conductance network of rGORs is shown in Figure S11. The current pathway between the ridges and conduction pathway in the rGO film acts as two resistors in parallel. According to the parallel circuit, the relationship between Rfilm, Rcontact, and total resistance R can be described by eq 1

much different on stretching in the axial and circumferential directions. For example, the mesh microstructure of rGORs-N becomes obviously orientated on stretching along the axial direction (rGORs-Na, Figure 6C), however, the orientation on stretching in the circumferential direction (rGORs-Nc, Figure 6D) is not so obvious. The different orientation degrees of stretched rGORs-N along the axial and circumferential directions result in the different resistance change, which will further lead to the difference in the sensitivities. That is why both the relative resistance change and gauge factor are much higher along axial direction than those along the circumferential directions. Because of the highly oriented patterns of rGORs-C, on stretching with a strain from 0% to 100% in the axial direction (rGORs-Ca, Figure 6E), the orientation remains stable but the width of GR2 is changing from 15.8 to 24.4 μm; on stretching in the axial direction (rGORs-Cc, Figure 6F), however, the orientation also remains stable, just with the GR2 ridges extending like springs. Consequently, the differences in the relative resistance change and gauge factor of rGORs-Ca and rGORs-Cc are relatively weak, even though their orientation is the most significant among the three types of rGORs. During

1/R = 1/R film + 1/R contact(1/R contact ∝ Acontact )

(1)

where A is the average inter-ridges contact area in the direction perpendicular to the stretching direction. In the stretching process up to 50% strain (interval I), the ridges reorganize and separate gradually and the contact area between the ridges decreases, which leads to a sharply increased Rcontact. Then there is a slow change stage (interval II) because of the slight change of contact points and contact area. In the stretching process H


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hydrophobic surface on the rigid substrate is easily destroyed under friction, while the hydrophobic microstructures on the elastic substrate are exceptional.47 The glove coated with rGORs was challenged with different mechanical treatments, including hand rubbing, ultrasonic cleaning and machine washing. Because GO was in situ reduced on the rough surface of the latex substrate, the interaction between GO and substrate is so strong that debonding or detachment can be avoided during mechanical treatment. As shown in Figure S12C−F, the morphologies of the glove remain stable, indicating the strain sensor is a very good wear-resistant and chemical-resistant candidate. Overall, these sensors have very good stretchable, wearable and washable properties.

beyond 250% strain (Interval III), the ridges come to the deep organization or partial fracture state, which leads to a sharply increased Rfilm. Eq 1 can be used to explain why the resistances along the circumferential direction (rGORs-Cc, rGORs-Nc) are significantly larger than the resistances along the axial direction (rGORs-Ca, rGORs-Na). Since the fabrication strain along the circumferential direction is much larger than that along the axial direction, there is a much larger film resistance (Rfilm = ρL/S) in this direction. We also carried out the electromechanical measurements to observe the response and recovery behaviors of the strain sensors under dynamic and static strains. The frequencydependence of the strain sensor is one of the major characteristics when utilized in the sensor fields. So the responses to different stretching frequencies were studied and the relative resistance changes of the strain sensor are shown in Figure 7A. Notably, the intensity and the signal shape of each response are identifiable with no obvious change at the same stretching frequency. Figure 7B shows that the variation of the relative resistance change synchronizes with the change of applied strain when the rGORs-based strain sensor suffers from a wide range of tensile strain during loading and unloading processes. During the loading process, the resistance significantly increased according to the decrease of the contact points. With the increase of applied strain from 10% to 50%, the relative resistance changes from ∼74% to ∼207%. Besides, the static state of electromechanical properties of the strain sensors was also investigated by stretching the strain sensor, holding it for about 10 s, and then releasing it. As illustrated in Figure 7C, the responses are reproducible under multiple stretching-releasing processes. It can be found in Figure 7D that the response is stable at the same strain state but varies with each other at different applied strains. The larger the applied strain, the higher the electrical response. This rGORs was first demonstrated as a sensor to detect finger movements. Gesture recognition sensor can convert different gestures into electrical signals and provide information about finger bending direction and bending angle. The rGORs strain sensors have high sensitivity, large strain range and excellent stability which meets well with the requirements for wearable electronic devices. As shown in Figure 7E, the strain sensor is fixed on the finger joint to recognize the cyclic bending action of the finger. In addition, the strain sensor can also be used to monitor the bending degree of the finger in real time (Figure 7F) and the resistance increases when the bending angle is larger. A gesture recognition sensor was designed on five fingers (Figure 7G), in which five identical strain sensors are connected in series. It can recognize various gestures including “Ok”, “Victory”, and “Fist” and convert it into different current signals. Subtle strain recognition is particularly important for the human health monitoring. Through the detection of swallowing, vocalization, and blood pulse, human health characteristics can be better obtained. As shown in Figure 7H, the rGORs-based strain sensor is directly attached to the skin on the vocal cord and the speech or swallowing actions will directly cause the skin to stretch, which can be detected and recorded. The sensitivity is also high enough to record blood pulse as a pulse monitor (Figure 7I). To evaluate the wearable and washable properties, a rGORsfully coated glove (Figure S12A) was prepared from a GO-fully coated inflated glove followed with deflation and reduction. The redeflated rGORs-coated glove can withstand DCM immersion for over 2.5 h without any leakage (Figure S12B). In general, the

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CONCLUSION

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METHODS

In this work, a facile dimensionally controlled 4D shrinking method was applied to fabricate highly buckling patterns using 2D GO as the building blocks, which makes it possible to generate hierarchical rGORs on the 3D curved substrates, either isotropic on spherical substrates or oriented on cylindrical substrates. The rGORs exhibits excellent superhydrophobic, solvent barrier and wear-resistant properties, potential for flexible, wearable, and chemical protection applications. Besides, the ridges contact with each other and the contact points between ridges act as numerous switches which can be tuned reversibly by the external strain. These distinct features enable the crack-free, hierarchical rGORs to be a good candidate for strain sensors for wearable electronics, gesture detectors, health monitors, and chemical protections, which is a good choice for firemen uniforms or chemical protecting clothing.

Fabrication of GORs and rGORs. Spherical and cylindrical latex balloons were purchased from Xiongxian Pengshuai Latex Products Co., Ltd., and rinsed with ethanol before using. GO suspensions with various concentrations were prepared by a modified Hummer’s method, purified, and characterized as described previously.33 The GO suspension was coated on the inflated balloon substrate through a rotation-dip coating method (Figure S1). The prestretched balloon is rotating at the rate of 30 rpm when dipped into GO suspension and after leaving from the GO suspension, continues to rotate until completely dried at the room temperature. Highly buckled hierarchical GORs were induced via deflation slowly and then reduced in the vapor of hydrazine hydrate at ∼100 °C to obtain rGORs. The deflation rate is typical ∼30 mL/s controlled using a flow meter and the reduction time is typical 15 h, enabling mostly recovery of the graphene sheets. Detailed fabrication parameters are listed in Tables S1 and S2. Characterizations of GORs and rGORs. The surface morphologies of GORs and rGORs were characterized using a JSM-6700F scanning electron microscope (SEM). The samples were sputter coated with a layer of gold before observation to increase the image quality. Fourier transform infrared spectroscopy (FT-IR) spectra were obtained using a Bruker TENSOR-27 Fourier transform infrared spectrophotometer. Raman spectra were obtained using a laser confocal Raman spectrometer (LABRAM-010) in the range from 400 to 2000 cm−1. The static water contact angle measurements were performed by the JC2000 × 1 contact angle and surface tension analysis system with water droplet size of 5 μL. The electrical measurements were performed using a CHI760E electrochemical workstation. Fabrication and Characterization of rGORs-Based Strain Sensors. The rGORs were cut into the rectangular shape with the size of 1 cm × 2 cm (width × length) and then two pieces of glasses were fixed on the ends of tailored rGORs with glue. Two copper electrodes were fixed on the rGORs with conductive silver paste to form a strain sensor. The electrical resistance and strain sensing measurements of I



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rGORs-based strain sensors were carried out on a CHI760E electrochemical workstation. Fabrication and Characterization of Partially or Fully rGORsCoated Gloves. The gloves were ultrasonic cleaned in ethanol for 15 min and then inflated. After that, GO suspension was partially or fully coated on the inflated gloves and dried at room temperature. The rGORs-coated gloves were obtained after deflation and reduction. Copper electrodes were fixed on the rGORs-partially coated gloves with conductive silver paste to detect the finger movements through the electrochemical workstation. The rGORs-fully coated gloves were reinflated and immersed into dichloromethane (DCM) and recorded the withstanding time. The gloves were cut into the square shape with the size of 1 cm × 1 cm to test the wear-resistance to various treatments, including ultrasonic cleaning, hand rubbing and machine washing treatment. The square sample was rubbed by two hands for 5 min and denoted as the hand-rubbed sample. In the ultrasonic cleaning and machine washing processes, the samples were immersed in deionized water and then ultrasonic cleaned or magnetic stirred for 10 min. The morphologies of the mechanical treated samples were observed using SEM.

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ACKNOWLEDGMENTS The project was financially supported by Hunan Provincial Natural Science Foundation for Distinguished Young Scholars (No. 14JJ1001) and Advanced Research Project of NUDT (JC11-01-01).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b18143. Abbreviated names of the samples, schematic illustration of the fabrication procedure of rGORs, relationship between the average thickness of rGO film and the concentration of GO suspension, SEM images of the balloon surface coated with and without GO, SEM images of rapid deflated rGORs, anisotropic shrinking process on the cylindrical substrate, effect of reduction time on the chemical, micro-structural and electrical properties of rGO, schematic illustration of the denoted axial and circumferential directions, solvent barrier property of various balloons directly contacting with DCM, schematic illustration of the rGORs-based strain sensor, influence of fabrication strain on the relative resistance changes of rGORs-S, resistor model proposed for the conductance network of rGORs, washable and wearable properties of a rGORs-fully-coated glove, fabrication parameters of rGORs-S, and fabrication parameters of rGORs-N and rGORs-C (PDF) Natural shrinking of a cylinder balloon (AVI)

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Research Article

Controlled shrinking of a cylinder balloon (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zengyong Chu: 0000-0001-7430-6027 Author Contributions

Y.S. and Y.T. contributed equally to this work. Z.Y.C., J. S., and Y.T. conceived and designed the experiments. J. S. and Y.T. carried out all the experimental fabrications and characterizations, who contributed equally to this work. All the authors contributed to important discussions regarding the research. J. S. and Y.T. wrote the original paper. All the authors took part in the rewriting of the manuscript and approved the final version. Notes

The authors declare no competing financial interest. J


Research Article

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