- Email: [email protected]
Three-dimensional printing of a tunable graphene-based elastomer for strain sensors with ultrahigh sensitivity
Accepted Manuscript Three-dimensional printing of a tunable graphene-based elastomer for strain sensors with ultrahigh sensitivity Kai Huang, Shaoming Dong, Jinshan Yang, Jingyi Yan, Yudong Xue, Xiao You, Jianbao Hu, Le Gao, Xiangyu Zhang, Yusheng Ding PII:
S0008-6223(18)31025-X
DOI:
https://doi.org/10.1016/j.carbon.2018.11.008
Reference:
CARBON 13624
To appear in:
Carbon
Received Date: 1 October 2018 Revised Date:
30 October 2018
Accepted Date: 4 November 2018
Please cite this article as: K. Huang, S. Dong, J. Yang, J. Yan, Y. Xue, X. You, J. Hu, L. Gao, X. Zhang, Y. Ding, Three-dimensional printing of a tunable graphene-based elastomer for strain sensors with ultrahigh sensitivity, Carbon (2018), doi: https://doi.org/10.1016/j.carbon.2018.11.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Three-Dimensional Printing of a Tunable Graphene-Based Elastomer for Strain Sensors with Ultrahigh Sensitivity
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Kai Huanga,b,c, Shaoming Donga,b,*, Jinshan Yanga,b,†, Jingyi Yana,b,c, Yudong Xuea,b,c, Xiao Youa,b,c, Jianbao Hua,b, Le Gaoa,b, Xiangyu Zhanga,b, Yusheng Dinga,b
a
State Key Laboratory of High Performance Ceramics & Superfine Microstructure, Shanghai
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Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b
Structural Ceramics and Composites Engineering Research Center, Shanghai Institute of
University of Chinese Academy of Sciences, Beijing 100039, China
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c
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Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
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Corresponding author. Tel: +86-21-69906030. Email: [email protected]
†
Corresponding author. Tel: +86-21-69906032. Email: [email protected]
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Abstract A graphene-based elastomer for sensors with tunable and high sensitivity was fabricated by using three-dimensional printing, in which a printable ink was developed by homogenizing
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graphene and polydimethylsiloxane (PDMS). To make the elastomer tunable and highly sensitive, different microstructures of three-dimensional graphene-PDMS (3DGP) can be formed.
Attributed to its well-interconnected scaffolds and designed microstructures, 3DGP demonstrates a series of multifunctional properties, such as excellent stability and a large gauge factor (up to
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448 at 30% strain). 3DGP has continuously stable piezoresistive behavior, even after 100
compress-release cycles under 10% strain. By considering the essential properties of 3DGP
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scaffolds, such as filament diameter, interaxial angle and interlayer space, the printed 3DGP structure can be tunable and highly sensitive. The controllable design and scalable fabrication of the 3DGP advanced functional material suggests that tunable strain sensors and wearable devices have great potential for different applications, which is a finding that can be referenced by future
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studies on 3D graphene-based sensors.
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1. Introduction Flexible and sensitive sensors that can be stretched or compressed to a large extent have aroused much attention for their potential applications in a great number of fields, including
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wearable devices[1-4], health monitoring[5-8], smart robotics[9-11], and human-machine interfaces[12-14]. Among those noted, the piezoresistive sensor capable of converting pressure into a resistance signal has the advantages of simple fabrication, wide application and fast
response[15-17]. Recently, the combination of nanomaterials, such as carbon nanotube (CNT)
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[18-21], Ag nanowires or nanoparticles[12, 22], and graphene or graphene oxide (GO) [23-25], with a polymer as a highly stretchable strain sensor has been a hot topic. Graphene is considered
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to have a considerable potential for strain sensor applications since its first preparation by Geim et al. in 2004[26-30]. There have been an increasing number of approaches to develop a stretchable sensor by virtue of the superior mechanical and electrical properties of graphene. By mixing tissue paper with GO solution, a reduced GO paper for pressure sensors can be prepared when followed by thermal reduction[31]. The laser-induced artificial graphene throat that
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integrates the functions of generating and detecting sounds indicates a promising prospect in voice control and wearable electronics.[24] Recently, given its high porosity and inter-connected network, 3D graphene for highly sensitive strain sensors can be obtained by chemical vapor deposition (CVD) [32-35], self-assembly[20, 36], and freeze-casting[37-39]. However, the
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large-scale preparation of highly stretchable and sensitive 3D graphene with desired structures using a simple and inexpensive method is still challenging.
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Direct ink writing (DIW), a 3D printing technology, is a rapidly developing additive manufacturing technology for large-scale production. 3D structures can be fabricated layer by layer by extruding a slurry of functional materials via a nozzle[40-43]. Besides the scalability, one of the important features of 3D printing is the tunability that enables various morphologies, which further helps to achieve the various performances to meet different requirements (i.e., size, shape and sensitivity of the strain sensor) in different application fields. Therefore, 3D printing is preferred to achieve the high-quality and tunable performances of strain sensor with different
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structures by adjusting the printing model, in which the tunable sensitivity is attributed to the change in the conductive pathway in the printing model. In previous approaches, PDMS has typically served as the substrate where the conductive materials are inserted inside or on the
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surface, which limits the utmost stretchability of the sensor[44-46]. Therefore, it is significant to incorporate PDMS into the nanosheets as part of the composite to build 3D assembly rather than as substrate, which could make full use of the superior elasticity of PDMS for highly stretchable sensor[47].
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In our research, a homogeneous ink of graphene and PDMS is developed for 3D printing. Printed 3D graphene-PDMS (3DGP) shows excellent mechanical properties, as well as high
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sensitivity and fast real-time response, under high strain (up to 50%) and durability (100 compressive cycles under 30%). The 3D sensor can achieve tunable sensitivity by designing the printing parameters and the component proportions. This suggests the possibility of applications in many fields, such as wearable devices, electrical skin, and health monitoring. 2. Experimental Section
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2.1 Preparation of 3DGP ink for 3D printing
Commercial graphene (0.6 mg, GT-G03, Xiamen Knano Graphene Technology Co., China) was dispersed in ethanol (200 ml) with the assistance of the surfactant ethylene glycol butylether (EGB, AR, 99%, Aladdin industrial Co., Shanghai, China) and dibutyl phthalate (DBP, 99%,
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Aladdin industrial Corporation, Shanghai, China) with a weight ratio of graphene:EGB:DBP = 3:2:1. The well-dispersed graphene supension was obtained by ultrasonication (Scientz-IID,
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Ningbo Scientz Biotechnology Co., China) ofthe above mixed solution at 200 W for 2 h. Commercial PDMS (Sylgard 184, Dow Corning Co. USA) and a curing agent were then successively added to graphene suspensions with weigh ratios of graphene:PDMS:curing agent = 1:5:0.5, 1:6:0.6, 1:7:0.7, 1:8:0.8. Each addition was followed by mechanical agitation and ultrasonication at 200 W for 2 h. The printable graphene-PDMS inks were obtained after ethanol was removed by naturally drying. 2.2 Fabrication of 3DGP by 3D printing
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Various 3DGP cubes with customized diameters were printed at room temperature (Regenovo 3D bioprinter V2.0, Regenovo Biotechnology Co., China). The nozzle diameter was set to 0.3, 0.4, or 0.5 mm, the pressure applied for extrusion was 0.4-0.6 MPa, and the X-Y
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motion speeds were 0.5-5 mm/s. The as-prepared 3DGP were obtained after printing, followed by curing at 120 °C for 30 min. 2.3 Characterization of 3DGP
The rheologyical properties of the graphene-PDMS inks with different proportions and
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PDMS with the curing agent were tested by a stress-controlled rheometer (Universla stress rheometer SR5, Rheometric Scientif, USA). The compressive cycle was conducted by a
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universal testing machine (Instron-5566, UK) and the elastic modulus for each was determind using a 0.2% strain offset linear slope method. Electrical resistance changes in 3DGP were measured by a digital multimeter (Keysight 34461A Truevolt 6.5, Shenzhen Junda Times Instrument Co., China). A laser micro-Raman spectrometer (Thermo Nicolet, USA), and aFourier infrared spectrometer (NICOLET Is10, Thermo Scientific, USA) were used to characterize the
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printed samples and raw materials. The micromorphology of 3DGP was observed by a field emission scanning electron microscope (FESEM; Hitachi SU8220, Japan). 3. Results and Discussion
3.1 Rheological Properties of Graphene-Based Ink
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The typical 3DGP fabrication process is shown in Fig. 1a. Preparation of printable ink is the primary step for 3D printing. To develop a uniform and printable ink, graphene was dispersed
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into ethanol with a dispersant consisting of ethylene glycol butylether and dibutyl phthalate, followed by sonication. Then, PDMS and the curing agent were successively introduced into the suspension, which was also followed by sonication according to the method employed previously for printing CNT electrodes[48].
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Fig. 1 (a) Schematic of the preparation of 3DGP. (b) Design diagram of 3DGP with different parameters, D: diameter of filaments, θ: interaxial angle, L: interlayer space. (c) Viscosity as a function of the shear rate for 3DGP inks with different proportions and PDMS with the curing
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agent, graphene: PDMS=1:5, 1:6, 1:7, 1:8, respectively. (d) Storage modulus (G′) and loss modulus (G″) as a function of shear stress for inks. (e) and (f) Digital images of the 3DGP ink in
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the syringe and ink-extruding, respectively.
Printable inks were obtained after the ethanol was removed by drying. To achieve the tunable sensitivity of 3DGP by using 3D printing, the essential factors including filament diameter, interaxial angle and interlayer space were controlled according to Fig. 1b. Inks with different graphene contents (graphene: PDMS = 1:5, 1:6, 1:7, 1:8) were designed in accordance with the optimized experimental results. When a higher graphene content (graphene: PDMS > 1:5) is maintained, the viscosity of ink will increase to a point at which it is difficult for the ink to extrude from nozzle smoothly. At a lower graphene content (graphene: PDMS < 1:5), the
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viscosity is too low to preserve the 3D-printed structure, and the electrical conductivity will greatly decrease. When printing 3DGP, the rheological properties of the inks are critical to the printing
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process and their performance. The viscosity of the inks and PDMS as a function of the shear rate is shown in Fig. 1c. It is suggested that the viscosity of PDMS basically stayed the same, which demonstrates that PDMS is Newtonian fluid and cannot be printed. After combining the graphene with PDMS, the viscosity of the inks decreases with the increase of shear rate, which
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suggests that these inks are non-Newtonian fluids with similar shear-thinning behavior and can be used in 3D printing[49]. It can be explained that the adding of graphene into PDMS block the
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flowing of PDMS by the hydrogen bond interaction between graphene and PDMS (as shown in FTIR spectra in Fig. 4), leading to a relatively viscous ink. However, with the increasing of shear stress, the viscosity of ink will decrease due to the breaking up of the hydrogen bond. Low viscosities (10-1~100 Pa·s) at high shear rates make inks easy to extrude. In addition, high viscosities (104~105 Pa·s) at low shear rates (10-2 s-1) help to preserve 3D shapes after printing.
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The storage modulus (G′) and loss modulus (G″) of the inks are investigated as function of shear stress in Fig. 1d. For these four inks, the storage modulus is higher than the loss modulus at low shear stress where the system resides in the linear viscoelastic region. This suggests that the inks are stiff with solid-like properties. After the crossover point of storage and loss modulus (yield
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stress), the storage modulus decreases sharply and is lower than the loss modulus. This suggests that the inks exhibit a liquid-like response and the flow should increase[50, 51]. The plateaus in
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the storage modulus increases orderly when the proportion is adjusted to 1:10 from 1:5, which is explained by the scaling relationship[52]. In addition, this contributes to the retention of the 3D-printed shape, given that all of the inks have a high storage modulus (105~106 Pa) and yield stress (> 102 Pa). The digital image of ink loaded in the syringe is shown in Fig. 1e, and the filament of 3DGP extruding from the nozzle is shown in Fig. 1f, which proves the printability of inks. 3.2 Mechanical property analysis of 3DGP
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The printing process is shown in Fig. 2a. 3DGP can be built by extruding the GPFs on the top of the underlying layer via a nozzle with a diameter of 500 µm. 3DGP of different shapes (cube, heart shape) and sizes can be created as shown in Fig. 2b, which indicates the excellent
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printability of graphene-PDMS inks. The printed 3DGP can be compressed and released in the direction of height by 50% strain and recover quickly with little deformation after the pressure is released, as shown in Fig. 2c. Additionally, when compressed in the direction along the diagonal of the bottom, 3DGP also rapidly recovers its shape, showing superior elasticity (Fig. S1). Fig.
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S2 shows that 3DGP (8×8×10 mm) can support 100 g with little deformation, which is 250
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times its own weight of 0.4 g, demonstrating the stiffness of 3DGP.
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Fig. 2 (a) Digital image of 3D printing. (b) Various 3DGP samples with different shapes and sizes. (c) The compress-release process of 3DGP along its height.
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To further investigate the mechanical properties of 3DGP, the compressive test was preformed to measure the compressive stress as a function of strain. The 3DGP sample used for the compressive test was 8×8×10 mm3. The compressive stress-strain curves of 3DGP with different proportions under different strains are shown in Fig. 3a-d. Fig. 3a suggests that the succeeding loading curve rises back to the max stress of the preceding cycle and maintains the trend of the preceding loading curve, which shows a strain memory effect and a the stable mechanical properties of 3DGP. The max stress increases from 85 kPa to 2000 kPa with an increase in strain from 10% to 50% (Fig. 3e). The excellent elasticity of 3DGP can also be
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confirmed by the few unrecoverable deformations (≤5%) under the large strain. Similar results are shown in Fig. 3b-d. The maximum deformation under 50% strain remains less than 10%, even though larger deformations are found as the graphene content decreases from 1:5 to 1:8.
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This suggests the excellent mechanical properties of the 3DGP (Fig. S3). Furthermore, all of the stress-strain curves under 50% strain have three regions: the first linear region, the second short region with a steeper slope, and the third linear region with a smaller slope after a breakdown at 35~40% strain. The second region that is typically flattened under heavy bending (30% strain) is
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possibly attributed to the high membrane stresses on the graphene pore walls. The third region with a smaller slope can be explained by the torn pore walls, which reduces the strength of the
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graphene walls[53]. It is concluded that the compressive modulus of 3DGP reaches a maximum at a ratio of 1:7 (Fig. 3f), which means that a higher or lower graphene content harms the compressive modulus. It can be concluded that a lower graphene content results in a less robust support. A higher graphene content means less PDMS filling between the graphene sheets, which forms an unconsolidated structure inside the 3DGP. To verify the durability of 3DGP,
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compressive cycling for 90 times under 30% strain was conducted, and the results are shown in Fig. 3g,h. The local enlarged region (Fig. 3h) shows that the first cycle is slightly higher than the next cycle at a given strain for the irreversible deformation. The successive cycles basically overlap from the second cycle to the 90th cycle. The max compressive stress as a function of
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cycle number is shown Fig. 3i. The max stress shows nearly no changes for 90 cycles, which suggests an excellent mechanical performance. It can be explained by the penetration of PDMS
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into the 3D graphene network, which is similar to the cartilages attached to hard bone joints. PDMS acts as the lubricant with high elasticity to hold back the restacking of graphene sheets effectively when anchored to the graphene[25]. Therefore, the 3DGP can achieve the remarkable recovery after lots of compress-release cycles.
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Fig. 3 (a-d) The compressive stress-strain curves of 3DGP under different max strains: graphene:
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PDMS = 1:5, 1:6, 1:7, 1:8, respectively. (e,f) Max strains and compressive modulus. (g,h) Compressive stress-strain curves of 1:5 3DGP for 90 cycles and an expanded region. (i) Retention of max stress after 90 compressive cycles. 3.3 Chemical Structure and Micro Morphology of 3DGP
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The chemical structures of the 3DGP, graphene and PDMS are characterized using a Raman and Fourier transform infrared (FTIR) spectrometer. The Raman spectra (Fig. 4a) of 3DGP and
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graphene have three distinct peaks at approximately 1350 cm-1, 1580 cm-1 and 2700 cm-1, which are attributed to D, G and 2D bands, respectively[54, 55]. The quite low and similar ID/IG values were obtained (0.135, 0.156, 0.127, 0.134, 0.133 for 1:5, 1:6, 1:7, 1:8, graphene, respectively) by comparing the ratio of the intensity between D and G bands (ID/IG). This indicates that the 3DGP samples have high crystallization, and the preparation process (printing and fabrication of the inks) does not damage the samples[56]. Furthermore, the characteristic peaks of 3DGP at 495 cm-1 and 2950 cm-1 are attributed to Si-O-Si and -CH2 stretching in PDMS, respectively. It is obvious that the peak at 495 cm-1 in 3DGP tends to broader and weaker than the corresponding
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peak in PDMS (Fig. 4b). It is dependent on the graphene nanosheets inserted into the PDMS, which suppresses the chain mobility and decreases the crystallinity[25, 57]. The FTIR spectra (Fig. 4c,d) show characteristic peaks at approximately 789-796 cm-1, 1020-1074 cm-1, and
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1255-1259 cm-1, which are respectively attributed to -CH3 rocking and Si-C stretching in Si-CH3, Si-O-Si stretching and CH3 deformation in Si-CH3[58-60]. Furthermore, the peaks at 1385 and 3432 cm-1 and at 1624 and 2970 cm-1 are attributed to the vibrations from C-OH and C-H
deformations, respectively, which provide more evidence for the existence of a hydrogen bond
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interaction between graphene and PDMS[38].
Fig. 4 a,b) Raman spectra of 3DGP with different proportions (graphene: PDMS = 1:5, 1:6, 1:7, 1:8, respectively), graphene and PDMS. c,d) FTIR spectra of 3DGP and the expanded region. SEM is employed to characterize the microstructure of 3DGP, as shown in Fig. 5. Printed 3DGP exhibits a clear woodpile-shape pattern with excellent bonding of the filaments between the adjacent layers (Fig. 5a). As the enlarged region of filament surface (Fig. 5b) suggests, the
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graphene is coated uniformly by PDMS, and the bright parts representing the edges of the graphene nanosheet are observed in Fig. 5c. The continuous 3D structure is strengthened during penetration by PDMS, which can help improve the mechanical properties of 3DGP. Graphene
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nanosheets make up the walls of the pores, and liquid PDMS penetrates the interconnected pores of the 3DGP network, resulting in low surface energies and viscosity[15]. The cross-section shown in Fig. 5d demonstrates the porous structure with an interconnected conductive network inside the filament. The size of the pores is determined from the enlarged region to be on the
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scale of several micrometers (Fig. 5e,f), which is necessary to achieve the strain sensing of
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3DGP.
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Fig. 5 (a-c) The SEM images of 3DGP surface with different magnifications. (d-f) The SEM images of the cross-section.
3.4 Piezoresistive Behavior of 3DGP The excellent conductivity and mechanical properties of graphene enable the application of 3DGP in the strain sensor. The 3DGP with a graphene: PDMS = 1:5 proportion was selected for characterization, and ∆R/R under different strains are shown in Fig. 6. The maximum ∆R/R increases from 0.5 to 17 in the range of 10% to 50% strain (Fig. 6a), and the peaks have slight decay for several cycles under different strains. The enlarged region (Fig. 6b) suggests that the
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∆R/R curves are symmetrical during the compress-release cyclic process, which provides evidence for reversible structure-properties of 3DGP. Furthermore, it is noteworthy that the compression process has two regions: the resistance of 3DGP first decreases and then increases
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until the strain reaches the maximum. This can be explained by the variations in the macro and micro structures of 3DGP during the process of compression and release. Fig. S4a shows that the decreasing interspace between adjacent layers under compression first results in a larger
contacted area between the filaments, which lowers the contact resistance. Then, the contact
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resistance becomes saturated at a larger strain, and the influence of the microstructure on
electrical resistance is higher than that of the macrostructure. The PDMS between graphene
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sheets will expand outward during the compression process (Fig. S4b), which will decrease the contact area between the graphene sheets and increase electrical resistance[59]. Fig. 6c gives the relative resistance changes (∆R/R) of 3DGP at different compressive frequencies under 10% strain. These data suggest that the electrical resistance of 3DGP shows stable durability at frequencies ranging from 0.0102 to 0.102 Hz. The ∆R/R curves nearly
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overlap under different frequencies (Fig. 6d), which could be attributed to the strong adhesion between graphene and PDMS[61]. Fig. 6e shows the ∆R/R of 3DGP with different graphene contents under 10% strain. The ∆R/R peaks increase with the decrease in the graphene content. This is attributed to the less conductive pathways among the 3D continuous network caused by
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the contacted area between graphene nanosheets[62, 63]. Furthermore, the lower ∆R/R curve of 1:8 3DGP under the initial compression demonstrates higher sensitivity than the other 3DGP
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samples, as shown in Fig. 6f.
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Fig. 6 (a,b) Resistance change under different strains and an expanded region of one cycle. (c,d) Resistance change of 3DGP at different compressive rates under 10% strain. (e,f) Resistance change of 3DGP with different proportions (graphene: PDMS = 1:5, 1:6, 1:7, 1:8, respectively). (g,h) Durability test under 30% strain for 100 cycles: resistance change-strain and stress-strain curves.
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The resistance and strain signals are detected simultaneously to test the stability during strain sensing in 3DGP. The relative resistance variation in 3DGP under 30% strain for 100 cycles and the corresponding stress-strain curves are shown in Fig. 6g,h, respectively. As the
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results demonstrate, though the maximal ∆R/R tends to decrease at first, the ∆R/R becomes constant and shows superior reversibility after several compress-release cycles. This
phenomenon could be attributed to the microcracks induced at the beginning of the compressive process[32, 53, 64]. The 3D structure, which is unstable at first, becomes constant after several
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cycles and outputs a stable relative resistance change, which is supported by the stress-strain curves (Fig. 6h). For several starting cycles, the successive cycle is lower than the previous cycle,
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yet the following cycles nearly overlap, which is consistent with the ∆R/R curves. Furthermore, the enlarged region in Fig. 6g shows good correspondence between ∆R/R and strain, indicating a potential application in strain sensors.
The 3DGP with different structures were prepared by 3D printing to show the tunability of their piezoresistive behavior in line with Fig. 1b. The gauge factor (GF) and optical images of
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3DGP are shown in Fig. 7. GF is defined as the following:
where ∆R, R0 and ε denote the variation in resistance, the resistance at 0% strain, and the applied
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strain, respectively[5]. GF is employed to measure the sensitivity of the strain sensor, which depends on both the intrinsic properties and the structural features of the sensor. It has been reported in previous studies that graphene-based strain sensors have low sensitivities (only 1.9 for suspended graphene) due to their rigid and stable structure[61, 65, 66]. Thus, 3D printing is an effective approach to build and design 3D structures that increase the GF of graphene-based strain sensors.
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Fig. 7 (a) Gauge factor as a function of strain in 3DGP with different filament diameters. (b-d)
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Optical images of 3DGP with diameters of 0.3, 0.4, and 0.5 mm. (e) Gauge factor as a function of strain in 3DGP with different interaxial angles. (f-h) Optical images of 3DGP with angles of 30°, 60°, and 90°; (i) Gauge factor as a function of strain in 3DGP with different interlayer spaces. (j-l) Optical images of 3DGP with interlayer spaces of 0.35, 0.4, and 0.45 mm.
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The impact of filament diameters on GF is shown in Fig. 7a and the relative resistance change-strain curve is shown in Fig. S5. The optical images of the corresponding 3DGP are
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illustrated in Fig. 7b-d. It can be concluded that the GF of 3DGP increases during the application of strain. For a given strain, the GF of 3DGP with 0.5 mm filaments, which is 448 at 30% strain, is several times higher than that of other two 3DGP samples (160 for 0.4 mm and 91 for 0.3 mm). This observation can be explained by the relatively less conductive pathway in the 0.5 mm filament due to the comparatively incompact structure and lower contact area between the graphene sheets[62, 67]. This is attributed to the lower pressure when the filament is extruded from the larger nozzle. Furthermore, Fig. 7e reveals that the larger interaxial angle can increase the GF. Considering the woodpile pattern of 3DGP shown in Fig. 7f-h, 3DGP with a 30°
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interaxial angle has more filaments and forms a denser structure. As a result, a more conductive pathway is formed to obstruct the changes in electrical resistance. Hence, building a more simple structure can improve the GF of 3DGP. The investigation of the interlayer space is shown in Fig.
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7i, and the optical images of 3DGP with various interlayer spaces, 0.35, 0.4, and 0.45 mm, are shown in Fig. 7j-l, respectively. The curve under small strain (inserted in Fig. 7i) indicates that the GF-strain curves for 0.35 and 0.4 nearly interlay with each other, but both are higher than that of 0.45 mm. For the 3DGP with a 0.45 mm interlayer space, the contact area (Fig. 7i) is
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smaller than that of the other two 3DGP samples, which increases the sensitivity to the same strain. In addition, fewer layers caused by a larger interlayer space gives rise to a less conductive
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pathway, which suggests a higher GF. To compare with other graphene-based strain sensors, the GF is shown in Fig. 8 and in Table S1[16, 20, 37, 38, 61, 68-76]. It is illustrated that most graphene-based strain sensors have low GF values (< 100), which are surpassed by the printed, tunable 3DGP with a GF of 448 in this paper. To summarize, the printed structure is designed to achieve tunability sensitivity and high GF in 3DGP, which introduces the possibility for using
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3DGP as a high-performance strain sensor in electronics.
Fig. 8 Comparison of the GFs for graphene-based sensors. 4. Conclusion
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In conclusion, 3DGP for strain sensors with ultrahigh sensitivity and strain was fabricated by 3D printing. The strain sensing of 3DGP remains stable even after 100 cycles of compress-release under 10% strain, which is important for its practical application. The
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as-prepared 3DGP can withstand 50% strain with few deformation (≤5%), which demonstrates its excellent mechanical properties. Printed 3D structures were investigated to confirm the
tunability of 3DGP for strain sensors. As the results suggest, decreasing the graphene content resulted in less conductive pathways and higher strain-sensitivity. Additionally, the
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graphene-PDMs filament with high stretchability was successfully employed to monitor the human body, which included monitoring of joint bending and the subtle muscular motions in the
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throat. The facile preparation of 3DGP with designable high performance allows 3DGP to serve as a strain sensor, which is a finding that will be useful for future studies of 3D graphene-based sensors. Acknowledgments
This project was sponsored by National Key Research and Development Program of China (NO.
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2017YFB0703200), National Natural Science Foundation of China (No. 51772310), CAS Pioneer Hundred Talents Program, Shanghai Pujiang Program (No. 17PJ1410100), Young Elite Scientist Sponsorship Program by CAST (No. 2017QNRC001) and Shanghai Institute of Ceramics Innovative Funding.
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Appendix A. Supplementary Data
Supporting Information Available: The digital photos of compress-release process of 3DGP in
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different directions, images of 3DGP supporting 100 g without shape distortion, Schematics of macro and micro structure under compression. References
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DOI:
https://doi.org/10.1016/j.carbon.2018.11.008
Reference:
CARBON 13624
To appear in:
Carbon
Received Date: 1 October 2018 Revised Date:
30 October 2018
Accepted Date: 4 November 2018
Please cite this article as: K. Huang, S. Dong, J. Yang, J. Yan, Y. Xue, X. You, J. Hu, L. Gao, X. Zhang, Y. Ding, Three-dimensional printing of a tunable graphene-based elastomer for strain sensors with ultrahigh sensitivity, Carbon (2018), doi: https://doi.org/10.1016/j.carbon.2018.11.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Three-Dimensional Printing of a Tunable Graphene-Based Elastomer for Strain Sensors with Ultrahigh Sensitivity
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Kai Huanga,b,c, Shaoming Donga,b,*, Jinshan Yanga,b,†, Jingyi Yana,b,c, Yudong Xuea,b,c, Xiao Youa,b,c, Jianbao Hua,b, Le Gaoa,b, Xiangyu Zhanga,b, Yusheng Dinga,b
a
State Key Laboratory of High Performance Ceramics & Superfine Microstructure, Shanghai
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Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b
Structural Ceramics and Composites Engineering Research Center, Shanghai Institute of
University of Chinese Academy of Sciences, Beijing 100039, China
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c
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Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
*
Corresponding author. Tel: +86-21-69906030. Email: [email protected]
†
Corresponding author. Tel: +86-21-69906032. Email: [email protected]
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Abstract A graphene-based elastomer for sensors with tunable and high sensitivity was fabricated by using three-dimensional printing, in which a printable ink was developed by homogenizing
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graphene and polydimethylsiloxane (PDMS). To make the elastomer tunable and highly sensitive, different microstructures of three-dimensional graphene-PDMS (3DGP) can be formed.
Attributed to its well-interconnected scaffolds and designed microstructures, 3DGP demonstrates a series of multifunctional properties, such as excellent stability and a large gauge factor (up to
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448 at 30% strain). 3DGP has continuously stable piezoresistive behavior, even after 100
compress-release cycles under 10% strain. By considering the essential properties of 3DGP
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scaffolds, such as filament diameter, interaxial angle and interlayer space, the printed 3DGP structure can be tunable and highly sensitive. The controllable design and scalable fabrication of the 3DGP advanced functional material suggests that tunable strain sensors and wearable devices have great potential for different applications, which is a finding that can be referenced by future
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studies on 3D graphene-based sensors.
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1. Introduction Flexible and sensitive sensors that can be stretched or compressed to a large extent have aroused much attention for their potential applications in a great number of fields, including
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wearable devices[1-4], health monitoring[5-8], smart robotics[9-11], and human-machine interfaces[12-14]. Among those noted, the piezoresistive sensor capable of converting pressure into a resistance signal has the advantages of simple fabrication, wide application and fast
response[15-17]. Recently, the combination of nanomaterials, such as carbon nanotube (CNT)
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[18-21], Ag nanowires or nanoparticles[12, 22], and graphene or graphene oxide (GO) [23-25], with a polymer as a highly stretchable strain sensor has been a hot topic. Graphene is considered
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to have a considerable potential for strain sensor applications since its first preparation by Geim et al. in 2004[26-30]. There have been an increasing number of approaches to develop a stretchable sensor by virtue of the superior mechanical and electrical properties of graphene. By mixing tissue paper with GO solution, a reduced GO paper for pressure sensors can be prepared when followed by thermal reduction[31]. The laser-induced artificial graphene throat that
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integrates the functions of generating and detecting sounds indicates a promising prospect in voice control and wearable electronics.[24] Recently, given its high porosity and inter-connected network, 3D graphene for highly sensitive strain sensors can be obtained by chemical vapor deposition (CVD) [32-35], self-assembly[20, 36], and freeze-casting[37-39]. However, the
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large-scale preparation of highly stretchable and sensitive 3D graphene with desired structures using a simple and inexpensive method is still challenging.
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Direct ink writing (DIW), a 3D printing technology, is a rapidly developing additive manufacturing technology for large-scale production. 3D structures can be fabricated layer by layer by extruding a slurry of functional materials via a nozzle[40-43]. Besides the scalability, one of the important features of 3D printing is the tunability that enables various morphologies, which further helps to achieve the various performances to meet different requirements (i.e., size, shape and sensitivity of the strain sensor) in different application fields. Therefore, 3D printing is preferred to achieve the high-quality and tunable performances of strain sensor with different
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structures by adjusting the printing model, in which the tunable sensitivity is attributed to the change in the conductive pathway in the printing model. In previous approaches, PDMS has typically served as the substrate where the conductive materials are inserted inside or on the
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surface, which limits the utmost stretchability of the sensor[44-46]. Therefore, it is significant to incorporate PDMS into the nanosheets as part of the composite to build 3D assembly rather than as substrate, which could make full use of the superior elasticity of PDMS for highly stretchable sensor[47].
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In our research, a homogeneous ink of graphene and PDMS is developed for 3D printing. Printed 3D graphene-PDMS (3DGP) shows excellent mechanical properties, as well as high
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sensitivity and fast real-time response, under high strain (up to 50%) and durability (100 compressive cycles under 30%). The 3D sensor can achieve tunable sensitivity by designing the printing parameters and the component proportions. This suggests the possibility of applications in many fields, such as wearable devices, electrical skin, and health monitoring. 2. Experimental Section
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2.1 Preparation of 3DGP ink for 3D printing
Commercial graphene (0.6 mg, GT-G03, Xiamen Knano Graphene Technology Co., China) was dispersed in ethanol (200 ml) with the assistance of the surfactant ethylene glycol butylether (EGB, AR, 99%, Aladdin industrial Co., Shanghai, China) and dibutyl phthalate (DBP, 99%,
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Aladdin industrial Corporation, Shanghai, China) with a weight ratio of graphene:EGB:DBP = 3:2:1. The well-dispersed graphene supension was obtained by ultrasonication (Scientz-IID,
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Ningbo Scientz Biotechnology Co., China) ofthe above mixed solution at 200 W for 2 h. Commercial PDMS (Sylgard 184, Dow Corning Co. USA) and a curing agent were then successively added to graphene suspensions with weigh ratios of graphene:PDMS:curing agent = 1:5:0.5, 1:6:0.6, 1:7:0.7, 1:8:0.8. Each addition was followed by mechanical agitation and ultrasonication at 200 W for 2 h. The printable graphene-PDMS inks were obtained after ethanol was removed by naturally drying. 2.2 Fabrication of 3DGP by 3D printing
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Various 3DGP cubes with customized diameters were printed at room temperature (Regenovo 3D bioprinter V2.0, Regenovo Biotechnology Co., China). The nozzle diameter was set to 0.3, 0.4, or 0.5 mm, the pressure applied for extrusion was 0.4-0.6 MPa, and the X-Y
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motion speeds were 0.5-5 mm/s. The as-prepared 3DGP were obtained after printing, followed by curing at 120 °C for 30 min. 2.3 Characterization of 3DGP
The rheologyical properties of the graphene-PDMS inks with different proportions and
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PDMS with the curing agent were tested by a stress-controlled rheometer (Universla stress rheometer SR5, Rheometric Scientif, USA). The compressive cycle was conducted by a
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universal testing machine (Instron-5566, UK) and the elastic modulus for each was determind using a 0.2% strain offset linear slope method. Electrical resistance changes in 3DGP were measured by a digital multimeter (Keysight 34461A Truevolt 6.5, Shenzhen Junda Times Instrument Co., China). A laser micro-Raman spectrometer (Thermo Nicolet, USA), and aFourier infrared spectrometer (NICOLET Is10, Thermo Scientific, USA) were used to characterize the
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printed samples and raw materials. The micromorphology of 3DGP was observed by a field emission scanning electron microscope (FESEM; Hitachi SU8220, Japan). 3. Results and Discussion
3.1 Rheological Properties of Graphene-Based Ink
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The typical 3DGP fabrication process is shown in Fig. 1a. Preparation of printable ink is the primary step for 3D printing. To develop a uniform and printable ink, graphene was dispersed
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into ethanol with a dispersant consisting of ethylene glycol butylether and dibutyl phthalate, followed by sonication. Then, PDMS and the curing agent were successively introduced into the suspension, which was also followed by sonication according to the method employed previously for printing CNT electrodes[48].
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Fig. 1 (a) Schematic of the preparation of 3DGP. (b) Design diagram of 3DGP with different parameters, D: diameter of filaments, θ: interaxial angle, L: interlayer space. (c) Viscosity as a function of the shear rate for 3DGP inks with different proportions and PDMS with the curing
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agent, graphene: PDMS=1:5, 1:6, 1:7, 1:8, respectively. (d) Storage modulus (G′) and loss modulus (G″) as a function of shear stress for inks. (e) and (f) Digital images of the 3DGP ink in
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the syringe and ink-extruding, respectively.
Printable inks were obtained after the ethanol was removed by drying. To achieve the tunable sensitivity of 3DGP by using 3D printing, the essential factors including filament diameter, interaxial angle and interlayer space were controlled according to Fig. 1b. Inks with different graphene contents (graphene: PDMS = 1:5, 1:6, 1:7, 1:8) were designed in accordance with the optimized experimental results. When a higher graphene content (graphene: PDMS > 1:5) is maintained, the viscosity of ink will increase to a point at which it is difficult for the ink to extrude from nozzle smoothly. At a lower graphene content (graphene: PDMS < 1:5), the
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viscosity is too low to preserve the 3D-printed structure, and the electrical conductivity will greatly decrease. When printing 3DGP, the rheological properties of the inks are critical to the printing
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process and their performance. The viscosity of the inks and PDMS as a function of the shear rate is shown in Fig. 1c. It is suggested that the viscosity of PDMS basically stayed the same, which demonstrates that PDMS is Newtonian fluid and cannot be printed. After combining the graphene with PDMS, the viscosity of the inks decreases with the increase of shear rate, which
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suggests that these inks are non-Newtonian fluids with similar shear-thinning behavior and can be used in 3D printing[49]. It can be explained that the adding of graphene into PDMS block the
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flowing of PDMS by the hydrogen bond interaction between graphene and PDMS (as shown in FTIR spectra in Fig. 4), leading to a relatively viscous ink. However, with the increasing of shear stress, the viscosity of ink will decrease due to the breaking up of the hydrogen bond. Low viscosities (10-1~100 Pa·s) at high shear rates make inks easy to extrude. In addition, high viscosities (104~105 Pa·s) at low shear rates (10-2 s-1) help to preserve 3D shapes after printing.
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The storage modulus (G′) and loss modulus (G″) of the inks are investigated as function of shear stress in Fig. 1d. For these four inks, the storage modulus is higher than the loss modulus at low shear stress where the system resides in the linear viscoelastic region. This suggests that the inks are stiff with solid-like properties. After the crossover point of storage and loss modulus (yield
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stress), the storage modulus decreases sharply and is lower than the loss modulus. This suggests that the inks exhibit a liquid-like response and the flow should increase[50, 51]. The plateaus in
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the storage modulus increases orderly when the proportion is adjusted to 1:10 from 1:5, which is explained by the scaling relationship[52]. In addition, this contributes to the retention of the 3D-printed shape, given that all of the inks have a high storage modulus (105~106 Pa) and yield stress (> 102 Pa). The digital image of ink loaded in the syringe is shown in Fig. 1e, and the filament of 3DGP extruding from the nozzle is shown in Fig. 1f, which proves the printability of inks. 3.2 Mechanical property analysis of 3DGP
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The printing process is shown in Fig. 2a. 3DGP can be built by extruding the GPFs on the top of the underlying layer via a nozzle with a diameter of 500 µm. 3DGP of different shapes (cube, heart shape) and sizes can be created as shown in Fig. 2b, which indicates the excellent
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printability of graphene-PDMS inks. The printed 3DGP can be compressed and released in the direction of height by 50% strain and recover quickly with little deformation after the pressure is released, as shown in Fig. 2c. Additionally, when compressed in the direction along the diagonal of the bottom, 3DGP also rapidly recovers its shape, showing superior elasticity (Fig. S1). Fig.
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S2 shows that 3DGP (8×8×10 mm) can support 100 g with little deformation, which is 250
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times its own weight of 0.4 g, demonstrating the stiffness of 3DGP.
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Fig. 2 (a) Digital image of 3D printing. (b) Various 3DGP samples with different shapes and sizes. (c) The compress-release process of 3DGP along its height.
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To further investigate the mechanical properties of 3DGP, the compressive test was preformed to measure the compressive stress as a function of strain. The 3DGP sample used for the compressive test was 8×8×10 mm3. The compressive stress-strain curves of 3DGP with different proportions under different strains are shown in Fig. 3a-d. Fig. 3a suggests that the succeeding loading curve rises back to the max stress of the preceding cycle and maintains the trend of the preceding loading curve, which shows a strain memory effect and a the stable mechanical properties of 3DGP. The max stress increases from 85 kPa to 2000 kPa with an increase in strain from 10% to 50% (Fig. 3e). The excellent elasticity of 3DGP can also be
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confirmed by the few unrecoverable deformations (≤5%) under the large strain. Similar results are shown in Fig. 3b-d. The maximum deformation under 50% strain remains less than 10%, even though larger deformations are found as the graphene content decreases from 1:5 to 1:8.
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This suggests the excellent mechanical properties of the 3DGP (Fig. S3). Furthermore, all of the stress-strain curves under 50% strain have three regions: the first linear region, the second short region with a steeper slope, and the third linear region with a smaller slope after a breakdown at 35~40% strain. The second region that is typically flattened under heavy bending (30% strain) is
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possibly attributed to the high membrane stresses on the graphene pore walls. The third region with a smaller slope can be explained by the torn pore walls, which reduces the strength of the
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graphene walls[53]. It is concluded that the compressive modulus of 3DGP reaches a maximum at a ratio of 1:7 (Fig. 3f), which means that a higher or lower graphene content harms the compressive modulus. It can be concluded that a lower graphene content results in a less robust support. A higher graphene content means less PDMS filling between the graphene sheets, which forms an unconsolidated structure inside the 3DGP. To verify the durability of 3DGP,
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compressive cycling for 90 times under 30% strain was conducted, and the results are shown in Fig. 3g,h. The local enlarged region (Fig. 3h) shows that the first cycle is slightly higher than the next cycle at a given strain for the irreversible deformation. The successive cycles basically overlap from the second cycle to the 90th cycle. The max compressive stress as a function of
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cycle number is shown Fig. 3i. The max stress shows nearly no changes for 90 cycles, which suggests an excellent mechanical performance. It can be explained by the penetration of PDMS
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into the 3D graphene network, which is similar to the cartilages attached to hard bone joints. PDMS acts as the lubricant with high elasticity to hold back the restacking of graphene sheets effectively when anchored to the graphene[25]. Therefore, the 3DGP can achieve the remarkable recovery after lots of compress-release cycles.
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Fig. 3 (a-d) The compressive stress-strain curves of 3DGP under different max strains: graphene:
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PDMS = 1:5, 1:6, 1:7, 1:8, respectively. (e,f) Max strains and compressive modulus. (g,h) Compressive stress-strain curves of 1:5 3DGP for 90 cycles and an expanded region. (i) Retention of max stress after 90 compressive cycles. 3.3 Chemical Structure and Micro Morphology of 3DGP
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The chemical structures of the 3DGP, graphene and PDMS are characterized using a Raman and Fourier transform infrared (FTIR) spectrometer. The Raman spectra (Fig. 4a) of 3DGP and
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graphene have three distinct peaks at approximately 1350 cm-1, 1580 cm-1 and 2700 cm-1, which are attributed to D, G and 2D bands, respectively[54, 55]. The quite low and similar ID/IG values were obtained (0.135, 0.156, 0.127, 0.134, 0.133 for 1:5, 1:6, 1:7, 1:8, graphene, respectively) by comparing the ratio of the intensity between D and G bands (ID/IG). This indicates that the 3DGP samples have high crystallization, and the preparation process (printing and fabrication of the inks) does not damage the samples[56]. Furthermore, the characteristic peaks of 3DGP at 495 cm-1 and 2950 cm-1 are attributed to Si-O-Si and -CH2 stretching in PDMS, respectively. It is obvious that the peak at 495 cm-1 in 3DGP tends to broader and weaker than the corresponding
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peak in PDMS (Fig. 4b). It is dependent on the graphene nanosheets inserted into the PDMS, which suppresses the chain mobility and decreases the crystallinity[25, 57]. The FTIR spectra (Fig. 4c,d) show characteristic peaks at approximately 789-796 cm-1, 1020-1074 cm-1, and
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1255-1259 cm-1, which are respectively attributed to -CH3 rocking and Si-C stretching in Si-CH3, Si-O-Si stretching and CH3 deformation in Si-CH3[58-60]. Furthermore, the peaks at 1385 and 3432 cm-1 and at 1624 and 2970 cm-1 are attributed to the vibrations from C-OH and C-H
deformations, respectively, which provide more evidence for the existence of a hydrogen bond
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interaction between graphene and PDMS[38].
Fig. 4 a,b) Raman spectra of 3DGP with different proportions (graphene: PDMS = 1:5, 1:6, 1:7, 1:8, respectively), graphene and PDMS. c,d) FTIR spectra of 3DGP and the expanded region. SEM is employed to characterize the microstructure of 3DGP, as shown in Fig. 5. Printed 3DGP exhibits a clear woodpile-shape pattern with excellent bonding of the filaments between the adjacent layers (Fig. 5a). As the enlarged region of filament surface (Fig. 5b) suggests, the
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graphene is coated uniformly by PDMS, and the bright parts representing the edges of the graphene nanosheet are observed in Fig. 5c. The continuous 3D structure is strengthened during penetration by PDMS, which can help improve the mechanical properties of 3DGP. Graphene
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nanosheets make up the walls of the pores, and liquid PDMS penetrates the interconnected pores of the 3DGP network, resulting in low surface energies and viscosity[15]. The cross-section shown in Fig. 5d demonstrates the porous structure with an interconnected conductive network inside the filament. The size of the pores is determined from the enlarged region to be on the
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scale of several micrometers (Fig. 5e,f), which is necessary to achieve the strain sensing of
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3DGP.
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Fig. 5 (a-c) The SEM images of 3DGP surface with different magnifications. (d-f) The SEM images of the cross-section.
3.4 Piezoresistive Behavior of 3DGP The excellent conductivity and mechanical properties of graphene enable the application of 3DGP in the strain sensor. The 3DGP with a graphene: PDMS = 1:5 proportion was selected for characterization, and ∆R/R under different strains are shown in Fig. 6. The maximum ∆R/R increases from 0.5 to 17 in the range of 10% to 50% strain (Fig. 6a), and the peaks have slight decay for several cycles under different strains. The enlarged region (Fig. 6b) suggests that the
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∆R/R curves are symmetrical during the compress-release cyclic process, which provides evidence for reversible structure-properties of 3DGP. Furthermore, it is noteworthy that the compression process has two regions: the resistance of 3DGP first decreases and then increases
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until the strain reaches the maximum. This can be explained by the variations in the macro and micro structures of 3DGP during the process of compression and release. Fig. S4a shows that the decreasing interspace between adjacent layers under compression first results in a larger
contacted area between the filaments, which lowers the contact resistance. Then, the contact
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resistance becomes saturated at a larger strain, and the influence of the microstructure on
electrical resistance is higher than that of the macrostructure. The PDMS between graphene
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sheets will expand outward during the compression process (Fig. S4b), which will decrease the contact area between the graphene sheets and increase electrical resistance[59]. Fig. 6c gives the relative resistance changes (∆R/R) of 3DGP at different compressive frequencies under 10% strain. These data suggest that the electrical resistance of 3DGP shows stable durability at frequencies ranging from 0.0102 to 0.102 Hz. The ∆R/R curves nearly
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overlap under different frequencies (Fig. 6d), which could be attributed to the strong adhesion between graphene and PDMS[61]. Fig. 6e shows the ∆R/R of 3DGP with different graphene contents under 10% strain. The ∆R/R peaks increase with the decrease in the graphene content. This is attributed to the less conductive pathways among the 3D continuous network caused by
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the contacted area between graphene nanosheets[62, 63]. Furthermore, the lower ∆R/R curve of 1:8 3DGP under the initial compression demonstrates higher sensitivity than the other 3DGP
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samples, as shown in Fig. 6f.
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Fig. 6 (a,b) Resistance change under different strains and an expanded region of one cycle. (c,d) Resistance change of 3DGP at different compressive rates under 10% strain. (e,f) Resistance change of 3DGP with different proportions (graphene: PDMS = 1:5, 1:6, 1:7, 1:8, respectively). (g,h) Durability test under 30% strain for 100 cycles: resistance change-strain and stress-strain curves.
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The resistance and strain signals are detected simultaneously to test the stability during strain sensing in 3DGP. The relative resistance variation in 3DGP under 30% strain for 100 cycles and the corresponding stress-strain curves are shown in Fig. 6g,h, respectively. As the
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results demonstrate, though the maximal ∆R/R tends to decrease at first, the ∆R/R becomes constant and shows superior reversibility after several compress-release cycles. This
phenomenon could be attributed to the microcracks induced at the beginning of the compressive process[32, 53, 64]. The 3D structure, which is unstable at first, becomes constant after several
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cycles and outputs a stable relative resistance change, which is supported by the stress-strain curves (Fig. 6h). For several starting cycles, the successive cycle is lower than the previous cycle,
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yet the following cycles nearly overlap, which is consistent with the ∆R/R curves. Furthermore, the enlarged region in Fig. 6g shows good correspondence between ∆R/R and strain, indicating a potential application in strain sensors.
The 3DGP with different structures were prepared by 3D printing to show the tunability of their piezoresistive behavior in line with Fig. 1b. The gauge factor (GF) and optical images of
∆ /
(1)
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=
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3DGP are shown in Fig. 7. GF is defined as the following:
where ∆R, R0 and ε denote the variation in resistance, the resistance at 0% strain, and the applied
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strain, respectively[5]. GF is employed to measure the sensitivity of the strain sensor, which depends on both the intrinsic properties and the structural features of the sensor. It has been reported in previous studies that graphene-based strain sensors have low sensitivities (only 1.9 for suspended graphene) due to their rigid and stable structure[61, 65, 66]. Thus, 3D printing is an effective approach to build and design 3D structures that increase the GF of graphene-based strain sensors.
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Fig. 7 (a) Gauge factor as a function of strain in 3DGP with different filament diameters. (b-d)
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Optical images of 3DGP with diameters of 0.3, 0.4, and 0.5 mm. (e) Gauge factor as a function of strain in 3DGP with different interaxial angles. (f-h) Optical images of 3DGP with angles of 30°, 60°, and 90°; (i) Gauge factor as a function of strain in 3DGP with different interlayer spaces. (j-l) Optical images of 3DGP with interlayer spaces of 0.35, 0.4, and 0.45 mm.
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The impact of filament diameters on GF is shown in Fig. 7a and the relative resistance change-strain curve is shown in Fig. S5. The optical images of the corresponding 3DGP are
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illustrated in Fig. 7b-d. It can be concluded that the GF of 3DGP increases during the application of strain. For a given strain, the GF of 3DGP with 0.5 mm filaments, which is 448 at 30% strain, is several times higher than that of other two 3DGP samples (160 for 0.4 mm and 91 for 0.3 mm). This observation can be explained by the relatively less conductive pathway in the 0.5 mm filament due to the comparatively incompact structure and lower contact area between the graphene sheets[62, 67]. This is attributed to the lower pressure when the filament is extruded from the larger nozzle. Furthermore, Fig. 7e reveals that the larger interaxial angle can increase the GF. Considering the woodpile pattern of 3DGP shown in Fig. 7f-h, 3DGP with a 30°
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interaxial angle has more filaments and forms a denser structure. As a result, a more conductive pathway is formed to obstruct the changes in electrical resistance. Hence, building a more simple structure can improve the GF of 3DGP. The investigation of the interlayer space is shown in Fig.
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7i, and the optical images of 3DGP with various interlayer spaces, 0.35, 0.4, and 0.45 mm, are shown in Fig. 7j-l, respectively. The curve under small strain (inserted in Fig. 7i) indicates that the GF-strain curves for 0.35 and 0.4 nearly interlay with each other, but both are higher than that of 0.45 mm. For the 3DGP with a 0.45 mm interlayer space, the contact area (Fig. 7i) is
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smaller than that of the other two 3DGP samples, which increases the sensitivity to the same strain. In addition, fewer layers caused by a larger interlayer space gives rise to a less conductive
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pathway, which suggests a higher GF. To compare with other graphene-based strain sensors, the GF is shown in Fig. 8 and in Table S1[16, 20, 37, 38, 61, 68-76]. It is illustrated that most graphene-based strain sensors have low GF values (< 100), which are surpassed by the printed, tunable 3DGP with a GF of 448 in this paper. To summarize, the printed structure is designed to achieve tunability sensitivity and high GF in 3DGP, which introduces the possibility for using
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3DGP as a high-performance strain sensor in electronics.
Fig. 8 Comparison of the GFs for graphene-based sensors. 4. Conclusion
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In conclusion, 3DGP for strain sensors with ultrahigh sensitivity and strain was fabricated by 3D printing. The strain sensing of 3DGP remains stable even after 100 cycles of compress-release under 10% strain, which is important for its practical application. The
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as-prepared 3DGP can withstand 50% strain with few deformation (≤5%), which demonstrates its excellent mechanical properties. Printed 3D structures were investigated to confirm the
tunability of 3DGP for strain sensors. As the results suggest, decreasing the graphene content resulted in less conductive pathways and higher strain-sensitivity. Additionally, the
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graphene-PDMs filament with high stretchability was successfully employed to monitor the human body, which included monitoring of joint bending and the subtle muscular motions in the
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throat. The facile preparation of 3DGP with designable high performance allows 3DGP to serve as a strain sensor, which is a finding that will be useful for future studies of 3D graphene-based sensors. Acknowledgments
This project was sponsored by National Key Research and Development Program of China (NO.
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2017YFB0703200), National Natural Science Foundation of China (No. 51772310), CAS Pioneer Hundred Talents Program, Shanghai Pujiang Program (No. 17PJ1410100), Young Elite Scientist Sponsorship Program by CAST (No. 2017QNRC001) and Shanghai Institute of Ceramics Innovative Funding.
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Appendix A. Supplementary Data
Supporting Information Available: The digital photos of compress-release process of 3DGP in
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different directions, images of 3DGP supporting 100 g without shape distortion, Schematics of macro and micro structure under compression. References
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