Printing Carbon Nanotube-Embedded Silicone Elastomers via Direct

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Printing Carbon Nanotube-embedded Silicone Elastomers via Direct Writing Bin Luo, Yu Wei, Hualing Chen, Zicai Zhu, Peng Fan, Xuejie Xu, and Baojun Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18614 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on December 2, 2018

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ACS Applied Materials & Interfaces

Printing Carbon Nanotube-Embedded Silicone Elastomers via Direct Writing Bin Luo1,2(#), Yu Wei2(#), Hualing Chen1,2(*), Zicai Zhu2, Peng Fan1,2, Xuejie Xu2, Baojun Xie3 1State

Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an

Jiaotong University, Xi’an, 710049, P. R. China. 2School

of Mechanical Engineering, Xi’an Jiaotong University, Xi’an, 710049, P. R.

China. 3College of Mechanical and Electrical Engineering, Jiaxing University, Jiaxing, 314001,

P. R. China.

ABSTRACT: Direct writing techniques for the printing of colloidal multiwalled carbon nanotubes (CNTs) embedded in polydimethylsiloxane (PDMS) were developed herein to fabricate complex structures including woodpiles, tetragonal scaffolds, and gradient mesh structures. The multiwalled CNTs served as a conductive filler and thickening agent for the printing ink. A suitable rheological behavior was obtained by mixing the CNTs with PDMS dissolved in an isopropyl alcohol solvent. A 7 wt.% CNT loading in the PDMS was optimum for printing gap-spanning features at a nozzle speed of 20 mm/s. The printed structures, including a woodpile and gradient mesh structure, were capable of detecting changes in external mechanical pressure. Printed CNT/PDMS strips exhibit electrical actuation with good mechanical performance (strain of 8.9%) at a low actuation voltage (60 V). The performance characterization

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and application display demonstrated the possibility of developing custom complex CNT/PDMS structures for a broad range of applications, including soft robots and flexible electronic devices.

KEYWORDS:

Direct

writing,

multiwalled

carbon

nanotube

(CNT),

polydimethylsiloxane, rheological behavior, gap spanning features

*Corresponding author email: [email protected] #These

authors contributed equally to this work.

1. INTRODUCTION Carbon nanomaterials including carbon nanotubes (CNTs) and graphene have favorable electrical, optical, thermal, and mechanical properties, and are therefore promising candidates in applications such as wearable electronics and flexible actuators.1-4 Further, stretchable conducting nanomaterials fabricated by embedding CNT networks into silicone elastomers such as polydimethylsiloxane (PDMS) have received great attention for sensor and actuator applications.5-7 For example, Jung et al. have manufactured a CNT/PDMS complex-based long-term wearable sensor resistant to motion disturbances and perspiration that is easily connected to common electrocardiograph devices.8 Further, Choi et al. have reported a CNT/PDMS composite-based senor to measure large mechanical deformations,9 while Li et al. have fabricated stretchable capacitive sensors based on CNT/PDMS.10 Chen et al. have


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reported an electrothermal CNT/PDMS actuator that can be applied in environments ranging from dozens to thousands of volts by adjusting the CNT content.11 Hu et al. reported a soft jumping robot that mimicked a somersaulting gymnast based on rolled CNT/PDMS bilayer composite actuators.12 However, most of the reported CNT/PDMS structures that depend on photolithographic, template, or casting methods in their preparation are limited to 2D structures.10 Further, these preparation methods typically yield structures with limited versatility and involve complex processes. To overcome these problems, three-dimensional (3D) printing methods are promising. Li et al. have proposed 3D printing CNT/PDMS via a direct writing technique. Direct writing has recently been used to construct stretchable sensors, complex scaffolds, 3D periodic graphene aerogels, and self-healing materials, glasses, and hydrogels.4,13-19 The linchpin of the direct writing technique is to create a kind of ink that can be extruded as a fiber and retain the printed shape by quickly solidifying, even while spanning gaps across underlying layers.20 ， 21 The rheology of the ink is a decisive element in the printing process and is vital for the manufacturing of unsupported geometric parts, because in practice the printed object must withstand its own weight with minimal deformation.21-25 Previous works by Li et al. have included few reports regarding rheological research, and only 2D CNT/PDMS shapes have been actually manufactured via direct writing. Herein, we optimize the rheological properties of CNT/PDMS ink for direct writing, and the development of the ink is presented. Different kinds of inks with various carbon nanotube contents were compared for optimal extrusion and minimal



deformation of the printed structures. Some complicated structures such as woodpiles, tetragonal scaffolds, and gradient mesh structures were fabricated. The printing process allowed the preparation of ink that exhibited shear thinning, self-supporting characteristics and high conductivity, which are well-suited for direct writing. The produced CNT/PDMS structures were dried and heated for solidification. These solidified CNT/PDMS structures can be designed as sensors and actuators, and the feasibility of the process was demonstrated.

2. EXPERIMENTAL 2.1. Materials and PDMS/CNT Composite Ink Preparation The multi-walled CNTs (Time Nano China, TSW3, diameter 10‒20 nm, length 0.5‒2 µm, >98%) were obtained from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences, P. R. China. All of the PDMS components were purchased from Dow Corning (Sylgard 184, Midland, MI, USA). The isopropyl alcohol (IPA) and other organic solvents (>99.9%) were obtained from（Sigma-Aldrich, St. Louis, MO, USA.） Figure 1 illustrates the preparation of the CNT/PDMS ink for printing, wherein the multi-walled CNTs were dispersed in IPA at an IPA:CNT weight ratio of 50:1. The resulting dispersion was then ultrasonicated for 30 min, whereupon The base of the Sylgard 184 silicone elastomer （ PDMS-A ） was added and the mixture was ultrasonicated for another 30 min. The IPA was then evaporated from the dispersion using a magnetic stirrer-heater at a temperature of 80 °C and a stirring speed of 800 rpm. The curing agent from the Sylgard 184 silicone elastomer （PDMS-B） was


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then added and the mixture was stirred well, whereupon the resulting mixture was transferred to a vacuum desiccator to remove any remaining bubbles. Using the above process, a series of CNT/PDMS composite inks containing a varying amount of CNTs from 5 to 9 wt.% were obtained.

Fig. 1. Schematic of the experimental process of ink preparation and the direct-writing process. (a) The multi-walled CNTs (MCNT) and PDMS-A were mixed in IPA; (b) the IPA was evaporated by mechanical stirring and heating; (c) the CNT/PDMS composite was prepared by thoroughly mixing in PDMS-B for cross-linking; (d) the direct-writing process using the CNT/PDMS composites ink.

2.2. Characterization of PDMS/CNT Composite Inks. The rheological behavior of the inks herein were analyzed at 25 °C by a rheometer (MCR302, Anton Paar, Austria) fitted with a parallel-plate geometry (PP35Ti, 35 mm diameter, 1 mm gap). The viscosity of the CNT/PDMS composites was measured as a function of shear rate (  ) using rotational stress from 1 S-1

to 100 S-1. Further, the

storage modulus (G′) and loss modulus (G″) were measured as the rotational frequency,



and was increased from 0.1 to 100 rad‒1.

2.3. Fabrication of Complex 3D Structures by Direct Writing with CNT/PDMS Composite Ink For this work, we modified an additive manufacturing device to meet the technical requirements of direct writing. As shown in Figure 2, the device contained a 3D positioning platform and an extrusion system comprising an air pump, a pressure adjusting valve, and an injection syringe. In the printing process, the first step was to design the CNT/PDMS composite structures via computer-aided design (CAD) by 3D modeling, to generate print paths from these models, and to translate these print paths into G-codes. In the second step, the pump was used to apply an appropriate air pressure into the injector to extrude ink from the needle. The X-Y-Z platform executed code commands in conjunction with the extrusion system to form the structures. The network structures designed herein were a woodpile structure with hollow features, a 3D arched bridge with gap-spanning features and a gradient mesh structure where the number of meshes changed with the ordinal number of its layer. These structures were then created via a layer-by-layer deposition process and were cured at 80 °C for about 2 hours.


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Fig. 2. Experimental set-up for the direct-write printing process

2.4. Characterization of PDMS/CNT Composite Structure Morphological analysis of the printed specimens was obtained using an optical microcamera and a scanning electron microscope (SEM; Zeiss Gemini SEM 500, Germany) equipped with energy dispersive X-ray spectrometry (EDS). The diameters of the printed fibers and the gap-spacing displacement of the CNT/PDMS ink were measured using an optical microscope (Nikon AZ100, Japan). Tensile tests of the printed CNT/PDMS materials were performed to characterize their mechanical properties. Standard test samples were printed via direct writing, whereupon a universal testing machine (CMT2000, MTS Ltd, USA) was used for the tensile modulus tests. The tensile tests were carried out at a gauge length of 35 mm and a stretch velocity of 5 mm/s.

2.5. Performance Characterization of PDMS/CNT Composite Structures The electromechanical driving capacity of rectangular specimens was measured



using the experimental configuration shown in Figure 3. In the setup, metal clips were adhered to both ends of the specimen and connected to a voltage source (Maisheng, direct current (DC) supply, MS305D, China).

Fig. 3. Configuration of the electromechanical driving test before (left) and after (right) a DC voltage is appliedA laser beam was held perpendicular to the underlying plate, where the laser spot was located on the center line of the specimen. The displacement of the midpoint was then measured with the laser rangefinder after a DC voltage was applied. Thermal images and the temperature distribution on the sample were observed simultaneously with an infrared thermal imager (Fluke Ti32, USA). The force output at the midpoint of the strip was measured using a micro-force transducer (Transducer Techniques, GSO-10).

3. RESULTS AND DISCUSSION 3.1. Printability of CNT/PDMS Inks The multiwalled CNTs herein served as a conductive filler and a rheological tailoring agent for the ink. Figure 4(a) shows the electrical conductivity of CNT/PDMS


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composites with varying CNT loadings. The CNT loading was observed to increase the electrical conductivity increased significantly. The composite loading with 10% mass fraction of CNTs can achieve a high electrical conductivity of 173.76 S/m. The viscosity of the ink was tailored by varying the CNT content in solution from 5 to 7 wt.% (Figure 4(b)). Both pristine PDMS and CNT/PDMS composites exhibited behavior typical of a shear-thinning non-Newtonian fluid, which is desirable for controllable extrusion during direct printing.4 Increasing the CNT loading to 5 wt.% resulted in an order-of-magnitude increase in the apparent viscosity, which further improved printability.18 The dynamic oscillatory frequency sweep of the inks was studied ranging from 0.1 to 100 rad‒1, and the measured storage modulus (G′) and loss modulus (G″) are plotted in Figures 4(c) and 4(d). Frequency-dependent behavior consistent with that of pure PDMS ink (i.e., a CNT loading of 0 wt.%) was observed, in which G′ and G″ increased dramatically as the frequency was increased from 0.1 to 100 rad‒1. Further, G′ was smaller than G″, which suggests that the liquid-like properties would not be conducive for direct writing. Increasing the CNT loading above 5 wt.% resulted in increases in G′ and G″. Specifically, G′ dramatically exceeded G″ as the frequency was increased, indicating solid-like behavior. Figure 4(e) demonstrates extruding ink containing 5 wt.% CNTs that was pulled upward into a stretched filament that could support itself, signifying that 3D printing is possible with this ink. For printing inks, the G′ and G″ also affect the ability of the filaments to span gaps during printing.21 This characteristic of the ink could be directly measured by printing the established model using the printer used in this work (Figure S1, Supporting Information).




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Fig. 4. Properties and printability of the CNT/PDMS ink: (a) Electrical conductivity as a function of CNT loading in the PDMS matrix. (b) Apparent viscosity as a function of shear rate for inks with varying CNT content. (c, d) Storage and loss moduli as a function of frequency at 1% strain for the (c) pristine PDMS and (d) CNT/PDMS ink. (e) Extension behavior of yield-stress fluids, where the composite containing 5 wt.% CNTs supported itself in the form of a printed cylindrical ring. (f) CNT/PDMS printability phase diagram. (g) Gap spanning performance as a function of CNT loading and nozzle speed. (h) Gap span model.

Filaments extruded using inks with higher CNT contents (i.e., 7‒9 wt.%) retained their extruded shapes well, which allowed the printing of complex structures with gapspanning features such as scaffolds. These are represented as the complex structures in Figure 4(f). Filaments extruded using inks with lower CNT contents (i.e., 5‒6 wt.%) tended to merge into nonporous monolithic structures, which allowed printing of only simple laminated structures such as the Xi’an Jiaotong University logo and circular columns. These are represented as the simple structures in Figure 4(f). The ink could be printed as 3D gap-spanning structures when the CNT loading was 7‒9 wt.% and the pressure exceeded a certain threshold. When the CNT loading was 9 wt.%, the ink could not be printed owing to liquid spreading and nozzle clogging, respectively. When the printing pressure was below the threshold value, the ink would not ﬂow out of the nozzle. The CNT/PDMS ink gap spanning displacement (L) that could be achieved at various nozzle speeds is shown in Figure 4(g). The value for L reached 3



mm when the CNT loading was 7 wt.% and the nozzle speed was 20 mm/s. The printed filaments readily collapsed when the nozzle speed was 30 mm/s. The optimum nozzle speed for printing gap spanning features was therefore considered to be 20 mm/s.

3.2. Printing of Complex CNT/PDMS Structures Various 3D architectures were then printed using the CNT/PDMS composite with a CNT loading of 7 wt.%. These architectures include a 2D network structure (Figure 5(a)), a freestanding 3D woodpile with hollow features (Figure 5(b)), a 3D scaffold with gap-spanning features (Figure 5(c)), and a gradient mesh structure (Figure 5(d)) wherein the mesh number varies with the ordinal number of its layer. These were all printed using a nozzle with an internal diameter of 290 µm, a pressure of 0.39 MPa, and a nozzle speed of 20 mm/s. A printed 3D computer-aided design model is shown in Figure S2 (see Supporting Information).


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Fig. 5. Digital photographs of the printed CNT/PDMS composite, showing (a) 2D network structure; (b) 3D woodpile structure; (c) 3D scaffold structure; and (d) gradient mesh structure

The internal structure of the printed CNT/PDMS composite was observed using SEM images of fractured cross-sections. Figure 6(a) shows that the CNTs possessing a high aspect ratio were well dispersed in the PDMS matrix, and that the CNTs formed a conducting network in the matrix. The gap spanning structure is observed in the SEM image in Figure 6(b), which shows the small feature size and spanning elements that could be achieved via direct writing. We obtained EDS measurements to determine the elemental compositions of the samples, which indicated that the three main constituent elements were Si, O, and C. The EDS cross-sectional maps of the gap-spanning and



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woodpile structures are shown in Figures 6(c) and 6(d), respectively, where a comparison of the EDS maps of these two different structures show a similar carbon content. This indicates that the CNTs were uniformly dispersed in the PDMS. Further, the cross-print path ensured a uniform dispersion. (a)

(c)

(d)

(b)

Si-map

Si-map

O-map

O-map

C-map

C-map

Fig. 6. (a) Cross-sectional SEM image of the CNT/PDMS composite in the woodpile structure. (b) SEM image of the printed 3D scaffold structure. EDS spectra and element maps of Si (left), O (center), and C (right) for the (c) woodpile (42.36% C, 19.35% O, 38.29% C) and (d) gradient (40.87% C, 25.87% O, 33.20% Si) structures.


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3.3. Mechanical Properties of Printed CNT/PDMS Material The tensile strength of the CNT/PDMS composite (7.0 wt.% CNT content) is shown in Figure 7(a), where the average elastic modulus of the printed CNT/PDMS composite is 1.7 MPa. For the printed composite, the tensile behavior of force versus displacement are found to be linear in nature (Figure 7(b)). Therefore, the printed material has a good linear elasticity and can be beneficial in the fabrication of resistance sensors or soft actuators.

Fig. 7. Mechanical properties of the printed CNT/PDMS material with 7.0 wt.% CNT content, showing the (a) stress vs. strain curves; (Inset) Photograph of the samples used for the stress-strain measurement; and the (b) load vs. displacement curves.

3.4. Performance Characterization and Application Display The resistance measured between the upper and lower faces of the woodpile structure was found to be 90 Ω, demonstrating that the printed CNT/PDMS has good electrical conductivity. We fabricated complete DC circuits composed of a lightemitting diode (LED) and the woodpile structure (Figure 8(a)) and the gradient



structure (Figure 8(b)), wherein the current sufficient to illuminate the LED occurred at 2.3 and 3.5 V, respectively. Increasing the compression force on both sides of the gradient mesh structure led to brighter LED illumination (see Movie S1, Supporting Information), which was attributed to a decreased distance between the CNTs in the

PDMS with compression that induced more interconnections. The response to stepwise increases in pressure by the woodpile structure was then investigated. The resistance of the woodpile structure progressively decreased as weights ranging from 20 to 200 g were placed on its surface (Figure 8(c)), demonstrating the sensitivity of CNT/PDMS to external pressure. This result suggests that the CNT/PDMS can be used to fabricate resistance sensors with varying structures producing varying sensitivities. Increasing the applied pressure resulted in a corresponding increase in the conductivity of the gradient mesh structure, as shown in Figure 8(d). The sensitivities of three different surfaces on the structure (marked A, B, and C in Figure 8(d)) to the external pressure was found to differ. This result demonstrates the feasibility of fabricating sensors with different sensitivities according to application requirements.


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Fig. 8. (a, b) Complete circuit composed of an LED and the (a) high conductivity woodpile structure and (b) the gradient structure, showing the LED response to pressure applied at surface A indicated in (d). (c) Change in resistance of the woodpile structure under different applied weights. (d) Conductivity of the gradient structure against compression strength applied at surfaces A, B, and C.

The printed CNT/PDMS materials were also found to serve as electrothermal actuators. A printed CNT/PDMS strip 40 mm × 5 mm × 1 mm in size was used to qualitatively analyze the actuator ability. The CNT/PDMS composite was placed on a glass substrate and fixed between two metallic clips, as shown in Figure 9(a). When 60 V was applied to the CNT/PDMS composite, the strip expanded along the direction of the applied current. The expansion was restricted by the clips, so the strip developed a



hump in the middle section, as shown in Figure 9(b). In Figure 9(c), the displacement and temperature both rose to a maximal value with time after the DC voltage was applied, and the values did not subsequently change with the constant voltage. When the voltage was turned off, the displacement and temperature returned to their original values. The displacement exhibited the same trend as the temperature and showed good cyclic repeatability. An output force of about 50 was generated under the voltage as the result of a real-time mechanical output test, which is shown in Figure 9(d). To quantify the actuator properties, the strain of the response was measured at 8.9%, which was larger than those reported in previous studies.11, 12

Fig. 9. Qualitative analysis of the actuation of a strip of CNT/PDMS showing (a) apparent linear strain under 60 V of applied voltage, (b) Deformation of each point in the CNT/PDMS strip along the length direction. (c) Actuated displacement (blue line)


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and temperature variation (black line) of the printed CNT/PDMS with the applied 60 V DC voltage off and on (red line). (d) The real-time force output of the CNT/PDMS composite during 60 V voltage-induced actuation

4. CONCLUSIONS In summary, we used direct writing to fabricate CNT/PDMS complex geometries exhibiting high conductivity. The CNT/PDMS ink with tailored rheological properties was used for direct writing, successfully fabricating a woodpile structure, a 3D arched bridge, and a gradient structure using CNT/PDMS ink containing 7 wt.% CNTs. The printed CNT/PDMS structures could be applied in strain and pressure sensing devices, and an electrothermal actuator was demonstrated. The 3D printing of functional materials and devices opens new routes for fabricating sensors for wearable electronics and actuators in soft robots.

Acknowledgements The authors acknowledge financial support from the National Natural Science Foundation of China (Grant No. 91648110). The authors thank Mr. Zijun Ren (Xi’an Jiaotong University) for carrying out SEM measurements. The authors thank Liwen Bianji, Edanz Group China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.



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European Ceramic Society 2017, 37 (6), 2481-2489 [22] Franchin, G.; Wahl, L.; Colombo, P. Direct ink writing of ceramic matrix composite structures. Journal of the American Ceramic Society 2017, 100 (10), 43974401. [23] Smith, P. T.; Basu, A.; Saha, A.; Nelson, A. Chemical modification and printability of shear-thinning hydrogel inks for direct-write 3D printing. Polymer 2018, 152, 42-50. [24] Zhu, P.; Yang, W.; Wang, R.; Gao, S.; Li, B.; Li, Q. Direct Writing of Flexible Barium Titanate/Polydimethylsiloxane 3D Photonic Crystals with Mechanically Tunable Terahertz Properties. Advanced Optical Materials 2017, 5, 1600977.



Graphical Abstract


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• Printing Carbon Nanotube-embedded Silicone Elastomer via Direct Writing •

Bin Luo1,2(#)，Yu Wei2(#) ，Hualing Chen1,2(*)，Zicai Zhu2，Peng Fan1,2， Xuejie Xu2,3, Baojun Xie4 1.State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong University, Xi’an, 710049, China. 2.School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an,710049, China. 3.College of Mechanical and Electrical Engineering, Jiaxing University, Jiaxing， 314001, China.

*Corresponding author email: [email protected] # These authors contributed equally to the work ACS Paragon Plus Environment


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Figure 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


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Figure 4(a)

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Figure 4(b)



Figure 4(c)

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Figure 4(d)



Figure 4(e)

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Figure 4(f)



Figure 4(g)

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Figure ACS Paragon 4(h) Plus Environment


ACS Paragon Plus Environment Figure 5

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Figure 6Plus(a) ACS Paragon Environment



Figure 6(b)

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Figure6(c)



Figure6(d)


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Figure7 ACS Paragon Plus Environment


Figure 8(a) ACS Paragon Plus Environment

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Figure8(b) ACS Paragon Plus Environment


Figure8(c)


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Figure8(d)


Figure9ACS Paragon Plus Environment

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Printing Carbon Nanotube-Embedded Silicone Elastomers via Direct

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