Multimaterial 3D Printing of Highly Stretchable ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23573−23583

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Multimaterial 3D Printing of Highly Stretchable Silicone Elastomers Lu-yu Zhou,†,‡ Qing Gao,†,‡ Jian-zhong Fu,†,‡ Qian-yong Chen,† Jia-pei Zhu,§ Yuan Sun,†,‡ and Yong He*,†,‡,∥ State Key Laboratory of Fluid Power and Mechatronic Systems, School of Mechanical Engineering, and ‡Key Laboratory of 3D Printing Process and Equipment of Zhejiang Province, School of Mechanical Engineering, Zhejiang University, Hangzhou 310027, China § State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Changsha 410082, China ∥ Key Laboratory of Materials Processing and Mold, Ministry of Education, Zhengzhou University, Zhengzhou 450002, China Downloaded via BUFFALO STATE on July 18, 2019 at 14:56:28 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: 3D printing of silicone elastomers with the direct ink writing (DIW) process has demonstrated great potential in areas as diverse as flexible electronics, medical devices, and soft robotics. However, most of current silicones are not printable because of their low viscosity and long curing time. The lack of systematic research on materials, devices, and processes during printing makes it a huge challenge to apply the DIW process more deeply and widely. In this report, aiming at the dilemmas in materials, devices, and processes, we proposed a comprehensive guide for printing highly stretchable silicone. Specifically, to improve the printability of silicone elastomers, nanosilica was added as a rheology modifier without sacrificing any stretching ability. To effectively control print speed and accuracy, a theoretical model was built and verified. With this strategy, silicone elastomers with different mechanical properties can all be printed and can realize infinite time and high speed printing (>25 mm/s) while maintaining accuracy. Here, superstretchable silicone that can be stretched to 2000% was printed for the first time, and complex structures can be printed with high quality. For further demonstration, prosthetic nose, data glove capable of detecting fingers’ movement, and artificial muscle that can lift objects were printed directly. We believe that this work could provide a guide for further work using the DIW process to print soft matters in a wide range of application scenarios. KEYWORDS: silicone elastomers, soft matters, 3D printing, flexible electronics, soft robotics

1. INTRODUCTION Silicone elastomers, due to their excellent flexibility, resilience, adaptability, and biocompatibility, have been widely used in flexible electronics,1−3 soft robotics,4−6 and smart medical devices.7,8 However, the slow cure speed of these silicone materials constrains the fabrication of objects to only traditional methods,9 such as casting,6,10 spin coating,11,12 mechanical removal,13,14 and so forth, which adds cost and limits their applications. Nowadays, numerous researchers have tried to use 3D printing process, that can build complex structures rapidly, to apply silicone elastomers, such as developing new UV curable elastomers,15,16 configurating unique suspension inks,17 embedded printing,18,19 designing special inkjet-printing processes,20,21 and so forth. Among them, the direct inkwriting (DIW) process, in which silicone inks are extruded out of nozzles to form a printed silicone fiber, is a promising solution because of its outstanding advantages in multimaterial printing22,23 and has achieved remarkable progress in flexible electronics,24−26 soft robotics,27−29 organ chips,30−32 biomedical implants,33 and smart composites.34−36 However, limited by the material properties of silicone elastomers (low viscosity and slow cure speed), the previously reported silicone inks © 2019 American Chemical Society

available for the DIW process are limited to several silicone elastomers with good thixotropic properties but ordinary mechanical properties, for instance, Dow curing SE 1700.31,34,37 At the same time, the lack of research on printing devices suitable for the curing characteristics of silicone elastomers and printing process also limits the printable time38 and print speed39 of silicone printing. In short, it is generally believed that the current DIW process for printing silicone materials faces three technical dilemmas: (1) materials: many types of silicone elastomers with excellent mechanical properties are hard to print due to their low viscosity and slow curing speed, while these highperformance materials are viewed as more potential solutions in many application scenarios.31,34 (2) Devices: short printable time caused by that the silicone should be mixed before printing. The most widely used silicone rubber in the DIW process are two-component heat-curing silicones, which are needed for mixing the two components together to cure. In existing reports, traditional syringes were used to store silicone Received: March 19, 2019 Accepted: June 11, 2019 Published: June 11, 2019 23573

DOI: 10.1021/acsami.9b04873 ACS Appl. Mater. Interfaces 2019, 11, 23573−23583

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ACS Applied Materials & Interfaces

Figure 1. Research purposes, main contents, and application prospects of this comprehensive guide.

process, the diameter of the printed fiber can be accurately predicted and controlled. Also, by choosing appropriate nozzle and air pressure, the printing speed can be significantly improved while maintaining the resolution of the printed fiber. With this comprehensive guide which covers material configuration, device design, and print implementation, multimaterial complex structures composing silicone rubbers with different mechanical properties can be printed directly, which could be an ideal method for medical prosthesis. What is more, some flexible circuits/sensors and artificial muscles were printed directly using this guide, which demonstrates their application prospects in the fields of flexible electronics and soft robotics.

inks during the printing process,29−35 where silicone inks were mixed beforehand. However, silicone would still cure at room temperature leading to inks’ rheological properties unstable during printing, which in turn limits the pintable time and the size of the printed structure. (3) Processes: fixed resolution of the printed fiber and slow printing speed are limited by the nozzle diameter. In most DIW printing processes, whether to print silicone rubber26,30,35 or other soft matters such as hydrogels,40−42 print parameters are mostly determined by trial and error and rarely change as needed during printing. This is caused by the lack of more systematic exploration of factors affecting fiber resolution and printing speed during printing, making these adjustable performances limited by the nozzle diameter. Here, we proposed a comprehensive guide for multimaterial 3D printing of highly stretchable silicones, as shown in Figure 1. With this guide, silicone rubbers with different mechanical properties can all be adjusted to be printable and realize infinite time and high speed printing (>25 mm/s) while maintaining printing resolution. The super-stretchable silicone rubber that can be stretched by up to 2000% was printed for the first time here. First, in order to improve the printability of silicone rubber, nanopowder was added as a rheology modifier, and furthermore, the theoretical analysis and experiments of rheology improvement mechanism show the versatility of this strategy. What is more, the addition of nano additive not only can improve the printability of silicone rubber but also can effectively improve the mechanical properties of printed silicone structures. Second, a novel printing device was used to realize a printing process of printing while mixing, where two components of silicone ink were stored separately and mixed while printing, thereby achieving an infinite extension of the printable time. Subsequently, a theoretical model for printing process was established and experimentally verified. Under the guidance of this model, by simple calibration

2. RESULTS AND DISCUSSION 2.1. Material Modification. As a kind of ink suitable for the DIW process, silicone ink should possess the correct rheological property in order for printing the parts to retain their shape before they are cured effectively. This rheological property is called thixotropy or shear thinning.43,44 However, common silicone rubbers do not have such characteristics, which makes them difficult to apply in the DIW process to print silicone materials. Nowadays, some trials are reported by adding nanorheology modifiers into unprintable materials to improve their rheological properties.41,45,46 Among various of nanorheology modifiers, nanosilica (NS), a cheap nanoadditive usually used to reinforce rubbers in industry, has been proven to improve rheological properties of many materials, including water,47 polyurethane,48 and resins.49 Lewis and her co-workers also added NS into silicone rubbers to make them an ideal hyperelastic body matrix in embedded printing27,50 which requires good shape retention of matrix materials. In some state-of-the-art reports, researchers have tried to use silica to improve the printability of silicone elastomers and used it to print superhydrophobic mem23574


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Figure 2. Principle and implementation of 3D printing of silicones. (A) Mechanism of rheological modification of silicone rubbers. (B) Rheology behavior of the investigated composite silicone inks: rheological data obtained under (I) steady-shear and (II) oscillatory conditions for inks with increasing concentration of 0, 4, and 8 wt % NS. (C) Optical photograph of the extruded fiber and printed structure using silicone inks with different components. Scale bar, 5 mm. (D) Novel mixing device for 3D printing silicones.

the fluid changes from purely Newtonian to a viscoelastic behavior with significant shear thinning effect (Figure 2B(I)). Increasing the NS content from 0 to 8 wt % allows us to increase the low-shear viscosity of the silicone ink by two orders of magnitude, reflected in the state of the fluid, that is, the system gradually changes from the sol state to the gel state (Figure 2B(II)). As shown in Figure 2C, silicone inks with different NS concentrations were extruded to form fiber and print structures. Results show that the viscoelastic nature of inks with high NS concentrations is crucial to prevent distortion of printed objects. As analyzed before, this rheological modification process is totally physical and versatile which can also be proven by the rheological test of more silicone rubbers. More complex silicone structures printed using modified silicone inks are shown in Figure S3. Therefore, with this strategy many different silicone rubbers with different mechanical properties were successfully printed, which demonstrates the versatility of this guide. 2.2. Printing Device. After preparing the silicone ink, how to design a printer suitable for silicone rubber to make the printing process stable is another issue. Some work added other curing retardants into epoxy-based inks to extend the printable time.45 However, the addition of curing retardants may have unpredictable effects on the performance of silicone elastomers. Therefore, it is better to design a new device that can combine the mixing process and printing process. Here, in this report, in addition to a common three-axis motion platform with multiple independent nozzles and a pneumatic feeding system, a novel mixing device was designed and used according to the curing characteristic of the twocomponent mixing of the silicone rubber. As shown in Figure 2D, the two components of silicone rubber are stored separately, and when air pressure is applied to the piston, these two components will be extruded and mixed in the mixing tube where the orthogonally arranged curved mixing blades continuously cut the fluid for complete mixing. Then, the mixed silicone ink will be extruded out of a nozzle to form

branes51 and shape-shifting soft materials.52 These studies are more concerned about the practical application in one field but did not investigate the mechanism of modification, the printing settings, and the mechanical properties after modification. In this report, a comprehensive study is carried out which can help to understand the influence of the addition of NS on the physical properties of silicone elastomer and could become a guide for more extensive applications. Based on these, hydrophobic fumed NS (as shown in Figure S1) was added into common silicone rubbers to make them printable. As shown in Figure 2A, the microscopic mechanism for improving the rheological properties of silicone rubbers with NS can be summarized as: (1) the interactions between the reactive hydroxyl groups on the silica and organic macromolecular chains. (2) The interaction between silica particles.53,54 When silica particles are sufficiently dispersed in the silicone, the “bridges” formed by the silica−silicone−silica will form a uniform three-dimensional network structure, thereby increasing the viscosity of the system significantly. Meantime, the connections of this network structure are mostly physical, which means that they are broken easily while being subjected to mechanical force (shear force), leading to a decrease in viscosity of the system. When the shear force is removed, these physical connections are reformed again, making the viscosity of the system to recover. Based on this, silicones with NS will exhibit a sol state when extruded to form a silicone fiber, while will exhibit a gel state to keep their shape when printed as complex silicone structures. Further, the effect of the addition of NS on the rheological behavior of an exemplary silicone rubber was studied through oscillatory and steady-state measurements. (For convenience, the exemplary silicone rubber is used here, and the rheological characterization of more silicone rubbers is shown in Figure S2). From the rheological data obtained, the steady-state viscosity and the state of the fluid were qualified, as depicted in Figure 2B. On the addition of increasing concentration of NS to this exemplary silicone rubber, the rheological response of 23575


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Figure 3. Theoretical model to predict and control the diameter of the printed fiber. (A) Schematic illustration of typical parameters for DIW 3D printing. (B) Experiments of the theoretical model for printed fiber diameter: (I) the optical photograph of the fiber shape as a function of speed. Scare bar, 1.5 mm and (II) comparison of theoretical and experimental data. (C) Simplified theoretical model compares with experimental data at (I) different air pressure P and (II) different nozzle diameter D.

a printed fiber steadily and continuously. This device for simultaneously mixing materials while printing can keep inks’ rheological properties stable during printing and extend the printable time indefinitely. This printing device is not only suitable for a certain silicone rubber. Most silicone rubbers need to mix their two components before use,29−35 which makes this device suitable for various kinds of silicones. Also, in the printing of complex structures and further applications below, different silicone rubbers were printed using this device, which also proves their versatility. 2.3. Theoretical Model. In the DIW process, printing process of complex structures can be divided into two steps: the formation of fibers and the deposition of extruded fibers on the three-axis motion platform. This is a typical polymer extrusion and deposition process, which has been widely studied for several decades.38,55,56 However, in current reports of 3D printing of silicone elastomers, a single setting of printing parameters was obtained by trials and errors and did not change during printing,26,30,35 which greatly limits the speed and accuracy of printed silicone structures. This is because that there is still a big gap between complex basic theory and practical operations. Here, a comprehensive analysis of the printing process was created, based on which, the printing process can be easily predicted and controlled. The typical parameters for DIW 3D printing are shown in Figure 3A. To form a continuous filament, the silicone ink is extruded out of the nozzle with a diameter of D at a speed of c by applying an air pressure of P. As a viscoelastic ink, the extrusion of silicone ink will lead die-swelling of the extruded fiber,55,56 which makes the extruded fiber diameter αD different with the nozzle diameter D. Long-term studies55−57 have shown that the die-swelling ratio α is related to polymer material properties, air pressure, die diameter, and even

extrusion equipment shape, which makes it too hard to predict α directly with controllable printing parameters. Meantime, the nozzle moves at a speed of V and a height of H (from the substrate or the printed layer) to deposit the extruded fibers. During the deposition of extruded fibers, as the speed V increases, the shape of the printed fiber will form three different types: curve fiber, straight fiber, and discontinuous fiber, as shown in Figure 3B. The straight fiber, which is viewed as the ideal one used in the DIW process, is achieved when the speed is higher than a critical move speed (CMS). Here, CMS equals to c. Further, the straight fibers will be towed and stretched during printing, resulting in the diameter of printed fiber d to be smaller than that of the extruded fiber αD. As the stretch increases, the diameter of the printed fiber will change from that larger than nozzle diameter D (die-swelling fiber) to equal (equal-diameter fiber) and then smaller (thinned fiber). When the speed V reaches the limiting move speed (LMS), the printed fiber will break. Also, LMS is determined by the material property.58 Here, for the exemplary silicone ink, it approximately equals 3.25c, given by experimental measurements. More specifically, during the printing of straight fiber, according to the volume conservation, the diameter of the printed fiber can be expressed as d = αD / V / c

(1)

where α and c cannot be quantitatively controlled, which makes this equation meaningless for guiding the actual operation. However, in the DIW method, printing speed V is the main controlled parameter because the nozzle diameter D is fixed and the change in air pressure P has hysteresis. Also, then the eq 1 can be transformed into 23576


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Figure 4. Effect of printing parameters on extruded fibers. (A) Effect of the air pressure P on (I) die-swelling ratio α, extrusion speed c, and (II) fiber diameter d. (B) Effect of the nozzle diameter D on (I) die-swelling ratio α, extrusion speed c, and (II) fiber diameter d.

Figure 5. Mechanical test and complex structures with different silicone printed. (A) Mechanical properties of (I) silicone specimens printed with different silicone rubbers and (II) modified and unmodified silicone specimens made by different methods. (B) Optical photograph with sample printed with super-stretchable silicones: (I)sample before stretching and (II) sample after stretching. Here the elongation at break is up to 2000%. (C) Sample printed with multimaterials: (I) optical photograph of the printed sample with multiple materials, (II) sample before stretching and (III) sample after stretching. (D) Complex 2D structures printed with exemplary silicones: (I) text of “3D Printing”, (II) flower and (III) negative Poisson’s ratio structure. (E) Complex 3D structures printed with exemplary silicones: (I) three-dimensional five-pointed star, (II) quadrangular pyramid, and (III) prosthetic nose. Scale bar, 6 mm.

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Figure 6. Flexible electronics printed with this guide. (A)Whole process of printing a strain sensor: (I) printing the base layer, (II) the circuit layer, and (III) the sealing layer and (IV) its optical photograph. (B) (I) optical photograph and (II−IV) performance experiments of a printed biaxial strain sensor: (II) plot of ΔR1 over time during a triangular x-direction strain cycle, (III) plot of ΔR2 over time during a triangular y-direction strain cycle, and (IV) plot of ΔR1/R10 vs strain. (C) (I) Optical photograph of a glove with embedded strain sensors and (II) electrical resistance change as a function of time for strain sensors within the glove at five different figures. Scale bar, 6 mm.

d 2 = d1

V1 V2

These phenomena indicate that by choosing appropriate nozzle and air pressure, the printing speed can be improved significantly, while maintaining the printing accuracy. It is worth mentioning that this theoretical model is not limited in a specific silicone material or under some certain parameters. The measurement of the diameter of printed fiber of one arbitrary speed (between CMS and LMS) can be viewed as a calibration operation, with which the printing resolution can be accurately predicted and controlled. What is more, by choosing appropriate nozzle and air pressure, the printing speed can be effectively improved while maintaining the fiber resolution. 2.4. Performance Testing and Printed Structures. The guide described above that whether material modification, printing devices, or theoretical models are versatile for various silicone rubbers. Based on this guide, standard tensile specimens (shown in Figure S4 and Video S1) were printed with different silicone rubbers to test the mechanical properties, which can also show the diversity of this guide in material selection. The formulas and other physical properties can be found in Table S1. Figures 5A(I) and S5 show the comparison of different printed silicones on three important mechanical properties: elongation, Young’s modulus, and Shore hardness. It can be seen that silicone rubbers with different mechanical properties can all be printed, and by appropriate combination, the mechanical properties can be also customized on demand (hard mixture and soft mixture). What is more, the printing direction and the diameter of printed fiber have no significant influence on the elongation and strength of printed silicone structures, as shown in Figure

(2)

where d2 is the intended fiber diameter, while d1 is the existing fiber diameter. Also, V1 and V2 are the speed values before and after the change, respectively. With this model, by measuring a simple basic diameter at one speed, the diameter d of all different speed (between CMS and LMS) can be easily predicted and controlled, which can be proved by Figure 3C. The results show that this model has great accuracy in different conditions, whose maximum error does not exceed 6%. What is more, to offer an empirical guide, it is necessary to study the relationship between the die-swelling ratio α, extrusion speed c, and the controllable printing parameters, that is, air pressure P and nozzle diameter D. Figure 4A shows the effect of the air pressure P. It can be observed that as the air pressure P increases, the die-swelling ratio α and extrusion speed c both increases, which is because that the increase in extrusion force causes a larger steady state speed and extrusion deformation, further leading to an increase in recovery swelling at the outlet.56 Based on this effect, printing speed V can increase significantly in the same fiber diameter d. Figure 4B shows the effect of the nozzle diameter D. As the nozzle diameter D increases, the extrusion speed c increases while the die-swelling ratio α decreases. Also under the combined effect of D, c, and α, in the same fiber diameter d, printing speed V can also increase significantly. 23578


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Figure 7. Artificial muscles printed with this guide. (A)Printing process of the artificial muscle: (I) printing base cylinder and (II) printing stiffener, and (III) its optical photograph after printing. (B) Geometry, simulation parameters, and finite element model. (C) Equivalent elastic strain and actual experimental shapes of the artificial muscle. (D) Axial contraction under different pressures obtained by the finite element model, analytical model, and experiments. (E) Optical photograph of using artificial muscles to lift objects. Scale bar, 13 mm.

similar phenomenon does not exist in the unmodified silicone, as shown in Figure S8B. What is more, after heating the stretched specimens, the flexibility can be restored. The stability of the strain−stress curve under multiple cycles also shows that the addition of NS will not influence the resilience of silicones. Therefore, the printed silicone structure can show a super stretchability (break elongation up to 2000%), as shown in Figure 5B and Video S2, which is reported for the first time. Figure 5C shows a printed multimaterial structure with different mechanical properties in different parts, which demonstrates the convenience of this guide in multimaterial printing. Figure 5D,E further shows more complex structures printed using this guide, including: the text of “3D Printing”, flower and negative Poisson’s ratio structure61 for 2D structures, and three-dimensional five-pointed star and quadrangular pyramid for 3D structures. What is more, very complicated structure such as the prosthetic nose can be printed owing to the adjustable rheological properties of silicone inks and stable and predictable printing process. The ability to build complex structures with multiple materials directly makes this guide a potential method for widespread use in the medical field, not only in organ chips30,31 but also in medical prosthesis.62−64 The whole printing process can be biocompatible because NS is an FDA-approved material that can be added in human implants and has little influence on the transmittance of silicone (Figure S9). There have been reports of direct printing of silicone materials to fabricate facial

S6. This is because although the silicone structures are printed fiber-by-fiber and layer−by-layer, the entire structure is printed as a solid and cured at the same time, which allows the structure to have nearly identical mechanical properties in different directions. Figure 5A(II) shows the comparison of unmodified pure silicones, cast silicones, and printed silicones. It can be observed that compared with unmodified pure silicones and cast silicones, printed silicone structures have better mechanical properties. This is because NS inside the silicone structure creates stronger interactions than the unmodified one, and the layer-by-layer manufacturing process builds a denser structure than the cast one which easily has defect inside due to the poor fluidity. It can be found that the addition of NS can improve the mechanical properties of silicone elastomers. The influence of additive concentration of NS on the mechanical properties of silicone elastomers is shown in Figure S7. It is easy to understand because more NS means stronger interactions in silicone elastomers. The better elongation of modified silicones can also be explained by the loading−unloading cycles. Addition of NS makes stronger interactions in the cured silicones. These interactions will be weakened or even broken in the stretch process, which dissipates energy, resulting in the hysteresis effect59 and facilitating system’s stretchability.38,60 Figure S8A shows an obvious effect of softening and hysteresis effect indicating the presence of energy dissipation.59 More specifically, in the loading−unloading cycles, the modulus drops significantly, especially from cycle 1 to cycle 2, while 23579


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ACS Applied Materials & Interfaces prostheses63 or other medical supplies.64 This comprehensive strategy that we proposed could be a guide for the potential applications in this field. To further demonstrate the potentials of this guide in flexible electronics, strain sensors, multilayer sensing devices, and strain sensors embedded in wearable objects were fabricated. Figure 6A shows the whole printing process. With this guide, packaged flexible strain sensors (Figure 6A(IV)) can be printed with a multimaterial 3D printer. Furthermore, more complex circuits, for example, multilayer sensing device, which is hard to fabricate with traditional processes, can also be printed directly, as shown in Figure 6B. Experimental data show that this biaxial strain sensor has excellent repeatability, consistency, and ideal linearity, which is necessary for practical applications. Adjustable mechanical properties of silicone rubber is a major advantage of this guide, reflected in the wearable device, that is, silicone elastomers with high flexibility and low hardness can be used, making the printed sensors readily attached to a variety of deformable surfaces including human skin. In another embodiment, strain sensors embedded in gloves were printed to monitor digital movement of user’s hands (Figure 6C). Specifically, cylindrical sensors, which are easy to mount on the fingers, were printed with soft-as-skin silicones. After placing it on the hand, we can monitor the movement of the fingers through significant changes in resistance. Collectively, these demonstrations show how this guide offer the ability to directly print complex flexible electronics using various types of silicones with adjustable mechanical properties in an efficient and reliable way. Further, bionic artificial muscles65 were printed to demonstrate the ability of this guide to be applied to soft robotics, as shown in Figure 7. Figure 7A shows the process of printing one kind of artificial muscle. Here, the versatility of our guide makes the multimaterial artificial muscle anisotropic, which is endowed with silicone rubbers with different mechanical properties. The anisotropic strain upon pressurization can lead to useful deformations, which can be predicted by the finite element analysis (FEA) model (Figure 7B). What is more, building an effective analytical model is meaningful for further understanding the working principle of this artificial muscle. Assuming that the radial stiffness is dominated by the soft silicone and the axial stiffness is dominated by the stiff silicone, the established lamination theory can be used to obtain the contraction of artificial muscle at different pressure ΔZ = L −

2L ij θ yz sinjj zz θ k2{

Based on this, by changing the applied pressure, this multimaterial artificial muscle will produce different degrees of axial contraction, which can be used to lift objects (Figure 7C,E). Also, the experimental and theoretical data under no load are shown in Figure 7D. It is worth mentioning that here this simple shape was deliberately chosen as an exemplary model, which can be studied in detail using the analytical model. Actually, this guide can be used to print more complex soft robots that goes far beyond this model, which can be seen in Figure S10 and Video S3.

3. CONCLUSIONS In summary, we report a comprehensive guide for multimaterial 3D printing of highly stretchable silicones. By modifying the rheological properties of the silicone inks with NS, almost all kinds of silicones with different mechanical properties (even super-stretchable one that can be stretched by up to 2000%) can be adjusted to be printable. To extend the printable time infinitely, a novel mixing device is designed and used. To make the printing process faster, finer, and more controllable, a theoretical model with all print parameters is set up, and based on this model, by choosing appropriate printing parameters, the printing speed can be effectively improved while maintaining printing accuracy. With this guide, complex 2D/3D structures composed of multiple materials can be directly printed, which can be an ideal promising solution to fabricate medical prosthesis, flexible electronics, and soft robotics. We believe that this work could provide a guide for further work using the DIW process to print soft matters in a wide range of application scenarios. 4. EXPERIMENTAL METHODS 4.1. Materials System. The silicone inks were obtained by mixing NS (Degussa AG, Germany) with different kinds of silicone rubbers, the suppliers and the main ingredient of silicone rubbers and the formulations of silicone ink can be found in Table S1. The two components of all silicone inks were mixed in an electric rotary agitator (JJ-1, Shanghai Qiuzo Scientific Instrument Co., Ltd, China) at 2000 rpm for 2 h and then vacuum-defoamed. For images in the figure, red pigment for silicone elastomers was added in 1 wt % of the total elastomer mixture. The exemplary silicone used in rheological characterization is HF7709. The conductive ink used in Figure 6 was prepared by homogenizing the as-received carbon conductive grease (AMKE Special Lubricants, China) for 2 min at 2000 rpm in the electric rotary agitator. 4.2. Rheology Characterization. Rheological properties of the silicone inks were measured by a rheometer (Discovery HR-2) equipped with a cone-plate with a diameter of 40 mm, a truncation gap distance of 50 μm, and a cone angle of 1.985°. To measure the viscosity, these silicone inks were loaded with steady rate sweeps within a shear rate range of 0.01−100 s−1. To measure the storage modulus (G′) and loss modulus (G″), frequency sweep tests were conducted in the linear viscoelastic region at a strain of 1.0%. The tests were all performed at 25 °C. 4.3. Print Procedure with Integrated Profiling. Printing was carried out using customized 3D printer EFL-BP6600 with an independent rotary axis from Suzhou Intelligent Manufacturing Research Institute Inc., Suzhou, China. Syringes, dispensing needles, and digital pneumatic regulator were used for extrusion, besides the mixing device. Nozzle specification: silicone inks: 22G, 23G, and 25G, conductive ink: 25G. After printing, all the samples were cured in a constant temperature drying oven at 80 °C for 30 min. 4.4. Sample Preparation and Characterization. The printed silicone specimens were directly printed using the customized 3D printer according to the dimensions in Figure S4. The cast silicone specimens were cast in 3D printed casting molds. Mechanical tests

(3)

where L is the original length of the artificial muscle and θ is the central angle of the arc formed by the warp after expansion, which can be obtained by the following transcendental equations 1+

PDa 2θ yzz D 1 ijj PDa jj zz = a − j z * 2t 8Lt { D0 Esoft k

Da = D0 +

2L jij i θ yzy j1 − cosjjj zzzzzz θ jk k 2 {{

(4)

(5)

where Da is the diameter of the largest diameter radial section circle, D0 is the original diameter of the radial section circle, P is the applied pressure, and t is the average thickness. (The details of the formula derivation can be found in the Supporting Information). 23580


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ACS Applied Materials & Interfaces were performed at 40 mm/min at a preforce of 0.01 N using electronic universal testing machine UTM2203 (Shenzhen Suns Technology Co., Ltd, China). Young’s modulus was determined between 0.5 and 5% strain. Reported values for ultimate tensile strength, Young’s modulus, and elongation at break were averaged over at least five samples. Also, a Shore A Durometer (Aice Instrument Co., Ltd, China) was used to measure the hardness of the silicone. 4.5. Sensor Performance. The experiment settings can be seen in Figure S11. High-precision digital multimeter Agilent 34401A (Agilent Technologies Inc., USA) was used to complete the resistance measurement. Two linear motors controlled by a high-precision multiaxis motion control card (GT2400-ACC2-VER2.4-AD-DA) was set to produce the mechanical movement, making the system motion error less than 0.01 mm. The sensors were mounted on this setup and were stretched at 0.5 mm/s for 50 times. 4.6. FEA Simulations. Finite element analyses were performed in ANSYS 14.0 to simulate the shape response of the artificial muscle to change in applied pressure. The superelastic material model was used to model the silicone materials, whose precise data were given by the mechanical testing. 4.7. Statistical Analysis. Data are presented as mean ± standard deviation of independent replicates. Statistical analysis is conducted using ANOVA, and single asterisk (*) indicates significant differences between groups (p < 0.05), and double asterisk (**) indicates extremely significant differences between groups (p < 0.01).

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LR17E050001), and the Fundamental Research Funds for the Central Universities.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04873.

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REFERENCES

SEM image of NS, complex structures printed using unmodified silicone inks and modified silicone inks, size and optical photographs of the tensile specimen, formulations and other properties of different types of silicones, strain−stress curve of different silicone specimens, effect of printing settings on the mechanical properties of tensile specimens, influence of concentration of NS on silicone elastomers’ physical properties, loading−unloading curves of silicone specimens, optical photograph of a bend actuator whose bend varies with air pressure, the experimental setup for sensor performance testing, and analytical model of the artificial muscles (PDF) Printing process of a tensile specimen (MP4) Stretching super-stretchable silicone sample (MP4) Bending actuator driven by air pressure (MP4)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lu-yu Zhou: 0000-0003-0568-0108 Yong He: 0000-0002-9099-0831 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper was sponsored by the National Nature Science Foundation of China (nos. 51622510, U1609207), the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (no. 51821093), the Nature Science Foundation of Zhejiang Province, China (no. 23581


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