3D Printing of Ultratough Polyion Complex Hydrogels - ACS Applied

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3D printing of ultra-tough polyion complex hydrogels Fengbo Zhu, Libo Cheng, Jun Yin, Ziliang Wu, Jin Qian, Jianzhong Fu, and Qiang Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09881 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 26, 2016

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ACS Applied Materials & Interfaces

3D Printing of Ultra-Tough Polyion Complex Hydrogels

Fengbo Zhu,3† Libo Cheng,1† Jun Yin, 1* Zi Liang Wu,2* Jin Qian,3* Jianzhong Fu,1 Qiang Zheng2

1. The State Key Laboratory of Fluid Power Transmission and Control Systems, Key Laboratory of 3D Printing Process and Equipment of Zhejiang Province, School of Mechanical Engineering, Zhejiang University, Hangzhou 310028, China 2. MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China 3. Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Department of Engineering Mechanics, Zhejiang University, Hangzhou 310027, China

† These authors contribute equally to this work. * Corresponding authors. E-mail: [email protected] (J.Y.), [email protected] (Z.L.W.), [email protected] (J.Q.)

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Abstract: Polyion complex (PIC) hydrogels have been proposed as promising engineered soft materials due to their high toughness and good processibility. In this work, we reported manufacturing of complex structures with tough PIC hydrogels based on three-dimensional (3D) printing technology. The strategy relies on the distinct strength of ionic bonding in PIC hydrogels at different stages of printing. In concentrated saline solution, PIC forms viscous solution, which can be directly extruded out of a nozzle into water, where dialyzing out of salt and counterions results in sol-gel transition to form tough physical PIC gel with intricate structures. The printability of PIC solutions was systematically investigated by adjusting the PIC material formula and printing parameters, in which proper viscosity and gelation rate were found to be key factors for successful 3D printing. Uniaxial tensile tests were performed to printed single fibers and multi-layer grids, both exhibiting distinct yet controllable strength and toughness. More complex 3D structures with negative Poisson's ratio, gradient grid and material anisotropy were constructed as well, demonstrating the flexible printability of PIC hydrogels. The methodology and capability here provide a versatile platform to fabricate complex structures with tough PIC hydrogels, which should broaden the use of such materials in applications such as biomedical devices and artificial tissues.

KEYWORDS: polyion complex hydrogel, 3D printing, tough hydrogel, printability, gelation, rheology

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1. Introduction Conventional hydrogels are often mechanically weak due to their highly swollen nature and heterogeneous network structure,1 and these hydrogels are limited to the applications in which the mechanical performance is not highly concerned. Recently, hydrogels have been applied to load-bearing components of actuation devices, medical implants,2 and artificial tissues,3-5 thus new demands are set to achieved enhanced mechanical properties for hydrogels.6 For example, large extensibility and high toughness are required for hydrogels to mimic the nature of biological tissues such as muscle and tendon, and it is desirable for the medical implant hydrogels to match the stiffness of surrounding biological tissues in a wide range from kPa- to GPa-level to avoid inflammation.6,7 Tremendous progress has been made in developing tough and flexible hydrogels for a variety of engineering applications.8-14 Inspired by the structures of cartilages, Gong et al. developed double-network (DN) hydrogels with extraordinary strength, which can sustain force and deformation to large extents without failure.15 More strategies and concepts have been developed for hydrogel materials in terms of different toughing mechanisms, including nanocomposite hydrogels, topological hydrogels, and dual-crosslinked hydrogels.16-20 However, these tough hydrogels suffer a major disadvantage of poor processibility due to their permanent network structure.21 For practical applications with diverse and complex configurations, such as those in biomedical devices and artificial tissues, hydrogels with both good processibility and mechanical performance are desired.22 Recently, one type of tough supramolecular hydrogels was obtained by polymerizing one charged monomer in the presence of oppositely charged polyelectrolyte with equivalent charge, termed as polyion complex (PIC) hydrogels.23 Excellent mechanical performance has been achieved due to the wide spectrum of bonding strength in PIC: strong associations serve as quasi-permanent crosslinks to maintain the integrity of the material, whereas weak associations behave as dynamic and sacrificial bonds to dissipate energy.23,24 Furthermore, these PIC hydrogels exhibit distinct mechanical strength after swelling them in saline solution with different 3



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concentrations, implying different status of ionic bonding.25 The tough PIC gels were found to be dissolved in highly concentrated saline solution, forming transparent viscous solutions, which were gelled into opaque tough hydrogels after being immersed into water to dialyze out the salt, resulting in recovery of ionic strength.25 Based on this reversible sol-gel transition, continuously extruding of PIC solutions into water produces tough PIC gel fibers.26-28 If a post-formed gel fiber faithfully reflects the imposed shape and strongly adheres to the pre-formed ones, tough physical gels with intricate structures are expected to be fabricated using three-dimensional (3D) printing technology. The objective of this study is to provide a promising 3D printing technology for fabricating tough hydrogels into complicated 3D structures. As an emerging technology for fabricating complicated structures, 3D printing shows its potential to quickly produce 3D objects with soft hydrogels for applications in tissue engineering, artificial organs, soft actuators, etc.29-33 During 3D printing, the pre-gel materials are usually patterned into a designed structure and subsequently fixed by a physical or chemical gelation process.30,34 Tough double network hydrogel comprised of poly(ethylene glycol) and sodium alginate was printed for cell encapsulation,22,35 but post photo-initiated polymerization was needed after the printing process to form the covalent network and toughen the hydrogel. In this study, we demonstrate an extrusion-based 3D printing of tough physical hydrogels into complex structures through direct sol-gel transition, without requiring post

photo-treatment.

PIC

precipitates

from

mixed

poly(3-(methacryloylamino)propyl-trimethylammonium chloride) and poly(sodium p-styrenesulfonate) (PMPTC and PNaSS) were plasticized by saline water and used as raw materials for 3D printing. These raw materials, initially weak and viscoelastic, were easily extruded out of nozzle into water with prescribed shapes. The sequential gelation of pre-gel solution after dialyzing out the salt and counterions of PIC produces tough equilibrated gels with the imposed shapes. We investigated the effects of material properties and printing parameters on the printability of PIC pre-gel and mechanical properties of printed hydrogels. Compared to previous work of processing 4


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tough gels, the present 3D printing based on direct sol-gel transition of tough hydrogels is facile yet powerful to fabricate complicated structures, which should be applicable to other tough physical hydrogels with dynamic noncovalent bonds.

2. Materials and Experiments 2.1. Materials and PIC solution preparation Sodium p-styrenesulfonate (NaSS; anionic monomer, 90 wt% purity), 3-(methacryloylamino)propyl-trimethylammonium

chloride

(MPTC;

cationic

monomer, 50 wt% aqueous solution), 2-oxoglutaric acid (photoinitiator) were purchased from Sigma-Aldrich and used as received. NaCl was received from Sinopharm Chemical Reagent Co., Ltd. Millipore deionized water was used in all the experiments. The detailed procedure of preparing the PIC solutions has been described in our previous work.25 In brief, PNaSS and PMPTC were synthesized by polymerizing the precursor aqueous solutions of 1 M NaSS and 1 M MPTC, respectively, in the presence of 0.05 mol% (relative to the monomer) 2-oxoglutaric acid under UV light irradiation (365 nm wavelength, 7.5 mW/cm2) for 8 h. Thus, resultant viscous liquids were precipitated in ethanol and dried in the oven to obtain PNaSS and PMPTC powders. The PNaSS and PMPTC polymers were dissolved in deionized water to prepare solutions with prescribed concentrations. Then, PNaSS and PMPTC solutions with equal volume were slowly dripped into 250 mL deionized water and stirred for 30 min, forming compact PIC precipitates. The precipitates with different charge ratio of PNaSS/PMPTC were collected and dried in the oven at 110 oC and made into powder. A certain amount of 4 M NaCl solution was added to the powder to plasticize the PIC, which was stirred at 90 oC for 24 h to obtain a homogeneous solution with prescribed PIC polymer content.

2.2. 3D printing of PIC hydrogels

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Figure 1. The schematic of 3D printing system and sol-gel transition mechanism. The viscous PIC solution was loaded into a syringe and extruded pneumatically into deionized water via a customized 3D printing platform. In water, the viscous PIC solution rapidly gelated into tough solid gel by dialyzing out the salt and counterions of PIC.

A 3D printing system was customized to print PIC hydrogel structures. The 3D printing system mainly consists of an extrusion-based dispersion system, and a 3D positioning stage (Figure 1). The extrusion system consists of an air pump, a pressure controller, and heated syringes with temperature control, which can be equipped with multi-nozzles. During the printing process, saline water plasticized PIC solution was loaded into selected syringe and pneumatically extruded into deionized water reservoir under room temperature, where the PIC solution gelated into solid gel rapidly. To characterize the feasible conditions for printability, PIC solutions with different polymer contents and charge ratios of PNaSS/PMPTC were tested under certain printing setup (Table 1). Then, several representative structures were constructed to demonstrate the capability of the present method, which were printed using the PIC solution with PNaSS/PMPTC charge ratio of 1.1:1 and wPIC/VNaCl of 1/15 g/mL, wPIC and VNaCl being the mass of PIC in the solution and the volume of 4 M saline solution, respectively. The printed gels were swelled further in water for at least 5 days and pictured by the digital camera or optical microscope (Nikon AZ100). Grid samples were freezing-dried and coated with a thin layer of gold for the observation under scanning 6


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electron microscope (Hitachi SU8010).

Table 1. Materials and printing parameters Parameter Charge ratio of PNaSS/PMPTC wPIC/VNaCl of PIC solution (g/mL) Extrusion pressure (kPa) Nozzle moving speed (mm/s) Nozzle inner diameter (mm) Syringe temperature (°C) Room temperature (°C)

Value 0.95:1, 1.0:1, 1.1:1, 1.15:1, 1.2:1 1/10, 1/12, 1/15, 1/17, 1/20 206.8 - 379.2 8, 10, 12 0.26 20, 60 20

2.3. Rheological and mechanical tests The rheological behaviors of PIC solution were analyzed using an ARG-2 rheometer (TA instruments, USA) equipped with a cone-plate (40 mm, 1.99°), and the shear rate was varied from 0.01 to 1000 s-1. The influence of temperature on the viscosity of PIC solution was measured by standard rheological tests at 20, 40, 60 and 80 oC. The rheological test of equilibrated gel prepared from PIC solution with wPIC/VNaCl = 1/15 g/mL and PNaSS/PMPTC charge ratio of 1.1:1 was performed by using the rheometer equipped with 20 mm parallel plates. The frequency-sweep were performed within the range of 0.01~100 Hz at 20 oC, in which the strain amplitude was fixed at 1% (within the linear viscoelastic region). Tensile tests of the printed PIC gel fibers and grids were performed to characterize their mechanical properties. The diameter and length of tested gel fibers were 0.6 mm and 25 mm, respectively. The grid samples were cut into rectangular shape (length: 25 mm; width: 5 mm). The tensile tests were carried out at a gauge length of 15 mm and a stretch velocity of 100 mm/min using a commercial tensile tester (Reger Co., Ltd, RWT10) at room temperature. To characterize the interfacial strength of contacting fibers, cross-fiber structure was printed to investigate its tensile behavior, and the stretch velocity was also set at 100 mm/min.

3. Results and Discussion 7



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3.1. Printability of PIC hydrogel solution Due to the ionic shielding and dilution effects, PIC powder is greatly plasticized into viscous solution by suitable amount of 4 M saline water. During 3D printing, the PIC solution was extruded directly into deionized water. Once the solution contacts with the water, the salt and counterions of PIC rapidly diffuse out due to the high concentration gradient, leading to the enhancement of ionic bonding between PMPTC and PNaSS, thereby forming tough PIC hydrogel. The rheological behaviors of transparent viscous solution and corresponding whitish tough gel were measured by frequency sweeps at room temperature (Figure 2). The PIC solution had loss modulus G'' larger than storage modulus G' when the frequency was less than 10 Hz, indicating the liquid state of the "ink" for printing. Both G' and G'' increased with the frequency, and G' slightly surpassed G'' when the frequency became larger than 10 Hz, indicating the highly viscoelastic gel state; at this investigated time scale, the chain relaxation speed cannot keep up with the shear frequency. On the other hand, the corresponding gelated gel had G' three orders of magnitude larger than the PIC solution. G' was slightly larger than G'', indicating that the whitish sample is in stable gel state with predominantly elastic property, which has been observed in previous studies on different PIC systems.25,36,37 The viscosity and gelation rate of PIC solution were found to be crucial factors for the production of well-defined gel fibers. The proper viscosity can suppress the surface tension, keeping the extruded PIC solution as fibers before gelation, rather than breaking or forming droplets. The sol-gel gelation rate has to be sufficiently fast to maintain the designed structure after PIC solution extrusion into water; however, if the gelation speed is too fast, the extruded solution readily becomes a gel without adhering to existing gel fibers, which may impair the integrity of the printed structure.

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Figure 2. Frequency sweep of PIC solution and corresponding equilibrated PIC hydrogel at room temperature, showing the frequency dependence of storage modulus (G') and loss modulus (G''). Here charge ratio of PNaSS/PMPTC = 1.1:1, wPIC/VNaCl = 1/15 g/mL and strain amplitude = 1%. The insets show the appearance of PIC solution and equilibrated hydrogel.

Since the viscosity of PIC solution is a key factor to influence the printability and gelation rate, rheological tests were conducted for different PIC solutions. As shown in Figure 3a, the increase in temperature T led to the decrease in viscosity. When T ≥ 60 oC, the influence of temperature was not so evident as that at low temperature. Therefore, in the following rheological tests and 3D printing, the temperature was set at 60 oC due to better fluidity of PIC solution unless stated otherwise; the same PIC solution can be successfully printed at room temperature (20 oC) using lower concentration of PIC or/and higher extrusion pressure (Figure S3). The PIC solution showed weak shear-thinning behavior at high shear rate (> 10 s-1), corresponding to the disentanglement of polymer chains. The concentration of PIC solutions also influenced their rheological behaviors, which showed similar shear-thinning behavior (Figure 3b). When wPIC/VNaCl ≥ 1/15, the viscosity dramatically increased with the increasing concentration of PIC solution (i.e. the increasing value of wPIC/VNaCl). The effect of charge ratio of PNaSS/PMPTC on the viscosity is shown in Figure 3c. The sample with charge ratio of 0.95:1 had the maximum viscosity, while that of 1.1:1 had the minimum viscosity. This is because the apparent charge ratio of 1.1:1 corresponds to the charge balance of PIC system, which has been identified due to the reagent 9



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impurity of NaSS.25 Any deviation from the charge balance point leads to the increase in viscosity, consistent with Wang and Schlenoff who showed that PIC solution has viscosity one magnitude lower than that of pure polyanion or polycation with twice of the individual concentration.38 In the PIC solution with concentrated salt, some ionic bonds might survive, resulting in the formation of segregated aggregates and thus a decrease in the exclude volume of polymer chains. Therefore, the viscosity of PIC solution is lower than that with relatively weak ionic bonding; deviation from the charge balance point leads to less or looser aggregates and thus slight increase in the viscosity of PIC solution.

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Figure 3. The effects of (a) temperature, (b) PIC concentration and (c) charge ratio of PNaSS/PMPTC on the viscosity of PIC solutions. (a) wPIC/VNaCl = 1/15 g/mL, charge ratio = 1.1:1; (b) charge ratio = 1.1:1, temperature = 60 oC; (c) wPIC/VNaCl = 1/15 g/mL, temperature = 60 oC. The concentration of saline solution was 4 M.

The printability of PIC solutions was directly verified by printing gel fibers at prescribed conditions, as listed in Table 1. The results were summarized in Figure 4a. The solutions with wPIC/VNaCl ≥ 1/12 g/mL were unprintable, because the viscosity of PIC solutions were too high to be extruded out of the nozzle (Figure 3c). When wPIC/VNaCl ≤ 1/17 g/mL (except the sample with charge ratio of 1.1:1 that was printable), the viscosity of PIC solutions was too low to retain the fibrous shape before sufficient gelation, causing the printed fibers to collapse into gel particles (Figure S1). We found when wPIC/VNaCl = 1/15 g/mL, all samples with different charge ratios showed good printability. When wPIC/VNaCl = 1/17 g/mL, only the sample with charge ratio of 1.1:1 was printable, which has relatively low viscosity (Figure 3c). Therefore, this solution should have a fast gelation rate to readily retain the printed fiber shape. According to our previous work,25 the equilibrated PIC gel with charge ratio of 1.1:1, which is near the charge balance, has the best mechanical properties, indicating the formation of most compact PIC. This combination of printability and high strength of PIC should correspond to fast gelation rate, which avoids shape collapse during the printing process. We should note that the printable region will be affected by other printing parameters, including the nozzle size, the extrusion pressure, the concentration of saline solution, etc. For example, the nozzle size will affect the gelation rate, because the gelation of inner PIC solution may be impeded to some extent by solidification of outer gel. However, we have identified that for the nozzle size used in our printing (radius: 130 µm), the gelation of inner solution occurs within 3 seconds and is not significantly affected by the preformed outer gel (Figure S2). In the following, the PIC solution with charge ratio of 1.1:1 and wPIC/VNaCl = 1/15 g/mL (4 M saline solution) was selected as the raw material for printing.

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(a) Unextrudable

Printable

Diameter (mm)

(b)

wPIC/VNaCl (g/mL)

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0.32 0.28 0.24 Nozzle moving speed (mm/s) 8 10 12

0.20

Irregular

0.16

0.95 1.00 1.05 1.10 1.15 1.20

207 224 241 259 276 293 310 328 345

Charge ratio of PNaSS/PMPTC

(c)

Pressure (kPa)

(d)

(e)

500 µm

5 mm

500 µm

Figure 4. Printability of PIC solutions. (a) Phase diagram of printable PIC solutions; (b) effects of extrusion pressure and nozzle moving speed on the diameter of printed fibers; (c) top view of a 14-layer grid; (d) SEM image of the grid (top view); (e) SEM image the grid (side view). Error bars in (b) represent standard deviation of the mean.

Spatial resolution (i.e. the diameter) of printed gel fibers is another concerned parameter in constructing 3D structural elements, which was found to be effectively controlled by 3D printing setup, including the extrusion pressure and moving speed of the nozzle. As shown in Figure 4b, the diameter of printed single gel fibers (at equilibrated state) can be manipulated by varying the extrusion pressure, corresponding to the flow rate of PIC solution. Fast moving speed of the nozzle also resulted in the decrease in fiber diameter due to the traction effect. At the lowest extrusion pressure and fastest moving speed that have been tested, the minimum fiber diameter of 180 µm was achieved. Multi-layer grids with a 0-90° lay-down pattern were also manufactured to evaluate the capability of our 3D printing platform for PIC solutions (Movie S1). Figure 4c shows a 14-layer grid (fiber diameter: 180 µm; spacing: 250 µm). SEM images from the top and side of the sample confirms the well-defined microstructure of the printed gel (Figures 4d and 4e).

3.2. Mechanical properties of printed gels 12


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The bulk PIC hydrogels exhibited excellent mechanical properties, as reported in the literatures.23,25,28 Whether or not the printed gel fibers and other 3D structures can maintain high toughness is of great interest. Figure 5a shows the uniaxial tensile curves of a printed single fiber (with diameter of 0.2 mm) and that of bulk PIC hydrogels.25 The Young's modulus (E), tensile strength (σb) and breaking strain (εb) of the printed fiber were 2.6 MPa, 1.2 MPa and 530%, respectively, which are found to be comparable to those of bulk PIC hydrogels. For the printed hydrogel grids, the tensile behaviors of force versus displacement showed that the stiffness of the grid structures can be nicely tuned through the geometric setting in printing (Figure 5b). Our printed multi-layer grids also exhibit much enhanced mechanical properties than most of 3D printed structures using hydrogels in previous studies.

(b) High density Intermediate density Low density

2.0

1.5

Printed fiber Printed stacked fibers Bulk hydrogel

Load (N)

Nominal stress (MPa)

(a)

1.0

1.5 1.0

0.5

0.5 0.0

0

1

2

3

4

5

6

0.0

7

Strain

(c)

0

10

20

30

40

50

Displacement (mm)

(d) 1.2

First cycle

relax

5 mm

15 mm

stretch

Load (N)

1.0

5 mm

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.6

Waiting time (min) 30 120

0.4 0.2 0.0 0

5

10

15

20

25

30

35

40

Displacement (mm)

Figure 5. Mechanical properties of printed objects. (a) Stress-strain curves of the printed single fiber, cross-fiber and bulk hydrogel, the insets present the morphology of tested samples; (b) load-displacement curves of the printed grids with different grid size, where the insets present the morphology of tested samples (grid size: 0.5 mm (upper), 1.0 mm (middle), 1.5 mm (lower); sample size: 5 mm by 15 mm; fiber diameter: 260 µm; layer number: 10); (c) self-recovery behavior of printed PIC hydrogel grids (scalar bars: 5 mm); (d) cyclic tensile curves of the printed grid after 13



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different waiting time. Similar to the bulk PIC gel,23,25 the printed grid also shows good self-recovery ability after being stretched up to 300% of its original length, as shown in Figure 5c. The deformed grid can almost recover to its initial state after 20 minutes of relaxation. Furthermore, the cyclic tensile curves (Figure 5d) quantify the strain recovery of the printed grid after the first loading-unloading loop. With more waiting time, the subsequent loading-unloading loops gradually approached the first cycle, and the loop after 120 min waiting time almost reproduced the first cycle. Interfacial bonding of gel fibers between neighboring layers matters for the integrated mechanical performance of the printed gel structures. The strength of interfacial bonding was characterized by tensile test of two cross bonded fibers, as shown in Figure 5a. The sample can sustain 560% strain without breaking, which is very close to the breaking strain of printed single fibers. Furthermore, the failure does not take place at the interface (Figure S4). The result indicates strong interfacial bonding between connected fibers, guaranteeing the good mechanical properties of integrated printed gels. Such strong interfacial bonding should be related to the proper level of gelation rate, which allows the formation of strong ionic bonds between the extruded fiber undergoing gelation and those pre-formed fibers.

3.3. Printing of complicated 3D structures Based on the good shape fidelity and strong interfacial bonding of PIC gel, various complex 3D structures can be constructed by the present 3D printing technology. Figure 6a shows a gradient grid printed with nozzle moving speed of 10 mm/s and extrusion pressure of 224 kPa. A multi-layer structure consisting of sinusoidal curves was printed with nozzle moving speed of 6 mm/s and extrusion pressure of 207 kPa, which indicates the versatility of printing path control (Figure 6b). The structural gel expands its width to some extent when being stretched, exhibiting negative Poisson’s ratio macroscopically (Movie S2). Moreover, the strategy can be readily extended to multi-material printing using multiple nozzles, 14


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demonstrated by multi-layer heterogeneous grids with alternative dyes of PIC solutions (Figures 6c). Figure 6d shows a multi-layered quadrangular frustum pyramid of 10 mm height, which shows the capability of constructing 3D structures in the out-of-plane direction.

Figure 6. Examples of printed structures. (a) A gradient grid; (b) a multi-layer structure with negative Poisson’s ratio; (c) an anisotropic grid printed with two different materials; (d) a quadrangular frustum pyramid. (The scalar bars are 5 mm for all the panels) The present 3D printing of tough physical hydrogels is based on the sol-gel transition of PIC and requires no post chemical reaction. This feature is different from previous hydrogel printing,22 and greatly simplifies the printing process in obtaining intricate 3D structures of tough hydrogels. The proper gelation speed and dynamic nature of ionic bonds also facilitate the interfacial bonding between different gel fibers, guaranteeing the integrity and toughness of printed gels. The PIC hydrogels have good biocompatibility,23,24 which endows the potential applications of printed PIC gels in structural biomaterials. Furthermore, the present printing approach based on sol-gel transition should be applicable to other types of tough physical hydrogels with

different

noncolvalent

interactions

such

as

15


hydrogen

bonding,


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metal-coordination bonds, hydrophobic interactions, etc.6,10,18

4. Conclusions In this work, an extrusion-based 3D printing platform was successfully developed to fabricate tough PIC hydrogels into various 3D structures. The viscous PIC solution was directly extruded out of nozzle into water, and tough physical hydrogels with imposed 3D structures were achieved after sol-gel transition caused by dialyzing out of salt and counterions of PIC. The printability of PIC solution was investigated by adjusting the material formula and printing parameters. The dynamic nature of ionic bonds in PIC favors the interfacial bonding between fibers and layers during the printing process, ensuring the integrity and toughness of printed gels. The printed PIC structures show excellent mechanical properties in terms of extensibility, strength and toughness. The present 3D printing technology based on sol-gel transition provides a facile approach to fabricate tough PIC hydrogels into complex structures, which may be used to other types of tough physical hydrogels as well. This work is of immediate use in promoting the practical applications of tough hydrogels in structural biomaterials, soft actuators, etc.

Acknowledgments This work was supported by the Thousand Young Talents Program of China, the National Natural Science Foundation of China (No. 11321202, No. 11402056 and No. 51403184), and the Zhejiang Provincial Natural Science Foundation of China (No. LR16A020001).

Supporting Information Figure S1: Morphologies of printed PIC hydrogels of different polymer concentrations under optical microscope. Figure S2: Gelling behavior of the PIC solution (wPIC/VNaCl = 1/15 g/mL). Figure S3: 3D printing of layered grids under room temperature (20 oC). Figure S4: Tensile behavior of printed cross fibers. 16


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Movie S1: 3D printing process. Movie S2: A multi-layer structure with negative Poisson’s ratio.

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3D Printing of Ultratough Polyion Complex Hydrogels - ACS Applied

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