3D printing of tough hydrogels based on metal coordination with a two-step crosslinking strategy

Abstract

We demonstrate the self-supporting 3D printing of complex hydrogel structures based on simultaneous crosslinking reactions while printing. The printing strategy is based on the Schiff base reaction and metal coordination with a two-step crosslinking process. The printing ink was first prepared by dispersing oxidized sodium alginate (OSA) and adipic dihydrazide (ADH) in poly(acrylamide-co-acrylic acid) (P(AAm-co-AAc)) polymer solutions, and was mixed and printed into 3D structures with an extrusion-based coaxial printing platform. Because of the rapid chemical crosslinking reaction between the aldehyde group in OSA and the hydrazide group in ADH, the printed structures can be solidified quickly, and are further crosslinked by forming carboxyl-Fe³⁺ coordination complexes to enhance their mechanical properties. The dynamic time-sweep rheological properties of the gel composed of different proportions of OSA and ADH were systematically investigated for the characteristic gelation time, and compression tests were carried out to measure the mechanical properties of the gel composed of OSA and ADH. Combining the gelation time and mechanical properties of the gel, the weight ratio of OSA and ADH was selected as 1 : 0.44 for an optimized setting in the subsequent printing. To evaluate the printability of inks, the material formula and printing parameters were systematically varied. The ink exhibited a wide printing range, self-supporting properties, and good printability. Tensile tests of the printed single fiber crosslinked by Fe³⁺ show that its strength and toughness are tunable. Complex 3D structures such as pyramids, cylinders, and noses were constructed to demonstrate the printability of the ink. This printing method provides a facile approach for tough hydrogel fabrication without changing the rheological properties of the ink or sacrificing the ultimate mechanical properties of the printed materials.

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1. Introduction

Hydrogels are often used in tissue engineering, soft robotics, actuators, etc.^1–3 However, the mechanical properties of traditional hydrogels are usually weak, which severely limits their use in load-bearing applications. To promote the applications of hydrogels as biomedical materials and structural elements, new types of gels with excellent mechanical properties are desired, achieved by different network structures and energy dissipation mechanisms, such as those in double network hydrogels, nanocomposite hydrogels, macromolecular microsphere composite hydrogels, slip ring hydrogels, polyion complex (PIC) hydrogels, ionically crosslinked hydrogels, etc.^4–13 Most of these hydrogels have poor processing performance due to the permanent crosslinking networks. In addition, polymerization and gelation are often completed simultaneously in the preparation process of these tough hydrogels, resulting in the dependence of gel shape on the reaction mold. A simple and effective method to realize the fabrication and structural construction of tough hydrogels is of great importance to broaden the applications of hydrogels in flexible devices, artificial organs, and other fields.^14–16 3D printing, also known as additive manufacturing, is a rapid prototyping technology that can quickly print various materials into complex structures,¹⁷ and has grown vigorously due to its significant advantages over traditional manufacturing techniques since first proposed by Hull in 1986.¹⁸ In the past few decades, 3D printing methods have been developed based on solid, liquid, and powder.^19–21 Among them, direct ink writing (DIW) based on liquid extrusion was the most commonly used strategy for hydrogel printing. In order to improve the printing efficiency and accuracy, additives such as nanoclay, nanocellulose, carrageenan, and other substances have been employed to adjust the viscosity and rheological properties of gels.^22–25

Zhao and coworkers²⁶ reported a highly stretchable and tough hydrogel by combining Ca²⁺ crosslinked alginate and covalently crosslinked poly(ethylene glycol)diacrylate (PEGDA), which exhibited a fracture energy of 1500 J m⁻². The toughening mechanism relies on the reversible crosslinking of Ca²⁺ and alginate in dissipating energy, while the covalent crosslinking of PEG maintains the integrity of the hydrogel. In addition, they developed a 3D printable tough hydrogel for the first time by adding nanoclay to control the viscosity of the pre-gel solution.²⁶ Wang and coworkers²⁷ also reported an ultra-strong 3D printed hydrogel for artificial meniscus, which consisted of cellulose nanocrystals, hard phenyl acrylate, and soft acrylamide. Due to the synergistic effect of the hydrophobic (phenyl acrylate) and hydrophilic (acrylamide) components, the hydrogel reported by Wang et al.²⁷ exhibited excellent mechanical properties with a tensile strength of ∼4.4 MPa, a compressive strength of ∼20 MPa, and a toughness of ∼6000 kJ m⁻³, which were comparable or even superior to the performance of human meniscus. Liu and coworkers²⁸ developed a 3D printable hybrid bioink composed of a hydrogen bonding monomer (N-acryloyl glycinamide) (NAGA) and nanoclay. The printed NAGA-clay pre-gel was cured under ultraviolet light to form a PNAGA-clay composite hydrogel scaffold with high strength. Owing to the dual amide hydrogen bonding interaction and physical crosslinking between the nanoclay and polymer chains, PNAGA-clay composite hydrogels exhibited extraordinary mechanical properties.

However, the introduction of tackifiers may adversely affect the biological functions of the gels and their constructs, imposing a potential risk for biomedical applications.²⁹ To solve this problem, Zhou et al.³⁰ reported a biocompatible physically crosslinked hydrogel with high strength and anti-swelling properties. This hydrogel composed of polyvinyl alcohol (PVA) and carrageenan showed outstanding shear-thinning rheological behavior and could be printed into various complex structures by DIW. The printed structure was physically crosslinked by the crystallization of PVA through the freeze–thaw process. The obtained gel was not only mechanically tough but also non-toxic, so it can be widely used in fields such as tissue engineering, drug delivery, etc.

To achieve real-time shape-controllable printing of high-strength hydrogels without changing the rheological properties of the ink or sacrificing the mechanical properties of the material, Zhu and coworkers^31,32 fabricated gel constructs with complex structures by printing plasticized polyion complex (PIC) solution in deionized water, where fast sol–gel transition occurred to form ultratough PIC hydrogel grids with excellent mechanical performance. Zheng et al.³³ developed an approach to obtain a 3D printable metal-coordination tough physical hydrogel, in which the poly(acrylamide-co-acrylic acid) (P(AAm-co-AAc)) polymer solution was immediately immersed into FeCl₃ aqueous solution to coordinate with Fe³⁺ after extrusion printing, and then soaked into water to reach an equilibrium state to obtain strong hydrogel structures. Moreover, the P(AAm-co-AAc) gel was combined with responsive poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAm-co-AAc)) to form 3D printed gel composites, which showed programmable shape morphing ability with high response speed and large output force in response to saline solution.³³

Here, we report a self-supporting 3D printed tough hydrogel by a two-step crosslinking strategy. The hydrogel ink was prepared by dispersing OSA and ADH in P(AAm-co-AAc) polymer solution, respectively, and mixed to form various complex structures by 3D coaxial extrusion printing. The first step of crosslinking occurs in the OSA/ADH mixture during the printing process. The aldehyde group in OSA reacts with the hydrazide group in ADH to form a crosslinked network with sufficient strength, endowing the printed ink with self-supporting properties. The second step of crosslinking occurs after soaking the printed hydrogel structures into FeCl₃ solution (Fig. 1). The carboxyl groups in the printed hydrogel form coordination bonds with Fe³⁺ to further improve the mechanical properties of the gel structures. During the printing process, OSA and ADH were uniformly mixed by the mixer in the nozzle, reacted rapidly to form a gel after extrusion, and provided sufficient mechanical strength to maintain the integrity of the printed structures. The entire printing process did not require an external force to secure the printed structures. The printed structures were crosslinked further with Fe³⁺ coordination. The final hydrogel showed superior and tunable mechanical performance, with a Young's modulus E of 0.72–3.43 MPa, a breaking stress σ_b of 1.24–2.26 MPa, a breaking strain ε_b of 246–614%, and a water content q of between 78.3% and 89.4%. This strategy of printing is suitable for other ion coordination hydrogels whose rheological performance and mechanical strength can be tuned by sequentially harnessing the internal crosslinking of the materials at different printing stages.

Fig. 1 Schematic of the 3D printing of tough hydrogel constructs by a two-step crosslinking strategy. OSA and ADH were dispersed in P(AAm-co-AAc) solutions using a double-tube syringe, and were uniformly mixed in a static mixer during extrusion printing. The first crosslinking occurred during the mixing of OSA and ADH based on the Schiff base reaction, forming a self-supporting scaffold after printing. The printed scaffold was then soaked into 0.1 mol L⁻¹ FeCl₃ solution for the second crosslinking, through metal coordination, and then immersed in deionized water to reach the equilibrium state with enhanced mechanical properties.

2. Experimental section

2.1 Materials

Sodium alginate (SA, alginic acid sodium salt from brown algae), acrylamide (AAm), and acrylic acid (AAc) were purchased from Sigma-Aldrich. 2-Hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (2959, photo-initiator) was purchased from Shanghai Macklin Biochemical Co., Ltd. Adipic acid dihydrazide (ADH) and sodium periodate (NaIO₄) were used as received from Aladdin Chemistry Co., Ltd. Iron chloride hexahydrate (FeCl₃·6H₂O) and sodium hydroxide (NaOH) were obtained from Sinopharm Chemical Reagent Co., Ltd, China.

2.2 Synthesis of OSA

In a 250 mL round-bottomed flask wrapped with aluminum foil, SA (10 g) was dispersed in ethyl alcohol (50 mL) with stirring. 50 mL of sodium periodate solution (10 g) was then added. The mixture was magnetically stirred to react for 6 h in a dark environment at room temperature.³⁴ Ethylene glycol (10 mL) was then added to the reaction mixture to reduce unreacted sodium periodate. The reaction was stirred for 2 h at ambient temperature, and the solution was dialyzed (Spectra/Por membrane, MWCO 14 000) for 3 days. The water was changed at least three times a day. The solution was then concentrated under reduced pressure to 100 mL and freeze-dried under reduced pressure to yield a white fluffy product (6.1 g, 61%).

2.3 Preparation of the OSA–ADH hydrogel

0.1 g dried OSA was dissolved in 1.5 mL PBS (pH = 7.4) within a transparent glass bottle. 1.5 mL solutions with different contents of ADH were added and shaken to form a homogeneous mixture. The mixture formed an OSA–ADH gel in a few minutes.

2.4 Synthesis of the P(AAm-co-AAc) copolymer

P(AAm-co-AAc) was synthesized by free radical polymerization. Prescribed amounts of AAc, AAm, and 2959 (0.5 mol%) were dissolved in deionized water. The molar fraction of AAc was kept constant as 10 mol%, and 1 mol L⁻¹ NaOH solution was used to maintain the pH at 7 during the synthesis of P(AAm-co-AAc). After degassing for 20 min by argon bubbling and curing with ultraviolet light for 1 h, highly viscous liquids of P(AAm-co-AAc) with different polymer contents were obtained.

2.5 Preparation of the 3D printed self-supporting hydrogel

A customized 3D printing system, which consists of an extrusion-based dispersion system and a 3D positioning stage, was utilized to print the ink. Before printing, OSA and ADH were dispersed into P(AAm-co-AAc) solutions using a dual-tube syringe, respectively. During the printing process, the dispersed polymer solutions were mixed/reacted in a static mixer with a nozzle, and extruded onto the substrate at a given flow rate of 0.5 mL min⁻¹. The printed structures were soaked in the selected FeCl₃ solutions (0.1 mol L⁻¹) for the second crosslinking, and then soaked in deionized water to achieve the equilibrium state with enhanced mechanical properties.³⁵

2.6 Preparation of the gel sample for the tearing test and cyclic stretch

The sample was prepared by dissolving OSA and ADH in an aqueous solution containing 2959, AAm and AAc (10 wt% P(AAm-co-AAc)), respectively, which were mixed into a homogeneous solution quickly and injected into the mold immediately. After the Schiff base reaction between OSA and ADH was complete, the sample was then exposed to ultraviolet light for 2 h, crosslinked with 0.1 mol L⁻¹ Fe³⁺ solution, and soaked in water to reach equilibrium before the tearing test and cyclic stretch.

2.7 Characterization

Fourier transform infrared (FTIR) spectra were recorded with a Nicolet iS 10 FTIR spectrometer in the region of 4000–400 cm⁻¹ with a resolution of 2 cm⁻¹. Solid samples were prepared as KBr pellets.

The mechanical properties of the gel with different compositions were examined by tensile tests at room temperature using a commercial tensile tester (Instron 3343). The stretch rate was 100 mm min⁻¹. The nominal stress and strain were recorded, and Young's modulus E was calculated from the slope of the stress–strain curves for the strain range below 10%. Tensile breaking stress σ_b and breaking strain ε_b were extracted from the stress–strain curves by averaging at least three separate tests.

Tearing tests were performed at room temperature to characterize the fracture energy of the obtained hydrogel. The hydrogel samples were cut into a rectangular shape (35 mm × 12 mm) with a 10 mm initial notch at the midpoint of the short edge. Two arms of the sample were clamped, and the two clampers were separated at a tearing rate of 100 mm min⁻¹.

The gelation time of the gel because of OSA–ADH crosslinking was measured by using an ARG-2 rheometer (TA Instruments) equipped with a cone-plate (40 mm, 1.99°). The measurements of storage/loss modulus were performed at room temperature with a shear rate of 1 s⁻¹.

3. Results and discussion

3.1 Synthesis and characterization of OSA

The cis-o-diol can be oxidized by sodium periodate, and the C–C bond between the two hydroxyl groups was broken to form two molecules of aldehyde.³⁶ Using this reaction mechanism, OSA can be synthesized by oxidizing sodium alginate (SA) with sodium periodate.³⁴ This reaction was experimentally verified by FTIR (Fig. S1a, ESI†): a new peak at 1732 cm⁻¹ was observed, attributed to the symmetric vibrational band of aldehyde groups (H–C=O), indicating the successful preparation of OSA.

3.2 Gelation and mechanical properties of the OSA–ADH gel

After the solutions of OSA and ADH were mixed, the mixed solution quickly formed a gel. The gelation mechanism was rooted in the rapid formation of acylhydrazone linkages via the Schiff's base reaction between hydrazide groups on ADH and aldehyde groups at the backbone of OSA. We used PBS solution to maintain a neutral environment (pH ≈ 7) for the system during the preparation of the OSA–ADH hydrogel, because the aqueous solution of ADH is alkaline while the Schiff base reaction needs a neutral environment. In printing 3D hydrogel structures, the pre-gel solution was acidic due to the existence of P(AAm-co-AAc), so we also used NaOH to adjust the pH of the system to maintain a neutral environment. The gelation time and mechanical properties of the obtained OSA–ADH gel were systematically investigated, serving as the critical inputs for the selection of optimized printability and solidification of the ink. A longer gelation time led to the collapse of printing structures, while a shorter gelation time often resulted in the clog of printing needles. Additionally, the OSA–ADH gel with insufficient mechanical strength could cause the collapse of the printed structures for multiple-layer printing.

The weight ratios between OSA and ADH were systematically varied, and dynamic time-sweep rheological tests were conducted to measure the gelation time of the OSA–ADH gel. The curves in Fig. 2a and Fig. S2 (ESI†) show the plot of the storage modulus and loss modulus versus time, defining a characteristic gelation time that corresponds to the cross-point of the storage modulus and loss modulus. This gelation time decreased from 121 to 80 seconds when the weight ratio between OSA and ADH varied from 1 : 0.35 to 1 : 0.52 (Fig. 2b). When the ratio between OSA and ADH became smaller than 1 : 0.44, the gelation time approached a plateau value. The chemical structure of the resultant hydrogel was confirmed by FTIR spectroscopy, as shown in Fig. S2 (ESI†). The amide bending vibration (N–H) at 1534 cm⁻¹ and the stretching vibration (C=O) at 1635 cm⁻¹ derived from ADH were detected in the spectrum of the OSA–ADH gel. Also, the peak at 1732 cm⁻¹ attributed to the symmetric vibration of the aldehyde group (C=O) on OSA almost disappeared, whereas a new peak at 1660 cm⁻¹ corresponding to the stretching vibration of imine (C=N) was observed, indicating the successful formation of acylhydrazone linkages between OSA and ADH.

Fig. 2 (a) Time-sweep rheological properties of the OSA–ADH gel during the gelation process. (b) Gelation time of the OSA–ADH gel with different weight ratios of OSA to ADH. (c) Compressive stress–strain curves of the OSA–ADH gel at different weight ratios of OSA to ADH. (d) Breaking strain and Young's modulus of the OSA–ADH gel at different weight ratios of OSA to ADH, extracted from the stress–strain curves in (c).

The mechanical properties of the obtained OSA–ADH gel were characterized by compression tests as well, as plotted in Fig. 2c and d. The compressive modulus of the gel increased first and then decreased as the content of ADH increased, and the maximum compression modulus was 47.9 kPa when the ratio between ADH and OSA was 1 : 0.44. This biphasic trend can be understood by the fact that too low or too high ADH content led to a decrease of the crosslinking density, which decreased the compressive modulus of the gel. Combining the measurements of gelation time and mechanical properties, the optimized ratio between OSA and ADH was selected as 1 : 0.44 for the following 3D printing process.

3.3 3D printing of the tough self-supporting hydrogel

Previous studies have shown that P(AAm-co-AAc) solution can be printed as a viscous liquid by extrusion printing, and strengthened by subsequent gelation in FeCl₃ solution to form metal coordination complexes.³³ However, the printed gel grids were easily levelled after being placed for no more than 1 minute due to liquid spreading, as shown in Fig. S3 (ESI†), limiting multi-layer printing. To solve this problem, here we adopted a method of involving the first crosslinking reaction that occurred during the printing process to fix the printed structures: OSA and ADH were dispersed into P(AAm-co-AAc), and then placed in a double-tube syringe for extrusion printing. The highly viscous polymer solutions, containing OSA and ADH, respectively, were uniformly mixed in a static mixer before being extruded through the nozzle (inner diameter: 0.41 mm) onto a glass substrate with a programmable printing path. The gel formed by OSA–ADH crosslinking acted as temporary support for the printed structures. In order to enhance the mechanical properties of the printed gel, the printed gel structures were soaked into Fe³⁺ solution to form the second crosslinking in the polymer network, and then soaked into deionized water to reach the equilibrium state. The concentration of P(AAm-co-AAc) and content of OSA of the ink were essential for printability and structural retention capacity.

To evaluate the printability of the ink, the P(AAm-co-AAc) concentration and OSA content were systematically varied. When the P(AAm-co-AAc) concentration and OSA content were too low, the viscosity of the ink was too low to sustain the shape of the printed structures before the gelation occurred, leading to the scenario of liquid spreading; when the P(AAm-co-AAc) concentration and OSA content were too high, the gelation was too fast, resulting in nozzle clogging (Fig. S4, ESI†). The ink could be printed into 3D structures only when the concentration of OSA and P(AAm-co-AAc) fell into an intermediate range. In experiments, we found that the ink was suitable for printing when the concentration of P(AAm-co-AAc) was between 8 and 11 wt%, and the content of OSA was within 2.6–4.4 wt%, as shown in Fig. 3a.

Fig. 3 Printability of the ink. (a) Printability diagram illustrating the regions of P(AAm-co-AAc) and OSA contents suitable for printing 3D gel structures. Global and top views of a 3D printed cuboid (b) before and (c) after being crosslinked with Fe³⁺.

With a determined parameter setting for the self-supporting property and gel printability, the ink with 10 wt% P(AAm-co-AAc) and 3.2 wt% OSA was chosen to produce a 50-layer cuboid, as shown in Fig. 3b. The length, width, and height of the printed cuboid were 30 mm, 30 mm, and 12 mm, respectively. The cuboid could maintain its shape for at least 1 h, creating a sufficient time window for the second reaction of Fe³⁺ coordination. The printed cuboid was transparent initially (Fig. 3b) and turned yellow after soaking in Fe³⁺ solution for 3 days and in deionized water for another 3 days (Fig. 3c). The global view of the 3D printed cuboid demonstrated a good self-supporting property, and the zoom-in image of the structure showed the shape fidelity of the treatment with Fe³⁺ solution and water. It should be noted that temperature has a profound effect on the reaction rate between OSA and ADH, the viscosity of P(AAm-co-AAc), and water evaporation time of the system. With an increase in temperature, we expect that the reaction rate becomes larger, the viscosity of P(AAm-co-AAc) decreases, and the water evaporation rate increases, which in turn affect the printability of the system. All the gel structures in this study were printed at room temperature, with the parameter settings from the printability window obtained at room temperature. For other temperatures, different series of printability diagrams (similar to Fig. 3) are needed, which will be pursued in future studies.

3.4 Mechanical properties of the printed gel

The printed hydrogel exhibited not only excellent self-supporting printing properties, but also good mechanical performance after being coordinated with Fe³⁺, as indicated in Fig. 4. Five groups of the gel after Fe³⁺ coordination were prepared with different contents of P(AAm-co-AAc) and OSA, and the content of AAc in P(AAm-co-AAc) was fixed at 10 mol%, referring to the work of Zheng et al.³³ The contents of P(AAm-co-AAc) and OSA significantly influenced the ultimate properties of the equilibrated hydrogel: the Young's modulus E increased from 0.93 to 1.20 MPa, the breaking stress σ_b increased from 1.24 to 2.26 MPa, the breaking strain ε_b increased from 317% to 614%, and the water content q decreased from 89.4% to 80.7%, as the P(AAm-co-AAc) content increased from 9 wt% to 11 wt% (for a fixed OSA content: 3.2 wt%). Similarly, as the OSA content increased from 2.6 wt% to 3.8 wt% (for a fixed P(AAm-co-AAc) content: 10 wt%), the ε_b and q values of the equilibrated gel decreased from 587% and 86.5 wt% to 246% and 78.3 wt%, respectively, and E increased from 0.72 MPa to 3.43 MPa, because of the increased crosslinking density for a higher OSA content (Table 1).

Fig. 4 Tensile stress–strain curves of the printable hydrogel after Fe³⁺ coordination with different P(AAm-co-AAc) and OSA contents.

Table 1 Young's modulus E, breaking stress σ_b, breaking strain ε_b, and water content q of the printable hydrogel after Fe³⁺ coordination with different P(AAm-co-AAc) and OSA contents

Samples		ε b (%)	σ b (MPa)	E (MPa)	q (wt%)
P(AAm-co-AAc) (wt%)	OSA (wt%)
9	3.2	317	1.24	0.93	89.4
10	2.6	587	1.48	0.72	86.5
10	3.2	451	1.60	1.07	82.2
10	3.8	246	1.44	3.43	78.3
11	3.2	614	2.26	1.20	80.7

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In addition to tensile tests, tearing tests were also performed to characterize the fracture resistance of the obtained gels. A representative sample for tearing tests is shown in Fig. 5a, where w is the thickness of the sample, ranging from 0.6 mm to 0.8 mm, and the typical sample after tearing is shown in Fig. 5b. The curves of the normalized force (applied force divided by the thickness of the gel) versus displacement exhibited an interesting tearing pattern with periodic stick–slip instability (Fig. 5c), and each period contained a moderate increase (positive slope) and an abrupt decrease (negative slope) in the force response. The positive value of the force slope indicates that the crack front is steady, and the negative value of the force slope indicates that the crack grows. The tearing fracture energy, G_av, can be calculated through G_av = 2 F_av/w,⁹ in which F_av is the averaged stretch force over the periodic undulation in tearing. The hydrogel showed a G_av value of about 220–530 J m⁻², which decreased with an increase in the OSA content for increased crosslinking density (Fig. 5d).

Fig. 5 Toughness of the 3D printed hydrogel. (a) Schematic of the tearing test on a pants-like sample. (b) The hydrogel sample after the tearing test. (c) Tearing behaviors of the equilibrated Fe³⁺ crosslinked hydrogel with 10 wt% P(AAm-co-AAc) and different contents of OSA, performed at a tearing rate of 100 mm min⁻¹, and (d) the corresponding averaged fracture energy G_av from the force-displacement curves in (c).

We investigated the mechanical hysteresis of the hydrogel by conducting cyclic tensile tests on the gel with 10 wt% P(AAm-co-AAc) and 3.2 wt% OSA. The preparation process of the cyclic stretch sample was the same as that for tearing tests, but the material was cut into a standard dumbbell shape for cyclic stretch. The stress–strain curves of the hydrogel exhibited a large hysteresis loop between loading and unloading paths, as shown in Fig. 6a, indicating vast energy dissipation attributed to the breakage of internal crosslinking points within the gel. Notable residual strain was observed when the sample was unloaded to zero stress, as quantified by the dependence of residual strain on the waiting time (Fig. 6b). With the elapsing waiting time, the residual strain gradually decreased and finally vanished after 4 h, and the subsequent loading and unloading cycle approached the first cycle, indicating that the viscoelastic deformation was almost recovered. The hysteresis ratio, calculated as the area ratio between the second hysteresis loop and the first one, became larger with more waiting time and reached 70.7% for 4 h of waiting, as shown in Fig. 6b. Such recovery behavior of the hydrogel is related to the reformation of the dynamic coordination bonds within the material.

Fig. 6 Hysterestic behaviors of the 3D printed hydrogel. (a) Cyclic tensile loading-unloading curves of the equilibrated gel with 10 wt% P(AAm-co-AAc) and 3.2 wt% OSA crosslinked with Fe³⁺ after different waiting times, and (b) the corresponding residual strains and hysteresis ratios. The sample was loaded to a prescribed strain of 100% and then unloaded to zero stress, with the same deformation rate of 100 mm min⁻¹.

3.5 Printing of complicated 3D structures

The involvement of OSA and ADH greatly enhanced the self-supporting property of the printing ink, so that various complex 3D structures can be produced. To demonstrate the excellent printability and shape fidelity of the material, diverse shapes with a hollow structure and high aspect ratio, such as pyramids (Fig. 7a), cylinders (Fig. 7b), and human noses (Fig. 7c), were printed. The printing process is described in the Experimental Section, and all the objects were printed from the ink with 10 wt% P(AAm-co-AAc) and 3.2 wt% OSA. Being transparent and colourless at the stage of the first OSA–ADH crosslinking, the printed constructs turned yellow and opaque after treatment with FeCl₃ solution for 3 days and with deionized water for 3 days. Those printed structures inherited the good self-supporting property and excellent mechanical performance of the ink. For the printed hollow cylinder with an imposed compressive strain of 90%, it recovered to its original shape within 10 seconds after the imposed compression was removed (Fig. 7d).

Fig. 7 Various 3D printed structures: (a) pyramid, (b) hollow cylinder, and (c) human nose. The scale bar was 10 mm. (d) The printed hollow cylinder underwent a compressive strain of 90% and returned to its original shape after a relaxation of 10 seconds.

4. Conclusions

In summary, we report a new method for 3D printing tough hydrogels with a coaxial 3D printing platform through a two-step crosslinking strategy, in which the ink was prepared by dispersing OSA and ADH in P(AAm-co-AAc) polymer solution, respectively. The first crosslinking step forms acylhydrazone linkages in the OSA/ADH mixture during the extrusion process, and the second crosslinking step forms carboxyl-Fe³⁺ coordination after soaking the printed hydrogel into FeCl₃ solution. The gelation times and mechanical properties of the OSA–ADH gel were systematically characterized to select the optimized ratio between OSA and ADH for the ink. Gel fibers printed by this two-step crosslinking strategy exhibited excellent mechanical properties with a Young's modulus E of 0.72–3.43 MPa, a breaking stress σ_b of 1.24–2.26 MPa, and a breaking strain ε_b of 246–614%. Various structures were successfully produced by this coaxial 3D printing of metal coordination hydrogels, demonstrating the self-supporting 3D printability of the ink. This printing approach should be useful in engineering a broad range of tough hydrogels that can rapidly be crosslinked by ion coordination, such as alginate hydrogels and chondroitin sulfate hydrogels, by harnessing the self-supporting printability of the ink, and toughening process of the printed constructs, thereby opening opportunities for the development of gel-based biomaterials and soft actuators with sophisticated structures.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Numbers 12125205, 12072316, 91748209, 12132014, and 51973189), the Key Research and Development Program of Zhejiang Province (2021C01183), and the Fundamental Research Funds for Central Universities of China (2020XZZX005-02).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, photos of the ink printability, and movies of the printing process. See DOI: 10.1039/d1tb02529e

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