An anti-pressure, fatigue-resistant and rapid self-healing hydrogel based on a nano-micelle assembly

Zhiang Shao a, Weimin Cheng a, Xiangming Hu *ab, Yanyun Zhao c, Peng Wang c, Mingyue Wu a, Di Xue a, Jiaoyun Hou a and Susu Bian a
aCollege of Mining and Safety Engineering, Shandong University of Science and Technology Qingdao, Shandong 266590, China. E-mail: xiangming0727@163.com
bCollege of Resources and Environmental Engineering, Binzhou University Binzhou, Shandong 256603, China
cCollege of Chemical and Environmental Engineering, Shandong University of Science and Technology Qingdao, Shandong 266590, China

Received 5th December 2019 , Accepted 23rd February 2020

First published on 26th February 2020


Herein, we found that a trace amount of acetic acid can strengthen DMC-NaSS micelles, resulting in a novel self-healing hydrogel with multiple non-covalent crosslinks. The hydrogel has both excellent self-healing capabilities and mechanical properties.


Hydrogels are soft materials with three-dimensional, crosslinked network structures containing a large amount of free water.1 In recent years, hydrogels possessing excellent characteristics such as environmental benignness,2,3 biological compatibility,4,5 responsiveness to external stimuli,6,7 and adhesiveness8 have been successively designed. These unique properties make them promising alternatives to plastics. Whether they are for engineering or medical applications, hydrogels must have satisfactory durability to withstand loading, including tension, compression, bending, and folding.9 The best way to ensure durability is to endow the hydrogel with satisfactory self-healing capability. Self-healing hydrogels mainly rely on electrostatic,10,11 hydrophobic,12,13 and host–guest interactions14,15 along with π–π conjugation,16,17 hydrogen bonding,18,19 and other dynamic crosslinked networks.20 In particular, hydrogels produced in recent years based on electrostatic interactions have shown both excellent mechanical and self-healing properties. For example, polyampholyte hydrogels developed by Gong's group show high toughness with MPa-scale Young's moduli along with nearly 100% self-healing ability.21 To be suitable as replacement materials for human cartilage tissue, hydrogels should have good anti-pressure and self-healing abilities along with softness similar to human cartilage; however, reported anti-pressure hydrogels are generally very hard (high storage modulus), which may limit their application as graft materials for human cartilage tissue.22 Therefore, a gel material that combines superior compression resistance and softness will result in great application prospects.

Herein, a novel rapidly self-healing hydrogel was created by copolymerizing acrylamide with self-assembled nanoparticles synthesized based on multiple non-covalent interactions, including dipolar, hydrophobic, and electrostatic interactions. The maximum compressive strength of the hydrogel reached 26 MPa, but its storage modulus was only 4230 Pa. In addition, the hydrogel showed an excellent self-healing rate (∼81% in 1 min) and extraordinary fatigue resistance. This work provides a new route to synthesize high-toughness hydrogels.30–32

To create this novel gel, we selected two functional monomers: methacryloyloxyethyltrimethylammonium chloride (DMC) as a cationic monomer and 4-styrenesulfonic acid sodium (NaSS) as an anionic monomer. The strong electrostatic interaction between the quaternary ammonium of DMC and the sulfonic acid group of NaSS23 can form a DMC-NaSS dimer (hereinafter referred to as DMC-NaSS; Scheme 1a). The hydrophilic amide group of DMC and the hydrophobic benzene group of NaSS provide DMC-NaSS with amphiphilicity, allowing the formation of nano-micelles in water (Scheme 1b). To further increase the stability of the micelles, a small amount of acetic acid (HAc, ∼0.22 mM) was added to the reaction system. The carboxyl group of HAc has a large dipole moment27 and can be connected with the quaternary ammonium of multiple dimers through dipole interactions (Step 2). This narrows the distance between the dimers and leads to the splitting of micelles (Scheme 1c). Finally, AM monomers disperse in the aqueous phase and penetrate the DMC-NaSS micelles to copolymerize with hydrophobic styrene or with the surrounding hydrophilic amide groups (Scheme 1d), promoting hydrogel formation. Our preliminary experiments revealed that hydrogels were only fully formed in the presence of a small amount of HAc. As shown in Scheme 1e, the pure DMC + AM + NaSS reaction system afforded a colorless, transparent, and highly viscous sol; when 0.22 mM HAc was added, a light-blue transparent soft gel with excellent self-healing capability was formed.


image file: c9py01839e-s1.tif
Scheme 1 A schematic diagram of the micellar copolymerization of DMC-NaSS dimers and acrylamide. (a) The synthesis and structural characterization of DMC-NaSS dimers. The DMC-NaSS dimers self-assembled into micelles in aqueous solution (b) without HAc and (c) with HAc. (d) The micellar copolymerization of acrylamide and DMC-NaSS micelles. (e) The morphologies of hydrogels with and without HAc.

To verify that DMC-NaSS self-assembled into nano-micelles in aqueous solution, and that the micelles split under the action of HAc, the DMC-NaSS solutions with different HAc concentrations were evaluated through dynamic light scattering (DLS) experiments. Fig. 1a shows the size distribution of DMC-NaSS micelles in solutions containing 1 M DMC-NaSS and different concentrations of HAc. The particle sizes of the micelles in DMC-NaSS solution without HAc were concentrated around 150 nm, while the particle sizes in the solutions containing a small amount of HAc were concentrated around 20 nm. This verifies our hypothesis that acetic acid can cause the micelles to split. In addition, as the HAc content increased, the particle sizes of the micelles tended to decrease slightly. When the HAc concentration reached 0.11 mM, the particle size distribution showed two peaks. This suggests that when a critical concentration of HAc is reached, the micelles are completely converted from large particle diameter to small particle diameter. Based on Fig. 1a, this critical concentration is between approximately 0.11 and 0.22 mM. To further investigate this phenomenon, DMC-NaSS solutions and hydrogels containing different HAc concentrations were observed by TEM (Fig. 1c–h and Fig. S6). Fig. 1c and d show images of a DMC-NaSS solution without HAc at different magnifications; Fig. 1e and f are images of a DMC-NaSS solution containing 0.22 mM HAc at the same magnifications. The DMC-NaSS micelles in the HAc-free solution were large and irregular, indicating poor micelle stability. In contrast, the DMC-NaSS micelles in the solution containing 0.22 mM HAc were much smaller and evenly distributed, indicating better micelle stability. Furthermore, TEM analysis indicated that a large number of micelles existed in the DMC-NaSS gel containing HAc (Fig. 1h). This suggests that the micelle structure was restored after copolymerization with AM, which might be related to the longer gelation time (≥24 h).29 Micelles were not observed inside the HAc-free DMC-NaSS gel (Fig. 1g), indicating that the polymerization completely destroyed the hydrophobic structures of the micelles. Fig. 1i shows the 1H NMR spectrum of DMC-NaSS gel (without MBA). The NMR spectrum indicates the distribution of three monomers on the polymer chain, demonstrating the dynamic cross-linking structure described in Scheme 1d (see S11 for details). Fig. 1b shows the dynamic viscosities of DMC-NaSS solutions with different HAc concentrations as functions of shear rate. In the early stage, DMC-NaSS solution exhibited obvious shear-thinning rheological behavior similar to general surfactant solutions.24,25 The dynamic viscosity of DMC-NaSS solution gradually increased with increasing concentration of HAc. This may be because HAc was added to the DMC-NASS micelles before they were fully formed, and the dipole effect of HAc reduced the micelle diameter and increased the micelle number, leading to an increase in solution viscosity.26 Moreover, at low shear rate and especially zero shear, the dynamic viscosity of the DMC-NaSS solution increased significantly with increasing HAc concentration. We propose that this is caused by two factors: on one hand, there may be hydrogen bonds between the HAc molecules embedded in different DMC-NaSS micelles, resulting in a damping effect; on the other hand, the addition of HAc can improve the micelle stability and thus significantly increase the viscosity of the solution.28


image file: c9py01839e-f1.tif
Fig. 1 (a) The size distributions of DMC-NaSS micelles in solutions with different HAc concentrations. (b) Dynamic viscosities of DMC-NaSS solutions with different HAc concentrations. TEM images of DMC-NaSS micelles in solutions (c and d) without HAc and (e and f) with HAc at different magnifications. TEM images of DMC-NaSS micelles in the hydrogels (g) with HAc and (h) without HAc. (i) The 400 MHz 1H NMR spectrum of DMC-NaSS gel in D2O at 30 °C.

After removing the hydrogel samples from the sampling bottles (2 cm in diameter and 5 cm in height), the samples were subjected to tensile (Fig. 2a and b) and compressive tests (Fig. 2e). Fig. 2d illustrates the standard stress–strain curves of the samples with different DMC-NaSS concentrations (MDMC–NaSS/Vwater). With increasing DMS-NaSS concentration, the fracture stress strength and elongation at break of the hydrogel increased (Fig. 2c), indicating that the content of DMC-NaSS micelles in the gel affected its toughness. In addition, the hydrogel containing 1 M DMC-NaSS showed exceptional stretchability (>19). Unless otherwise mentioned, this hydrogel was used for all measurements and experiments. Although the nominal stress of the DMC-NaSS gel was not high, its toughness and compressive strength were extraordinary. As shown in Fig. 2e and f, even when a high stress (26 MPa) was exerted on the DMC-NaSS hydrogel, the hydrogel was not crushed. However, pure PAM hydrogel is crushed at just 9 MPa, indicating that the DMC-NaSS hydrogel can sustain a larger load. PAM hydrogel with a monomer concentration of 3 M served as the control. Fig. 2e shows the outstanding compressive toughness and recovery ability of the DMC-NaSS gel; the DMC-NaSS gel was tough enough to withstand a series of compressive deformations of 97% and fully recover after removing the pressure. The fatigue resistance of the DMC-NaSS gel was investigated by successive cyclic compression tests at 90% strain. As shown in Fig. 2d, the stress–strain curves show evident hysteresis and almost coincide with first cycle, and the recovery ratio reached 95% after four cycles (inset of Fig. 2d). This demonstrates the extraordinary compression recovery and fatigue resistance of the DMC-NaSS gel. The excellent compressive capacity of the gel is mainly derived from the multiple non-covalent interactions in the acrylamide skeleton and DMC-NaSS micelles. In particular, the HAc-reinforced DMC-NaSS micelles are stable nanoparticles that effectively enhance the mechanical properties of the gel.


image file: c9py01839e-f2.tif
Fig. 2 (a) The hydrogel specimen used for tensile testing. (b) The stretched shape of the hydrogel at 1600% strain. (c) Typical tensile stress–strain curves of hydrogels with different DMC-NaSS content amounts. (d) Typical successive loading–unloading compression tests of the DMC-NaSS hydrogel for four cycles at 90% strain without resting intervals (the inset shows the corresponding recovery ratio for each cycle). (e) PAM and DMC-NaSS gel specimens before, during, and after compression. (f) Typical compression stress–strain curves of the PAM and DMC-NaSS hydrogels.

To study the effect of healing time on the self-healing ability of the hydrogel, several identical gel samples (2 cm in diameter and 10 cm in a height) were each cut into two halves. After 10 s, 1 min, or 30 min, the two halves were combined without any external force or additional action. The original sample and combined gel sample after different healing periods were subjected to conventional tensile tests. The hydrogel was found to have strong self-healing capability; the gel achieved almost complete self-healing (∼81%) at 60 s after the damage was inflicted (Fig. 3c). The self-healing ability (HE) of a gel is defined as the ratio of the tensile stress of the healed gel to that of the initial gel.21 To further investigate the hydrogel's distinct self-healing capability, crushing–splicing tests were conducted on the gel samples. First, the gel was cut into three sections and then spliced again; its tensile properties were then tested one minute later (Fig. 3a). Next, the hydrogel was completely crushed, and the pulverized gel units were poured into a 25 mL beaker. After 1 min, the hydrogel was removed after complete healing; no evidence of splicing traces was observed on the gel, and its tensile properties remained excellent (Fig. 3b). The results of the alternate step-strain test (strain = 1% and 1000%) also verify the hydrogel's self-healing capability (Fig. 3d).8 When a small amplitude oscillatory shear (strain = 1%) was applied to the gel, the elastic modulus was greater than the loss modulus, and the two moduli did not change with time, indicating that the gel maintained a complete crosslinked network structure under small oscillation strain. Subsequently, when the gel was subjected to large amplitude oscillation shear (strain = 1000%), the elastic modulus (G′) and loss modulus (G′′) values were immediately reversed with a sharp drop, indicating that the gel was converted into a sol state due to the destruction of the gel network. By switching the strain from a large strain of 1000% to a small strain of 1% at a fixed frequency (1.0 Hz), the gel-like character (G′ > G′′) was recovered instantaneously without any significant decrease during each repeatable cycle of the recovery. Therefore, the rapid sol–gel transition with complete recovery of the gel network after disruption corroborated the excellent self-healing capability of the DMC-NaSS gel. The self-healing mechanism was the reconstruction of reversible DMC-NaSS micelles in the gel network system.


image file: c9py01839e-f3.tif
Fig. 3 Tear–heal testing of (a) the separated hydrogel and (b) the pulverized hydrogel. (c) Typical tensile stress–strain curves of healed hydrogels after different healing times. (d) Dynamic rheological experiments of the DMC-NaSS hydrogels. Illustration of the molecular structures of the DMC-NaSS gel under different extents of deformation: (e) original state, (f) small deformation, and (g) large deformation.

In a series of quantitative self-healing tests, we found that DMC-NaSS hydrogel has a very strong self-healing ability, even in the presence of chemically crosslinked structures. This may be due to the multiple non-covalent interactions, including dipolar, hydrophobic, and electrostatic interactions, in the hydrogel. During tensile testing, the weak dipolar, hydrophobic, and electrostatic interactions of the gel were lost, resulting energy dispersal and elongation (separation of DMC-NaSS micelles and dimers; Fig. 3e and f). During the healing process, the recombination of micelles and dimers resulted in the restoration of the gel's mechanical properties. Remarkably, the self-healing process of our gel is completely spontaneous and does not require any outside intervention.

Conclusions

In summary, we developed a new type of rapid self-healing hydrogel based on multiple non-covalent interactions. The hydrogel possesses extraordinarily fast self-healing capabilities (healing efficiency of 81% at 60 s), and excellent elongation at break (≈1900%) and tensile strength (≈8 kPa). The elasticity, strength, toughness, and self-healing time of the hydrogel can be easily adjusted by varying the proportions of the three monomers. In addition, the hydrogel has a soft texture (G′ ≈ 4230 Pa) along with an extremely high compressive strength (>26 MPa). Therefore, our gel has exceptional potential for biomedical applications, such as being used in artificial cartilage materials, wound care materials, and cell culture scaffolds.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China [51674038 and 51874193]; the National Key R&D Program of China [2018YFC0807900]; the Shandong Province Natural Science Foundation [ZR2018JL019 and ZR2017PEE024]; Qingchuang Science and Technology Program of Shandong Province University [2019KJG008]; the Shandong Province Science and Technology Development Plan [2017GSF220003]; the Key Program of the National Natural Science Foundation of China [51934004]; Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents [2017RCJJ010 and 2017RCJJ037]; Shandong Province First Class Subject Funding Project [01AQ05202]; and the SDUST Research Fund [2018TDJH102].

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Footnote

Electronic supplementary information (ESI) available: Materials and experimental procedures, FT-IR, 1H NMR, TEM, SEM, swelling tests, rheological analysis or additional details. See DOI: 10.1039/c9py01839e

This journal is © The Royal Society of Chemistry 2020