Highly tough and puncture resistant hydrogels driven by macromolecular microspheres

Abstract

Traditional hydrogels with poor mechanical properties have the largest barrier for extensive practical applications, such as artificial tendons, cartilage, skin and so on. In this work, a novel design strategy is proposed and demonstrated to improve the mechanical behavior of hydrogels by introducing ductile macromolecular microspheres (MMs) as crosslinking centers. Firstly, the MMs are synthesized, using butyl acrylate as the main component and dicyclopentyl acrylate as an intermolecular crosslinker, by a conventional emulsion polymerization method. Then acrylamide (AM) and hexadecyl methacrylate (HMA) are crosslinked by the MMs in water to form MM crosslinked poly(acrylamide-co-hexadecyl methacrylate) (P(AM/HMA)–MM) hydrogels. From the tensile measurements, the P(AM/HMA)–MM hydrogels exhibit dramatic enhancement of fracture stress σ_f (0.555 MPa) and fracture strain ε_f (5533%) when compared to the original P(AM/HMA) hydrogels. Furthermore, the P(AM/HMA)–MM hydrogels also have excellent puncture resistant properties. Based on traditional mechanisms of rubber-toughened plastics, it is clear that MMs can not only prevent the further development of cracks but can also be stretched to deform and absorb a large amount of energy. It is envisioned that this novel strategy, inspired by a toughening mechanism, will be an effective approach to enhance the mechanical properties and broaden the range of applications for hydrogels.

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Introduction

Hydrogels with a three-dimensional network structure can hold a large amount of water and simultaneously keep their shape. They had inspired considerable attention because of their wide range of applications, such as tissue scaffolds,^1,2 drug delivery vehicles,^3–5 cell supports,⁶ and so on. However, the conventional hydrogels had poor mechanical properties because of the heterogeneous network structure, which limited the application range of tissue engineering, such as artificial cartilage, tendons, muscles and blood vessels.⁷ Therefore, a large number of investigations have been focused on developing new mechanisms for toughening hydrogels, including double network hydrogels,⁸ slide-ring hydrogels,⁹ nanocomposite hydrogels,^10–13 tetra-poly(ethylene glycol) (PEG) hydrogels,¹⁴ clay composite hydrogels,¹⁵ macromolecular microsphere (MM) composite hydrogels^16–18 and hydrophobic association hydrogels.^19–22

The hydrophobic association hydrogels were composed of hydrophilic components and hydrophobic segments. The hydrophobic chains could be assembled into micelle-like aggregates by molecular entanglement at physical crosslinking points in hydrogels. When the hydrogels were loaded and deformed, the curled hydrophobic chains could slide and be stretched so that large amounts of energy would be dissipated. Jiang et al. and Yang et al. had reported a method of obtaining hydrophobic association hydrogels with a new hydrophobic molecule of octylphenol polyoxyethylene ether acrylate.^21,22 Introducing the hydrophobic monomers into hydrogels seemed to be a feasible way to increase the mechanical strength. However, the simple physical interaction force between the hydrophobic chains was weak so that the tensile strength was not high enough when compared to that of hydrogels with chemical bonds.

MMs were usually environmentally sensitive and mainly used in a drug delivery system. However, it was difficult to form bulk hydrogels. Even if there were some hydrogels formed with the MMs, the mechanical properties of the hydrogels were poor. Subsequently, microspheres had been introduced to bulk hydrogels to improve their strength.^23–26 For example, peroxidized microspheres with a diameter 100 nm acted as both initiators and crosslinkers, grafting poly(acrylic acid) chains onto the surface of microspheres. As a result, the hydrogels exhibited a good compressive performance, but the tensile and tearing properties, which truly reflected resistance against crack propagation, had not been reported.¹⁷

In this paper, a novel method of MM crosslinked hydrogels with high toughness and puncture resistant properties is reported. The MMs were prepared with butyl acrylate (BA) as the main component and dicyclopentyl acrylate (DCPA) as an intermolecular crosslinker and a one-step emulsion polymerization method was used.^27,28 Subsequently, the highly tough hydrogels were prepared by using acrylamide (AM) and hexadecyl methacrylate (HMA) as main chains and MMs as a chemical crosslinker. Inspired by traditional mechanisms of rubber-toughened plastics,^29,30 MMs would not only prevent the crack development, but also stretch after the deformation of substrates to enhance the break elongation. It was envisioned that MM crosslinked hydrogels would exhibit high tensile strength and long elongation at break. The structure and formation mechanism for MM crosslinked P(AM/HMA) hydrogels are shown in Fig. 1. This novel strategy should open up new pathways for toughening hydrogels and be an effective approach to broaden the biomedical applications of hydrogels.

Fig. 1 Structure and formation mechanism of MM crosslinked P(AM/HMA) hydrogels.

Experimental

Materials

AM (99.0%), 4-arm PEG acrylate (4-arm PEGAA, molecular weight of 2000), sodium dodecyl sulfate (SDS, ≥97%), potassium persulfate (KPS, 99.5%), sodium chloride (NaCl, 99.5%), BA, DCPA (99.5%), and sodium carbonate (Na₂CO₃, >99.0%) were supplied by Aladdin (Shanghai, China). HMA (95%) was supplied by Zhejiang Kangde New Materials Co., Ltd, China. BA was distilled under reduced pressure, and was stored at −18 °C. Deionized water was used in the experiment.

Preparation of MMs

A series of experiments were carried out to determine the different particle sizes of the MMs. The following variables were investigated: the SDS molar percentage was changed in the range of 0.55 mol% to 2.75 mol% for BA, the Na₂CO₃ molar percentage was investigated in the range of 0.6 mol% to 3.0 mol% for BA, the DCPA and KPS molar percentages were also investigated for BA. The fundamental recipe used to prepare the MM latexes is listed in Table 1.

Table 1 Recipe for preparation of MMs latexes

MMs	BA/water (wt%)	DCPA (mol%)	SDS (mol%)	KPS (mol%)	Na2CO3 (mol%)
a	40/60	2	2.75	0.0064	0.6
b	40/60	2	2.20	0.0064	1.2
c	40/60	2	1.65	0.0064	1.8
d	40/60	2	1.10	0.0064	2.4
e	40/60	2	0.55	0.0064	3.0

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Deionized water, SDS, Na₂CO₃ were added to a three-necked boiling flask and allowed to dissolve. SDS and Na₂CO₃ were used as emulsifier and electrolyte, respectively. Then, the monomer (BA) and crosslinking agent (DCPA) were added to the reactor and kept at 70 °C, and nitrogen (N₂) gas was flushed through the system to remove the oxygen (for about 30 min). Initiator (KPS) was added into the reactor after dissolving it in some deionized water. The rate of N₂ bubbling was decreased to minimize evaporation. The reaction time zero was taken at this point. The reaction time was set up to 120 min in order to obtain a high conversion for the experiments. Finally, the MM latexes were obtained.

Preparation of hydrogels

Preparation of P(AM)–MM hydrogels. NaCl and SDS were dissolved at room temperature in deionized water with constant stirring to prepare an aqueous solution of 5% SDS/0.4 M NaCl. After the solution turned transparent, the MM emulsion was added into the solution and stirred for 2 h to completely disperse MMs. KPS and AM were added into the beaker and the mixture was stirred for 10 min; the KPS molar percentage was 0.01 mol% for AM. Then, the solution in the beaker was poured into a mold consisting of two parallel glass plates and a 3 mm silicone spacer and then this was kept at 70 °C for 3 h to obtain P(AM)–MM hydrogels.

Preparation of P(AM/HMA) hydrogels. NaCl and SDS were dissolved at room temperature in deionized water with constant stirring to prepare an aqueous solution of 5% SDS/0.4 M NaCl. After the solution turned transparent, HMA was added into the solution and stirred for 2 h to completely disperse the HMA. KPS and AM were added into the beaker and the mixture was stirred for 10 min; the KPS molar percentage was 0.01 mol% for AM. Then, the solution in the beaker was poured into a mold consisting of two parallel glass plates and a 3 mm silicone spacer and then this was kept at 70 °C for 3 h to obtain the P(AM/HMA) hydrogels.

Preparation of P(AM/HMA/PEGAA) hydrogels. NaCl and SDS were dissolved at room temperature in deionized water with constant stirring to prepare an aqueous solution of 5% SDS/0.4 M NaCl. After the solution turned transparent, HMA was added into the solution and stirred for 2 h to completely disperse the HMA. AM, 4-arm PEGAA and KPS were added into the beaker and the mixture was stirred for 10 min. The mass fraction of AM in the aqueous medium was 25%, the KPS molar percentage was 0.01 mol% for AM, and the 4-arm PEGAA molar percentage was 0.05 mol% for AM. Then, the solution in the beaker was poured into a mold consisting of two parallel glass plates and a 3 mm silicone spacer and then this was kept at 70 °C for 3 h to obtain P(AM/HMA/PEGAA) hydrogels.

Preparation of P(AM/HMA)–MM hydrogels. NaCl and SDS were dissolved at room temperature in deionized water to prepare an aqueous solution of 5% SDS/0.4 M NaCl. After the solution turned transparent, the HMA and MM emulsion (the mass content of MMs in the added monomers was 0 wt%, 0.5 wt%, 1 wt% and 1.5 wt%) were added into the solution and stirred for 4 h to disperse completely the HMA and MMs. KPS and AM were added into the beaker and the mixture was stirred for 10 min; the KPS molar percentage was 0.01 mol% for AM. Then, the solution in the beaker was poured into a mold consisting of two parallel glass plates and a 3 mm silicone spacer and then this was kept at 70 °C for 3 h to obtain P(AM/HMA)–MM hydrogels. The formulations of the P(AM/HMA)–MM hydrogels were denoted as P(AM_x/HMA_y)–MMs_(j,k). Here, x was the mass fraction of AM in an aqueous medium, y was the mole fraction of HMA for AM, j was the mass fractions of MMs for AM and HMA, and k was the particle size of the MMs. The detailed compositions of the hydrogels are shown in Table 2.

Table 2 The components of MMs crosslinked hydrogels

P(AMx/HMAy)–MMs(j,k)	AM (wt%)	HMA (mol%)	MMs (wt%)	MMs (Diameter/nm)
P(AM25/HMA0)–MMs(1.0,349)	25	0	1.0	349
P(AM2/HMA2)–MMs(0,0)	25	2	0	0
P(AM25/HMA2)–MMs(0.5,349)	25	2	0.5	349
P(AM25/HMA2)–MMs(1.0,349)	25	2	1.0	349
P(AM25/HMA2)–MMs(1.5,349)	25	2	1.5	349
P(AM25/HMA2)–MMs(1.0,103)	25	2	1.0	103
P(AM25/HMA2)–Ms(1.0,214)	25	2	1.0	214
P(AM25/HMA2)–MMs(1.0,300)	25	2	1.0	300
P(AM25/HMA2)–MMs(1.0,424)	25	2	1.0	424

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Fourier-transform infrared (FTIR) spectroscopy detection

The chemical structure of the MMs was determined using a Nicolet iS-50 FTIR spectrometer (Thermo Scientific) using 64 scans at a resolution of 4 cm⁻¹. Before the measurements were made, the latex containing the MMs was freeze-dried in a FDU-2110 freeze vacuum drier (Eyela).

Dynamic light scattering measurement

The particle size and particle size distribution of MMs were characterized using a NanoBrook 90Plus particle size analyzer (Brookhaven Instruments, USA). The laser light scattering measurements were set at 90°. The samples of MMs were diluted with deionized water to 1/1000th of the original concentration before analysis. Five tests were made for each sample. Then the average particle size and the size distribution were obtained.

Particle morphology

The particle morphology and distribution were observed using a Jem-1210 transmission electron microscope (TEM; Jeol, Japan). The accelerating voltage was set at 100 kV. Samples were prepared by diluting the latex to an appropriate concentration (about 0.1 mg mL⁻¹) with deionized water and then dropping a sample of this onto the copper wire mesh. The magnification was 50 000 times and the scale was 500 nm in the images shown later.

Mechanical properties

The tensile tests on the hydrogels were carried out using a AGS-X 100N tensile tester (Shimadzhu, Japan) at room temperature. The samples were cut into a dumbbell shape (length 30 mm, gauge length 12 mm, width 4 mm, thickness 3 mm). The dumbbell shaped samples were clamped and stretched at a constant velocity of 40 mm min⁻¹. The fracture stress, σ_f, and the fracture strain, ε_f, were determined from the tensile curves. For hysteresis measurement, the samples were first stretched to a maximum extension ratio (λ = 15) and then unloaded. The dissipated energy (U_hys) was estimated using the area below the stress–strain curves or between the loading–unloading curves. Tearing testing was performed using the same tensile tester. The gel samples were cut into a trouser shape (40 mm in length, 12 mm in width, and 3 mm in thickness) with an initial notch of 20 mm. The two arms of the sample were clamped, and one arm was fixed, while the other one was pulled at 40 mm min⁻¹. The tearing energy (T) was estimated by:⁷
where F_ave is the average force of peak values during steady-state tear, and w is the thickness of the samples.

Rheological testing

Dynamic rheological measurements were performed using a physical AR 2000ex rheometer (TA Instruments). The dynamic viscoelastic properties (storage modulus, G′, and loss modulus, G′′), of the hydrogels were determined at different frequencies between 0.01 and 100 Hz. Frequency sweep tests for the samples were carried out at 25 °C using parallel plate geometry (25 mm diameter).

Morphology observation

The fracture section of the hydrogels was examined using a JSM 6510 scanning electron microscope (SEM; Jeol). The samples were freeze-dried in an FDU-2110 freeze vacuum drier (Eyela) and then immediately put into liquid N₂ for 3 minutes to break them. All the samples were sputtered with platinum before measurement and the magnification factor was 2000 or 5000.

Results and discussion

Characterization of MM crosslinked hydrogels

In this approach, the uniform sized MMs were synthesized by free radical polymerization, using BA as the main component and DCPA as a crosslinker in an aqueous medium (method shown in Fig. 1(a)). BA could be reacted with DCPA by using KPS as an initiator and MMs with a certain degree of crosslinking were obtained. The aggregation and coalescence of MMs could be avoided because of the presence of SDS as an emulsifier. Furthermore, the size of the MMs could also be controlled by adding different quantities of emulsifiers and electrolytes (Na₂CO₃). DCPA with a double-loop structure gave sufficient steric hindrance so that the unreacted carbon double bonds of DCPA would exist on the surface of the MMs. As a result, MMs with pendent double bonds could be synthesized using other vinyl monomers as chemical crosslinking agents. The chemical structure of the MMs was characterized using FTIR spectroscopy and the results are shown in Fig. 2. It was found that the wavenumbers at 1654.2 and 1571.4 (cm⁻¹) showed the characteristic peaks of the pendent double bonds of DCPA in the FTIR patterns. The particle size and distribution of the MMs were also obtained using particle size analysis and TEM, and the results are shown in Fig. 3. The mean diameters of the MMs were 103 nm, 214 nm, 300 nm, 349 nm and 424 nm.

Fig. 2 FTIR spectrometry results for the MMs.

Fig. 3 Particle size and distribution of MMs obtained using particle size analysis and TEM.

The formation of MM crosslinked hydrogels was carried out using radical polymerization, meaning that the monomers would form growing chains after initiation. If the growing chains encountered the residual carbon double bonds of the MMs, the hydrogels would be prepared using the MMs as chemical crosslinkers. Meanwhile, the hydrophobic side chains from HMA would be self-assembled into physical crosslinked micelles. Furthermore, if the hydrophobic side chains encountered the hydrophobic components of MMs, the physically crosslinking centers would also be formed in the hydrogels. Therefore, the MMs would become chemical crosslinkers and form partly hydrophobic associated points with hydrophobic components from HMA because of their intermolecular interaction. Based on this hypothesis, a schematic structure of MM crosslinked hydrogels is shown in Fig. 1(b).

Mechanical properties of the hydrogel

The high toughness of MM crosslinked P(AM/HMA) hydrogels was demonstrated by stabbing, knotted stretching, loading and compressing experiments. Fig. 4(a) shows the puncture resistant properties of hydrogels and the results indicated that there was no damage or penetration of the sheet shaped hydrogels. The knotted stretching experiment of the cylinder shaped hydrogels is shown in Fig. 4(b) and indicates the excellent stretching property (10 times its original length) without any fracture. Furthermore, Fig. 4(c) shows the loading experiment of the cylinder shaped hydrogels and 350 g of hollow iron pipe could easily be lifted. Also, the compression of the hydrogels was tested, as shown in Fig. 4(d), and it was found that the hydrogel could still recover its original dimensions after unloading. All these experiments confirmed the elasticity and toughness and puncture resistant properties of P(AM/HMA)–MM hydrogels.

Fig. 4 Demonstration of the toughness of MM crosslinked P(AM/HMA) hydrogels through (a) stabbing, (b) knotted stretching, (c) loading and (d) compressing experiments (MM content was 1.0 wt% of added monomers and the diameter was 349 nm after adding monomers).

Subsequently, the compared tensile properties between PAM–MM, P(AM/HMA), P(AM/HMA/PEGAA) and P(AM/HMA)–MM hydrogels were compared. As shown in Fig. 5(a), the PAM–MM hydrogels with a chemically crosslinking network exhibited poor extensibility and strength. The fracture stress, σ_f, and fracture strain, ε_f, were 0.035 MPa and 552%, respectively. In contrast, P(AM/HMA) hydrogels with a single hydrophobic associated network exhibited longer elongation of break and higher mechanical strength. The fracture stress, σ_f, and fracture strain, ε_f, were 0.207 MPa and 3233%, respectively. P(AM/HMA/PEGAA) hydrogels with a hydrophobic associated network and chemical crosslinking network also exhibited higher mechanical properties. The fracture stress, σ_f, and fracture strain, ε_f, were 0.367 MPa and 4285%, respectively. However, it was obvious that the mechanical properties of P(AM/HMA)–MM hydrogels were significantly enhanced by adding a small amount of MMs (1.0 wt% for AM and HMA). The fracture stress, σ_f, and fracture strain, ε_f, were 0.555 MPa and 5533%, respectively. This means that MMs could not only become a chemical crosslinker but also act as a synergistically hydrophobic crosslinking center in hydrogels. The tearing mechanical properties of P(AM/HMA), P(AM/HMA/PEGAA) and P(AM/HMA)–MM hydrogels were also investigated. Fig. 5(b) shows that the tearing energy of the P(AM/HMA)–MM hydrogels was about 800 J m⁻², which was much higher than that of P(AM/HMA) hydrogels (about 350 J m⁻²) and P(AM/HMA/PEGAA) hydrogels (about 550 J m⁻²). This also indicated that the MMs could enhance the fracture propagation and puncture resistance of the hydrogels. The hydrophobic side chains of HMA would be entangled with the hydrophobic components of the MMs to form a micellar structure. The physically crosslinking centers with curled hydrophobic chains would slide, stretch and dissipate a large amount of energy when the hydrogels were loaded and deformed. Comparison of the stress–strain curves of the three types of hydrogels during a loading–unloading cycle revealed that their energy dissipation capacity, the sliding of the chain could dissipate energy. As shown in Fig. 5(c), at λ = 15, the dissipated energies of P(AM/HMA)–MM hydrogels (350 kJ m³) were much higher than that of P(AM/HMA) hydrogels (223 kJ m³) and P(AM/HMA/PEGAA) hydrogels (230 kJ m³). The physically crosslinking centers with curled hydrophobic chains would slide during the deformation, therefore, MM crosslinked P(AM/HMA) hydrogels exhibited excellent mechanical properties.

Fig. 5 (a) Tensile curves of PAM–MMs, P(AM/HMA), P(AM/HMA/PEGAA) and P(AM/HMA)–MM hydrogels. (b) Tearing energies of P(AM/HMA), P(AM/HMA/PEGAA) and P(AM/HMA)–MM hydrogels. (c) The first cyclic loading–unloading stress–strain curves of P(AM/HMA), P(AM/HMA/PEGAA) and P(AM/HMA)–MM hydrogels (MM content was 1.0 wt% for AM and HMA, and the diameter was 349 nm with added monomers).

Then, the effect of the MM content on the mechanical properties of P(AM/HMA)–MM hydrogels was measured and the results are shown in Fig. 6. With the increase of MM contents from 0 wt% to 1.5 wt%, the tensile stress of the hydrogels would constantly increase and then decrease. The maximum yield stress of P(AM/HMA)–MMs hydrogels (1.0 wt% of MMs for AM and HMA) achieved was 0.55 MPa. Furthermore, the fracture elongation of hydrogels (1.0–1.5 wt% of MMs for AM and HMA) demonstrated more than 55 times its original length. This was because of the chemical crosslinking and hydrophobic molecular slippage of MMs in hydrogels. One interesting phenomenon was observed that an obvious buffering platform existed in the tensile curve for P(AM/HMA)–MM hydrogels (1.0–1.5 wt% of MMs for AM and HMA) when the stress reached a critical yield stress. The stress was almost unchanged in the buffering zone with the increase of strain. When the stress was close to the yield stress, MMs could produce an enormously elastic deformation because of the performance of the rubbery particles themselves. Therefore, MMs embedded in hydrogels could play multiple roles of chemically crosslinking, physically hydrophobic entanglement and rubber-like toughening zones.

Fig. 6 Tensile curves of P(AM/HMA)–MM hydrogels with different MM contents (MM diameter was 349 nm with added monomers).

The particle size of the MMs also had an important effect on the mechanical properties of the hydrogels and the tensile properties are shown in Fig. 7. The fracture stress of the hydrogels showed a slight trend of first increase and then decrease with the increase of particle size of the MMs as shown in Fig. 7(a). The maximum stress was 0.55 MPa for P(AM/HMA)–MMs with MMs of 349 nm diameter. This means that at the same content of MMs the mechanical properties were determined by the effectiveness of each crosslinker and the crosslinker number. The effectiveness of each crosslinker was proportional to the particle size, and the crosslinker number was inversely proportional to the particle size. The combined effect showed a trend of first increase and then decrease with the increase of particle size of the MMs and the optimum value of the combined effect appeared in the particle with a diameter of 349 nm. Furthermore, the fracture strain exhibited a similar trend for P(AM/HMA)–MMs with different particle sizes of MMs as shown in Fig. 7(b).

Fig. 7 Fracture stress and fracture strain of P(AM/HMA)–MM hydrogels with different particle size of MMs (MM content was 1.0 wt% for AM and HMA in added monomers).

Rheology properties of hydrogels

In order to reveal the relationship between the microstructure and the macroscopic mechanical properties, the effect of the MM content on the rheological properties of the hydrogels was investigated³¹. As shown in Fig. 8, it is clear that introducing MMs had an obvious effect on the storage modulus (G′) and loss modulus (G′′), and all samples were strong elastic materials because of G′ exceeding G′′ in the measured frequency range. With the increase of MM content, the storage modulus (G′) decreased, demonstrating that the hydrogels became more flexible because of of the leak interaction of hydrophobic MMs and the hydrophilic matrix. Otherwise, loss modulus (G′′) increased, indicating that the number of unstable or reversible crosslinking points increased. The crosslinking points were easily destroyed by external force, leading to movement of molecular chains and this produced viscous deformation. As a result, the hydrogels exhibited high fracture strain in the tensile test. This result was consistent with those shown in Fig. 6 and also indicated that the fracture strain of the hydrogel could be easily enhanced by introducing MMs. Furthermore, the ratio of G′′/G′ (tan δ) indicated the viscosity of the material³². As shown in Fig. 9, the ratio of tan δ also increased with the increase of content of MMs because of the increasing unstable or reversible crosslinking points.

Fig. 8 Storage modulus (G′) and loss modulus (G′′) of P(AM/HMA)–MM hydrogels at different frequencies (mean diameters of MMs were 349 nm in added monomers).

Fig. 9 Ratio of loss modulus (G′′) to storage modulus (G′) of P(AM/HMA)–MM hydrogels at different frequencies (mean diameters of MMs were 349 nm in added monomers).

Observation of morphology

To confirm the microstructure of MMs in P(AM/HMA) hydrogels, SEM was employed to observe the surface of brittle failure for P(AM/HMA)–MM hydrogels and the image obtained is shown in Fig. 10. It is clearly seen that MMs could be embedded in the P(AM/HMA)–MM hydrogel matrix, indicating that the forces between MMs and the matrix were very strong. MMs as chemical crosslinkers and hydrophobic associated centers can be dispersed uniformly in the hydrogel matrix that the homogeneous network structure could be formed in hydrogels. Furthermore, the MMs themselves could produce enormously elastic deformation under the forces. As a result, P(AM/HMA)–MM hydrogels exhibit excellent mechanical properties in the tensile test.

Fig. 10 SEM image of MM crosslinked P(AM/HMA) hydrogels (MM content was 1.0 wt% in added monomers and the diameter was 349 nm in added monomers).

Conclusions

In summary, highly tough and puncture resistant P(AM/HMA)–MM hydrogels were successfully prepared by the introduction of MMs as chemical and physical crosslinking centers. The MM crosslinked hydrogels exhibited an extraordinary tensile strength and elongation at break compared to traditional hydrogels toughened by simple hydrophobic segments. Furthermore, MMs could produce enormously elastic deformation themselves because of the nature of the rubbery particles, leading to long fracture elongation of over 55 times their original length. Also, the P(AM/HMA)–MM hydrogels exhibited excellent puncture resistance. Therefore, the novel strategy, inspired by the toughening mechanism of rubber-enhanced plastics, proved to be effective in toughening hydrogels and could open new pathways for extensive biomedical applications of toughened hydrogels, such as in artificial tendons, cartilage, skin and so on.

Acknowledgements

This research was supported by a grant from National Natural Science Foundation of China (NSFC) (No. 51473023 and 51103014).

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