Poly(vinyl alcohol) Nanocrystal-Assisted Hydrogels with High

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Polyvinyl Alcohol Nanocrystal Assisted Hydrogels with High Toughness and Elastic Modulus for 3D Printing Ang Li, Yi Si, Xiaohan Wang, Xianjing Jia, Xuhong Guo, and Yisheng Xu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01786 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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ACS Applied Nano Materials

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Polyvinyl Alcohol Nanocrystal Assisted Hydrogels with High Toughness and

2

Elastic Modulus for 3D Printing

3

Ang Li1, §, Yi Si2,§, Xiaohan Wang1, Xianjing Jia3, Xuhong Guo1,4,5, Yisheng Xu1,4,5,*

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1State

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Technology, Shanghai 200237, China

6

2Department

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Shanghai, 200032, China

8

3Lab

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Ministry of Education, School of Materials Science and Engineering, East China University of

Key Laboratory of Chemical Engineering, East China University of Science and

of Vascular Surgery, Zhongshan Hospital Fudan University, 180 Fenglin Road,

of Low-Dimensional Materials Chemistry, Key Laboratory for Ultrafine Materials of

10

Science and Technology, Shanghai 200237, China

11

4Engineering

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University, Shihezi 832000, China

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5International

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University of Science and Technology, Shanghai 200237, China

Research Center of Xinjiang Bingtuan of Materials Chemical Engineering, Shihezi

Joint Research Center of Green Energy Chemical Engineering, East China

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*Corresponding Author: [email protected] (Yisheng Xu)

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ABSTRACT

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Flexibility, diversity and applicability in complicated situations are urgently required for next

3

generation of tough hydrogels with good processability. To achieve this good performance,

4

complicated chemical polymerization is conventionally involved in the preparation of tough

5

hydrogels, which is tedious, energy-consuming, detrimental to environment and hard to scale up.

6

In contrast, physical crosslinking primarily the electrostatic force is always adopted as a

7

complementary to chemical crosslinking. Here we proposed a simple, non‐polymerization method

8

to develop a novel type of dual physically crosslinked tough hydrogel, which consists of

9

polyvinyl alcohol crystallite crosslinked network and hyaluronic acid–Fe3+ physically crosslinked

10

network. Instead of using electrostatic interaction, nanosized PVA crystallites were chosen as

11

major crosslinking sites for the primary network of the hydrogel. By annealing the freeze−thawed

12

hydrogel followed by the Fe3+-carboxylic group complexation to construct the second network,

13

extraordinary mechanical performance including excellent tensile strength (~8 MPa), remarkable

14

toughness (~19.6 MJ/m3) and high elastic modulus (~10 MPa) was successfully achieved.

15

Especially, the precursor solution with viscoelastic properties was demonstrated to as a “new”

16

type of ink for 3D printing with no UV curing is required. Such design provides a simple and new

17

avenue for the preparation of tough hydrogels featured with 3D printing processability and we

18

believe that the design can be potentially applied for building future soft devices.

19 20

KEYWORDS: polyvinyl alcohol, crystallization, hydrogel, physical crosslink, 3D printing

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1


INTRODUCTION

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Hydrogels have attracted extensive attentions due to its wide ranging applications, such as drug

3

delivery system,1-3 soft actuators4-5 and multifunctional sensors.6-7 However, most of the synthetic

4

hydrogels suffer from insufficient mechanical strength and toughness,8 which limit their further

5

applications. To address these issues, a series of mechanically strong hydrogels have been

6

developed through various reinforced strategies, including double-network (DN) hydrogels,9-12

7

nanocomposite (NC) hydrogels13-15 and slide-ring hydrogels.16-17 Nevertheless, it is noteworthy

8

that polymerization is always necessary to form strong and tough hydrogels,18-20 which is tedious,

9

detrimental to environment, and unfavorable for large scale fabrication.

10

Meanwhile, strong electrostatic crosslinking is often used for tough hydrogels or introduced

11

into hydrogel to construct a double network to improve the mechanical performance, such as

12

polyacrylic acid (PAA)–Fe3+ crosslinking and alginate–Ca2+ crosslinking.9-10,21 For example, Hu

13

et al. developed a PAA–PAAm based tough hydrogel which was physically crosslinked by clay

14

nanosheets

15

self-recoverability.22 However, in the aforementioned case, physical crosslinking mainly serves as

16

a supplementary reinforcement and chemical polymerization is still inevitable in the preparation

17

for functional polymeric chains such as PAA–PAAm,22-23 for physically crosslinking sites.

18

Apparently chemical crosslinking is always necessarily involved for tough hydrogel preparation

19

and a tough hydrogel simply by complete physical crosslinking was seldom reported probably due

20

to the relatively weak interactions between the polymer chains.

and

Fe3+,

and

featured

with high

stretchability,

toughness,

and

good

21

As an alternative to chemical and electrostatic crosslinking, crystallites can serve as effective

22

physical crosslinking used for tough hydrogel preparation. One representative example is the 3 ACS Paragon Plus Environment


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polyvinyl alcohol (PVA) hydrogel,24-29 which was extensively applied in the cell encapsulation,30

2

metal ion probes31 and drug sustained release.32 Typically, freeze−thaw method is involved in the

3

preparation of PVA hydrogel, during which the PVA crystals are formed and serve as physical

4

cross-linking sites.33 In PVA hydrogel, the crystals are usually smaller than 10 nm (~100 Å).34-35

5

The nanosized PVA crystals were found to be able to dramatically improve the mechanical

6

properties of PVA hydrogel. However, the hydrogel prepared through this method cannot achieve

7

a satisfied mechanical strength.36-37 In order to provide more crystallites for crosslinking,

8

annealing is commonly adopted in tuning the inner structure of PVA, such as increasing the

9

stiffness of the electrospun PVA fibers38 or enhancing the density of PVA-based cation exchange

10

membrane.39 Annealing with heating a material above its recrystallization temperature,

11

maintaining a suitable temperature followed by cooling, can increase the ductility of the material

12

by altering the physical or chemical properties. Therefore, the increase of crystallinity of PVA

13

hydrogel can be achieved through annealing40-41 but only at the expense of a lower water content.

14

On the basis of the progress in PVA hydrogel, we assume that it could be possible to acquire

15

tough hydrogel without any chemical reaction during the preparation process.

16

Herein, we report a simple method to prepare a dual physical-crosslinked hydrogel, which

17

consists of nanosized PVA crystallite crosslinked network and hyaluronic acid–Fe3+ network. No

18

polymerization reaction process is involved in the preparation, which is eco-friendly and suitable

19

for large scale production. Through annealing, the hydrogel shows strong mechanical strength

20

with a maximal toughness of 19.6 MJ/m3 and an elastics modulus of 10 MPa. Especially, the

21

viscous precursor solution with decent viscoelastic properties can be processed by 3D printing to

22

form tough gel structures with different patterns. The 3D printing processibility provides much 4 ACS Paragon Plus Environment

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convenience in the subsequent applications such as soft devices. Above all, we propose a totally

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chemical reaction free method for tough hydrogel fabrication which could be a remarkable

3

progress in the field. Such dual physically crosslinked hydrogel with high mechanical strength

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and 3D printing processability can be a promising material with wide applications for future

5

intelligent or soft devices.

6

EXPERIMENTAL SECTION

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Materials

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Polyvinyl alcohol (PVA, 98.0-99.0 mol% degree of hydrolysis, Mw ≈ 75 000 g/mol) was

9

acquired from Greagent. Hyaluronic acid (HA, Mw ≈ 900 000 g/mol) was purchased from

10

Shandong Freda Pharmaceutical Company and used as received. FeCl3 and hydrochloric acid was

11

obtained from Shanghai Lingfeng Co. Ltd. Millipore deionized water was used in all the

12

experiments.

13

Preparation of PVA/HA–Fe3+ hydrogel

14

In a typical run, HA (1 wt %, w/w) was dissolved in deionized water overnight to get a

15

homogeneous solution. Then, PVA (14 wt %, w/w) was added to the prepared solution with

16

vigorous mechanical stirring at 90 °C. The total polymer concentration was settled to 15 wt %.

17

After 6 h, the viscous polymer solution was ultrasonically treated to remove air bubble and then

18

poured into a Teflon mold. Subsequently, the Teflon mold was allowed to a freeze–thaw cycle

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(−20 °C for 24 h and 30 °C for 1 h) to acquire the PVA/HA hydrogel. The hydrogels were

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dehydrated by heating in an oven at 30 °C for 24 h and annealing at 130 °C for 1 h. At last, the

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dried samples were immersed in 0.1 mol/L FeCl3 solution for 48 h and equilibrated in DI water

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for 24 h. The PVA/HA–Fe3+ freeze–thaw hydrogel was prepared by 1–3 freeze–thaw cycles 5 ACS Paragon Plus Environment


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respectively but without annealing treatment, and the other treatments were the same as above.

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To prepare the PVA/HA–Fe3+ hydrogel of different polymer contents, the weight ratio of HA

3

was varied from 0 wt % to 1.5 wt % at an interval of 0.25 wt %. The total weight ratio of polymer

4

was fixed at 15 wt % so the PVA concentration was changed from 15 wt % to 13.5 wt % after

5

varying HA content.

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To prepare the PVA/HA–Fe3+ hydrogel at different annealing temperatures, the annealing

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temperature was set from 100 °C to 150 °C at an interval of 10 °C. The PVA/HA–Fe3+ hydrogel

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without annealing treatment was also prepared as blank sample.

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To prepare the PVA/HA–Fe3+ hydrogel with different ionic crosslinking degrees, the dried

10

hydrogels were immersed in FeCl3 of different concentrations (0.01 mol/L, 0.05 mol/L, 0.1

11

mol/L, 0.5 mol/L and 1 mol/L).

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To prepare the PVA/HA–Fe3+ hydrogel at different pH, the hydrogel was equilibrated in acidic

13

water at different pHs (pH 2, pH 3, pH 4 and pH 5) after immersing in FeCl3 solution.

14

Mechanical properties

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The tensile strength was tested on Hengyi HY–0580 testing instrument with a 100 N loading

16

cell at a crosshead speed of 100 mm/min. The samples were made to dumbbell shape (initial

17

gauge length: 12 mm, width: 2 mm). The elastic modulus was calculated from the initial slope

18

according to the stress–strain curve. The toughness and dissipated energy were determined by

19

integrating the area below the stress–strain curve. Each sample was tested at least five times to

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obtain average data.

21

Characterization

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The physical crosslinking between Fe3+ and HA was determined by FT‐IR spectroscopy 6 ACS Paragon Plus Environment

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(Nicolet 6700 instrument) and UV–vis spectroscopy (Shimaduz UV–2550). For the UV–vis

2

spectroscopy test, 20 μL of 0.1 mol/L FeCl3 solution was added to 3 mL of 5 mg/mL PVA/HA

3

(wPVA/wHA=14:1) mixed solution or 3 mL deionized water respectively. The SEM images of

4

hydrogel were obtained from NOVA Nano SEM 450 with an accelerating voltage of 5 kV.

5 6

The mass of dry state hydrogel (md) and wet state hydrogel (ms) was weighed to determine the water content (Wc) and equilibrium swelling ratio (ESR). Wc =

7

ms ― md

ESR =

8

ms ms md

× 100%

× 100%

9

X–ray diffraction (XRD) data were recorded on a Bruker D8 XRD diffractometer with Cu Kα

10

radiation (40 kV, 40 mA) at a scan rate of 10 °/min, in a range of angles corresponding to 2θ =

11

10–80°. The step size was set to 0.02°. The crystallite size (L) was calculated by the Scherrer

12

equation: L =

13

Kλ 𝛽cos (𝜃)

14

where β is the line broadening at half the maximum intensity, K (K = 1) is the morphology

15

constant, λ is the wavelength of X-ray and θ is the Bragg angle.42-43 DSC tests were performed via

16

a TA Q2000 instrument by heating from 20 to 250 °C with a heating rate of 10 K/min under N2

17

flow. TA Instruments Universal Analysis 2000 software was employed to analyze the melting

18

temperature and enthalpy. The degree of crystallinity (Xc) was calculated from the equation

19

below:

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Xc% =

∆H × 100% ∆Hc

ΔHc (138.6 J/g) is the thermodynamic enthalpy of melting of a 100% crystalline PVA.44-45 All 7 ACS Paragon Plus Environment


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the DSC measurements were repeated three times to obtain average data.

2

Rheological measurements were performed on a MCR501 rheometer (Anton–Paar Physical,

3

Austria) with a parallel–plate geometry (25 mm) in oscillation mode. Strain sweep was conducted

4

at 25 °C from 0.001% to 1000% at a fixed frequency of 1 Hz. Frequency sweep was scanned from

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0.1 to 100 Hz at 25 °C with a constant strain amplitude of 1%.

6

3D printing of processable hydrogel

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The precursor solution was printed by a 3D printing system (HTS-400, Fochif). At first, the

8

polymer precursor solution was loaded into an extrusion cartridge as printing ink. Subsequently,

9

the ink was extruded out of a flat tip needle (diameter: 0.5 cm) controlled by a screw extruder at a

10

speed of 5 mm/s and the precursor solution was printed for 3 layers. After the same subsequent

11

treatment as the preparation of hydrogel, the printed solution was made to tough hydrogel films.

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The microscope image of 3D printing hydrogel was acquired on a LEIKA DM 2500P microscope.

13

RESULTS AND DISCUSSION

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Preparation of PVA/HA–Fe3+ hydrogel

15

PVA/HA–Fe3+ double network hydrogel was prepared as demonstrated in Figure 1. HA and

16

PVA were firstly dissolved in deionized water to acquire a viscous solution. The polymer mixed

17

solution was then processed with a freeze–thaw treatment, forming the PVA/HA hydrogel due to

18

the crosslinks by nanosized PVA crystals. The hydrogel was dried at 30 °C and annealed at

19

130 °C to further crystallize the PVA chains. It has been proved that the annealing treatment can

20

serve as a more efficient method as a complement for freeze–thaw method to increase PVA

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crystallites.44,46 Considering the combination strength and stability with carboxylic group,23 Fe3+

22

was chosen for the crosslinking of HA. The dry hydrogel was immersed in FeCl3 solution for 48 h 8 ACS Paragon Plus Environment

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to realize the physical crosslinking. Since excess Fe3+ may produce the mono–crosslinking and

2

bis–crosslinking,22-23 but not the tris–crosslinking with the carboxylic groups, the redundant Fe3+

3

was removed in the final step by equilibrating the hydrogel in deionized water for 24 h. The tough

4

PVA/HA–Fe3+ double network hydrogel was obtained after full re-hydration.

5 6

Figure 1. Schematic illustration of the preparation of PVA/HA–Fe3+ hydrogel.

7 8

The prepared hydrogel as shown in Figure 2a displays a high performance in stiffness and

9

ductility. Figure 2b shows the SEM of lyophilized hydrogel. A compact structure with rough

10

surface rather than a porous structure can be clearly seen from the SEM result. The tough

11

hydrogel displays a prominent mechanical strength as found in Figure 2c. The hydrogel with a

12

diameter of 1.8 mm can afford a load of 4 kg, exhibiting an excellent mechanical strength. Figure

13

2d demonstrates the hydrogel can afford high level deformations such as knotting.



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1 2

Figure 2. (a) Photographs of the prepared PVA/HA–Fe3+ hydrogel. (b) SEM image of the

3

lyophilized hydrogel. (c) The hydrogel with a diameter of 1.8 mm can lift up 4 kg barbell disks.

4

(d) The hydrogel shows certain elasticity by knotting.

5 6

The formation of PVA crystal networks is confirmed by XRD pattern of the dry hydrogel

7

shown in Figure 3a. The two crystalline peaks at 2θ = 19.8° and 40.8° corresponding to the

8

characteristic peaks of PVA crystalline can be clearly identified.47 According to the Scherrer

9

equation, the crystallite size of PVA in the obtained hydrogel is about 12 Å, a little smaller than

10

that of previous reports.37 The physical crosslinking between Fe3+ and carboxylic groups is

11

reflected in the UV–vis spectra (Figure 3b) and FT–IR spectra (Figure 3c). Both the polymer

12

solution and FeCl3 solution show almost no absorbance in the range of 400–600 nm. However,

13

the mixture of polymer solution and FeCl3 solution exhibits a broad absorbance peak at ∼450 nm,

14

which demonstrates the complexation between Fe3+ and HA.23,48 It’s noteworthy that, if the

15

concentration of PVA/HA solution and Fe3+ increased, upon mixing the solutions, flocculent 10 ACS Paragon Plus Environment

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precipitate was clearly observed, which reflected the complexation visually. As shown in FT–IR

2

spectra, after the introduction of FeCl3, the peaks at 1706 and 1560 cm-1 diminish, indicating the

3

combination between Fe3+ and carboxylic group after the introduction of Fe3+.

4 5

Figure 3. (a) XRD profile of the dried PVA/HA–Fe3+ hydrogel. (b) UV–vis spectra of 5 mg/mL

6

PVA/HA mixed solution, 3 mL of water or polymer precursor solution after addition of 20 μL of

7

0.1 M FeCl3 solution. (c) FT–IR spectra of dried PVA, PVA/HA and PVA/HA–Fe3+ hydrogels.

8

(d) Enlarged view of the FT–IR spectra from 1000 cm-1 to 2000 cm-1.

9 10

Mechanical properties of PVA/HA–Fe3+ hydrogel

11

The PVA/HA–Fe3+ hydrogel consisting of PVA crystal crosslinked network and HA–Fe3+

12

physical-crosslinked network, and the dual crosslinked structure endows the system with

13

extraordinary mechanical performance. By modulating experimental conditions including HA 11 ACS Paragon Plus Environment


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content, annealing temperature, Fe3+ concentration and pH, the mechanical performance of the

2

PVA/HA–Fe3+ hydrogel can be well-tuned and the possible mechanism involved in the

3

performance variation is investigated accordingly.

4 5

Figure 4. (a) Typical stress–strain curves of PVA/HA–Fe3+ hydrogels prepared with different HA

6

contents. (b) The stress and strain of the prepared hydrogels as a function of HA content.

7

Toughness (c) and elastic modulus (d) of PVA/HA–Fe3+ hydrogels prepared with different HA

8

contents.

9 10

The hydrogel was prepared with a systematic increase of weight ratios of HA to PVA from 0 to

11

1.5 wt %. The total weight ratio of the polymer was fixed at 15 wt %. Due to the high molecular

12

weight, PVA couldn’t be fully dissolved in a viscous precursor solution in the presence of

13

excessive amount of HA. The maximum weight ratio of HA was therefore set to be 1.5 wt %.

14

Figure 4 shows the influence of HA content on the mechanical performance of the hydrogel. It 12 ACS Paragon Plus Environment

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1

can be clearly observed that the overall mechanical performance was improved significantly as

2

the HA content was raised from 0 wt % to 1 wt % due to the incorporation of HA–Fe3+

3

networking. Compared with the pure PVA hydrogel, the toughness of PVA/HA hydrogel was

4

shown to have a ~400% increase, from ~3.8 MJ/m3 to ~19.6 MJ/m3 while the elastic modulus was

5

also increased by ~170% from ~1.8 MPa to ~4.9 MPa. However, once the content of HA was

6

above 1 wt %, the mechanical performance dropped dramatically because excess HA polymer

7

chains hindered the crystallization of PVA, leading to a less dense structure of the PVA network.

8

Therefore, the dependence of mechanical performance on HA content is non‐monotonic with an

9

optimal value appearing at around 1 mg/mL.

10 11

Figure 5. Water content (a) and DSC curves (b) of PVA/HA–Fe3+ hydrogels prepared with

12

different HA contents.

13 14

Because of the water retention capability of HA,49 the water content increased with HA

15

content, but the overall water content was less than 45% due to the treatment of annealing.46 Thus,

16

the hydrogel became more ductile by the incorporation of HA and the final equilibrium swelling

17

ratio (Figure S1) of the hydrogel can be tuned by controlling the weight ratio of HA. To further

18

investigate the effect of HA content on PVA crystallization, PVA/HA–Fe3+ hydrogels prepared 13 ACS Paragon Plus Environment


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with different HA contents were then analyzed by DSC as presented in Table 1. The degree of

2

crystallinity (Xc) was calculated from the endothermic peak of the DSC curve with the equation

3

below:

4

Xc% =

∆H × 100% ∆Hc

5

In which ΔHc is the thermodynamic enthalpy of melting a 100% crystalline PVA (138.6 J/g). ΔH

6

can be calculated from the DSC curves in Figure 5b by integrating the area in the range of 190 °C

7

to 240 °C. In contrast to the pure PVA hydrogel, the crystallinity of PVA in PVA/HA–Fe3+

8

hydrogel decreased gradually with HA content, which is consistent to the previous reports.50

9

Combined with previous results, it can be demonstrated that the reduction of crystallinity leads to

10

less crosslinking for PVA and accounts for a lower mechanical strength. That is, a proper amount

11

of HA helps to improve the ductility of hydrogel leading to a high strain whereas excess amount

12

of HA results in an opposite effect. Therefore, a balance between the PVA crystallite crosslinked

13

network and HA–Fe3+ network needs to be considered to acquire optimal mechanical

14

performance of the hydrogel. Meanwhile, the introduction of HA also leads to a lower melting

15

point (Tm) of the hydrogel, probably due to the defective crystals formed in the HA chains.47,51 As

16

a result, a modest HA content (1 wt %) was used to prepare PVA/HA–Fe3+ hydrogels with

17

optimal toughness and elastic modulus.

18 19 20 21 22


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Table 1. Parameters obtained from DSC curves of PVA/HA–Fe3+ hydrogels prepared with

2

different HA contents. HA content%

Tm/°C

ΔH/(J g-1)

Crystallinity%

0 wt %

222.0 ± 0.8

71.1 ± 5.8

51.8 ± 7.8

0.25 wt %

218.7 ± 1.2

65.8 ± 3.2

47.5 ± 4.2

0.5 wt %

213.9 ± 0.3

61.7 ± 0.5

44.5 ± 0.4

0.75 wt %

213.7 ± 0.8

58.4 ± 2.8

42.2 ± 2.0

1 wt %

213.4 ± 0.3

55.30 ± 1.6

39.9 ± 1.1

1.25 wt %

212. 2 ± 1.8

50.3 ± 1.9

36.3 ± 1.4

1.5 wt %

200.0 ± 4.5

48.1 ± 2.1

34.6 ± 1.5

3

4 5

Figure 6. (a) Typical stress–strain curves of PVA/HA–Fe3+ hydrogels prepared at different

6

annealing temperatures. (b) Annealing temperature-dependent changes of stress and strain of the 15 ACS Paragon Plus Environment


Page 16 of 38

1

prepared hydrogels. Toughness (c) and elastic modulus (d) of PVA/HA–Fe3+ hydrogels prepared

2

at different annealing temperatures.

3 4

As seen above, the crystallinity plays a significant role on the crosslinking of the hydrogel. In

5

order to enhance the crystallinity of PVA, annealing was employed in the preparation of hydrogel

6

since it has been proven to generate PVA crystals with higher density compared to the freeze–

7

thaw process (Figure S2). The prepared PVA/HA hydrogel was treated under different

8

temperatures, ranging from 100 to 150 °C. A yellowish dry gel (Figure S3) was formed after

9

annealing. As shown in Figure 6, it can be observed that the annealing treatment at 130 °C led to

10

an improved mechanical performance of the hydrogel, with tensile strength increasing from 1.87

11

MPa to 7.15 MPa and toughness increasing from 3.46 MJ/m3 to 19.62 MJ/m3. Moreover, as the

12

annealing temperature was increased, the overall mechanical performance was enhanced and the

13

water content decreased accordingly (Figure S4). The DSC results (Figure S5) and XRD results

14

(Figure S6) hence confirm the contribution of annealing to the enhancement of crystallization for

15

PVA. According to the DSC results in Table S1, raising the annealing temperature is believed to

16

facilitate the crystallization process, therefore leading to a much denser PVA crystallite

17

crosslinked network. Once the annealing temperature is above 130 °C, the stress increases due to

18

the higher crystallinity, but correspondingly the reducing water content leads to less stretchability.

19

A too high crystallinity is deleterious for the elasticity and makes the gels more fragile. Therefore,

20

the overall mechanical toughness shows a slightly decreasing tendency. As discussed above, the

21

overall mechanical strength can be increased dramatically upon annealing treatment. Considering

22

the very low water content (< 40%) under high temperature (140 °C and 150 °C), we chose 16 ACS Paragon Plus Environment

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130 °C as a suitable temperature to prepare the hydrogel in the subsequent study of mechanical

2

performance.

3

In addition to the PVA crystallite network, the electrostatic interaction between Fe3+ and

4

carboxylic group also has a strong influence on the mechanical performance of the hydrogel.

5

Therefore, the mechanical properties of hydrogels prepared under different Fe3+ concentrations

6

were studied. As shown in Figure S7, the overall toughness and elastic modulus of the hydrogel

7

increased with Fe3+ concentration and reached maximum values at CFe3 + = 0.1 M, after which

8

the sharp decline of overall mechanical strength was obviously observed. This phenomenon is

9

ascribed to the influence of the Fe3+ concentration on the crosslinking degree between Fe3+ and

10

carboxylic groups. Initially, Fe3+ and COO- crosslinked insufficiently so the mechanical

11

properties could be enhanced after increasing CFe3 + . However, excessive presence of Fe3+ will

12

transform the Fe3+-COO- interaction state from tridentate to bidentate, or even monodentate and

13

the mechanical performance could be weakened by so. Therefore, a moderate Fe3+ concentration

14

is essential for construction of a secondary cross-linking network.

15

The secondary crosslinking degree was further modulated by adjusting the equilibration pH of

16

the hydrogel. The hydrogel was equilibrated in acidic water at different pHs (pH 2, pH 3, pH 4

17

and pH 5, respectively) instead of DI water for 24 h. By changing the pH of soaking solution, the

18

complexation patterns and mechanical properties were altered as shown in Figure S8. At pH 2,

19

HA inclined to form the mono–complex with Fe3+. However, in slightly acidic environment (pH

20

4-5), tris-carboxyl–Fe3+ complexation with the highest stability dominated, leading to higher

21

toughness and modulus. The different complexation modes could be clearly reflected by the color

22

change from light yellow to brown of hydrogel as pH was increased as shown in the insert of 17 ACS Paragon Plus Environment


Page 18 of 38

1

Figure S8b. Overall, the hydrogel became softer and more ductile at lower pH, along with a lower

2

tensile strength. This provides a way to tune the mechanical properties of the hydrogels by pH.

3 4

Figure 7. Ashby charts for mechanical properties of various soft materials, including the

5

PVA/HA–Fe3+ hydrogel in current work, Alginate gel,9 Alginate–PAAm hydrogel,9 PVA–PAAm

6

hydrogel,46 PAAm hydrogel,9 PVA hydrogel,52 P(AAm–co–AAc) gel,23 NC gel,53 tetra–PEG

7

gel,54 cartilage and skin.55 Elastic modulus-dependent changes of tensile strength (a) and fracture

8

energy (b).

9

The experimental results have demonstrated that PVA nanocrystal assisted tough hydrogels

10

with tunable properties can be developed simply through two physical crosslinking networks

11

without any chemical polymerization process. The Ashby chart in Figure 7 shows a visualized

12

comparison of various soft materials and our hydrogel. The presented mechanical strength of

13

PVA/HA–Fe3+ hydrogel is adopted under four different annealing temperatures (100 °C to

14

130 °C). The PVA/HA–Fe3+ hydrogel performs better than most of the hydrogels in terms of the

15

tensile strength and elastic modulus and shows a comparable performance to skin and cartilage in

16

terms of elastic modulus and fracture energy. Especially, under the condition of similar water

17

content, the PVA/HA–Fe3+ hydrogel is apparently superior than the PVA–PAAm hydrogel in all

18

conditions studied in our experiments. 18 ACS Paragon Plus Environment

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1


Potential application as 3D printing hydrogel

2

The hydrogel was further studied for processing as a 3D printing material and it was shown that

3

the material could be processed into different shapes and formed without UV curing which can be

4

potentially utilized for various applications including vascular networks or tissue engineering

5

scaffolds.56-57 The introduction of HA of high molecular weight endowed the precursor solution

6

with highly viscoelastic properties. After being extruded from the nozzle, the viscous solution

7

could maintain the initial shape during the 3D printing process and the precursor solution could

8

potentially serve as an “ink” for 3D printing.

9 10

Figure 8. (a) Strain-dependent and (b) frequency-dependent changes of the storage (G', solid

11

symbol) and loss (G", open symbol) moduli as well as (c) shear rate-dependent changes of

12

viscosity of the precursor solutions with different HA contents.

13 14

The rheology behavior of the precursor solution was studied before printing. Judged from the

15

strain sweeps in Figure 8a, the linear viscoelastic (LVE) region was determined to end at the

16

strain of ~30%. Moreover, the precursor solution showed a reduced mobility and a higher

17

viscosity (Figure 8b and c) as the HA concentration was increased. When the HA content was 1.5

18

wt %, the G’ was even larger than G’’, making it difficult to extrude the solution out of the nozzle

19

while the precursor solution can’t maintain the shape after being extruded out of the nozzle under 19 ACS Paragon Plus Environment


Page 20 of 38

1

HA content lower than 1 wt %. So a modest HA concentration of 1 wt % was chosen for the

2

subsequent 3D printing.

3

Figure 9a demonstrates the 3D printing process in which the precursor solution could be

4

printed into different shapes. After the same treatment as the hydrogel above, the viscous solution

5

became tough hydrogel films of different shapes, such as regular hexagon, square and circle in

6

Figure 9b-d. Figure 9e shows a microscopic image of the printed hydrogel film. The white arrows

7

indicated overlapped interface of different printing layers, confirming the maintenance of the

8

initial shape for the viscous solution during the 3D printing process.

9 10

Figure 9. (a) Schematic illustration of 3D printing process. The precursor solution was printed

11

into different patterns for three layers, including regular hexagon (b), square (c) and circle (d). (e)

12

The microscopic image of the 3D printing hydrogel.

13 14

CONCLUSIONS

15

In summary, a PVA assisted tough hydrogel prepared through dually physical crosslinking

16

shows an outstanding toughness and elastic modulus. The hydrogel compromises nanosized PVA 20 ACS Paragon Plus Environment

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1

crystal crosslinked network and HA–Fe3+ ionically complexed network, and no polymerization

2

reaction process is involved, which is eco-friendly and suitable for large scale production. The

3

mechanical performance can be well tuned by the variation of the polymer weight ratio, annealing

4

temperature, Fe3+ concentration and pH. By optimizing the crystallization degree of PVA and the

5

complexation strength between HA and Fe3+, the extraordinary toughness (fracture energy: ~19.6

6

MJ/m3) and hardness (elastic modulus: ~10 MPa) for PVA/HA–Fe3+ hydrogel is successfully

7

achieved. Furthermore, the introduction of high molecular weight HA endows the precursor

8

solution with superior viscoelastic properties, so that the precursor solution can serve as an “ink”

9

for 3D printing. The dual physically cross-linked hydrogel with great mechanical strength and 3D

10

printing processability can therefore be a promising material with broad applications such as soft

11

devices.

12 13 14

Supporting Information

15

The following files are available free of charge via the Internet at http://pubs.acs.org. The

16

mechanical

17

PVA/HA−Fe3+ freeze-thaw hydrogels. The images, water contents, XRD curves and DSC curves

18

of PVA/HA−Fe3+ hydrogel annealed at different temperatures.

properties

comparison

between

PVA/HA−Fe3+

annealing

hydrogels

and

19 20

Author Contributions

21

§A.

L. and Y. S. contributed equally to this work.



Page 22 of 38

1 2

ACKNOWLEDGMENT

3

The authors gratefully acknowledge the financial support from the National Science Foundation

4

of China (NSFC; No. 21676089, 5171101370). This work was also sponsored by Shanghai Talent

5

Development Fund (2017038), Interantional One belt One Road Collaboration Project of

6

Shanghai (18490740300), the Fundamental Research Funds for the Central Universities

7

(222201717013, 22221818014), and 111 Project Grant (B08021).

8 9 10

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3 4 5 6

TABLE OF CONTENTS

7



Figure 1. Schematic illustration of the preparation of PVA/HA–Fe3+ hydrogel. 319x167mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2. (a) Photographs of the prepared PVA/HA–Fe3+ hydrogel. (b) SEM image of the lyophilized hydrogel. (c) The hydrogel with a diameter of 1.8 mm can lift up 4 kg barbell disks. (d) The hydrogel shows certain elasticity by knotting. 236x118mm (300 x 300 DPI)



Figure 3. (a) XRD profile of the dried PVA/HA–Fe3+ hydrogel. (b) UV–vis spectra of 5 mg/mL PVA/HA mixed solution, 3 mL of water or polymer precursor solution after addition of 20 μL of 0.1 M FeCl3 solution. (c) FT– IR spectra of dried PVA, PVA/HA and PVA/HA–Fe3+ hydrogels. (d) Enlarged view of the FT–IR spectra from 1000 cm-1 to 2000 cm-1. 189x138mm (300 x 300 DPI)


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Figure 4. (a) Typical stress–strain curves of PVA/HA–Fe3+ hydrogels prepared with different HA contents. (b) The stress and strain of the prepared hydrogels as a function of HA content. Toughness (c) and elastic modulus (d) of PVA/HA–Fe3+ hydrogels prepared with different HA contents. 202x134mm (300 x 300 DPI)



Figure 5. Water content (a) and DSC curves (b) of PVA/HA–Fe3+ hydrogels prepared with different HA contents. 188x68mm (300 x 300 DPI)


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Figure 6. (a) Typical stress–strain curves of PVA/HA–Fe3+ hydrogels prepared at different annealing temperatures. (b) Annealing temperature-dependent changes of stress and strain of the prepared hydrogels. Toughness (c) and elastic modulus (d) of PVA/HA–Fe3+ hydrogels prepared at different annealing temperatures. 200x133mm (300 x 300 DPI)



Figure 7. Ashby charts for mechanical properties of various soft materials, including the PVA/HA–Fe3+ hydrogel in current work, Alginate gel,9 Alginate–PAAm hydrogel,9 PVA–PAAm hydrogel,46 PAAm hydrogel,9 PVA hydrogel,52 P(AAm–co–AAc) gel,23 NC gel,53 tetra–PEG gel,54 cartilage and skin.55 Elastic modulusdependent changes of tensile strength (a) and fracture energy (b). 198x68mm (300 x 300 DPI)


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Figure 8. (a) Strain-dependent and (b) frequency-dependent changes of the storage (G', solid symbol) and loss (G", open symbol) moduli as well as (c) shear rate-dependent changes of viscosity of the precursor solutions with different HA contents. 285x67mm (300 x 300 DPI)



Figure 9 (a) Schematic illustration of 3D printing process. The precursor solution was printed into different patterns for three layers, including regular hexagon (b), square (c) and circle (d). (e) The microscopic image of the 3D printing hydrogel. 232x117mm (300 x 300 DPI)


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Poly(vinyl alcohol) Nanocrystal-Assisted Hydrogels with High

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