Hydrogel with Ultra-Fast Self-Healing Property both in Air and

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Hydrogel with Ultra-Fast Self-Healing Property both in Air and Underwater Weipeng Chen, Dezhao Hao, Wanjun Hao, Xinglin Guo, and Lei Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17118 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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ACS Applied Materials & Interfaces

Hydrogel with Ultra-Fast Self-Healing Property both in Air and Underwater ∥

AUTHOR NAMES: Wei-Peng Chen†‡, De-Zhao Hao§ , Wan-Jun Hao*†, Xing-Lin Guo*‡, Lei Jiang§ AUTHOR ADDRESS: † Hainan University, Hainan tropical island resources ministry of education key laboratory of advanced materials, Haikou 570228, PR China. ‡ Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China. §Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China. ∥School of Future Technology, University of Chinese Academy of Sciences, Beijing, 100190, P. R. China. KEYWORDS: ultra-fast; dynamic borate bond; smart self-healing; underwater; double-network hydrogels

ABSTRACT: Self-healing hydrogels have a great potential utilization in 3D printing, soft robotics, and tissue engineering, due to the ability to expand the useful lifespan. There has been a large number of successful strategies in developing hydrogels which exhibit rapid and

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autonomous recovery. However, a gel with an excellent self-healing performance within several seconds is still an enormous challenge. Here, an ultra-fast self-healing hydrogel based on an agarose/PVA double network is presented. The gel utilizing dynamic borate bond exhibits 100% cure in strength and elongation after healing for 10 seconds in air, and this hydrogel shows an excellent self-healing property underwater as well. In addition, the agarose/PVA double network hydrogel exhibits a smart self-healing property of an in-situ priority recovery, ensuring the shape and the function is the same as original one. With the combination of self-healing properties, such a hydrogel could be applied to a board range of areas.

Introduction Hydrogels with remarkable physical properties and biocompatibility are one of the crucial "soft and wet" materials reserving quantity of water in their 3D network structure, thus leading to a wide utilization in many areas, such as contact lenses, drug delivery, substitutes for skin, tendons and cartilage.1-5 In these years, self-healing hydrogels, which shows an ability to repair the damages, have attracted extensive attention.6-10 As their network structures and original properties are maintained, the life-time and reliability of the gels will be improved.6-7 A lot of efforts have been made to fabricate self-healing hydrogels, the research works have been focused on non-covalent reactions and dynamic covalent reaction.11-18 The former involves hydrogen bonding,11 ionic bonding,12 supramolecular interactions13 and chain entanglement,14 while the latter deals with phenylboronate esters,15 disulfide bonds16 and acylhydrazone bonds.17 There are a number of self-healing hydrogels are stimulus independence,11-14 however, in applications, self-healing hydrogels are often plagued with problems such as the long selfhealing time22-27 and stimulus dependence.11-14 The specific repair environment is another serious


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question for the utilization of self-healing hydrogels, but there is rarely research focused on healing in different circumstances, especially in wetness environment or underwater. It is still challenging to prepare an ultra-fast self-healing hydrogel (without external stimuli) applied in wetness environment and underwater.28-30 In this work, we report a new type of double network (DN) hydrogel with one network being chemically crosslinked and the other being physically crosslinked, consisting of a hydrogen bond-associated agarose gel as the first network and a dynamic borate bond-associated PVA gel as the second network (named as agarose/PVA DN hydrogels). The dynamic PVAborate network will provide the hydrogel with a self-healing property,31-32 and the agarose network will make an additional platform to bear stress and firm the structure, thus to improve strength, toughness and stability.33-35 As a result, the agarose/PVA DN hydrogels show excellent self-healing ability. Firstly, these hydrogels perform extremely fast self-healing capability (almost 100% recovery of initial strength and elongation in 10 seconds) at room temperature without external stimulus. Secondly, the hydrogels exhibit an outstanding self-healing performance underwater, which will have a great significance in broadening the application fields of the hydrogels. Thirdly, the agarose/PVA DN hydrogels show a smart self-healing property, it gives priority to repair the damage, which has a great benefit to the maintenance of materials performance. Besides, hydrogels exhibit a recycle shapeable property (formation of many shapes after gelling repeatedly). And both PVA and agarose are widely used as medical materials for their exceptional biocompatibility.34,

36-38

We believe that the agarose/PVA

hydrogels with multiple advantages could be generally applied to a broad range of applications, from biomaterials to underwater engineering. Experimental Section


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Materials and chemicals. Polyvinyl alcohol (degree of hydrolysis 99%, degrees of polymerization are 2800) is purchased from Sinopec Sichuan Vinylon Works. Agarose (congealing temperature between 34.5 - 37.5℃, Loss on Drying ≤ 10%) is purchased from Sigma-Aldrich Inc. Borax (analysis of pure, Na2B407·10H20) is purchased from Sinopharm Chemical Reagent Co., Ltd.. All reagents are directly used without any purification. Synthesis of PVA-borax hydrogel and agarose/PVA DN hydrogels. In the experiment, PVA is dissolved in hot water (water bath at 98 degrees for 2h) to form a transparent solution of 20 wt% in concentration. 50 ml PVA solution is mixed with 50 mL borax solution (0.04 mol L-1) under stirring (water bath at 90 degrees) for 1 hour. Then the hydrogel is put in a cylindrical mold “A” (with borax solution). And mold “B”, which is slightly smaller than the inner diameter of the cylindrical model, is pressed on the surface of hydrogel. At last, a pressure of 2.5kg is applied to press for 2 hours. In order to obtain samples which are suitable for thickness, the sample is cut into small pieces (5cm * 5cm), and put in the mold “C”. A pressure of 2.5kg is also applied to press for 1 hours. The pressed step is applied to remove the bubbles and heal the cracks in hydrogels. The agarose/PVA solution is prepared through the same process of PVA solution, and in the prepared solution, the PVA contents are 20 wt% and the agarose contents are range from 1 wt% to 3 wt%. Then 50ml agarose/PVA solution is mixed with 50 mL borax solution (0.04 mol L-1) under stirring (water bath at 90 degrees) until the gel is obtained. And the hydrogel is put in the mold with borax solution and pressed for 3 hours. Characterization. Mechanical Measurements. The mechanical properties of the hydrogels are measured with a universal testing machine and the tensile machine is 100N range. Firstly, hydrogels are prepared in a size of 8cm×1cm×0.5cm. While testing, the samples are nipped to


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tensile machine and pulling rate is 100 mm/min. And the data is the average strength of five samples in the same group. Self-Healing Experiments. The hydrogels (8cm×1cm×0.5cm) are cut into halves, and then the two separate hydrogels are contacted in air and underwater. There is no other stress or outside stimulate applied during the healing process. After self-healing, the tensile test is measured again to calculate the healing efficiency. Rheological Measurements. The rheological behaviors of the agarose/PVA DN hydrogels are analyzed with a modular compact rheometer (Anton Paar, MCR 302). Samples are prepared in the shape of cylinder with a diameter of 15 mm and a thickness of 6 mm. The frequency (ω) sweep test at ω = 0.1 - 100 rad/s, and strain (γ) = 0.5% of agarose/PVA DN hydrogel at 25 °C. The temperature (t) sweep test at t = 25 - 100°C, and strain (γ) = 0.5% of the hydrogel at 0.5 rad/s. Results and discussion The method for fabricating the double network (DN) hydrogel is convenient. As shown in Fig.1a, a network is formed through a dynamic covalent crosslinking reaction of PVA and borax. The borate ions can complex to the adjacent hydroxyl of PVA, the complexation of borate ions and PVA hydroxyl is reversible, allowing the fabrication of hydrogels with a self-healing property.31-32 Due to the invertible nature of the borate bond, the chain of PVA in the hydrogels still can move. This will lead to weak mechanical properties and limited stability of PVA-borax hydrogels which restrict its further applications severely.33, 39 Constructing another network is a promising strategy to address these limitations.33-35 Here, we employed agarose to form the network. When the resulting solution is gradually cooled down, the agarose crosslinked via


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hydrogen bond and formed the polymer network. Meanwhile, hydrogen bonds also form between PVA and agarose.33 The steadiness of the PVA hydrogel and the agarose/PVA DN hydrogels are shown in Fig.1b. Two types of cuboid-shaped gels are prepared, respectively. Then, the samples are kept in a wet environment (72% RH) at room temperature for 3 hours and the shape changes are observed. According to the result, after 3 hours, the shape of the PVA hydrogel changes a lot, which proves that the bonds of gel are dynamic. However, the shape of the agarose/PVA DN hydrogel keeps as initial, indicating the improvement of stability. The main reason for the higher stability of the DN hydrogel is that the movement of the PVA chain will be inhibited by the hydrogen bond between the PVA and agarose.40-41 It is no doubt that a better mechanical performance offers the hydrogels a wider range of applications. Then, we initially characterize the mechanical behavior and self-healing property of the PVA hydrogel and the agarose/PVA DN hydrogels. Fig.2a are the stress-strain curves of the PVA hydrogel and the agarose/PVA hydrogels with different contents of agarose (0.5%, 1%, 1.5%) and the curves of them after healing for 10 seconds, respectively. It can be seen that the elastic modulus of the PVA hydrogel is about 7.4 kPa, and the maximum original stress is 11.53 kPa at the strain of 337% (Fig.2b). The tensile test indicates that the PVA hydrogel is soft, and easy to be fractured. However, the elastic modulus and the tensile fracture stress of materials improve evidently through the addition of agarose. The tensile fracture stress of the hydrogel with 0.5 wt% agarose is 14.17 kPa, which has no significant difference from the PVA hydrogel, but the corresponding ruptured strain is 625%, which is almost 2 times higher than that of the PVA hydrogel. Increasing the agarose ratio from 0.5% to 1.5%, both the elastic modulus and the maximum stress increase. The hydrogels with a content of 1 wt% agarose shows a stress of 19.53 kPa, and an elastic modulus of 14.64 kPa. While the hydrogels with a content of 1.5 wt%


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agarose, the elastic modulus is 15 kPa and the maximum stress is up to 24.65 kPa, which is more than 2 times larger than the PVA hydrogel. The results indicate that the mechanical performance improves with the addition of agarose. To assess the self-healing behaviors of the hydrogels, the tensile test is measured to calculate the healing efficiency.42-44 The hydrogel is nipped to tensile machine, and just after the hydrogel was cut into two pieces, two parts of the gel are put together and contact for 10 seconds, then the healed hydrogel is stretched to test the tensile strength (Fig.2c). As summarized in Fig.2a, when the test starts after only 10 seconds healing time, the stress-strain curves of healed hydrogels almost coincide with the original one. Fig.2b shows the original stress and the healed stress of the PVA hydrogel and the agarose/PVA DN hydrogel with different agarose contents. It shows that the average stress of the PVA hydrogel is 11.51±1.56 kPa, and the stress of the gel after healing for 10 seconds is 10.9±1.02 kPa (test 5 times). The tensile stress recovers to 94.7 %, which indicates the excellent self-healing performance of dynamic borate-associated hydrogels. The complexation of borate ions and hydroxyl on adjacent polymer strands is extremely fast (0.33 sec), and the hydrogel heals when a number of hydroxyls complex with borate ion, so that the material can recover perfectly after contacting for seconds.11, 31-32 For the agarose/PVA DN hydrogels with agarose contents from 0.5 wt% to 1.5wt%, the average original stress is 14.07±2.10 kPa, 19.48±1.32 kPa, and 24.60±1.33 kPa, respectively, and the corresponding healed stress (after 10 seconds healing) is 13.96±1.81 kPa, 19.30±1.59 kPa, and 23.58±1.84 kPa. Among them, the average healing efficiency of the hydrogels with 1 wt% agarose is up to 99%. According to the research, hydrogels are all self-healing nearly 100%, although the strength of them improved with the increase of agarose content. This indicates that agarose network participates the healing reaction, but agarose hydrogel does not show a self-


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healing property (Fig.S1, Fig.S2). We believe that the agarose chains will be dragged by the PVA chains at the contact interface for the hydrogen bonds exist between PVA and agarose. Then, agarose chains will be tangled in another network, the hydrogen bonds form between the PVA chains and agarose chains, leading to a formation of the new network. The experiment proves that hydrogels show an extremely fast self-healing property. While the general times for self-healing of hydrogels are in the minute scale,25-27 this kind of hydrogels can heal completely in 10 seconds. Furthermore, the agarose/PVA DN hydrogels can also recover underwater. Fig.3a shows the healed stress of the agarose/PVA DN hydrogel with 1 wt% agarose (underwater). In the case of recovery times are 10 seconds, the average healed stress of the hydrogel is 4.45±0.76 kPa (test 5 times). And the stress of the gel after healing for 30 seconds is 8.53±1.25 kPa, while the stress is 13.72±1.41 kPa after a 60 seconds healing. The tensile stress of hydrogel after healing for 60 seconds recovers to 70 %, which indicates the agarose/PVA DN hydrogels with a fast selfhealing property underwater as well. As illustrated in Fig.3b, the hydrogel is cut and the two pieces (dyed red and blue) are immediately brought together underwater. After contact for 60 seconds, the self-healing bending can bear stretching, as shown in Fig.3b(III). As Fig.3c shows, the hydrogel fragments are put into a mold and healed underwater for one hour. And the fragments can be remolded into a hydrogel. This process can be explained by the possibility that the borate can diffuse in the water environment, the dynamic crosslinking of the PVA chains are still carried out and the agarose chain will tangle. Therefore, the hydrogels can still heal. And the slower self-healing process may result from the lower borate concentration and the insufficient contact of hydrogels. With the rapid self-healing property in multiple environments, the


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agarose/PVA DN hydrogels have a huge potential utilization, especially the engineering that requires a quick response or working in the water. To further evaluate the self-healing ability of the agarose/PVA DN hydrogel, another selfhealing test is also carried out. The contact of two fresh sections is defined as in-situ self-healing, while the contact of a fresh section and a non-fresh section is defined as ex-situ self-healing. As shown in Fig.4, a pillar shaped hydrogel is cut into two pieces (Fig.4a). When they contact insitu, the whole gel is lifted at the moment of touching (about 1 second) (Fig.4b). However, if the sections contact ex-stiu, the gels are still apart after the instantaneous contact (Fig.4c). According to the test, there is a great difference between the self-healing efficiency of contacts. Fig.4d indicates the experiment methods of in-situ self-healing and ex-situ self-healing and their stress and strain curves of the agarose/PVA DN hydrogels (containing 1 wt% of agarose). While testing, the desired shape hydrogel is sandwiched on the tensile tester and cut into two pieces. Insitu self-healing is to contact the two hydrogels along the section (two fresh surfaces), while exsitu is to insert another gel in the gap between the gels and put the three parts together (a fresh surface and an old surface). It is shown in Fig.4d, in the case of recovery times are 10 seconds, the efficiency of in-situ self-healing and ex-situ self-healing shows a great difference. In-situ self-healing for 10 seconds, the sample shows a tensile fracture stress of 18.32 kPa. The maximum stress of the sample after 10 seconds ex-situ self-healing is only 5.2kPa, less than onethird of the in-situ. Fig.4e shows the in-situ and ex-situ stress of the PVA hydrogel and the agarose/PVA DN hydrogels with different agarose contents. When the test starts after 10 seconds, the in-situ recovery stress of the PVA hydrogel is 11.51 kPa, and the ex-situ recovery stress is 6.98 kPa. The strength of the in-situ self-healing is 1.65 times larger than the ex-situ self-healing. For the agarose/PVA DN hydrogels, the difference becomes larger as the agarose


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ratio increases. The hydrogel containing 1.5 wt% of agarose shows a tensile fracture stress of 24.6 kPa after an in-situ healing, and it shows a maximum stress of only 4.5 kPa after an ex-situ healing. The maximum stress after an in-situ healing is 5.47 times larger than the ex-situ. The self-healing property of the agarose/PVA DN hydrogel is related to the dynamic covalent bonds which are formed through the complexation of borate ions and PVA hydroxyls. The hydrogel has the ability to enrich borate ions, which leading to the interior concentration of borate ion is larger than the exterior. 45-46 For fresh sections, the concentration of borate ions is consistent within and outside the interface. At this moment, the efficiency of self-healing is good for both the hydroxyl density and borate ion concentration of the section are higher. While the section is stale, the borate ion concentration on the surface of the gel tends to be smaller than that inside for the effect of agarose/PVA DN hydrogel accumulation of borate ion in solution. Besides, the interface of the hydrogel becomes stable, the diffusion of borate ion is inhibited. Therefore, the borate ion concentration of the section is lower, leading to less number of hydroxyls react and a poor self-healing performance (compared with the in-situ self-healing). With the increasing concentration of agarose, the ex-situ self-healing is less efficient. The reason is, the higher concentration of agarose, the more difficult it is for borate ion to move. Another phenomenon shows in Fig.4e is that the self-healing efficiencies of hydrogels are enhanced with the increase of self-healing time. After healing for 60 seconds, the maximum stress of the PVA hydrogel recovers to 87%. For ex-situ, with the increase of agarose concentration, the stresses of the agarose/PVA DN hydrogels recover to 77% (0.5 wt%), 58% (1 wt%) and 54% (1.5 wt%), respectively. The experiment indicates that the hydrogels exhibit a smart self-healing property of giving priority to in-situ healing. With the smart self-healing property, the shape and function of hydrogels are the same as initial after healing, leading to a more dependable usage and a longer


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lifespan. These hydrogels are conducive to the preparation of smart repair materials, such as skin. And the hydrogels provide a new thinking to imitate the healing ability of creatures. The rheological measurements are also carried out to gain insights into the structural arrangement by frequency sweep and temperature sweep. Fig.5a shows the storage modulus G' and the loss modulus G" of the hydrogel in the frequency range of 0.01 to 100 rad/s. As Fig.5a shows, the difference between the storage modulus and the loss modulus is getting bigger with the increase of frequency. It is reported that the modulus of dynamically cross-linked gels is frequency-dependent, while the permanently cross-linked gels demonstrate frequencyindependent modulus.47-48 And according to Fig.5b, the value of storage modulus G' is always bigger than loss modulus G". These indicate that agarose/PVA DN hydrogel as a whole is dynamically cross-linked, and the gelation state can be kept in the range of 25 to 100 °C. With the increase of temperature, the storage modulus decreases, which proves that the hydrogel is softer. Under a higher temperature, the hydrogen bonds are more unstable. It is 90 °C and the agarose cannot stay crosslink after reaching a certain temperature (Fig.S5), thus, do not have contribution to stabilize the network. The dynamic borate bonds still take effect, but the exposure reduces. Under this condition, the hydrogel is easy to be shaped. When the temperature decreases, the agarose network will gradually form and leads to the shape retention. This property is shown in Fig.5c. Firstly, a cylindrical shape (original shape) hydrogel is prepared. Then, after heating to 90 °C, the prepared hydrogel can be molded to other shapes, after cooling to room temperature, the shape was fixed. With this property, hydrogels can be prepared into many complex geometrical shapes to apply in different places, and it can be reused to avoid the waste of resources. Conclusions


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In summary, we prepared an agarose/PVA DN hydrogels, which show excellent selfhealing property both in air and underwater. The dynamic PVA-borate network provides hydrogel with an ultra-fast self-healing property (100% recovery in 10 seconds) without stimuli, and the agarose network improves the strength and stability of the hydrogel, meanwhile, leading a smart self-healing performance. Moreover, due to its unique reversible network structures, the materials are shapeable and recyclable. Therefore, we believe that the agarose/PVA DN hydrogel could offer a new gel platform for the development of smart self-healing devices and biomedical materials, and have a great potential application in areas from soft robotics to dynamic surface coatings.


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Fig.1. Structure of agarose/PVA DN hydrogel. a) Schematics and architecture of agarose/PVA DN hydrogel. b) The steadiness of PVA network hydrogels and agarose/PVA DN hydrogels.


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Fig.2. Tensile and self-healing test of PVA hydrogel and agarose/PVA DN hydrogel and photographs of self-healing process. a) Tensile stress–strain curves of original PVA hydrogel and agarose/PVA DN hydrogel with different contents of agarose (0.5 wt%, 1 wt%, 1.5 wt%) and the curves of healed PVA hydrogel and agarose/PVA DN hydrogel. b) Original stress and healed stress histogram of PVA hydrogel and agarose/PVA DN hydrogel with different contents of agarose (0.5 wt%, 1 wt%, 1.5 wt%). c) Strip-shaped agarose/PVA DN hydrogel with 1 wt% of agarose (I); the hydrogel is cut into two pieces (II); the hydrogel can heal automatically after contacting for 10 seconds at room temperature (III); the healed hydrogel is stretched (IV).


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Fig.3. Self-healing test and photographs of self-healing process underwater. a) Healed stress histogram of agarose/PVA DN hydrogel with 1 wt% of agarose. b) Agarose/PVA DN hydrogel with 1 wt% of agarose is cut into two pieces (one stained with rhodamine B and the other stained with methylene blue) (I); the hydrogel can heal underwater after contacting for 60 seconds at room temperature without external intervention (II); the healed hydrogel is stretched (III). c) Agarose/PVA DN hydrogel with 1 wt% of agarose is cut into pieces (I); the pieces are put in the mold and molding underwater for 1 hour (II); the gel pieces heal as an integral gel block (III).


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Fig.4. The different self-healing efficiency of agarose/PVA DN hydrogel. a) Cubic agarose/PVA DN hydrogel (with 1 wt% of agarose) which is cut into two pieces. b) Two parts of hydrogels are contact in-situ for 1 second(I), and the whole gel is lifted (II). c) Two parts of hydrogels are contact ex-situ for 1 second(I), and the hydrogels are still two parts (II). d) Tensile stress–strain


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curves of in-situ and ex-situ healed agarose/PVA DN hydrogel with 1 wt% of agarose. e) In-situ and ex-situ healed stress histogram of PVA hydrogel and agarose/PVA DN hydrogel with different contents of agarose (0.5 wt%, 1 wt%, 1.5 wt%).

Fig.5. Rheology behavior of agarose/PVA DN hydrogel. a) Frequency (ω) sweep test at ω = 0.1 100 rad/s, and strain (γ) = 0.5% of agarose/PVA DN hydrogel at 25 °C. b) Temperature (t) sweep test at t = 25 - 100°C, and strain (γ) = 0.5% of the hydrogel at 0.5 rad/s. c) The shapeable property of the hydrogel.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID Wanjun Hao：0000-0003-4441-2843 Xinglin Guo：0000-0003-2089-4825 Notes The authors declare no competing financial interest.


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AUTHOR INFORMATION ACKNOWLEDGMENT This research is supported by the National Research Fund for Fundamental Key Projects (2012CB933800, 2013CB933000, and 2012CB934100), National Natural Science Foundation (21121001, 21421061, 21434009, 21504098, 21127025, 21175140, 51073165 and 20974113), the Key Research Program of the Chinese Academy of Sciences (KJZD-EW-M01), the National High

Technology

Research

and

Development

Program

of

China

(863

Program)

(2013AA031903), Major projects in Hainan province (ZDZX2013015), Major science and technology support project in 12th Five-Year (2012BAJ02B08-3) by Ministry of science and technology.


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BRIEFS An ultra-fast self-healing hydrogel based on an agarose/PVA double network is presented. The gel utilizing dynamic borate bond exhibits almost 100% cure after healing for 10 seconds in air, and 70% cure after healing for 60 seconds underwater. In addition, hydrogels show a smart self-healing property, it gives priority to repair the damage.


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SYNOPSIS TOC Fig.


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Hydrogel with Ultra-Fast Self-Healing Property both in Air and

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