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Conductive, tough, transparent, and self-healing hydrogels based on catechol-metal ion dual self-catalysis Zhanrong Jia, Yan Zeng, Pengfei Tang, Donglin Gan, Wensi Xing, Yue Hou, Kefeng Wang, Chaoming Xie, and Xiong Lu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01498 • Publication Date (Web): 10 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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Chemistry of Materials

Conductive, tough, transparent, and self-healing hydrogels based on catechol-metal ion dual self-catalysis Zhanrong Jia1+, Yan Zeng1+, Pengfei Tang1, Donglin Gan1, Wensi Xing1, Yue Hou1, Kefeng Wang2, Chaoming Xie1* and Xiong Lu1*

1

Key Lab of Advanced Technologies of Materials, Ministry of Education, School of Materials

Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, Sichuan, China

2

National Engineering Research Center for Biomaterials, Sichuan University, Chengdu 610064,

China

+ These

authors contributed equally to this work

* Corresponding authors.

E-mail: [email protected]; [email protected]

1

ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract

Tough and conductive hydrogels are the promising materials for various applications. However, fabrication of these hydrogels at room or low temperatures, without external stimuli, is a challenge. Herein, a novel dual self-catalytic system composed of a variety of metal ions and catechol-based molecules was developed to efficiently trigger the free-radical polymerization of tough, conductive, transparent, and self-healing hydrogels at low temperature without any external stimuli. Ferric ions (Fe3+) and dopamine (DA) were chosen as model compounds, which form stable redox pairs that act as a dual self-catalytic system to activate ammonium persulfate to generate free-radicals. Consequently, the radicals could rapidly trigger the hydrogel self-gelation at low temperatures (6 °C) within 5 s. The dual self-catalytic system opens up a facile route to synthesize multifunctional hydrogels at mild conditions for a broad range of applications, especially in tissue engineering and wearable electronics.

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1. Introduction Conductive and tough hydrogels demonstrating excellent mechanical properties and electric conductivity offer great promise in the fields of tissue engineering, soft robotics, and stretchable bioelectronics1-4. A plethora of tough hydrogels have recently received a wealth of interdependent research, including double network tough hydrogels5, interpenetrating tough hydrogels6-7, nanocomposite polymer tough hydrogels8-9, host-guest tough hydrogel10 and ionically cross-linked tough hydrogels11. The majority of these tough hydrogels can effectively dissipate energy, which is attributed to the interpenetrated brittle and ductile networks12. The process involving the preparation of conductive and tough hydrogels is complicated, and typically comprises monomers, chemical cross-linkers, toxic auxililary agents and uniformly distributed conductive agents in the hydrogel network13-14. In particular, the gelation of hydrogels is tedious and dependent on external stimuli, such as UV irradiation or thermal initiation15. Thus, significant challenges remain to fabricate multifunctional hydrogels possessing high toughness and conductivity at room or low temperatures without external stimuli.

Recently, catechol chemistry-based hydrogels have become a significant area of research focus and catechol-containing molecules have been widely used to fabricate tough16-17, conductive18-19, and self-healing hydrogels20-21. Catechol groups can form strong covalent and noncovalent bonds with various inorganic/organic/metallic moieties that impart distinctive properties to hydrogels, such as self-adhesion, self-healing, and prominent mechanical properties22-24. Additionally, the catechol group can be converted to semiquinone/quinones by oxidation, and consequently activate 3



persulfate to efficiently produce sulfate radicals and hydroxyl radicals25. These radicals can function as an electron-shuttle in chemical and biochemical processes26. Therefore, catecholcontaining molecules, together with their oxidative products, quinone groups, form a self-redox system that is able to trigger the free-radical polymerization in the preparation of functional hydrogels.

Metal ions are widely used as functional components to endow a plethora of outstanding properties to hydrogels27. Incorporation of metal ions can endow a hydrogel with tunable and enhanced mechanical properties, because many metal ions can form reversible and sacrificial bonds with polymer backbones through coordination. For example, Caruso’s groups reported that metalphenolic dynamic coordination is a versatile platform for the development of functional hydrogels, which can respond to pH change28-30. In addition, metal-carboxyl coordination is another reversible bond that can respond to redox31. Metal ions have also been employed to produce conductive hydrogels because metal ions function as carriers for electronic conduction32. Furthermore, metal ions can form dynamic and reversible bonds at the damaged interface, which efficiently improve the self-healing properties of hydrogels33. Additionally, metal ions can activate persulfate through a one-electron transfer process to form radicals34. Recently, the free-radicals generated through the redox reaction between Fe2+ and S2O42- were utilized to initiate polymerization of hydrogel coatings35. In particular, metal ions could be coordinated by chelating reagents to form stable metal ion-chelate complexes to realize a continuous ion release, which is able to improve the efficiency of radical generation by the redox between the metal ions and S2O42- 34. Catechol groups have been 4


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reported to effectively chelate with metal ions to form reversible non-covalent complexes36. The basis of this research gave us inspiration to use catechol-containing molecules as chelating regents to form catechol-metal complexes, and thereafter, to utilize these complexes as polymerization catalysts so as to self-catalyze the gelation of hydrogels at room or low temperature without the assistance of external stimuli.

Herein, we report a novel dual self-catalytic system based on a catechol-metal ion complex that can be used to self-catalyze the gelation of tough, conductive, and self-healing hydrogels. The system comprises the catechol-containing molecules, such as dopamine (DA), tannic acid (TA) and tea polyphenol (TP), and transition metal ions, which can efficiently and rapidly activate ammonium persulfate (APS) to generate radicals and trigger the free-radical polymerization of various vinyl monomers, yielding multifunctional hydrogels. The transition metal ions, which act as a common oxidant, together with reductive catechol-containing molecules, to form redox pairs. The reduced metal ion is a good electron donor which can participate in single-electron transfer reactions (Figure 1a). On the other hand, the catechol groups coordinate with metal ions and act as an effective reducing agent. Together with their oxidized products, quinone and semiquinone, catechol groups also form a catechol-based catalytic system as an electron donor37 (Figure 1b).

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2. Results and Discussion 2.1. The mechanism of dual self-catalytic system. This dual self-catalytic system can effectively activate persulfate, thereby generating free radicals to trigger the polymerization of monomers, such as acrylic acid (AA) and acrylamide (AM), without using auxiliary agents and external stimuli, such as UV irradiation and thermal initiation. The gelation speed of the hydrogels can be easily tuned by varying the ratio of metal ions to catechol-containing molecules. This catalytic system is extremely reliable and efficient to trigger hydrogel gelation, and therefore leads to a facile hydrogel preparation method, in that the process only requires metal ions being mixed with catechol-containing compounds at mild conditions. In this work, a series of hydrogels composed of different catechol-metal pairs were screened. As a model material, the DA-Fe hydrogel was used to elucidate the gelation mechanism and demonstrate multiple functions.

2.2. The universality of dual self-catalytic system. To demonstrate the universality of the dual self-catalytic system, hydrogels composed of different catechol-metal ion pairs were produced (Figure 2a). The different catechol-metal ion pairs were able to induce the self-polymerization of vinyl monomers (AA and AM) with the presence of APS, revealing the universality of the dual self-catalytic system. A low temperature polymerization experiment of the DA-Fe-poly acrylic acid (PAA) hydrogel was conducted to verify the efficiency of this dual self-catalytic system. As shown in Figure 2b, the temperature of the AA, N,Nmethylenebis acrylamide, APS, DA, and Fe3+ solution was 6 °C, after keeping in a fridge for 30 6


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min. After the addition of the APS solution, a rapid exothermic gelation occurred, the hydrogel subsequently polymerized within 30 s, which demonstrates that the hydrogel can be polymerized at a low temperature without UV radiation or thermal initiation. Furthermore, gelation time decreased, from 50 s to 5 s, as a function of the increased Fe3+ concentration, demonstrating the controllable gelation speed of the dual self-catalytic system (Figure 2c). Additionally, the rapid polymerization of hydrogel was observed even after storing the DA and Fe3+ mixture solution for 3 days.

2.3. The characterization of free-radicals. The free-radicals generated by the DA-Fe3+ dual-catalytic system were characterized by the 5,5dimethyl-1-pyrroline N-oxide (DMPO)-trapped electron spin-resonance resonance (ESR) experiments (Figure 3a). The ESR spectrum of the dual-catalytic system with APS possessed a quartet of signals from the DMPO-OH adducts with relative intensities of 1:2:2:1 during the reaction, which was assigned to hydroxyl radicals36. The ESR spectrum exhibited the same peak after 3-day storage. These results demonstrated that the dual-catalytic system comprising the APS solution generates free-radicals and induces the gelation of the hydrogel, even after prolonged storage. Radical quenching experiments were conducted to further verify the role of hydroxyl radicals during the hydrogel polymerization. Tertbutanol and potassium iodide are typical radical quenching compounds for •OH and general free-radicals, respectively. After the addition of a radical quenching agent, hydrogel polymerization was hindered, demonstrating that •OH plays a crucial role in the reaction (Figure S1). 7



2.4. The redox reaction in dual self-catalytic system. Hydrogel self-polymerization is mainly attributed to the free-radicals generated from APS, which is activated by the metal ion-catechol redox dual-catalytic system. The redox process between Fe3+ and DA was investigated by cyclic voltammetry (CV) experiments and X-ray photoelectron spectroscopy (XPS) analysis. For the CV experiments, DA was coated on Ti foil and used as the working electrode, Ag/AgCl (saturated KCl) as the reference electrode, and Pt as the counter electrode. The solutions of FeCl3 solution (6 mg mL−1) were used as the electrolyte. CV experiments showed that the redox couple peaks appeared at −0.6 ~ 1.2 V, corresponding to quinone/catechol conversion of DA, caused by Fe3+ oxidation, which indicates a stable redox reaction between DA and Fe3+ (Figure 3b)38. To further investigate the redox reaction of Fe3+catechol/carboxylate complex, two electrochemical groups were designed. In the first group, DA was used as the working electrode, and AA solution was used as the electrolyte. The result showed that no redox reaction happened in the DA-AA system (Figure S2a). In another group, DA was used as the working electrode, and Fe3+/AA mixed solution was used as the electrolyte, respectively. The result revealed that the redox reaction happened after the addition of Fe3+ (Figure S2b). Moreover, DA and APS cannot form a redox system, while only Fe3+-DA system exhibit redox reaction (Figure S2 c-d).

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Generally, both oxidation and coordination exist in the catechol-metal ion based gelation chemistry, and the coordination effect is dominant under basic condition because metal ions can stabilize deprotonated catechol groups39. In the current dual catalytic system, the oxidation is dominant, which is attributed to two reasons. One is that the dual catalytic system is neutral. The catechol groups are only partially deprotonated in neutral condition40, resulting in the co-existence of oxidation and coordination. The another one is that a large excess of Fe3+ ions (the molar ratio of Fe3+ to DA is ~ 12.5:1) are used in the dual catalytic system. To investigate the oxidation and coordination of DA in the current dual catalytic system, three groups with different Fe3+ concentrations were characterized by UV-vis analysis. The results revealed that both oxidation and coordination existed in both groups (Figure S3a-c)

39.

Moreover, the absorbance of quinone

increased with the increase of the Fe3+ concentration, and the absorbance of catechol coordination decreased with the increase of the Fe3+ concentration. This is because the catechol groups are more susceptible to be oxidized at the high Fe3+ ion concentrations.

Additionally, XPS analysis demonstrated that the peaks were ascribed to Fe2+ and Fe3+ because the Fe3+ was reduced to Fe2+ by DA (Figure 3c). Furthermore, the XPS spectra also revealed that the Fe3+/Fe2+ ratio in the catechol-metal iron redox dual-catalytic system was increased as a function of the Fe3+ initial concentration (Figure S4). Meanwhile, the catechol groups of DA were oxidized to semiquinone/quinone groups, and therefore the peaks of O-C and O=C peaks were observed in the O 1s spectrum (Figure 3d). These results proved the redox process between DA and Fe3+. 9



2.5. Mechanical properties. The self-polymerized DA-Fe-PAA hydrogel exhibits outstanding stretchability and toughness, which is attributed to the interactions between Fe3+, catechol-groups, and PAA network. The hydrogel is highly stretchable and able to withstand high degrees of stretching (up to 2000%) without fracture (Figure 4a). Figure 4b shows typical tensile stress-strain curves of the hydrogel as a function of Fe3+ content. The hydrogel exhibited both high tensile strength (TS) and a large extension ratio (ER). The TS of the hydrogel was improved as the Fe3+ content increased from 0.4 % to 0.8 %, and the maximum TS was greater than 15 kPa (Figure 4c). However, when the Fe3+ content reached to 1 %, the hydrogel was fragile due to the high cross-linking density. The ER decreased with the increasing of the Fe3+ content and displayed a maximum value of 2600% (Figure S5). The maximum fracture energy was 2145 J m−2, when the Fe3+ content of the hydrogel was 0.6 wt. % (Figure 4d). The product of strength and ductility was as high as 21 MPa % (Figure 4e), which indicates that the interaction of Fe3+, DA, and PAA allows the chain deformation to absorb energy when subjected to the tensile test. Furthermore, the hydrogel could withstand a high compression strain of 80% without fracture (Figure 4f). The maximum compression strength is ~280 kPa (Figure 4g). SEM micrographs revealed that the internal structure of dried hydrogel is uniform and porous (Figure S6). These results demonstrate that hydrogels with superior mechanical properties have the potential to meet the mechanical needs of soft tissues related applications.

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We compared the mechanical properties of DA-Fe-PAA and TA-Fe-PAA hydrogels to investigate the effect of catechol contained molecules on the performance of the hydrogel. The compressive strength, tensile strength, fracture energy, and the product of strength and ductility of the TA-FePAA hydrogels are lower than that of DA-Fe-PAA hydrogels (Figure S7). These results demonstrate that the DA-Fe-PAA hydrogel possess superior mechanical properties. The main reason is that TA has more the catechol groups than DA in a same molar amount. Therefore, during polymerization, the TA system consumes more hydroxyl radicals than DA system, resulting in incomplete polymerization and low cross-linking level of TA-Fe-PAA hydrogel

41.

We also

compared the mechanical properties of TA-Fe-PAA, TA-Co-PAA, TA-Ni-PAA and TA-Zr-PAA hydrogels to investigate the effect of metal ions on the performance of the hydrogel. These results demonstrate that Fe-based hydrogel is the weakest while Zr-based hydrogel is the strongest (Figure S8). This is because the catalytic efficiency of the four kinds of metal ions is different. The catalytic efficiency of Fe3+ is the highest, which leads to the fastest polymerization rate. Consequently, Febased hydrogel has the short polymer chains, which results in weak mechanical properties42-43 . The mechanical properties of the current hydrogel is comparable with that of double network (DN) hydrogel, which is a typical class of tough hydrogel44. Compared with DN hydrogels, the toughness of the current hydrogels is weaker to some extent45-48. However, the extension ratio of the current hydrogel (2500 %) is higher than that of DN hydrogels (~1000 %) due to the dynamic non-covalent bonds in the hydrogel network49.

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2.6. The transparency, conductivity, and self-healing properties of hydrogel. In addition to outstanding mechanical properties, the DA-Fe-PAA hydrogel also exhibited high transparency, conductivity, and self-healing properties. The high optical transparency of the DAFe-PAA hydrogel is a critical feature for wearable electronic devices that require visualization50. The flower is clearly observed after covered by the hydrogel (Figure 5a1). The transmittance of DA-Fe-PAA at 650 nm wavelength is 91%, when the ferric ion content was 0.4 wt.% (Figure 5a2).

The DA-Fe-PAA hydrogel possess excellent conductivity, which is attributed to the ferric ions in the PAA network (Figure 5b1). The conductivity increased with increasing Fe3+ content and reached a maximum value of 38 S m−1 (Figure 5b2). This value surpasses that of the majority of previously reported conductive hydrogels1,

51-52

and demonstrates the material suitable for

wearable electronics.

The DA-Fe-PAA hydrogel also exhibits good self-healing properties (Figure 5c). The hydrogel was brought into contact after cut into two pieces, and self-healed into an integrated hydrogel 6 h later, prior to illuminating a LED (Figure 5c1). After healing for 6 h, the stretchability of the hydrogel recovered to 85% (0.6 wt.% Fe3+) (Figure 5c2). The maximum TS and ER of the hydrogel after 6 h healing recovered to ~90% (0.8 wt.% Fe3+) (Figure 5c3). The conductivity (1 wt.% Fe3+) nearly recovered to 98% (Figure 5c4). These results revealed that the self-polymerized DA-FePAA hydrogel retained good stretchability and conductivity after self-healing. Self-healing of the hydrogel was as a result of the efficient formation of dynamic crosslinking points between Fe3+ 12


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and carboxylic groups of PAA and catechol groups of DA at the hydrogel fracture interface (Figure 1).

The hydrogel exhibits superior light transmission and electrical conductivity, and therefore can be used as a sensor or bioelectrode. As shown in Figure 5d, the hydrogel attached to the skin and acted as an electrode to detect the electrocardiogram (ECG) (Figure 5d1) and electromyography (EMG) (Figure 5d2) signals about the human body. Furthermore, the hydrogel can detect slight human signals, such as facial movements when smiling (Figure S9).

3. Conclusion In this work, we demonstrated a novel, ultrafast self-catalytic system composed of a variety of metal ions and catechol molecules, which can be used to produce a series of tough, conductive, transparent, and self-healing hydrogels. Note that previous studies reported that low-valent transition metal ions, such as Fe2+, can catalyze the decomposition of persulfate. However, their reactivities are relatively unstable and the catalytic efficiency is low35. Conceptually different from previously reported catalytic systems containing only transition metal ions, this dual self-catalyzed system demonstrates that catechol-metal ion redox repair, in which both the catechol and metal ions have catalytic activity, possesses high catalytic efficiency toward APS. This system can selftrigger free-radical polymerization and other auxiliary conditions, such as light and heat, are not required. Furthermore, this system circumvents the inherent drawbacks of catechol-containing monomers (DA, TA, and TP), which are commonly regarded as radical scavenges41, 53 and cannot 13



be directly applied during free-radical polymerization. Taking these into consideration, such an ultrafast, dual self-catalytic system composed of transition metal ions and catechol-containing molecules shed new light on the development of hydrogels in the future.

Taking the DA-Fe complex as the model for the dual self-catalytic system, we fabricated a conductive and tough DA-Fe-PAA hydrogel. Hydrogel polymerization can occur at low temperatures (6 °C) without using UV irradiation or thermal initiation. The hydrogel exhibited superior mechanical properties, self-healing abilities, and high electrical conductivity. The maximum tensile strain of the hydrogels was 2600% and the conductivity was ~38 S m−1, which meets the requirements of commercial soft electronic devices. The excellent self-healing properties resemble that of human tissue, and the hydrogel can self-heal after mechanical damage over a short period of time. These superior properties demonstrate the potential of these hydrogels in various applications, such as soft tissue replacement materials, wearable electric devices, and smart biosensors.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental section, digital photo of radical quenching experiments, high-resolution XPS spectra, mechanical properties of the various hydrogel, CV curves of the DA electrode in different solutions, UV-vis spectra of DA/Fe3+ solution with different Fe3+ concentration, SEM micrographs, and signal detection. 14


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Acknowledgement This work was financially supported by the National Key Research and Development Program of China (2016YFB0700800), R&D Program in Key Areas of Guangdong (2019B010941002), NSFC (81671824, 31700841), Fundamental Research Funds for the Central Universities (2682019JQ03). The authors thank Dr. Chunchun Zhang and Dr. Hanjiao Chen in Analytical & Testing Center, Sichuan University for ESR test. The authors wish to acknowledge the assistance on materials characterization received from Analytical & Testing Center of the Southwest Jiaotong University.

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Figure 1. Schematic illustration of a catechol-metal ion dual self-catalytic system triggering the polymerization of hydrogels, and the probable redox reactions in this system. a) The metal ionbased catalytic system. b) The catechol-based catalytic system and the probable switch of three different redox states.

Figure 2. a) Digital photo of the gelation of AA and AM hydrogels polymerized by transition metal irons (Fe3+, Co2+, Ni3+, and Zr3+) and catechol-containing molecules (DA, TA, and TP) at room temperature. b) Low temperature gelation diagram of the DA-Fe hydrogel. c) Hydrogel gelation time as a function of Fe3+ concentration after storing the pre-polymerization solution for 1 and 3 days at room temperature.

Figure 3．a) DMPO spin-trapping ESR spectra of a 50 μL pre-polymerization solution (0.06 g mL−1 Fe3+, 0.003 g mL−1 DA, 0.02 g mL−1 APS, and 0.025 g mL−1 DMPO) after storing for 1 and 3 days. b) CV curves of the DA electrode in a FeCl3 solution. c−d) High-resolution XPS spectra of Fe 2p and O 1s regions for the DA-Fe-PAA hydrogel (the molar ratio of Fe3+ to DA was 12.5:1)

Figure 4. Mechanical properties of the DA-Fe-PAA hydrogel. a) Digital images of the hydrogel tensile test (0.6 wt. % Fe3+). b) Typical tensile stress-strain curves, c tensile strength, d) the fracture energy, e) strength and ductility product, f) typical compressive stress-strain curves, and g) compressive strength, of the hydrogels as a function of Fe3+ content.

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Figure 5．Conductive, transparent, self-healing, and physiological signal detection properties of the DA-Fe-PAA hydrogel. a) Hydrogel transmittance. a1) Digital photo of the transparent hydrogel covering on a flower. a2) UV-vis transmittance spectra of the hydrogels as a function of Fe3+ contents. b) Hydrogel conductivity. b1) Photos demonstrating the good conductivity of the hydrogel. b2) Hydrogel conductivity as a function of Fe3+ contents. c) Self-healing properties of the hydrogel. c1) Photo demonstrating the good conductivity of the hydrogel after self-healing. c2) Typical tensile stress-strain curves of the original and healed hydrogel (0.6 wt.% Fe3+). c3) Mechanical self-healing efficiency (TS and ER) of the hydrogels after 6 hours healing. c4) Healing efficiency of conductivity of the hydrogels as a function of Fe3+ contents after 6 hours healing. d) Hydrogel acting as the electrode to detect the d1) ECG and d2) EMG signals.

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Figure 1. 239x155mm (600 x 600 DPI)


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Figure 2．

140x143mm (600 x 600 DPI)



Figure 3． 141x111mm (600 x 600 DPI)


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Figure 4. 76x96mm (600 x 600 DPI)



Figure 5. 139x100mm (600 x 600 DPI)


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TOC 62x47mm (300 x 300 DPI)


Conductive, Tough, Transparent, and Self-Healing ... - ACS Publications

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