High-Performance Lignin-Based Water-Soluble Macromolecular

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High-Performance Lignin-based Water-Soluble Macromolecular Photoinitiator for the Fabrication of Hybrid Hydrogel Yuan Liu, Xing Huang, Kaichen Han, YuHua Dai, Xueqin Zhang, and Yuxia Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05357 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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ACS Sustainable Chemistry & Engineering

High-Performance Lignin-based Water-Soluble Macromolecular Photoinitiator for the Fabrication of Hybrid Hydrogel

Yuan Liu a, Xing Huang a ,b, Kaichen Han c, Yuhua Dai c, Xueqin Zhang a and Yuxia Zhao a,*

a

Technical Institute of Physics and Chemistry, CAS, 29 Zhongguancun East Road, Haidian

District, Beijing, 100190, China. E-mail: [email protected]. b

University of the Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing, 100049,

China. c

College of Material Science and Engineering, Beijing Institute of Petrochemical

Technology, 19 Qingyuan North Road, Daxing District, Beijing 102617, China. KEYWORDS: Lignin, macromolecular photoinitiator, water solubility, low migration, hybrid hydrogel

E-mail address of all authors: Yuan Liu: [email protected]

Xing Huang: [email protected]

ACS Paragon Plus Environment 1

ACS Sustainable Chemistry & Engineering 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

Kaichen Han: [email protected] Xueqin

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Yuhua Dai: [email protected]

Zhang: Yuxia Zhao: [email protected]

[email protected]


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ABSTRACT

Developing low-migration macromolecular photoinitiators (macro PIs) is significant to achieve the sufficient biosafety of photopolymers. In this study, for the first time lignin was introduced into photoinitiating systems. A novel lignin-based water-soluble macro PI (L-PEG-2959) was designed and synthesized by introducing water-soluble PEG chain and photoinitiating moiety into lignin backbone simultaneously. A series of characterizations were performed, including 1H NMR, FTIR, UV-Vis, GPC and TGA analysis, which indicated that LPEG-2959 has excellent light absorption property within 200400 nm, high initiating efficiency and superior water-solubility. Additionally, L-PEG-2959 could be compatible well with glycidyl methacrylate modified gelatin (Gel-GMA) and initiate the photopolymerization of Gel-GMA quickly under UV irradiation to form a chemical crosslinking hybrid hydrogel. Compared with the neat hydrogel made by Gel-GMA initiated with Irgacure2959, a commercial small molecular PI, the hybrid hydrogels presented controllable swelling property, increased mechanical strength, remarkably reduced precipitates and enhanced biosafety. The results indicated the great potential of lignin-based macro PIs in preparing biosafety photopolymers.



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INTRODUCTION Photopolymerization is a highly efficient method for curing coatings and printing inks. Over the last decades, three-dimensional (3D) printing based on photopolymerization has attracted wide attentions for its superior resolution, accuracy and good z-axis strength. It can build products from designed prototypes rapidly and has large potentials in fabricating personalized 3D structures 1. Hydrogels are highly absorbent natural or synthetic polymeric networks filled with a large amount of water, which is very similar to natural extracellular matrix (ECM). In recent years, it is becoming a research hotspot to construct scaffolds for tissue engineer with 3D printing hydrogels 2. As a medical material, its biological safety is a major concern. Generally, a photo-curable formulation contains photoinitiators (PIs), polymerizable monomers, solvents and other additives. After illumination, monomers can interconnect into a polymer network, which is normally much safer than monomer itself due to the loss of unsaturated double bonds with high reactivity. So, the potential toxicity comes mainly from the precipitation of PIs, solvents and residual monomers. In order to avoid the use of toxic organic solvents, developing water-soluble polymerization systems is a major trend. In addition, designing low-migration macromonomers or macro PIs is an effective strategy to increase the safety of end products 3-4.


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Studies on optimizing hydrogel formulations with increased biosafety have been widely reported. Through modification of polymerizable groups, such as acrylate group, many macromonomers based on natural polymers have been obtained and widely used in fabricating photocurable biomedical hydrogels due to their good biocompatibility and biodegradability 2. Among them, methacrylate modified gelatin (Gel-MA) is one of the most studied bio-based macromonomers. As reported, hydrogels based on Gel-MA have a variety of advantages, such as relative low cost, excellent water-holding ability, and similarity with human tissues. Meanwhile, they can promote vascularization and tissue formation, exhibiting great potential in tissue engineering

5-8.

However, (Gel-MA)-based

formulations are generally in gel state below 40°C 7. The photocrosslinking process of GelMA has to be conducted in vitro firstly followed by the surgical transplantation of hydrogels into the body, which limits its usage as injectable materials with minimal trauma. On the other hand, some commercial available small molecular PIs, such as Thioxanthone, Anthraquinone and Benzophenone, have been selected to develop macro PIs through grafted into linear, dendritic, hyperbranched polymers or nanoparticles to achieve reduced mobility and improved safety

9-12.

As the only water-soluble PI with high-

performance in market, Irgacure2959 is the first choice for aqueous photocurable systems. However, it is difficult to meet the needs of applications due to its limited water-solubility (≤ 0.5 wt%) and high mobility 13.



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Herein, we proposed to design a water-soluble macro PI based on Irgacure2959. Lignin is chosen as the biomacromolecule backbone to achieve this goal for the first attempt due to its various advantages, such as abundant in nature, intrinsic biosafety, multiple reactive sites 14. In addition, the 3D hyperbranched structure with plenty of benzene rings in the backbone of lignin made it an alternative reinforcing filler to strengthen biohydrogels, whose mechanical strength is usually weak

15-17.

Furthermore, as reported

recently, the researchers demonstrated that thermal stability and water retention properties of a hydrogel made by lignin-methacrylate copolymers could be enhanced

18.

Meanwhile,

using methacrylated lignin as a component in a photopolymer resin for 3D printing, a 4fold increase of ductility in cured parts was obtained 19. These exciting results also prompt us to carry out this study. In order to fabricate hybrid hydrogels containing lignin, the poor solubility of original lignin should be improved firstly. Polyethylene glycol (PEG) was selected to modify lignin in this study to achieve sufficient water-solubility and good compatibility 20-21. Afterwards, a photoreactive moiety based on Irgacure2959 was grafted onto the PEG modified lignin (L-PEG) through a coupling reaction. Finally, the target macro PI (L-PEG-2959) was successfully prepared. A series of characterizations were performed, including 1H NMR, FTIR, UV-Vis, GPC and TGA analysis. In addition, the photoinitiating capability, migration performance, and biosafety of L-PEG-2959 and the swelling property and mechanical strength of the hybrid hydrogels were investigated.


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EXPERIMENTAL SECTION Materials Lignin (Alkaline Lignin with 2.5 mmol/g -OH) of biochemical reagent grade was purchased from Sinopharm Chemical Reagent Co., Ltd., China. Gelatin (Type B, 300 bloom from porcine skin) was provided by Gelatin Research Group in Technical Institute of Physics and Chemistry, CAS. Glycidyl methacrylate (GMA) was purchased from Aladdin Industrial Corporation. Polyethylene glycol (PEG1000) was obtained from Guangdong GuangHua Chemical Factory Co., Ltd.(China). BF3-Et2O with 47% BF3 and 53% Et2O and Epichlorohydrin (ECH) were from J&K Scientific Ltd., China. Irgacure2959 was from 3A Chemicals Co., Ltd., China. Hydrochloride acid (HCl) and all solvents of analytical grade were purchased from Beijing Chemical Factory and used as received. Preparation of L-PEG-2959 The synthesis route of L-PEG-2959 are shown in Scheme 1. PEG-modified lignin (L-PEG) was prepared according to reference

18

and the detailed procedure was demonstrated in

supporting information (SI). Secondly, 2.24 g (0.01 mol) Irgacure2959 was dissolved in 50 mL ethanol in a flask with vigorous stirring. After it dissolved completely, the solution was heated to 55°C, 5 mg BF3-Et2O as catalyst was added and subsequently 0.925 g ECH was added dropwisely. The reaction continued over 2 h at 55°C. After excess ECH and ethanol were removed by distillation, chlorinated 2959 (2959-ECH) was obtained. Then, 2 g LPEG was solubilized in 50 mL ethanol, and 2959-ECH (1.16 g) was added. The pH of the mixed solution was adjusted to 89 using 1 M NaOH. The reaction was refluxed at 80°C for 4 h. After cooling down to room temperature, the pH of the solution was adjusted to 7



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using 1 M HCl, and ethanol was removed by vacuum rotary evaporation. By adding 20 mL of distilled water, some white powder was precipitated from the reaction solution. The brown filtrated solution was dialyzed against deionized water using 5 kDa cut-off dialysis membrane tubing for 1 week at 45°C to remove unreacted 2959, 2959-ECH, salts and other impurities. The purified liquid was lyophilized for 1 week to obtain L-PEG-2959. Formation of hydrogels Gel-GMA (Glycidyl methacrylate modified gelatin) was synthesized as described in reference 6 and the synthetic process was provided in SI. The photocurable formulation was composed of 1 g Gel-GMA as macromonomer, 0.01 g Irgacure2959 (sample 0) or 0.05, 0.1 and 0.15 g L-PEG-2959 (samples 13) as PI, and 4 g deionized water as solvent. The mixture was heated to 40°C and slowly stirred until all ingredients dissolved completely and no air-bubbles remained. Then, the viscous liquid was pured into a circular Teflon mold (Φ2.5×1 mm) and exposed under the irradiation of an ultraviolet (UV) light (365 nm, 30 mw/cm2) for 15 minutes. Finally, a hydrogel disc (Φ2.5×1mm) was prepared after demoulding. Characterizations 1H-NMR

spectra were recorded on a Bruker AIII400 MHz NMR spectrometer using

DMSO-d6 as the deuterated solvent. Fourier transform infrared spectroscopy (FTIR) were performed on a PerkinElmer Instruments Spectrum GX FTIR spectrometer. UV-Vis spectra were recorded on a Hitachi U-3900 Spectrophotometer. Thermogravimetric analysis (TGA) was carried on a Q800 thermogravimetric analyzer (TA instruments, USA). The molecular weight and polydispersity index (PDI) were analyzed by gel permeation


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chromatography (GPC). Morphologic analysis were conducted using HR-TEM (JEOL JEM 2100 instrument), AFM (Bruker Multimode 8 instrument), SEM (JEOL JSM 4800) and DLS (Malvern Instruments, Series 4700), respectively. Real time-FTIR (RT-FTIR) of the photopolymerization processes of diacryloyl poly(ethylene glycol) (PEGDA Mn=1000) initiated by Irgacure 2959 or L-PEG-2959 were detected on a FTIR spectrometer (Nicolet IS10). The swelling ration of hydrogels was evaluated by measuring the wet weight (W1) under water equilibrium state and dry weight (W0) under freeze-dried treatment of a hydrogel disc, and calculating the datum of (W1 − W0)/W0. The final value was the mean of three parallel samples per group. The migration property of PIs (Irgacure2959 or L-PEG-2959) from hydrogels was assessed by measuring the UV-Vis spectra of the extracted solutions of hydrogel discs made by different PIs under ultrasonication in ethanol for 3 days. The surface hardness of hydrogels was measured on a TI 950 TriboIndenter with a varied load at a constant displacement of 200 nm. For each sample, the final Young’s modulus was the mean of three hydrogel discs in parallel. All the above characterizations were performed at room terperature. More details are supplymented in SI. The cytotoxicity of PIs was evaluated using the MTT assay with L929 cell lines, which is mouse fibroblasts. The cells were incubated in the culture media of DMEM containing 10% FBS and 1% Penicillin/Streptomycin. L929 cells were seeded onto a 96 well plate at a density of 1×104 cells per well in 100 L culture media and incubated for 24 h for cells to be adherent in an incubator with a humid atmosphere with 5% carbon dioxide at 37°C. After that, 100 L culture media containing different PIs was added to achieve a series of final concentrations as 2.5 mg/mL for Irgacure2959 and 3.12, 6.25 and 12.5 mg/mL for L-



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PEG-2959 in wells, respectively. The normally cultured cells were used as control group. After another 24 h incubation, 20 L MTT reagent (dissolved in PBS, 5 g/L) was added into the plates for additional 4h incubation. Afterwards, the culture media was removed and all wells were washed with PBS. 150 L DMSO was added into the wells to dissolve the formazan crystals. The absorbance in each well was measured at 570 nm on a Multiskan FC instrument (Thermo scientific).

RESULTS AND DISCUSSION 1H

NMR, IR and UV-Vis spectra

1H

NMR spectra of lignin (L), L-PEG, L-PEG-2959 and Irgacure2959 are shown in

Fig.1A. After PEG modification, the strong signal of proton in the ethylene glycol unit (CH2CH2O) within 3.53.8 ppm clearly appears and almost submerges the signal of proton in the alkoxy group (such as CH3O) of lignin, which indicates a high substitution ratio of PEG in L-PEG. Meanwhile, the new generated signal of proton in the terminal hydroxyl group of PEG was presented within the region of 4.54.7 ppm in the 1H NMR spectrum of L-PEG. After grafting 2959 moiety, the appearance of two new doublet signals of aromatic proton at 8.2 and 7.0 ppm and the terminal methyl groups at 1.5 ppm proves the successful synthesis of L-PEG-2959. In the FTIR spectra of L, L-PEG and L-PEG-2959 (Fig.1B), the appearance of very intense bands at 1114 and 951 cm-1 in L-PEG are attributed to the C-O-C stretching vibration, demonstrating the successful introduction of PEG into lignin. The new peak of 1724 cm-1 in L-PEG-2959 is assigned to carbonyl group. Combined with the enhanced peak


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intensity of C-H stretching vibration at 2912 cm-1 and benzene ring at 1600 cm-1, the successful synthesis of L-PEG-2959 can be further confrimed. The UV-Vis absorption spectra are shown in Fig.1C. Due to their different solubility, lignin was dissolved in 1M NaOH solution, Irgacure2959, L-PEG and L-PEG-2959 were dissolved in distilled water with the concentration of 0.1 mg/mL, respectively. Lignin exhibits characteristic absorption peaks at 210220 and 281 nm 21. After PEG modification, no obvious change can be found in L-PEG. However, after grafting 2959 moiety, the absorption within 250350 nm increases substantially and its absorption peak around 280 nm is overlap well with that of Irgacure2959, which also indicates the successful synthesis of L-PEG-2959. Thermogravimetric analysis The thermal degradation behaviours of lignin and L-PEG-2959 were performed by TGA. As depicted in Fig.S2, lignin undergoes a two-step degradation process, which relates to the degradation of aliphatic (first step) and aromatic (second step) components, respectively, and a large proportion (62 wt%) of char residue is found

22.

L-PEG-2959

also exhibits a two-step degradation process, in which the first step corresponds to the degradation of aliphatic components from lignin and Irgacure2959 and the second step is from the PEG decomposition. Because PEG and Irgacure2959 can degrade completely at 650°C, the char residue of ~45 wt% in L-PEG-2959 mainly derives from lignin. According to the different char residues (45 wt% of L-PEG-2959 versus 62 wt% of L), the mass fraction of lignin in L-PEG-2959 can be caculated as 73 wt% by 45/62. Thus, the total mass fraction of PEG and Irgacure2959 can be deduced as ~27 wt%. As shown in UV-Vis



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spectra, the absorption peak at 280 nm are mainly due to lignin and 2959 moieties, by comparing different absorbance values of lignin, Irgacure2959 and L-PEG-2959 with the same concentration of 0.1 mg/mL at 280 nm, the mass fractions of PEG and 2959 in L-PEG-2959 can be estimated as 21 wt% and 6 wt%, respectively. All of these data are listed in Table S1. Consequently, the large amounts of PEG moiety (21wt%) provide sufficient water solubility of L-PEG-2959 (over 30 wt%, shown in Fig.S2). Molecular weight The molecular weight of a PI is an essential parameter in determining its extractability in photocured samples since the mobility of a molecule is generally inversely related to its molecular weight 23. As obtained from GPC data (listed in Table S1), the molecular weight of L-PEG-2959 (Mn=15,700) is dramatically enhanced compared with Irgacure2959 (Mn=224). Also, the major peaks of lignin before and after treatment are very similar (shown in Fig.1D), indicating that L-PEG-2959 still keeps the original hyperbranched structure of lignin with multiple functional active sites, which is beneficial to generate local high concentration of free radicals with a big block among them, may resulting in a high photo-initiating efficiency 24. Micromorphology analysis The morphology analysis of L-PEG-2959 was characterized by AFM and HR-TEM. As shown in Fig.2A, a homogeneous dispersion of nanoparticles (NPs) with diameter 60100 nm and average size of 75 nm (see DLS result in Fig.S4) is obtained. The height profile of the NPs indicate that their height is about 6080 nm (Fig.2B), suggesting the NPs are nanospheres. HR-TEM provides a similar result, in which a well-distributed nanospheres


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(Fig.2C) with lignin core and multiple arms of grafted PEG or PEG-2959 chains (Fig.2D) is presented. Such core-shell stuctures is benefical to endow L-PEG-2959 with good watersolubility, compatibility, low viscosity 12 as well as high photoreactivity 25. Homogeneity of hydrogels The formulations of a neat hydrogel (sample 0) and three hybrid hydrogels (samples 13) are listed in Table 1. Until now, most reported hybrid hydrogels composed of gelatin matrix and other organic or inorganic fillers, such as cellulose, chitosan, graphene and etc 26-30, had a common problem, that is limited dispersion homogeneity and stability even if the adding amount of fillers is less than 1 wt%. In this study, a superior compatibility between GelGMA and L-PEG-2959 was obtained (shown in Figs.3AB). Three hybrid samples before UV curing are transparent and the colour varied from orange to dark brown with the increasing dosage of lignin from 1 wt% to 3 wt%. Furthermore, such a phase stability can keep a long time (≥ 20 days at 40°C). The reason should be due to the good compatibility of PEG chains in the shell of these filled lignin nanospheres. In addition, the rheology property of three hybrid samples before UV curing was dramatically changed. As shown in Fig.S5, the gel state can turn into flowable state after adding L-PEG-2959 at room temperature (25℃), while the pure Gel-GMA (sample 0) is still gel state. This phenomenon illustrated that the strong hydrogen bonds within gelatin was destroyed by the addition of lignin nanospheres, which further indicated a robust interfacial interaction between the two components was established. This result may provide an effective method to tailor rheology property of gelatin and further to improve its injectable and printable properties.



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Sol-gel transition phenomenon was observed in the fabrication process of lignin-gelatin hybrid hydrogels under UV irradiation (30 mW/cm2, 365 nm), which proved the photoinitiation capability of L-PEG-2959. The fabrication process of lignin-gelatin hybrid hydrogels is illustrated in Scheme 2. Transparent hybrid hydrogels with good uniformity were obtained after UV curing (shown in Fig.3C). The photoreactivity of L-PEG-2959 was characterized by RT-FTIR. As shown in Fig.S7, samples 2-3 both exhibit a significantly higher degree of double bonds conversion compared with that of the reference sample 0 containing 0.01 g Irgacure2959. Due to the 6 wt% of 2959 in L-PEG-2959, the weight ratio of 2959 in the formulation of samples 1-3 are only 0.003, 0.006 and 0.009 g, respectively, which indicates the photoreactivity of L-PEG-2959 is very high. The freeze-dried hydrogels was observed by SEM. As shown in Fig.3D, dry samples 1-3 exhibit smooth internal porous networks, also proving a good compatibility between Gel-GMA and L-PEG-2959, in which the maximum mass fraction of lignin up to 8 wt% in dry sample 3 calculated by 0.15 g/1.15 g (L-PEG-2959/ (L-PEG-2959+Gel-GMA))  61 wt% (L/L-PEG-2959). Swelling property The bybrid hydrogels have very good swolling property. Meanwhile, they can maintain structure integrity after immersed in distilled water at 40°C for 20 days (shown in Fig.S6), which indicates a chemically crosslinked 3D polymer network was certainly formed. Fig.4A depicts the water retention ability of four hydrogels. The equilibrium swelling ratio of samples 0-3 is decreased with the increasing dosage of L-PEG-2959. The saturated water content of sample 3 decreases down to ~78 wt% while the datum of sample 0 is approximately 90 wt%. This result suggests that the water retention ability of hydrogels


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can be influenced by adjusting the amount of L-PET-2959, which should be due to the compact rigid structure of lignin with little holding water capacity. Migration behavior The UV-Vis spectra of the extracted solutions from samples 0-3 as shown in Fig.S8, which indicates the relative release amounts of extracted small molecules from these samples after reaching equilibrium state. It is clear that the precipitated PIs or the fragments of PIs from samples 1-3 are significantly reduced compared to that of reference sample 0. Moreover, as shown in Fig.4B, the release amount from hybrid hydrogels goes up with the increased dosage of L-PEG-2959, but the increase extent is very limited. It result proves an obviously lowered migration behavior of L-PEG-2959 compared with Irgacure2959. Mechanical Property As shown in the Fig.4C, all the hybrid hydrogels present gradually increased mechanical strength compared with the neat gelatin hydrogel due to the addition of L-PEG-2959. The Young’s modulus of sample 3 enhanced almost ten-fold compared with that of sample 0, indicating a robust strength effect of lignin. So far, many strategies have been attempted to reinforce gelatin hydrogel. A popular one is to introduce strong mechanial NPs as reinforce fillers, such as nano cellulose, chitosan, clay, laponite, graphene and etc 26-30. However, some NPs have potential toxicity or concentration limit. This study enables lignin NPs a new choice for developing hydrogel materials with high mechanical strength. Cell viability assay Since it is well-known that gelatin matrix has good biosafety 5, the potential toxicity of



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samples 03 comes mainly from PIs. We investigated the cyctotoxicity of Irgacure2959 and L-PEG-2959 with a same concentrations in accordance with hydrogel formulations in table S1. As showed in Fig.4D, the viability of L929 cells cultured in PBS containing LPEG-2959 is obviously enhanced compared with the control group containing Irgacure2959. It should be attributed to the intrinsic biocompatibility of lignin as well as the low migration property of L-PEG-2959. Moreover, the cell viability decreases with the increasing concentration of L-PEG-2959, indicating a dose-dependent toxicity, which is coincident with the enhanced extraction amount of precipitates from samples 1-3. More evaluation in vitro and in vivo will be conducted in the following study.

CONCLUSIONS In this study, a water-soluble lignin-based macro PI (L-PEG-2959) was designed and synthesized based on the chemical conbination of water-soluable lignin and commercial PI Irgacure2959. A series of characterizations were performed, including 1H NMR, IR, UVVis, GPC and TGA analysis. Excellent light absorption property under UV light and watersolubility were achieved for L-PEG-2959. Additionally, a superior compatibility between LPEG-2959 and Gel-GMA was found, while the rheology property of their mixture could be dramatically changed compared with the neat gelatin. Under UV irradiation, L-PEG-2959 can effectively initiate the photocrosslinking of Gel-GMA and form a homogeneous 3D network hybrid hydrogel. The photoinitating efficiency of L-PEG-2959 was found to be comparable with that of Irgacure2959. With the increase of L-PEG-2959 dosage, the hybrid hydrogels exhibited a decreased swelling property and enhanced mechanical strength. Furthermore, compared to the neat gelatin hydrogel containing Irgacure2959, the extraction


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amount of L-PEG-2959 from the hybrid hydrogels was trenmedously reduced. Through in vitro experiments, it was preliminarily showned that the cytotoxicity of L-PEG-2959 is obvioualy lower than that of Irgacure2959.The above results indicate L-PEG-2959 is a promising macro PI in fabricating biosafety hydrogels. ASSOCIATED CONTENT Additional experimental information and data included in the supporting information file.

AUTHOR INFORMATION Corresponding Author *[email protected]; phone +86 010-82543569，Fax +86 010-82543569 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial support from the Austrian-Chinese Cooperative R&D Projects (Grant No. GJHZ1720) and the Cross-training Program for High Level Talents in Beijing Colleges and Universities-Practical Ability Training Program. ABBREVIATIONS PI, photoinitiator; L, lignin; PEG, poly(ethylene glycol); ECH, epichlorohydrin; Irgacure2959, (2-hydroxy-1-[4[(2-hydroxyethoxy)phenyl]2-methyl-1-propanone]; L-PEG, PEG-modified



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lignin; L-PEG-2959, Irgacure2959 grafted L-PEG; PDI, polydispersity index; ECM, extracellular matrix; Gel-GMA, glycidyl methacrylate modified gelatin. REFERENCES (1) Guvendiren, M.; Molde, J.; Soares, R. M.; Kohn, J. Designing Biomaterials for 3D Printing. ACS Biomater Sci Eng. 2016, 2(10), 1679-1693, DOI 10.1021/acsbiomaterials.6b00121. (2) Van Vlierberghe, S.; Dubruel, P.; Schacht, E. Biopolymer-based hydrogels as scaffolds for tissue engineering applications: a review. Biomacromolecules. 2011, 12(5), 1387-1408, DOI 10.1021/bm200083n. (3) Corrales, T.; Catalina, F.; Peinado, C.; Allen, N. S. Free radical macrophotoinitiators: an overview on recent advances. Journal of Photochemistry and Photobiology A: Chemistry. 2003, 159(2), 103-114, DOI 10.1016/S1010-6030(03)00175-8. (4) Liang, Q.; Zhang, L.; Xiong, Y.; Wu, Q.; Tang, H. A facile method to prepare a polyethylene glycol modified polysilane as a waterborne photoinitiator. Journal of Photochemistry

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FIGURES.

Figure 1. Characterizations: (A) 1H NMR of (a) lignin (L), (b) L-PEG, (c) L-PEG-2959 and (d) Irgacure2959, using (CD3)2SO as deuterium reagent; (B) The FTIR spectra of L, L-PEG and L-PEG-2959; (C) The UV-Vis spectra of L in 1 M NaOH/water, Irgacure2959, L-PEG and L-PEG-2959 in water with the concentration of 0.1 mg/mL; (D) The GPC curves of L-PEG and L-PEG-2959.


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Figure 2. Morphology analysis: (A) AFM images of dispersed L-PEG-2959 nanoparticles (NPs); (B) The height profile of two NPs in Fig.2A; (C) HR-TEM images of the dispersed L-PEG-2959 NPs and (C) The core-shell structure of two NPs in Fig.2C.



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Figure 3. (A) Visual images of samples 03 before UV curing; (B) Microscope image of sample 1; (C) Visual images of samples 03 after UV curing; (D) SEM images of samples 03 after freeze drying. The formulation of samples 03 was listed in Table 1.


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Figure 4. (A) Water retention ability of samples 03; (B) Related release amount of PIs from samples 03; (C) Young’s modulus of samples 03; (D) Cell viability of Irgacure2959 and L-PEG-2959.



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SCHEMES.

Scheme 1: The synthesis route of L-PEG-2959


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Scheme 2: The fabrication process of lignin-gelatin hybrid hydrogels

TABLE. Table 1. Formulations of samples 0-3

SYNOPSIS



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A lignin-based water-soluble macromolecular photoinitiator was synthesized and showed great potentials for fabrication of lignin hybrid hydrogels with high

performance.


High-Performance Lignin-Based Water-Soluble Macromolecular

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