Facile fabrication of tough photocrosslinked polyvinyl alcohol hydrogels with cellulose nanofibrils reinforcement
Graphical abstract
Introduction
Polyvinyl alcohol (PVA) is one of the most significant, biocompatible synthetic polymers [1,2]. Because of the hydroxyl group presenting in each repeating unit, PVA exhibits highly hydrophilic and hydrogen bonding characteristics [3,4]. The hydrogen bonding interaction can induce physical gelation of PVA chains [2,5,6], and the existed hydroxyl groups can also be modified to produce the chemical crosslinked hydrogels [4,[7], [8], [9]]. PVA hydrogels exhibit biocompatible [2,6,10,11], biodegradable and resisting protein adsorption and cell adhesion [12,13], which have been utilized in several biomedical [13,14] and pharmaceutical application [8,15]. However, the urgently need of high mechanical strength hydrogel for suffering large stress in use, such as cartilage, tendon, ligament repair and other tissue engineering scaffolds [13,16,17], promoting to develop high strength and toughness PVA hydrogels.
Many efforts have been devoted to improve the mechanical strength of PVA hydrogels. The freeze-thaw method is experimentally straightforward and not require any chemical crosslinking agent, and the physical crosslinked PVA hydrogel prepared by this method [2,[18], [19], [20]] shows higher mechanical strength. However, the time-consuming freezing-thawing process and high energy consumption go against the low-carbon economy. Alternatively, exploration of chemical crosslinker to connect PVA chains has been found to be an efficient and feasible way to fabricate PVA hydrogels with better properties, such as glutaraldehyde [9,21,22] and borax [[23], [24], [25]] reinforced PVA hydrogels.
Cellulose nanofibrils (CNF), displaying excellent mechanical properties, high surface areas, readily biocompatibility and biodegradability [5,26,27], have been incorporated into the numerous polymer matrix to reinforce the hydrogels' [18,19,[27], [28], [29]] or aerogels’ [5,9,26,29] mechanical strength. CNF can be synthesized using various methods including mechanical shearing, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidation, enzymatic hydrolysis of macroscopic cellulose followed by high-pressure homogenization or ultrasonication [30,31]. The utilization of CNF in the preparation of enhanced PVA hydrogels has also been explored, such as CNF reinforced glutaraldehyde or borax crosslinked PVA hydrogels [28,29], or CNF reinforced cyclic freeze-thaw physical crosslinked PVA hydrogels [18,19]. Although the mechanical property of the composited PVA hydrogels were improved, the much higher mechanical strength of PVA hydrogels are urgently needed to accommodate the desired specific tissue engineering applications [[32], [33], [34]].
Moreover, photocrosslinked polymerization is a facile, biocompatible approach, which has been used to construct 3D cell culture matrix or tissue scaffold. The gelation could occur in minutes under light irradiation with a little amount photoinitiator [35,36].
Inspired by the previous work, we fabricated a tough PVA hydrogel by exploiting the advantage of photocrosslinking method and the reinforcement effect of CNF. As shown in Scheme 1, a photoinitiated functional methacrylate group was firstly grafted into PVA polymer upon reacting with the pendant hydroxyl groups of PVA chains to generate PVA-MA, subsequently the mixture solution of PVA-MA, CNF, photoinitiator I2959 were irradiated under UV light, facilely construct a higher mechanical PVA hydrogel. The tough mechanism is based on the hypothesis that the abundant hydroxyl groups in the surface of the crystalline CNF can increase the interaction of already photoinduced chemical crosslinked PVA chains via hydrogen bonding, which can increase the crystallinity of PVA hydrogels. The objective of this work is to prepare higher mechanical PVA hydrogel using a facile, biocompatible approach, and demonstrate the reinforced mechanism. By adjusting the incorporation content of CNF in hydrogel formulation, a high mechanical strength of 490 kPa (fracture compressive stress) is obtained which is higher than that of previously reported nanocellulose reinforced PVA hydrogels. And the synthetic optimized PVA hydrogel also exhibits excellent cyclic compressive performance.
Section snippets
Materials
Polyvinyl alcohol (average MW = 94899 g/mol) was purchased from Sinopharm Chemical Reagent Co., Ltd. Methacrylate glycidyl ether (GMA, 97%) was bought from Macklin. 4-dimethylaminopyridine (DMAP, 99%) was got from TCI. Dimethyl sulfoxide (DMSO, 99.7%) and acetone (99.7%) were purchased from Tianjin Damao Chemical Reagent factory. Cellulose powder was bought from Aladdin. 2,2,6,6-tetramethyl piperidine oxide (TEMPO, 98.5%), sodium hypochlorite (NaClO, 99.7%), sodium hydrate (NaOH, 97%), absolute
Characterization of CNF and PVA-MA
As shown in Fig. S2a, CNF dispersed well in water with the length ranging from one to several microns, which promoted the favorable dispersion of CNF into PVA-MA solution (Fig. S2b). Moreover, the insets of Fig. S2a and Fig. S2b depicted that the tyndall effect emerged by irradiating CNF suspension or PVA-MA/CNF solution with a laser pointer, which directly showed that CNF was dispersed well in aqueous solution or PVA-MA solution. The average particle size (267 nm, Fig. S3) and surface with
Conclusions
In conclusion, we have developed a convenient, fast method to fabricate tough PVA hydrogels with CNF reinforcement. Except avoiding using the small molecule chemical crosslinking agent and time-consuming freeze-thaw process, a much higher mechanical strength PVA hydrogel was successfully prepared by optimizing the CNF loading content, and the compressive fracture stress reaches 490 kPa which is higher than that of previously reported nanocellulose reinforced PVA hydrogels. Specifically, this
Acknowledgements
This work was supported by National Natural Science Foundation of China [31741022]; Natural Science Foundation of Guangdong Province [2018A030310146].
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These authors contributed equally to this work.