Cell interactions and cytotoxic studies of cellulose nanofibers from Curauá natural fibers
Graphical abstract
Introduction
Nanocellulose has been one of the most explored nanomaterials over the last years. It corresponds to a generical term to specify cellulose particles with dimensions in nanoscale. Basically, there are two main materials that can be obtained from lignocellulosic sources: cellulose nanocrystals (CNC) and cellulose nanofibers (CNF). CNC are rigid rod shape particles usually produced by strong acid hydrolysis (De France, Hoare, & Cranston, 2017). Meanwhile, CNF are longer and flexible, being also called as nanofibrillated cellulose (NFC) and normally obtained by mechanical shear (Charreau, Foresti, & Vazquez, 2012). This popularity arises from a combination of abundant sources, low cost and optimal physical properties, since these materials present a high aspect ratio allied to flexibility and toughness.
It’s first successfully trials as driver of outstanding reinforcement phase in nanocomposites resulted in an expansion in the number of potential applications of such materials. Some examples are given by its utilization to improve permeability control(Bardet et al., 2015), nanocomposites (Dufresne, 2017), rheological properties (Nazari, Kumar, Bousfield, & Toivakka, 2016), hydrogels (De France et al., 2017), preparation of special papers (Castro et al., 2018), aerogels (Mariano, Bernardes, & Strauss, 2018) and even support for conductive paints and batteries (El Baradai et al., 2016; Hoeng, Denneulin, & Bras, 2016).
Nowadays, the processing techniques to isolate and characterize these nanoparticles are already well established and an exponential grown in the number of different sources can be verified in literature (Cherian et al., 2011). The great aspect of this expansion is to provide biomass valorization, transforming residues from agriculture in nanoparticles with potential to present commercial value through a low cost process (Novo, Bras, Garcia, Belgacem, & Curvelo, 2015). Considering the abundance of biomass, the characteristics can significantly vary for each vegetal source, for instance, the size of nanocrystalline domains, crystallinity index, mechanical resistance (Jonoobi et al., 2015). The optimization of these properties and identification of the best source for each aimed application is a laborious work that is still progressing. Its importance lies in the need of solid knowledge of the surface properties and toxicity of these materials that needs to be enlightened.
Very recent studies shows nanocellulose-based systems are being tested for some biomedical applications as composites to bone replacement, artificial skins and scaffolds to tissue and cells growing (Jorfi & Foster, 2015; Smyth et al., 2017). These applications are based on nanocellulose non-toxicity. However, actual research state is still not totally conclusive, and many steps are required before clinical trials. Additional toxicity tests and in vivo studies (with three different animal models), are just some examples. Indeed, the toxicity of such materials depends on source, experimental procedures (chemical, mechanical or enzymatic), functionalization and aimed final application. In this scenario, there are only a few studies about cytotoxicity and adhesion tests of cellulose nanofibers obtained by mechanical shear from the grinder (Supermasscoloider Masuko®) from Curauá fibers.
Here, Curauá fibers were chosen to be studied mainly due to their high amount of cellulose and its unique superior mechanical properties that can be an advantage in preparation of several bio-based materials such as nanocellulose in general (Souza et al., 2010; Souza et al., 2014; Oliveira de Castro, Frollini, Ruvolo-Filho, & Dufresne, 2015). CNF from Curauá fibers was produced using mild reaction conditions and a smaller number of cycles through the mechanical grinder. Further physical-chemical characterization of the obtained CNF was extensively studied together with small angle X-ray scattering (SAXS) analysis, which gave insights on the native structure of nanofibrillated cellulose. The obtained CNF presented good mechanical properties, as a solid membrane, studied by stress strain tests, and as hydrogels studied by rheology. Both properties show that the obtained material can be used to prepare different scaffolds for biomedical applications Therefore, the toxicological behavior of cellulose nanofibers from Curauá fibers were investigated with fibroblasts cells (Vero cells as the cell lineage) by the direct and indirect contact assay. Later, the adhesion test on the CNF surface were also evaluated with same cells.
Section snippets
Cellulose nanofibers isolation
Curauá leaf fibers (Ananas erectifolius), originally from São Paulo State, were obtained from UNESP – Sao Paulo State University, Rural Engineering Department, where they are kept in a genetic bank. The fibers were milled and passed through a sieve with 4 mm mesh, dried at 60 °C overnight to remove moisture, and then treated with 0.05 N HCl solution for 2 h at 70 °C. To the resulting suspension was added NH4OH solution until reaching pH 9.5. Later, the pulping process was done using 4% NaOH
Fibers processing and morphology
In general, methods described in the literature to obtain CNFs are based on two processing steps: (i) lignocellulosic materials receive chemical (or enzymatic) treatments aiming to isolate cellulose fibers, which is followed by (ii) a mechanical defibrillation of the same fibers. In first one, most common chemical methods normally involve TEMPO-oxidation prior to mechanical shearing (Nechyporchuk, Belgacem, & Pignon, 2014). Resultant oxidation degree of these fibers is determinant for the high
Conclusions
Along this study, it was possible to obtain nanofibers from Curauá plant using less chemical and energy consumption in comparison to other natural fibers described in literature. It could be reached due to the high amount of cellulose in the fiber and their intrinsic morphology, consequently, reduced number of cycles into the grinder were used. The obtained nanoparticles showed a ribbon-like morphology that was able to provide an interesting rheological behavior in water. Since these materials
Conflict of interest
The authors declare no competing financial interest.
Acknowledgements
This study was financed by DFAIT (Canada) and CAPES (Brazil) in a Joint Program - Grant Number 7604/13-0. We also thank the Staff at the Centre For Nanostructure Imaging from Department of Chemistry at University of Toronto for the STEM images and LNNano (Brazilian Nanotechnology National Laboratory at CNPEM) for AFM imaging – Proposal AFM-23045.
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