Nanocellulose-alginate hydrogel for cell encapsulation
Introduction
Cell encapsulation has garnered attention as a technology to provide immunoprotection for transplanted cells. The cells can be protected from the immune systems by a semipermeable membrane that allows nutrients and secreted proteins to permeate while isolating the cells from hostile immune reactions. Therefore, the transplantation of encapsulated cells has been suggested as a promising cell-based treatment for a variety of diseases, such as diabetes, metabolic deficiencies, liver failure, cancer, and neurodegenerative and cardiovascular diseases (Orive et al., 2003, Rokstad et al., 2014, Schmidt et al., 2008; Zhang et al., 2013).
Alginate is a biopolymer that forms a hydrogel in the presence of divalent cations, such as Ca2+ (Draget, Steinsvag, Onsoyen, & Smidsrod, 1998). Because alginate hydrogels have excellent biocompatibility, they have been preferentially used to protect transplanted cells from immune rejection and as a matrix to increase the cell viability in cell encapsulation (Bratlie et al., 2012, Orive et al., 2006Tam et al., 2011, Rokstad et al., 2014). However, the mechanical and chemical stability of alginate is not sufficient to achieve long-term transplantation. Consequently, reinforcing materials need to be added to an alginate matrix to overcome this limitation (Chan et al., 2011, Cordoba et al., 2013, Santagapita et al., 2012).
Bacterial cellulose (BC) biosynthesized by Gluconacetobacter xylinus has a high aspect ratio of high crystalline nanofibers (Klemm et al., 2005, Park et al., 2013b, Watanabe et al., 1998). BC nanofibers show better mechanical properties and higher hydrophilicity than celluloses from other sources (Hu et al., 2011, Park et al., 2013a). In spite of its promising properties, BC nanofibers cannot be easily applied as a component of composites due to their inter-connected 3D structure based on a large number of hydrogen bonds. Recently, a 2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidation process was developed to modify the surface of cellulose, and well dispersed cellulose fibrils could be effectively obtained using the electrostatic repulsion of fibers under mild and environmentally friendly conditions compared to other methods (Isogai, Saito, & Fukuzumi, 2011; Saito, 2010).
In this study, TEMPO-mediated oxidized bacterial cellulose (TOBC) was used to improve the mechanical and chemical stability of an alginate hydrogel. TOBC and alginate have a similar chemical structure, and they both participate in the Ca2+ crosslinking process. The carboxyl groups on the surface of TOBC provided the possibility of participating in the construction of an alginate-based composite and played important roles in the structural, mechanical and chemical stability. The viability of cells encapsulated in the TOBC-alginate composite was investigated for possible biomedical application of the composite in the future.
Section snippets
Biosynthesis and purification of BC
G. xylinus (KCCM 40216) was obtained from the Korean Culture Center of Microorganisms. The bacterium was cultured on mannitol medium containing 2.5% (w/w) mannitol, 0.5% (w/w) yeast extract and 0.3% (w/w) bacto-peptone. Bacteria were introduced into Petri dishes containing culture medium at 28 °C for 5 days. After incubation, the BC membrane biosynthesized on the surface of the liquid culture medium was harvested and purified with 1 wt% NaOH (SAMCHUN Chemical, Korea) followed by washing with
Results and discussion
BC was successfully selectively oxidized at the C6 carbon using TEMPO (Fig. 1A). The asymmetric stretching band at 1602 cm−1 of the FTIR spectra indicated chains modified by carboxyl groups. Moreover, the stretching vibration band of the CH at 2896–2990 cm−1 and stretching vibration band of the OH groups near 3345–3539 cm−1 were considerably reduced after the modification in TOBC (Dong et al., 2013, Fujisawa et al., 2011, Lin et al., 2012).
The effect of oxidation on the crystalline structure of BC
Conclusions
We attempted to design alginate–cellulose composites to encapsulate cells stably. TEMPO-mediated oxidized bacterial cellulose (TOBC) improved the mechanical and chemical stability of the SA beads. The incorporation of TOBC and SA was confirmed via the analysis of the chemical structure and crystallinity of the composite. The cells encapsulated in the beads tended to aggregate and form a cluster. Specifically, the cells encapsulated in the TOBC/SA beads were more viable and proliferated more
Acknowledgments
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT & Future Planning (Grant number 2013023612).
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