Controlled production of soft magnetic hydrogel beads by biosynthesis of bacterial cellulose

https://doi.org/10.1016/j.jiec.2021.05.016Get rights and content

Abstract

It describes a simple process to create soft spherical ferrogels by the biosynthesis of bacterial cellulose (BC). Gluconacetobacter xylinus forms the enclosed structure of the ferrogel, which is a hydrogel containing magnetic nanoparticles (MNPs). The core ferrogel is prepared by incorporating MNPs with cellulose nanofibers (CNFs) in culture medium and a BC shell is produced by the bacteria that moves to the medium surface. Spherical BC ferrogels (SBCFs) are fabricated by dropping the core ferrogel with bacteria on a glass surface coated with poly(tetrafluoroethylene) microbeads followed by culture for seven days. The size of the SBCFs is controlled by the external magnetic force, CNF, and MNP concentrations. The CNF immobilizes MNPs and supports the spherical structure by filling the SBCF structure. Because a highly dense BC layer is formed at the surface of the medium droplets, a unique core–shell structure of SBCF is formed.

Introduction

Bacterial cellulose (BC) is a unique, networked structure of a layered hydrogel that is entangled with cellulose fibers in nanoscale [1], [2], [3]. Given its high mechanical strength and moisture content [4], [5], [6], BC can be used for the fabrication of 3D structure which is deformable and robust in the aqueous system. Moreover, the incorporation with magnetic nanoparticles (MNPs) proposes a unique material system of responsive and actuating functions by applying external magnetic field. A ferrogel is a hydrogel with viscoelastic and magnetic properties [7], [8], [9]. Ferrogels contain magnetic substances, such as colloidal MNPs in networked polymeric structures and their sensitive response to an external magnetic force causes a change in morphology or position in the system [7], [10], [11], [12], [13].

Typically, the shape of BC harvested from the cultivation bath is planar, and a sheet-like pellicle is produced because bacteria moves to the medium surface for oxygen, which is critical in the biosynthesis of cellulose [14], [15], [16], [17]. Several methods to prepare spherical or hollow BC structures are available using water-in-oil emulsion [18] and microfluidics [15], [19]. The rotationally agitated culture of bacteria routinely produces spherical BC structures [17], [20], [21], [22], [23]. Despite the simplicity of the process, relatively large size of spheres is prepared with the agitated culture and it is difficult to control the size precisely.

The exact volume of medium droplets with bacteria can produce the uniform size of BC spheres in air. Smaller droplets can be made by reducing the volume of drop through the needle. However, the capillary force of the needle surface restricts the volume of the drop until it falls. It is critical to suppress the capillary force and yield stress of the medium by applying an external force, such as magnetic field in addition to gravity.

Here, spherical droplets are formed at the highly hydrophobic surface and a spherical BC ferrogel (SBCF) can be produced by culture of medium droplets containing Gluconacetobactor xylinus (G. xylinus) and MNPs at the hydrophobic plate surface. The combination of MNPs and the medium accelerates the falling speed by an external magnet and the strength of the magnetic force determines the size of the droplets. To maintain the spherical shape of the medium droplets during the incubation time, the viscosity of the medium needs to be increased by adding biologically safe materials to the fluidic medium. Cellulose nanofibers (CNFs) are naturally derived and have the same chemical structure as BC [24], [25]. Since a CNF hydrogel shows a shear thinning rheological property, it is appropriate for extrusion through a needle and forming a gel-like structure at the substrate surface [26], [27]. The high porosity and nanofibrous structure of SBCFs are crucial in biomedical and environmental applications due to high specific volume and surface area. Moreover, the size controlled fabrication of magnetized BC beads will be attractive for the design of retrievable green systems such as heavy metal removal and water treatment, and precise location of cells or drugs in biomedical applications.

Section snippets

Preparation of a CM-CNF hydrogel

Prior to carboxymethylation and nanofibrillation of the pulp fibers, kraft pulp (79.4% ± 0.6% cellulose, 18.8% ± 0.2% hemicellulose and very little amount of lignin and byproducts, Moorim P&P, Ulsan, Korea) was beaten using a laboratory valley beater (DM-822, Daeil Machinery Co., Ltd., Daejeon, Korea) for 30 min at 500 rpm. Beaten pulp (dry weight, 70 g) was solvent-exchanged with ethanol and immersed in a solution of monochloroacetic acid (14 g, Sigma-Aldrich, St. Louis, MO, USA) in isopropanol (3200 

Results and discussion

Since G. xylinus was an aerobic bacterium that biosynthesized cellulose at the surface of a medium, the shape of the cellulose pellicle depended on the shape of the surface exposed to air. Because the droplets at the solid surface formed a round surface to air but the rest of the interfaces was blocked to air, only a planar structure of the cellulose pellicle was produced. For the formation of spherical cellulose pellicles, the droplets needed to be exposed to air in all directions. The

Conclusions

The movable SBCF beads were prepared by biosynthesizing cellulose pellicles at the droplet surface by G. xylinus. The MNPs were well distributed and combined in the CM-CNF hydrogel as evidenced by FE-SEM. Droplets of a hydrogel medium consisting of CM-CNF, culture medium, bacteria, and MNPs were prepared by applying an external magnetic force. The combination of CM-CNF and MNPs was critical to controlling the droplet size, where higher CM-CNF and MNP concentrations reduced the diameter of the

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2018R1D1A1B07049081). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2020R1A4A1018017).

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