Recent advances in nanoengineering cellulose for cargo delivery

Abstract

The recent decade has witnessed a growing demand to substitute synthetic materials with naturally-derived platforms for minimizing their undesirable footprints in biomedicine, environment, and ecosystems. Among the natural materials, cellulose, the most abundant biopolymer in the world with key properties, such as biocompatibility, biorenewability, and sustainability has drawn significant attention. The hierarchical structure of cellulose fibers, one of the main constituents of plant cell walls, has been nanoengineered and broken down to nanoscale building blocks, providing an infrastructure for nanomedicine. Microorganisms, such as certain types of bacteria, are another source of nanocelluloses known as bacterial nanocellulose (BNC), which benefit from high purity and crystallinity. Chemical and mechanical treatments of cellulose fibrils made up of alternating crystalline and amorphous regions have yielded cellulose nanocrystals (CNC), hairy CNC (HCNC), and cellulose nanofibrils (CNF) with dimensions spanning from a few nanometers up to several microns. Cellulose nanocrystals and nanofibrils may readily bind drugs, proteins, and nanoparticles through physical interactions or be chemically modified to covalently accommodate cargos. Engineering surface properties, such as chemical functionality, charge, area, crystallinity, and hydrophilicity, plays a pivotal role in controlling the cargo loading/releasing capacity and rate, stability, toxicity, immunogenicity, and biodegradation of nanocellulose-based delivery platforms. This review provides insights into the recent advances in nanoengineering cellulose crystals and fibrils to develop vehicles, encompassing colloidal nanoparticles, hydrogels, aerogels, films, coatings, capsules, and membranes, for the delivery of a broad range of bioactive cargos, such as chemotherapeutic drugs, anti-inflammatory agents, antibacterial compounds, and probiotics. SYNOPSIS: Engineering certain types of microorganisms as well as the hierarchical structure of cellulose fibers, one of the main building blocks of plant cell walls, has yielded unique families of cellulose-based nanomaterials, which have leveraged the effective delivery of bioactive molecules.

Keywords: Bacterial cellulose; Cancer therapy; Cellulose nanocrystals; Cellulose nanofibrils; Drug delivery; Hairy nanocellulose; Nanocellulose; Wound healing.

Figure 1.

Nanocelluloses and examples of their production methods. Plants and microorganisms are two main sources of cellulose. Cellulose fibrils (often chemically pre-treated, e.g., with TEMPO) are disintegrated into nanoscale fibrils, named cellulose nanofibrils (CNF) through rigorous mechanical processing (such as microfluidization and high-pressure homogenization) [13]. When cellulose fibrils are treated with a strong acid, such as sulfuric acid, the amorphous regions of fibrils are hydrolyzed, yielding needle-shaped crystals known as cellulose nanocrystals (CNC) [17]. Moreover, instead of removing amorphous regions, when they are solubilized through oxidation, cellulose fibrils fall apart into CNCs sandwiched between two protruding layers of functionalized cellulose chains. This class of newly emerged nanocelluloses is called hairy CNCs [8,9,18]. Some microorganisms, such as bacteria Acetobacter xylinum, also produce cellulose with a high purity, which may further be processed with a strong acid to yield bacterial CNCs (BCNC) [14,16]. Note that other sources of nanocelluloses are tunicates and algae [19].

Figure 2.

Various types of cellulose nanocrystal (CNC)-based biomaterials for nanomedicine and their main applications.

Figure 3.

(a) CNCs are readily decorated with polycations (e.g., polyethylenimine, PEI), imparting a positive charge to their surface, which promotes the electrostatic (physical) binding of siRNA killer. These self-assembled nanoparticles are internalized by C2C12 myoblasts (b), resulting in gene editing, silencing the expression of cell cycle genes, and promoting apoptosisdriven cell death [87]. (c) Within 24–72 h, cell death becomes evident by the formation of debris (shown with arrows), which is more pronounced for CNC-PEI-siRNA complexes, similar to the commercial transfection reagent (HiPerFect, Qiagen). Carbohydr. Polym., 164: 258–267, Copyright (2017), with permission from Elsevier.

Figure 4.

(a) Chemical modification of CNC surface with FITC and folic acid (FA) for visualizing the enhanced receptor-mediated uptake of the nanocelluloses by human DBTRG05MG, which is shown in (b) the fluorescence images of stained cells [80]. Reprinted with permission from S. Dong, H.J. Cho, Y.W. Lee, M. Roman, Synthesis and cellular uptake of folic acid-conjugated CNCs for cancer targeting. Reprinted with permission from (Biomacromolecules, 15:1560–1567). Copyright (2014) American Chemical Society.

Figure 5.

(a) Synthesis of pH-responsive magnetic CNC-based nanocarrier. The nanocellulose was reacted with tosylchloride for the functionalization with tris(2-aminoethyl)amine (AMFC), providing amino groups to effectively attract the carboxyl groups of methotrexate (MTX, an anticancer drug). The magnetic CNC-based MTX delivery system, fabricated by an in situ reaction with FeCl₂ and FeCl₃, provides AMFC-coated magnetic nanoparticles (AMFC@MNPs), which are responsive to pH. (b) MTX-loaded magnetic CNC carriers, benefitting from the similarity of MTX to folic acid, faciliatet the folate receptor-mediated cell internalization [82]. Reproduced from [82] with permission from the Centre National de la Recherche Scientifique (CNRS) and The Royal Society of Chemistry.

Figure 6.

Examples of common methods to make cellulose nanocrystal (CNC)-based cargo delivery systems. (a) Direct chemical modifications of CNC surface to conjugate Cltx via the Fisher esterification of the hydroxyl groups of CNC and the carboxylic acid groups of a drug in a Brønsted acid ionic liquid [90], reproduced from [90] with permission from the Centre National de la Recherche Scientifique (CNRS) and The Royal Society of Chemistry. (b) Solvent casting of CNC-alginate nanocomposites crosslinked using calcium ions. Carbohydr. Polym., 1:186–195, Copyright (2018), with permission from Elsevier [126]. This platform may be used to encapsulate active molecule of choice. (c) Self-assembly through the electrostatic adsorption of sulfuric acid-hydrolyzed CNC (bacterial) with amine-bearing polymers [127]. Reprinted with permission from [127]. Copyright (2018) American Chemical Society.

Figure 7.

Bacterial cellulose (BC)-based biomaterials, including colloidal nanoparticles, films, membranes, and hydrogels for the delivery of a broad range of cargos. Panel (a) is reprinted with permission from [146], Copyright © 2015, Springer Nature, Springer Science Business Media New York. Panel (b) is reprinted from [140] under Creative Commons Attribution 4.0 International License for the Open Access content. Panel (c) is reprinted with permission from [147]. © 2018 WILEY- VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 8.

(a) Mechanisms of cargo (e.g., BSA [136]) loading in BC-based carriers, including physical absorption/adsorption and chemical conjugation. The carrier may be used in hydrated or dry states. Supplementing BC with additives, such as glucose, provides remarkable shape recovery after rehydration [135]. (b) BC-based materials have been widely used for wound dressing, enabling the controlled release of bioactive (e.g., MTX) molecules [143]. BC may also be combined with antibacterial nanomaterials, such as ZnO for burn wounds [128]. Images in Panel (a, left) are reprinted from [136] Copyright (2014), with permission from Elsevier. Images in panel (a, right) are reprinted from [135] under Creative Commons Attribution 3.0 Unported Licence. Panel (b, left) is adapted from [128] Copyright (2017), with permission from Elsevier. Panel (b, right) is adapted from [143] Copyright (2018), with permission from Elsevier.

Figure 9.

Cellulose nanofibril (CNF)-based biomaterials, including hydrogels, aerogels, capsules, films, and inorganic nanocomposites for cargo delivery. Images of hydrogel, capsules, aerogel, composite films, and inorganic nanocomposites are reprinted from [193], [188], [194], [195], and [185] with permissions from The Royal Society of Chemistry, Copyright (2017) American Chemical Society, Copyright (2015) American Chemical Society, Copyright (2017) American Chemical Society, and Elsevier, respectively.

Figure 10.

Cellulose nanofibril capsules. (a) Fabrication of CNF microcapsules from colloidal CNF dispersions using sacrificial calcium carbonate cores undergoing the layer-by-layer (LbL) assembly of CNF, XyG, CNF, and AP [187]. (b) The cross section of capsules [188], indicating a calcium carbonate core (red arrow) coated with the multilayer CNF nanocomposite (white arrows). (c) Capsule pores change size as a response to ionic strength variation, providing a stimuli-responsive cargo carrier platform with tailored loading (in water)/trapping (in salt)/release (in water) capacities [187]. (d) Mechanically-stable liquid-core capsules [205] made up of a covalently crosslinked CNF/CNC shell with an inner aromatic polyurea layer. These capsules are about six times stiffer than aromatic polyurea capsules. Images of panels (a) and (c) are reprinted from [187], copyright (2018), with permission from Elsevier. Panel (b) is reprinted with permission from [188]. Copyright (2017) American Chemical Society. Panel (d) is reprinted with permission from [205]. Copyright (2014) American Chemical Society.

Figure 11.

CNF-based hydrogel beads for the controlled release of probiotics. (a) Cellulose fibrils undergo TEMPO-mediated oxidation, resulting in the carboxylate conjugation to the C6 of monomers (glucose). (b) The oxidized nanofibrils are mixed with cellulose fibers in an aqueous solution containing LiOH/urea/water and maintained at −20°C, followed by freeze-thawing and centrifugation. The product is added to a dilute hydrochloric acid solution to form the beads with various CNF-to-cellulose fiber ratios. (c) The bead pore size may be regulated by changing the concentration of CNF; the higher the CNF, the larger the pores. (d) The engineered porosity of gel beads and their pH-responsiveness originated from the carboxylic acid groups enable the controlled release of probiotics [190]. Reprinted with permission from [190]. Copyright (2018) American Chemical Society.

Figure 12.

An example of CNF production methods and CNF-based biomaterials for cargo delivery. (a) Extraction of CNF from trees via the physical disintegration of chemically-modified cellulose fibrils [248], Copyright (2011), with permission from Elsevier. (b) Hydrogels made up of carboxylated CNF and chitosan for the controlled release of 5-FU [189], reprinted with permission from [189], copyright (2018) American Chemical Society. (c) CNF nanocomposites for chemotherapeutic delivery via direct drug binding to Fe₃O₄-Ag₂O quantum dot-decorated CNF [185], copyright (2017), with permission from Elsevier. (d) CNF foams, providing a sustained release of active molecules (riboflavin) as a result of increased diffusion path [247], copyright (2016), with permission from Elsevier. (e) Concentrated CNF dispersions readily form a shear-thinning hydrogel, which may be used for the delivery of anti-cancer drugs, such as metformin (Met) [183] (Copyright (2017), with permission from Elsevier) and Dox [191] (Copyright (2018), with permission from John Wiley and Sons) for treating melanoma.