Nanocellulose, a tiny fiber with huge applications
Graphical abstract
Introduction
Increased demand for high-performance materials with tailored mechanical and physical properties, makes nanocellulose the most attractive renewable material for advanced applications. Cellulose is the product of biosynthesis from plants, animals, or bacteria, while the general term ‘nanocellulose’ refers to cellulosic extracts or processed materials, having defined nano-scale structural dimensions. Nanocellulose can be divided to three types of materials: (I) cellulose nanocrystals (CNCs), also referred to as nanocrystalline cellulose (NCC) and cellulose nanowhiskers (CNWs), (II) cellulose nanofibrils (CNFs), also referred to as nano-fibrillated cellulose (NFC), and (III) bacterial cellulose (BC). Different approaches are used to extract nanoparticles from cellulose sources, resulting in particles with varied crystallinities, surface chemistries, and mechanical properties [1]. See Figure 1 for electron microscope images of the three types of nanocellulose.
Currently, CNCs are mainly produced by acid hydrolysis/heat controlled techniques, with sulfuric acid being the most utilized acid. Extraction of the crystals from cellulose fibers involves selective hydrolysis of amorphous cellulose regions, resulting in highly crystalline particles with source-dependent dimensions, for example, 5–20 nm × 100–500 nm for plant source CNCs. Sulfuric acid hydrolysis grafts negatively charged sulfate half-ester groups onto the surface of the particles, which act to prevent aggregation in aqueous suspensions due to electrostatic repulsion between particles. Furthermore, the rod-like shape of CNCs leads to concentration-dependent liquid crystalline self-assembly behavior.
CNFs are micrometer-long entangled fibrils that contain both amorphous and crystalline cellulose domains, unlike CNCs which have near-perfect crystallinity (ca. 90%). Entanglement of the long particles gives highly viscous aqueous suspensions at relatively low concentrations (below 1 wt%). The extraction of CNFs from cellulosic fibers can be achieved by three types of processes: (I) mechanical treatments (e.g. homogenization, grinding, and milling), (II) chemical treatments (e.g. TEMPO oxidation), and (III) combination of chemical and mechanical treatments [2].
BC is produced extracellularly by microorganisms, with Gluconacetobacter xylinum being the most efficient amongst cellulose-producing microorganisms. Different from plant-source nanocellulose, which may require pre-treatment to remove lignin and hemicellulosics before hydrolysis, BC is synthesized as pure cellulose. BC nanofibers, characterized by average diameters of 20–100 nm and micrometer lengths, entangle to form stable network structures (see Figure 1).
The different types of nanocellulose exhibit distinct properties which dictate their applicability and functionality, that is, certain types of nanocellulose are better suited for specific applications than others. The unique properties of nanocellulose include high Young's modulus/tensile strength (e.g. 150 GPa/10 GPa for CNCs), a range of aspect ratios that can be accessed depending on particle type, and potential compatibility with other materials, such as polymer, protein, and living cells. Furthermore, the options for chemical and material processing of nanocellulose are extremely versatile, opening up a wide range of possibilities in terms of structure and function. The scope of this article encompasses the main types of nanocellulose outlined above, with a special focus on cellulose nanocrystals, presenting our opinion regarding important recent advances in nanocellulose research and the directions driving current technologies and future outlooks.
Section snippets
Nanocellulose photonics
Nanocellulose is of interest for photonic applications for reasons inherent to the material; first among these is the liquid crystalline behavior of CNCs which gives rise to iridescent films of defined optical character, secondly both CNCs and CNFs may form optically transparent stand-alone films. The versatility of these materials lies in the nature and surface chemistry of cellulose – with relatively little effort, nanocellulose can be made compatible with both hydrophilic and hydrophobic
Nanocellulose films and foams
CNC films have been widely researched, mainly for their chiral nematic organization and optical properties (see Nanocellulose Photonics), and also for their gas barrier [38, 46, 47], water sorption [47], and mechanical properties. Recently, the thermal conductivity of CNCs was studied by Diaz et al., from a single crystal to films with different degrees of alignment [48].
Another widely researched area is the alignment of CNC films by the application of external forces, such as magnetic,
Nanocellulose functionalization
The surface hydroxyl groups and relatively large specific surface area provide abundant active sites for nanocellulose modification. Covalent modifications, such as oxidation [69], esterification [70, 71], etherification [72], polymer grafting [73, 74], and silylation [75, 76], as well as noncovalent binding [77], are proposed to introduce functional groups onto nanocellulose surfaces or as precursors for further modification [75]. Compared to CNFs and BC, CNC functionalization is gathering
Nanocellulose in thermoplastic materials
Nanocellulose, including CNCs and CNFs, is extensively used as a filler in thermoplastic polymeric matrices to produce cost-effective, highly durable nanocomposite materials, with a ‘greener’ footprint. The native crystallinity, high strength, and moderate to high aspect ratio (ca. 10–1000 length/diameter; type dependent) of nanocellulose are relevant for stress-transfer and load-bearing in thermoplastics, such as starch, polyvinyl alcohol (PVA) [94], poly lactic acid (PLA) [95, 96, 97],
Nanocellulose-protein composites
The combination of nanocellulose and protein in nanocomposite materials aims to combine attractive qualities from each component in a synergistic fashion. In terms of material science applications, this strategy often falls under the umbrella of ‘biomimicry’, where nature inspires the fabrication of functional materials; in this case the approach may aspire to recreate the hard/soft composite mechanical motif that is prevalent in many natural organisms [130], casting nanocellulose in the role
Nanocellulose in medical applications
Nanocellulose is a promising biomaterial for medical applications owing to its good biocompatibility [141, 142, 143] and relatively low toxicity [144•], as well as distinct geometry, surface chemistry, rheology, crystallinity and self-assembly behavior [1, 145, 146]. While it is generally accepted that BC is non-toxic [141], the issue of biotoxicity is less resolved for other nanoparticles of cellulose, such as CNCs and CNFs. The toxicity of these materials depends on particle size, surface
Conclusions
Although the topic of nanocellulose (CNCs, CNFs, and BC) has been intensively researched over the past 2+ decades, the room for new developments, particularly in the fields of coatings and medical devices, clearly exists. Pushing the boundaries of nanocellulose further into flexible electronics, optical devices, and high performance functional plastics, to create organic materials with tunable, ‘smart’, and biomimetic characteristics will be of particular interest for the future, especially as
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgments
The authors thank David Ernst Weber for design of the table of contents graphic, and the Minerva Center for Biohybrid Complex Systems for support. TA is grateful to the Azrieli Foundation for the award of an Azrieli Fellowship, and YC and EA acknowledge the PBC post-doctoral fellowship for funding. This work was partially supported by a Minerva Grant and two FP7 programs: BRIMEE (608910) and NCCFOAM (604003-2). In addition, the authors would like to thank the Hebrew University Center for
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These authors contributed equally to this work.