Current international research into cellulose as a functional nanomaterial for advanced applications

Since ‘nanocellulose’ emerged onto the scene as a material there has been a proliferation of the terminology used to describe these materials. We will aim to be consistent in our description of nanocellulose and thereby conform to standards that have been recently laid out in another comprehensive review of techniques to analyze what should collectively be called cellulose nanomaterials (CNMs) [18]. This is the acronym we will use to refer to the different forms of ‘nanocellulose’. We will use the terms cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) to refer to the rod-like and fibrillar cellulosic materials. Confusingly CNCs have also been called whiskers, needles and nanocrystalline cellulose (NCC), and we will avoid such terms. Bacterial cellulose (BC) will also be used as a term, referring to the fibrillar material produced by the gram-negative bacteria Glucanobacter xylinum. We will conform to the international organization for standardization (ISO) on the terminology used for CNMs [19] where possible and to otherwise revert to commonly used terms.

Cells modify their behavior dependent on the cues that they perceive from their microenvironment [31‐33]. One strategy to render the surfaces of several types of nanocellulose more biocompatible and bioactive is through surface modification [25, 26]. Chemical moieties, biomolecules, bio-oligomers and biomacromolecules can be introduced physically or chemically at the surface of nanocellulose to provide cell signals. These include positive or negative surface charges as well as cell receptors such as the amino acid sequence arginine-glycine-aspartic acid. These signals aim at favoring cell uptake/adsorption at the surface of nanocellulose scaffolds, and their effect on cell adhesion, growth, proliferation, and morphology is typically studied.

Research involving the use of BC and CNCs for tissue engineering applications was reviewed by Dugan et al. in 2013 [34]. The surface modification of BC with bioactive peptide sequences using cellulose-binding domains was discussed as a strategy to enhance its bioactivity. The surface charge of CNCs has also been mentioned as a critical factor to provide their surface with biocompatibility and bioactivity. It is known that mammalian cells possess a net negative charge. As a result, materials with positively charged surfaces could potentially favor cell uptake by electrostatic attraction. On the other hand, materials with negatively charged surfaces would minimize cell uptake due to electrostatic repulsion. The first in vitro study of CNCs with living cells was by Roman et al., where the potential of CNCs as carriers in targeted drug delivery applications was demonstrated [35]. Mahmoud et al. then modified the surface of CNCs with fluorescein isothiocyanate or alternatively rhodamine B isothiocyanate, providing them with negative and positive surface charges, respectively [36]. CNCs modified with positive surface charges were found to be uptaken by human embryonic kidney 293 cells, whereas those modified with negative surface charges were not significantly taken up by both cell types at a physiological pH. The results were explained in terms of cell/material surface electrostatic interactions [36]. Cell uptake mechanisms are, however, more complex, and other scaffold features need to be considered including local nanofiber alignment among others, which are discussed in detail below.

The research group led by Ferraz at Uppsala University explored the effect of nanocellulose surface charge on human dermal fibroblast (HDF) cell culture 2D film scaffolds. In their first work, cationic CNFs and Cladophora nanocellulose were obtained by glycidyltrimethylammonium chloride condensation, whereas anionic CNFs and Cladophora nanocellulose were obtained by carboxymethylation and TEMPO-oxidation, respectively [37]. The results revealed that anionic CNF films possessed greater cytocompatibility than non-modified and cationic CNF films. On the other hand, anionic Cladophora films better promoted cell adhesion and viability compared to non-modified and cationic Cladophora films. The improved cell adhesion of HDF onto anionic Cladophora films was attributed to local nanofiber alignment.

In a subsequent study, non-modified, anionic, and cationic CNF films were evaluated as 2D scaffolds to direct monocyte/macrophage (MM) responses in the absence or presence of lipopolysaccharide [38]. The results suggested that MM cultured onto anionic CNF films experienced activation toward a proinflammatory phenotype. Non-modified CNF films, however, promoted a mild activation of THP-1 monocyte cells, whereas cationic CNF films behaved as a bioinert material. None of the materials were able to directly activate the MM toward an anti-inflammatory response.

In a third study by the same group, the effect of the negative surface charge density of TEMPO-oxidized Cladophora nanocellulose on the response of HDF and human osteoblastic cells was investigated [39]. From a carboxyl group amount ≥ 260 μmol g⁻¹, equivalent to a threshold ζ-potential value of -36 mV, TEMPO-oxidized Cladophora nanocellulose was found to be cytocompatible, demonstrating that bioinert nanomaterials can be turned bioactive by adjusting the magnitude of their surface charge density.

More recently, three primary works have studied the effect of surface charge on cell culture. Films composed of non-modified, anionic, and cationic CNFs were obtained by an evaporation-induced droplet-casting method [40]. Non-modified and cationic CNFs resulted in 2D surfaces with higher degrees of local nanofiber orientation compared to anionic CNFs. With respect to cell viability and proliferation, anionic and cationic CNF surfaces were found to perform similarly compared to a positive control surface. Although the use of fibronectin coating slightly enhanced cell response for all 2D surfaces, uncoated anionic and cationic CNF surfaces were found to support cell growth. Cationic CNF surfaces, along with the presence of CNF alignment, were found to guide cell growth toward a specific orientation direction [40]. In a subsequent study by Pajorova et al. [41], cellulose mesh 3D scaffolds were coated with either cationic, anionic or a 1:1 mixture of cationic and anionic CNFs. Cell adhesion, proliferation, spreading, and morphology were studied by seeding the 3D scaffolds with either HDF or adipose-derived stem cells (ADSC). Anionic CNFs promoted the proliferation of both HDF and ADSC, whereas cationic CNFs enhanced the adhesion of ADSC. The cationic and anionic CNF mixture resulted in promoting combined benefits arising from each CNF type [41]. Lastly, CNFs, CNCs and TEMPO-CNFs with variations in total surface charge were investigated as coatings for cell culture [42]. TEMPO-CNFs with a total surface charge of 1.14 mmol g⁻¹ were found to provide the highest cell viability and adhesion compared to the mechanically isolated CNFs without chemical pre-treatment, and CNCs, from which HDF cells were unable to adhere, leading to low viability [42].

Another strategy to render nanocellulose bioactive is by binding either amino acids, peptides, or proteins onto its surface. A first study covalently bound amino acids to the surface of commercial cellulose filter membranes [43]. Cationic amino acids including lysine and arginine were found to enhance cell adhesion, whereas anionic as well as small amino acids significantly reduced cell adhesion. In subsequent work, the surface of α-cellulose fibrous networks was modified by covalent grafting of hydrophilic, aliphatic and aromatic amino acids onto their surface by esterification [44]. Aromatic amino acids, and in particular tryptophan, resulted in enhanced fibroblast cell spreading. Immobilization of collagen peptides onto the surface of dialdehyde BC was proposed by Wen et al. [45] and found to promote enhanced fibroblast cell adhesion and attachment compared to non-modified BC. Another investigation by Barud et al. [46] used silk fibroin proteins to modify the surface of BC, forming a sponge-like scaffold. This was found to facilitate the attachment and growth of L-929 cells, where proteins acted as cell receptors [46].

The research group led by Franck Quero at the University of Chile have produced protein-functionalized cellulose fibrils from the tunic of Pyura chilensis by a top-down approach [47]. As illustrated in Fig. 3, the CNFs were used to produce films, which were subsequently evaluated as 2D scaffolds to culture mouse skeletal C2C12 myoblast cells [47]. Membranes having ~ 3.1% residual proteins at their surface were found to promote higher cell density and spreading as well as a more orientated shape cell morphology compared to membranes constituted of bleached CNFs. In another work by Zhang et al. [48], the real-time cell adsorption of cells onto non-modified and modified CNF films was monitored by multi-parametric surface plasmon resonance. The presence of either human recombinant laminin-L521 (natural protein of the extracellular matrix) or poly-L-lysine at the surface of the CNF films resulted in enhanced attachment of human hepatocellular carcinoma cells compared to non-modified CNF films.

Another method to render the surface of CNFs bioactive has been to modify their surface by polyion complex formation between negatively charged TEMPO-oxidized CNFs and positively charged basic fibroblast growth factors (bFGFS) [49]. In this way, Liu et al. mimicked the interactions that naturally exist between bFGFS and heparin sulfate in the extracellular matrix. The scaffold was found to release controlled amount of bFGFS, which was regulated by both CNF surface chemistry and enzymatic deconstruction of the scaffolds. This resulted in significantly enhanced fibroblast cell proliferation [49].

Instead of tailoring the surface chemistry of nanocellulose, other features have been reported to control cell response including multiscale topographical and multicomponent approaches.

These aspects and their specifics are presented in the next subsection and could potentially be translated toward new commercial products in the near future.

Three-dimensional biophysical and biochemical interactions between cells and scaffolds modulate the cell response in terms of proliferation, migration, differentiation, deposition of extracellular matrix proteins, and—ultimately—cell survival. These interactions are dependent on cell surface receptors and, when cell attachment occurs, are regulated by integrin pairs that have a defined nanometric spacing between them [51]. The control of cell alignment via topographical features has been known since the late 1980s/early 1990s [52‐54]. Microscale features with step changes > 10 µm and spacings > 2 µm inhibit cell migration and spreading [55], whilst nanoscale features with dimensions < 70 nm and spacings between 70 and 300 nm disrupt focal adhesions [56]. Thus, modification of the tissue engineering scaffold over multiple length scales is significant in defining the response of the cells.

This subsection briefly reviews one particularly promising and emerging area of nanocellulose tissue engineering, namely the design of scaffolds with topographical anisotropy at one, or more, length scales. There are, however, numerous examples in the literature of nanocellulose scaffolds that do not exhibit anisotropy that are well suited to biomedical applications (and tested explicitly in vitro or in vivo) as reviewed elsewhere [20, 25, 57‐66] with many newer reports focusing on 3D printing specifically [67‐71]. Some highlights in the general area of nanocellulose tissue engineering include: 3D scaffolds with controlled bioabsorbability [30, 72]; self-healing hydrogels for cell delivery [73]; scaffolds that maintain stem cell pluripotency [74], or guide cell differentiation [75‐79]; hydrogels that support organoid growth [80‐83]; and bioinks that enable 3D cell printing [84‐92].

Since all nanocellulose types are anisotropic, being fiber or filament-like with diameters at the nanoscale (e.g., BC and CNF), or spindle-like nanoparticles with all dimensions at the nanoscale (e.g., CNCs), the design of tissue scaffolds with, at least, short-range directionality and nano/micro features becomes possible. Furthermore, long-range directionality and mechanical anisotropy to direct cell response can be achieved via topographical patterning of the scaffold (e.g., soft lithography, 3D printing, templating, electrospinning) or through directed assembly to induce nanoparticle alignment.

In 2010, Dugan et al. [93] were the first to report that the nanoscale structure of CNCs could align myoblast muscle cells (Fig. 4a&b), despite being orders of magnitude smaller than the cells themselves. Straightforward 2D surfaces with spin-coated (radially oriented) CNCs and no other components were sufficient to demonstrate this effect [93]. Similarly, the anisotropic deposition of CNCs onto titanium surfaces guided fibroblast proliferation, offering new opportunities for integrating implants [94]. Although these features appear too small to enable cell attachment according to earlier literature, it has been recently demonstrated that integrin clusters can form across nanofeatures so long as the feature spacing is < 110 nm [95]. It is reasonable to expect that these integrin clusters arrange themselves in line with the features, enabling cell alignment. This work in simple 2D nanocellulose films set the stage for more complex 2D scaffolds and higher dimensionality structures with optimized physicochemical properties and a variety of topographies.

Soft lithographical techniques have been utilized in tissue engineering for almost 20 years [96], however, the first reported use with nanocellulose was demonstrated on BC by Bottan et al. [97] in 2015 (Fig. 4c&d). Since then, the patterning of BC is typically achieved using polydimethylsiloxane with designed structural features > 1 µm [97‐100]. Spacings of 10 µm between features have been used to direct growth of fibroblasts [97, 99], neuronal cells [98, 100] and muscle cells [100], whilst having a more limited impact on keratinocytes [97, 99]. Importantly, structural control of fibroblast growth via BC scaffolds can influence scar formation in vivo [101]. Jin et al. demonstrated that structured pure BC scaffolds with 10 µm stripes reduced fibroblast proliferation, which limited collagen accumulation. The reduction in collagen, in turn, reduced scar contraction and limited hypertrophic scar formation [101]. Likewise, Boni et al. [99] produced similarly structured BC scaffolds impregnated with silk sericin. The structural features enabled alignment of fibroblasts whilst the silk sericin enhanced fibroblast and keratinocyte proliferation in vitro. However, the structural features limited collagen deposition compared to structurally unmodified BC, which they suggested enhanced the potential to limit fibrosis and scar formation [99]. Thus, soft lithographical modification of BC offers opportunities to produce inexpensive scaffolds and wound dressings with improved healing properties.

Despite previous publications describing increased cell alignment with decreased feature spacing of BC-based materials [97], reports of directly controlling BC ‘ribbon’ alignment are limited. Wang et al. [102] have recently demonstrated that alignment could be achieved via wet-drawn stretching of the BC film. The gelatin impregnated, aligned films exhibited enhanced mechanical properties and significantly improved fibroblast alignment in vitro, with further enhancement achieved via electric field stimulation [102].

Alternatively, one may produce aligned fibers via electrospinning. He et al. produced aligned regenerated cellulose fibers (with ca. 200 nm diameters) loaded with CNCs and spun from lithium chloride/dimethyl acetamide solution [103]. This composition and processing led to improved tensile properties of the fibers and enabled aligned growth of dental follicle cells [103]. The same scaffolds impregnated with bone morphogenic protein-2 promoted osteogenic differentiation of mesenchymal stem cells in vitro, while anisotropic fiber orientation promoted cell alignment, no significant differences in biomarkers (alkaline phosphatase activity, calcium content) were observed between aligned and unaligned fibers [104]. However, in vivo, the aligned fibers enabled aligned collagen deposition and new cortical bone growth on the external face of the implant. These responses were not observed in the unaligned scaffolds, nor in a similar study using aligned poly(L-lactic acid) nanofibers. As such, the enhanced response in the CNC loaded sample was attributed to the improved mechanical properties observed by the incorporation (and alignment) of CNCs in the scaffold [104].

The improvement of mechanical properties and development of nanoscale anisotropy in multicomponent electrospun and 3D-printed scaffolds via CNC/CNF inclusion has only recently begun to be explored [105‐111]. For example, De France et al. reported on the facile production of ‘2.5D’ poly(oligoethylene glycol methacrylate) (POEGMA)/CNC scaffolds that enabled microscale control of ‘wrinkled’ features, similar to those produced using soft lithography, via controlled thermal shrinkage while independently controlling the alignment of the nanoscale features via electrospinning [109]. Uniaxially (micro) wrinkled features with parallel-orientated (nano) fibers resulted in aligned myoblast growth (Fig. 4e&f), whereby CNC content was used to control the compressive modulus and protein uptake [109]. Similarly, Huang et al. [110] demonstrated that they could align oxidized BC along the direction of extrusion in simple printed structures, which influenced the orientation of lung epithelial stem cells. While research into the nanoscale alignment of nanocellulose in 3D-printed architectures through control of shear is still in the initial stages [112‐114], it offers opportunities for the development of scaffolds that exhibit photoresponsive mechanical properties [115], or change shape depending on the degree of hydration [116].

Nanocellulosic 3D structures with directed cell growth due to anisotropic microscale topographies have been achieved via directional freezing to produce aligned pores within cryogels [117, 118] (as opposed to the anisotropy resulting from aligned nanocellulose itself). The effect of pore morphology on cell response is less well defined than that of 2D ridges/grooves. For example, Karageorgiou and Kaplan reported that the optimal pore size to promote osteogenesis was  > 300 µm [119], while the optimal pore diameter for neuronal cells has often been reported to be  < 100 µm [120‐122]. However, strategies to control pore size and morphology in nanocellulosic cryogels are known, including regulation of the gel composition [123‐125]; freeze-casting temperature and/or rate [123, 126]; sol pH [127]; and nanocellulose morphology [126]. Furthermore, Tetik et al. [128] have recently incorporated directional freezing with 3D printing, which enabled the production of controlled 3D geometries with aligned micropores.

Nanoscale anisotropy via alignment of nanocellulose is readily achieved via various techniques [129]. These include shear forces [112, 113, 130‐133]; magnetic fields [134‐137]; electric fields [138‐142]; and material stretching [143‐146]. With regard to magnetic alignment in 3D scaffolds, De France et al. [147] showed that anisotropy could be induced in POEGMA-CNC hydrogels with weak magnetic fields capable of aligning CNCs quickly, even within a polymer gel [136]. This enabled alignment of myoblasts in a 3D culture. Echave et al. [137] took this concept further to produce biphasic gelatin hydrogels that mimicked the tendon-bone interface with magnetically aligned CNCs in one section and hydroxyapatite in the other. Adipose-derived stem cells preferentially aligned and expressed tenascin-C (TNC), a tendon tissue-related biomarker, in the CNC section, while cells remained disordered and preferentially expressed osteopontin (OPN), an osteogenic differentiation-related biomarker, in the section containing hydroxyapatite [137].

Unfortunately, there are limited comparative studies investigating cell responses to different nanocellulose morphologies. Kummala et al. [42] have recently examined the response of dermal fibroblasts on different nanocellulose types (i.e., CNCs, a predominantly microfibrillated CNF, and two predominantly nanofibrillated CNFs). While true ‘nano’-CNFs supported cell attachment and proliferation, limited cell response was observed on the microfibrillated CNF and CNC surfaces. However, as the authors note, one must consider all the variables involved—including topographical features, surface chemical group type and degree of modification, fibril dimensions, tensile properties, and cell type—to be able to draw conclusions as to the cell response. This requires modeling to isolate cell response to specific parameters, such as the regression modeling performed by Johns et al. [148], and must also take the growth media into consideration as selective molecular adsorption will mediate cell-surface interactions [41, 118].

We conclude that topographically anisotropic nanocellulose scaffolds are promising biobased biomaterials for enabling cell alignment in both 2D and 3D, impacting—for example—wound healing in vitro and stem cell differentiation in vivo. Control of the topographic features at both the microscale and nanoscale through patterning and nanocellulose alignment offers new opportunities in regulating cellular response. Combining these structures with stimuli-dependent chemistry may present future opportunities in dynamic/bio-responsive tissue engineering.

Cellulose has a long history of application in pharmaceutics owing to ease of availability and a good compaction property, primarily as an excipient in oral formulations. Nanocellulose has been shown to play a variety of roles and offer several advantages in drug delivery applications, for example, for its release modulation, as a drug carrier, its improved mechanical properties, better compaction, and appropriate rheological modification [20, 23, 149‐151]. It has been shown that the addition of nanocellulose can control the release of incorporated drugs to significantly reduce consumption [152‐156]. Nanofibers offer an additional advantage in terms of mechanical support and improvement of shelf life by improving oxidative stability [29]. Novel drug delivery systems such as triggered and targeted forms have emerged using different types of nanocellulose [157, 158]. Most interestingly, nanocellulose allows easy incorporation of multiscale therapeutic agents such as nanoparticles, drug molecules, supramolecular organization and as a template for the production of other drug carriers [159‐162].

In this subsection, the most up-to-date and relevant findings in the literature are summarized in Table 1, describing the nanocellulose type and source, composition or formulation, mode of drug delivery, and key outcomes. The ability of nanocellulose to act as a drug delivery modulator may be attributed to various reasons: aggregation of nanocellulose, interactions between drug molecules and cellulose hydroxyl groups, as well as cellulose’s ability to modulate the microstructure and morphology of composite materials. It has been shown that nanocellulose can be successfully used for both water-soluble as well as poorly water-soluble drugs [24, 151, 163]. Nanocellulose can be modified, and in the case of BC, to hasten as well as delay the release of a drug and offer a combination of release profiles [164‐167]. Nanocellulose has been utilized for various routes of drug delivery—oral, transdermal, local. There are a few recent reviews on nanocellulose in drug delivery [24, 151, 163, 168]; however, an important emerging concept of triggered or actuated drug delivery remains less discussed [24, 151, 163, 168].

Several companies have been commercializing medical grade products based on BC (e.g., Membracel® by Vuelopharma, Epi Nouvelle⁺® naturelle by JeNaCell GmbH) and CNFs derived from tunicates (e.g., Ocean TuniCell®) and wood pulp (e.g., UPM Biomedicals). Most of these products are based on non-surface-modified nanocellulose but UPM Biomedicals now sells a cell culture media product named GrowDex®-A that consists of surface-modified CNFs with proteins or peptides. UPM Biomedicals also produces FibDex®, an advanced CNF wound care dressing that has proven to provide efficient wound healing in skin graft donor sites [209]. CELLINK have also developed a series of bioinks based on CNFs and alginate for the 3D printing of tissue engineering cell scaffolds. As such, commercial opportunities exist to move forward with the use of surface-modified nanocellulose toward new commercial products.

Surrounding wound dressings with transdermal drug delivery, application, product development and commercialization of nanocellulose continues to advance significantly. Some commercial products include Biofill®, Bioprocess®, Suprasorb X + PHMB® and Xcell®. Nanocellulose-based drug delivery products such as Gengiflex® membranes for dental implants exist; however, there is a need to push clinical trials and commercialization of nanocellulose further [23, 149]. For widespread acceptance of nanocellulose in drug delivery, a better understanding of the influence and regulation of drug release, interactions between drug molecules and nanocellulose, as well as possible reduction or destruction of drug activity and structure is required. In addition, toxicity needs to be further assessed, likely on a case-by-case basis [210].

Using locally available agricultural waste biomass including hemp, and corn stover, various approaches for the preparation of heavy metal adsorbents have been explored. In one study, cellulose-supported iron oxides were prepared and applied for the removal of As(III), As(V) and Cr(VI) ions. Notwithstanding that arsenic is considered carcinogenic even at very low concentrations, over 200 million people in 40 countries around the world are exposed to drinking water with As levels above World Health Organisation guidelines (10 µg L⁻¹) [227]. Although As(V) predominates (as H₂AsO₄⁻ and HAsO₄²⁻) in oxidizing conditions, in regions of previous gold mining activity which are often characterized by reducing conditions and high sulfate concentrations, arsenic exists predominantly in the more toxic As(III), as H₃AsO₃⁰ and H₂AsO₃⁻. Nevertheless, because of slow redox transformations, both forms often occur in either redox environment [228].

Anita Etale’s group has explored two strategies for the use of cellulose in water treatment applications. The first involves using cellulose as a support material for iron oxides which are excellent adsorbents for arsenic [229]. Preliminary work, however, showed them to be prone to dissolution so that the prepared iron oxides ‘leached’ into treated water compromising both the treatment process and adsorbent lifespan. To address this challenge, iron oxides were deposited on CNF extracted from hemp fibers. Importantly, the CNF are thought to be porous [219]. CNF porosity may be the result of dissolution of lignin and hemicellulose between the lamellae by the chemical pulping process, or mechanical treatments e.g., blending and sonication applied to increase fibril separation. CNF with average pore sizes of ~ 6 nm have recently been reported [219]. Manninen et al. [230] also reported a cumulative pore volume of 1.7 mL g⁻¹ from pore sizes that ranged from < 1 – 3 nm in kraft pulp fibers, and from 3 nm to an undefined maximum between the CNF fibers.

The approach to adsorbent synthesis has, therefore, involved exploiting these pore spaces as embedment sites for contaminant adsorbents. In one study, a cellulose-ferrihydrite composite (Fig. 5a) was synthesized by the deposition of iron oxide onto TEMPO-oxidized fibrils at pH 10.5. Iron oxides, including poorly crystalline ones such as ferrihydrite, have a strong affinity for both As(III) and As(V). Adsorption occurred primarily by ligand exchange with surface OH₂ and / or OH⁻ resulting in bidentate binuclear inner-sphere complexes [229, 231]. This adsorbent displayed efficiencies of > 99% for the removal of As in mine drainage contaminated water (Fig. 5b). Further, column experiments showed that 1 g of the adsorbent was needed to treat 1L of contaminated water. The reduction in absorbance intensity from the OH region (1000 cm⁻¹) on FTIR spectra (Fig. 5c) suggests that adsorption of both arsenic species involved some loss of OH as suggested by Jain et al. [229].

A similar approach was employed in Sillanpää’s group for the removal of Ni, Cd, Cu, as well as PO₄³⁻ and NO₃⁻ [232]. Removal of these ions by CNF-calcium hydroxyapatite composites was rapid with > 95% of metal ions adsorbed in the first five minutes of exposure. Removal efficiencies were also high for phosphate ions (> 85%) and nitrates (> 80%). Exposure of the same adsorbent to Cr(VI) contaminated water also revealed similarly high efficiencies between pH 5 and 7: 94% of Cr(VI) ions were removed from solution in the first 5 min [233]. In a separate study, succinic anhydride was deposited onto CNF, and the composite investigated for adsorption of a range of metals (Fig. 5d) [215]. The results showed maximum metal uptakes ranging from 0.72 to 1.95 mmol g⁻¹ and following the order Cd > Cu > Zn > Co > Ni. Adsorption was constant between pH 3 and 7 for Zn, Cu, and Cd (> 95%), and above 75% for Co and Ni at pH 7. Importantly, 96 -100% of the adsorption efficiency could be regenerated by sonicating the used composite in 1 M HNO₃ for 15 s.

A second approach has been through surface modification of cellulose e.g., via amination [216], thiolation [234], and cationization. This latter approach uses two quaternary ammonium salts: 3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CHPTAC) and glycidyltrimethylammonium chloride (GTMAC), and has been explored (Fig. 5e) and the resulting materials examined for Cr(VI) removal (Fig. 5f), and antibacterial activity. The results showed that at pH 4, 0.1 g of CHPTAC-modified cellulose removed up to 47% of Cr (VI) ions, while 72% was adsorbed by GTMAC-cationized cellulose. GTMAC-cationized also displayed considerable antibacterial effects, reducing the viability of Escherichia coli by up to 45% after just 3 h of exposure. However, Cr(VI) uptake in contaminated water (pH 2.7) was diminished to 22% likely due to competition from sulfate and selenate ions [235] which are abundant in mine drainage. Nevertheless, together, these results suggest that cationized cellulose can be applied in the treatment of Cr(VI)-contaminated mine water particularly if pre-treatments to reduce concentrations of other anions are applied.

Bacterial cellulose (BC) is a class of nanocellulose with a unique structure and properties, such as a three-dimensional network of intertwined cellulose nanofibers, remarkable mechanical properties, high porosity, and low density [236, 237]. In its original state, BC is a hydrogel with a significant water uptake volume (~99%) [237]. Freeze-drying removes the water content but still preserves the BC 3D nanofibrous network and transforms a hydrogel into an aerogel. Moreover, pyrolysis of the BC aerogel in an inert gas atmosphere leads to the carbonization of the BC nanofibers, thereby forming a carbon nanofiber aerogel. The carbonized BC (c-BC) aerogel inherits the BC precursor's merits, so it still preserves the 3D continuous architecture, the interconnected nanofibrous network, and an extremely high porosity. The surface area of the carbon nanofibers can reach > 400 m² g⁻¹, with a sizeable porous volume (ca. 3.00 cm³ g⁻¹) and an ultra-light weight (4–6 mg cm⁻³) [238]. Furthermore, high-temperature pyrolysis can induce the hydrophobic/oleophilic properties of the c-BC aerogel, which makes it an ideal material for oil sorption.

Oil pollutants are one of the leading global environmental problems. Every year, the annual spillage of petroleum compounds to the marine environment is over 1.4 million tonnes which has caused catastrophic effects on ecological systems [239, 240]. Therefore, a remedy for oil-spills is urgently needed. Although there are several approaches for treating oil-spills, the most effective approach is via physical adsorption, i.e., the use of oil sorbents, which has been proven to be energy-efficient, highly selective, environmentally friendly, fast, and recyclable [241, 242]. Research over the last decade has reported several types of oil sorbent materials. Carbon aerogels have received considerable attention as one of the most effective materials for adsorption, separation, and recovery of spilled oil [243, 244]. The carbon aerogels can be fabricated from various biomass-based products, such as cotton, bamboo, winter melon, or waste paper [241]. These aerogels have been applied for remedying oil-spills and have successfully demonstrated a large oil sorption capacity of up to 100 times of their weight. However, the preparation processes for such aerogels may involve severe mechanical and chemical pre-treatments, which are high in energy consumption, and subject to environmental concerns. Moreover, high-temperature pyrolysis may cause frangibility and brittleness to those biomass-based carbon aerogels [245], which hinders their practical use for oil recovery. On the other hand, carbon aerogels from carbonized BC (c-BC) have advantages over other biomass-derived materials. The fabrication of c-BC is simple and cheap without any use of harsh chemicals or complicated processes. Also, the natural 3D continuous nanofibrous architectures of BC make the c-BC material mechanically robust and flexible. The c-BC aerogel is typically obtained in a bulky macroscopic form, which is desirable for easy handling and recycling after oil sorption. Plus, the shapes and sizes of the c-BC aerogel are controllable via the bacteria cultivation process and scalable for industrial production.

The application of the c-BC aerogel for oil sorption was firstly reported by pioneering work done by Wu et al. [246]. They showed that pyrolyzing the BC aerogel under an argon atmosphere at 700–1300 °C led to a c-BC aerogel with a density of only 4–6 mg cm⁻³ and a high porosity up to ca. 99.7%. The c-BC aerogel exhibits hydrophobicity and can adsorb a wide range of oils and organic solvents with excellent recyclability by direct combustion. The sorption capacity reached up to 310 times the weight of the c-BC aerogel. Moreover, it was highly flexible and mechanically robust. It could also be compressed to a more than 90% volume reduction and almost recover to its original shape after release, making a ‘squeezing’ process to recover oil possible [246]. In a recent study, a c-BC aerogel pyrolyzed at 1200 °C was compressed to 99.5% strain, and it was able to be restored elastically to almost its original shape after release [247]. A detailed surface area and pore-size study showed that micropores and mesopores (2–100 nm) occupied a large portion of the pore-size distribution, providing huge spaces for oil sorption and leading to high oil sorption capacity. Furthermore, this c-BC aerogel was an excellent thermal insulator with extremely low thermal conductivity (0.025 W m⁻¹ K⁻¹) comparable to air [247].

Several efforts have been reported to increase the oil sorption capacity of c-BC aerogels by using a composite approach. c-BC was combined with carbon nanofibers derived from polyimide (PI) by freezing the mixture of BC and PI precursor suspension before imidization and pyrolysis [248]. The resultant carbon aerogel consisted of a 3D carbon skeleton with a cellular architecture from carbonized PI, decorated with 1D c-BC nanofibers. This hierarchical structure was advantageous for enhancing the compressive modulus leading to an improved shape-retention ability due to the effective crosslinking between the PI carbon skeleton and the c-BC nanofibers. The aerogel was so stiff that it supported a weight of 200 g without any noticeable deformation. Moreover, the combined 3D carbon skeleton and 1D c-BC nanofibers resulted in a reduced pore size and a narrow pore size distribution, which could be beneficial for oil sorption [248].

Wan et al. fabricated a c-BC aerogel nanocomposite with graphene [249]. To facilitate the uniform distribution of graphene in the BC network, an in situ biosynthesis route under agitated cultivation using a graphene-dispersed culture medium was employed. The spherical BC/graphene hydrogel was carbonized at 800 °C to form a sphere-like c-BC/graphene aerogel. The nanocomposite aerogel exhibited an open honeycomb-like surface pattern consisting of numerous ridges and large cavities, which increased the aerogel's specific surface area and porosity. The unique nanostructure of the sphere-like c-BC/graphene aerogel is the key to enhance the sorption capacity (up to 457 times of its weight) for a wide range of oils and organic solvents [249].

Reduced graphene oxide (rGO) has also been composited in a c-BC aerogel [250]. This was done by freeze-casting and freeze-drying of the GO and BC mixed suspension. The GO nanosheets and BC nanofibers were uniformly assembled into a porous and interconnected 3D network. Subsequent pyrolysis transformed it into rGO/c-BC aerogel with the preserved 3D nanostructure where the c-BC nanofibers were still coated on the rGO sheets. The aerogel density was easily controlled by varying the concentration of the precursor in the suspension and the ratio of GO/BC. The lowest density was found for a GO/BC ratio of 1:1. The oil sorption capacities of the rGO/c-BC aerogel ranged from ~ 300 to 1000 times of its weight, much higher than most carbon sorbents [250]. The ultrahigh sorption capacity was attributed to its low density and high porosity.

Luo et al. devised a new method of preparing c-BC aerogels containing rGO by a novel BC culturing process [251]. A thin BC pellicle was used as a substrate in the static culture. The solution containing a 2D few-layer rGO (FrGO) suspension was sprayed onto the BC substrate to form a thin layer, onto which BC was grown to consume the sprayed FrGO layer completely. The process was repeated to form a thick BC/FrGO layered structure, which was then freeze-dried and pyrolyzed. The c-BC/FrGO aerogel from the process exhibited an entangled nanostructure between the FrGO sheets and c-BC nanofibers, creating a multi-scaled porous structure and large specific area. As a result, the c-BC/FrGO aerogel exhibited mechanical robustness and an extremely high sorption capacity of 245–598 times its weight for a range of oils and organic solvents [251]. It also showed excellent reusability by both squeezing and combustion, with nearly the same sorption retention.

To assist the collection of a c-BC aerogel for reuse, regeneration, or recycling after oil sorption, functionalizing the aerogel with magnetic properties is a very useful and practical approach. A magnetic functionalized c-BC aerogel can be collected easily in large quantities with the aid of an applied magnetic field. Supree Pinitsoontorn and his team at the Institute of Nanomaterials Research and Innovation for Energy (IN-RIE), Khon Kaen University, have explored that concept by using in situ co-precipitation of magnetic Fe₃O₄ nanoparticles in the BC pellicle before converting it into a magnetic c-BC aerogel [252]. Interestingly, by controlling the concentration of the initial Fe₃O₄ precursors, the c-BC nanofibers were decorated with well-dispersed magnetic nanoparticles with a Fe/Fe₃O₄ core–shell structure (Fig. 6a&b). The core–shell structure increased the magnetization of the NPs due to the large magnetization of the Fe core. This, in turn, improved the magnetic attraction ability when subjected to external magnetic forces. Although magnetic NPs were impregnated in the c-BC structure, the outstanding mechanical properties of the c-BC aerogel were still maintained. The magnetic c-BC aerogel was able to be compressed up to 90% strain and return to its original shape after release. This process was repeated up to 100 successive cycles, and the shape of the aerogel was nearly unchanged from the original state. Moreover, even with the addition of magnetic NPs, the magnetic c-BC aerogels had an ultralightweight property with a density of only 7.4 mg cm⁻³, which is lighter than other magnetic carbon aerogels from several carbon sources [253, 254]. The oil sorption capacity of the magnetic c-BC aerogel was still very high (37–87 times of its weight), which is comparable to other carbon aerogels from various sources [254‐256]. Also, it can be rendered recyclable several times by dissolution. The highlight of the magnetic c-BC aerogel is its ability for magnetic retrieval from the liquid after sorption. As shown in Figs. 6c&d, the motion of the magnetic c-BC aerogel in a liquid can be controlled by an external magnet. The magnetic force can also lift it out of the liquid after use. This functionality is beneficial for manipulating the sorbents in a large area of polluted water, in a practical application of this technology.

The magnetic c-BC aerogel could not just be used for the remediation of an oil-spill but also be applied to other contaminant adsorption situations. Figure 7 shows a series of images demonstrating the dye (indigo carmine) sorption capability of the magnetic c-BC aerogel [257]. The vivid blue color gradually faded, and the water became clear just like before the dye was added, indicating the complete dye removal, as confirmed by UV–Vis analysis. After the process, the aerogel was magnetically removed from water by a permanent magnet. The dye-adsorbed c-BC aerogel can be regenerated by dissolution followed by oven-drying before reuse. In addition, the magnetic c-BC aerogel can still be utilized in water treatment applications for other contaminants such as bisphenol A and Cr(VI) [258, 259].

In recent years, various semiconductor/cellulose composite materials have been widely used in the degradation of organic dyes in printing and dyeing wastewater, such as metal oxides (TiO₂, ZnO, WO₃), metal sulfides (CdS, ZnS), bismuth-based semiconductors (BiOCl, BiOBr, BiOI, Bi₄O₅Br₂), silver-based semiconductors (AgBr, AgI, Ag₃PO₄, AgVO₄ and AgCrO₄) and non-metallic semiconductors (graphite, carbon nitride).

Some studies combine photocatalysis with other oxidation methods to further strengthen wastewater treatment. Rajagopal et al. [260] prepared microcellulose (MC) and TiO₂ composite materials, combined with hydrogen peroxide photocatalytic degradation (TiO₂ + MC + H₂O₂), to decolorized wastewater containing multiple dyes under sunlight. They showed that 99% of high-concentration methylene blue (MB) dye wastewater (200 mg/L) can be degraded within 150 min, and the removal efficiency of Chemical Oxygen Demand (COD) can reach 72%. It was highlighted that the synergy index of H₂O₂ assisted photocatalytic degradation is 3.54, which shows that the above process coupling has a positive synergistic effect. Other research that is being developed by Cai et al. [261] takes advantage of a nanocomposite strategy, rendering stable cellulose-based hybrid materials with diverse functionalities for micropollutant removal. Through synergistic oxidation, e.g., persulfate activation, photocatalytic water treatment has been proven to obtain practical value and can be further developed industrially [261].

As already discussed, heavy metal ions in water can be removed by adsorption. However, single physical adsorption can only enrich and transfer heavy metals but cannot completely remove them. Semiconductor photocatalysts have redox capabilities, which can change the chemical properties of heavy metal ions to reduce their toxicity. Taking TiO₂ as an example, the mechanism of photocatalysis to remove heavy metal ions is roughly as follows:1) nano-TiO₂ adsorbs heavy metal ions on its surface, 2) an ultraviolet lamp is excited to generate photogenerated electron–hole pairs, and the electrons transition to the conduction band and transfer to the TiO₂ surface, 3) photogenerated electrons reduce the adsorbed heavy metal ions to low valence states (such as chromium, mercury, lead) or elemental forms (such as silver), the metal ions in the lower valence state further generate compounds and precipitate (such as chromium) or further obtain electrons as elemental substances (such as lead, mercury) and deposit on the surface of TiO₂ particles. However, supporting TiO₂ (or other semiconductor photocatalyst) on the surface of cellulose can improve the removal efficiency of heavy metal ions. Although the adsorption of heavy metal ions by the hydroxyl groups in cellulose contributes to the removal of metal ions, the adsorption effect is weak. For this, researchers usually take advantage of chemical modification or graft copolymerization to introduce effective adsorption active sites on the surface of cellulose [262] such as carboxyl groups, amino groups, and sulfonic acid groups. These groups can selectively recognize and capture various heavy metal ions through electrostatic attraction or complexation and chelation coordination effects [263], thereby improving the photocatalytic removal efficiency.

Interfacial solar evaporation, which utilizes the abundant sunlight to evaporate saltwater, has gained significant attention as an environmentally benign and sustainable approach. Significant efforts have been made in realizing supporting substrates that provide optimal thermal management and unimpeded water transport to foster high-performance interfacial solar evaporation. The high degree of crystallinity of CNMs provides excellent mechanical stability, and their highly dense surface functional groups enable direct deposition or adsorption of various photothermal materials [264, 265]. After subjecting MoS₂/BC bilayered aerogels to vigorous mechanical agitation, no detachment of photothermal materials from a BC matrix was observed, and the bilayered structure remained intact [266]. Besides, CNMs can be easily processed into nanomicroporous structures, and this interconnected porous structure can enhance the light absorption of the photothermal materials loaded onto these structures because of the increased light reflection and scattering within the pores. Jiang et al. prepared carbon nanotube/cellulose composite aerogels as photothermal materials. Owing to the strong hydrogen bonding between the ample hydroxyl groups of CNFs and carboxylic groups of CNTs, these materials were found to be robust. The ultrahigh porosity of the CNF aerogel and high light absorption of CNTs led to a 97.5% light absorbance within the light range from 300 to 1200 nm [267]. In addition, owing to a low thermal conductivity they provide excellent thermal insulation. The thermal conductivity of a sophisticated solar evaporator designed by Li et al. was as low as 0.06 W m⁻¹ K⁻¹. This solar evaporator achieved high evaporation efficiency, up to 85.6% under 1 sun illumination [267].

Melt blending is an important manufacturing process, and its development is important for the commercialization of cellulose nanocomposites and associated products such as automotive parts and packaging films. However, many challenges need to be overcome to enable large-scale production [287]. One challenge is the high material cost and complexity of the manufacturing process, which further increases the total cost of the material. In addition, the environmental impact of the manufacturing process is a vital consideration. Since significant amounts of energy are required to manufacture promising biobased cellulose nanocomposites, this has negative implications for product cost and commercial production. For this reason, the use of cellulosic nanomaterials in thermoplastic polymers must be extensively explored. The improvement in mechanical properties associated with the addition nanocellulose has been a research focus; however, the addition of nanocellulose can also result in other benefits. Regardless of the impact on polymer properties, nanomaterial additives must be optimally dispersed, distributed, and, in some cases, oriented within the polymer.

Generally, the manufacturing of nanocomposites is challenging since nanomaterials have a large surface energy and tend to agglomerate. CNMs are usually fabricated via top-down processes in the presence of water; although these materials initially disperse well in the water, once they have dried, redispersal is difficult owing to strong interactions that may include hydrogen bonding amongst other interactions, but ultimately leading to what is commonly termed ‘hornification’. CNFs and CNCs form gels at low concentrations (1 wt.%) because of their tendency to form networks; however, gels derived from CNCs are less viscous owing their reduced length. Melt compounding is a high-temperature nanocomposite fabrication process in which polymers are heated above their melting temperature; this method restricts the use of CNMs as some are sensitive to high temperatures. These specific properties of nanocellulose materials make their use in melt compounding with thermoplastics even more challenging [287].

Many researchers have reviewed this topic; for example, Oksman et al., Wang et al., Zheng and Pilla, and Clemons and Sabo [288‐291]. These reviews explore the developments in the extrusion processing of cellulose nanocomposites; Oksman et al. [288] focused on the processing and properties of cellulose nanocomposites, Wang et al. [289] focused on potential industrial processes, Zheng and Pilla [290] focused on melt processing with CNCs, and Clemons and Sabo focused on the wet compounding of cellulose nanocomposites [291].

In melt blending processes, various components, such as polymers, additives, and nanocellulose (reinforcing agents), are mixed in a compounding extruder where high temperatures and high shear forces melt the polymer; a specific screw configuration enables the mixing of the components. Two types of extruders are used in compounding processes: co-rotating and counter-rotating twin-screw extruders. Co-rotating extruders are preferred because they are more effective at mixing and dispersing the components; they also allow a flexible screw design, i.e., screws can be tailored to maximize dispersive, distributive mixing, or minimize shear forces to preserve fiber length. Co-rotating extruders are also more effective at removing moisture and volatiles, which is important if liquids are used as processing aids. The main challenges in the melt processing of cellulose nanocomposites are the controlled feeding of nanocellulose materials into the extruder and the dispersion and distribution of these nanomaterials in the polymer without degrading the cellulose or polymer [287, 288]. Different approaches to the melt processing and associated dispersion of CNMs have been explored, namely, liquid-assisted extrusion or wet feeding, dry feeding, and single- or multi-step processing, including master-batch processing and solid-state processing.

Despite its high Young’s modulus and tensile strength, one of the main challenges in the commercialization of nanocellulose as reinforcement for various composite applications is price. A recent market study reported that most producers will sell CNF gel at a price of ~ US$100/kg [325]. Whilst the market price for BC is not widely reported, a recent techno-economic analysis on the large-scale production of BC using an energy-efficient airlift reactor with modified Hestrin-Schramn medium estimated that the breakeven price for manufacturing BC was US$25/kg (a wet BC pellicle containing 99% water) [326]. Nanocellulose is therefore not cost-competitive in the high-volume composite market, especially when a high loading fraction of nanocellulose (> 30 vol.-%) is required to achieve significant improvement in mechanical performance [278]. Cheaper sustainable reinforcing fillers, such as wood flour and natural fibers, are available for the composite industry [327‐330]. It can be anticipated, however, that the high cost of nanocellulose could be offset by designing high value composites containing only a low loading fraction but still offering dramatically improved mechanical performance that conventional materials cannot achieve. One such area where nanocellulose could make a significant impact is their use to enhance the performance of transparent polymeric amour. The entry level for transparent amours is either monolithic acrylic or laminated polycarbonate/acrylic systems. The next choice up is glass-clad polycarbonate, which offers a significantly higher level of impact protection but at the expense of added weight and cost. There are currently no lightweight polymeric transparent armor solutions that could bridge the gap between the two levels of impact protection.

The research group at Imperial College London is currently working on developing low loading fraction nanocellulose enhanced acrylic systems to close this property–performance gap. BC is an ideal candidate in this context due to its high single nanofiber tensile properties and its similarity to the refractive indices of acrylic resins [274], an essential requirement to achieve high level of optical transparency in a composite system [331]. Furthermore, BC is biosynthesized as pseudo-continuous cellulose nanofiber network [332‐334] with a high specific surface area (> 50 m² g⁻¹) [335‐337]. Therefore, the introduction of BC into an acrylic resin could create additional energy-dissipation mechanisms, including fiber-matrix and fiber–fiber debonding, as well as fiber re-orientation and fracture. To produce optically transparent BC-enhanced poly(methyl methacrylate) (PMMA) composites, Santmarti et al. [338] first press-dried BC pellicle (Fig. 11a) into a sheet of a BC nanopaper (Fig. 11b), followed by immersion and polymerization in a cell-cast mold containing a methyl methacrylate (MMA) syrup. Such a composite construct utilizes BC nanopaper as a two-dimensional reinforcement. While the starting BC nanopaper was not transparent, the resulting PMMA composite containing the BC nanopaper was optically transparent (Fig. 11c). The light transmittance of a 3 mm thick composite was found to be 73% at a wavelength of 550 nm. Such a high level of optical transparency was due to the low loading fraction of the BC nanopaper used (1 vol.-%), the filling up of the pores within the BC nanopaper structure with PMMA, the similarity between the refractive indices of BC and PMMA, as well as the small lateral size of the BC fibrils.

Significant improvement in the composite performance was achieved even though the BC loading was only 1 vol.-%. The tensile modulus of the resulting BC nanopaper-reinforced PMMA composite was measured to be 4.2 GPa, a 24% increase over the tensile modulus of neat PMMA. The BC nanopaper-reinforced PMMA composite was also found to possess significantly improved fracture resistance and flatwise Charpy impact strength. The initial critical stress intensity factor (\({K}_{\mathrm{Ic}}\)), a measure of resistance to fracture, of the BC nanopaper-reinforced PMMA composite was found to be 1.72 MPa m^0.5; a 20% increase over the \({K}_{\mathrm{Ic}}\) of neat PMMA. A 20% increase in flatwise Charpy impact strength of the composite was also observed when compared to neat PMMA. Santmarti et al. [338] also investigated whether such improvements could also be achieved if BC was uniformly embedded within the PMMA matrix (i.e., BC as a three-dimensional reinforcement). The BC pellicle was first solvent exchanged from water though acetone into MMA, followed by polymerization in a cell-cast mold. While the resulting composite was also transparent, the mechanical performance of such a composite performed poorly when compared to neat PMMA. The \({K}_{\mathrm{Ic}}\) and flatwise Charpy impact strength were found to be only 0.7 MPa m^0.5 and 4.7 kJ m⁻², respectively; a 50% decrease in \({K}_{\mathrm{Ic}}\) and 25% decrease in flatwise Charpy impact strength compared to neat PMMA.

The mechanical performance of a composite is the volume-weighted average between the mechanical properties of the matrix and the reinforcement [339]. In theory, embedding BC uniformly within a polymer matrix should lead to a PMMA composite with improved performance as the high modulus and strength of a single BC nanofiber can be effectively utilized. However, fractographic analysis revealed the presence of significant matrix embrittlement when BC was embedded uniformly within PMMA (Fig. 12a). When BC was used as a two-dimensional reinforcement in the form of BC nanopaper, the effect of matrix embrittlement was minimized due to a reduced BC-PMMA interface. In addition to this, the reinforcing effect of such laminated composite construct stemmed from the mechanical properties of the BC nanopaper (Fig. 12b), which was found to possess a high tensile modulus and strength of 19.6 GPa and 188 MPa, respectively, as well as a high \({K}_{\mathrm{Ic}}\) of 11.9 MPa m^0.5 [339], which is comparable to a single aramid fiber; reported to be 6.63 MPa m^0.5 [340]. The use of BC nanopaper also removed the need for complicated solvent exchange or drying steps, reducing the complexity of composite manufacturing. It should be noted that the long dewatering time of BC (or nanocellulose in general) to produce (bacterial) cellulose nanopaper, which is often cited as a bottleneck, could be addressed by reducing the grammage of the nanopaper or to induce flocculation of the nanocellulose by the addition of multivalent salts [341].

Since the loading fraction of BC required to achieve performance enhancement is low, the resulting BC nanopaper-PMMA laminated composite construct is expected to be cost-competitive, increasing its market uptake for advanced composite applications. However, there are still some outstanding issues in using BC nanopaper to enhance the properties of acrylic resins for impact protection that still need to be addressed, especially the effect of moisture on the long-term durability of the laminated construct as moisture is known to cause unpredictable delamination even in commercial transparent armor laminates under ambient service conditions [342]. Furthermore, the transparency of the BC nanopaper-acrylic laminated construct at elevated temperatures is another major issue requiring further research.

Cellulose biocomposites are often considered as environmentally friendly materials. The introduction of nanocelluloses is motivated by similar arguments, yet the energy associated with defibrillation of wood fibers into nanocellulose (process energy and energy for chemicals) is very high [343]. Wood substrates offer some advantages in this respect, provided the intrinsic cellulosic nanostructures in wood can be preserved and exploited. The category of wood-polymer composites, however, is not new; wood has been impregnated with formaldehyde-based thermosets and commercialized (Impreg®, Compreg®), and monomers for thermoplastics have also been impregnated and polymerized in wood substrates [344]. The new aspect of many recent studies is that the cellulose nanostructure in the cell wall of the wood substrate has been preserved and targeted for functionalization [345].

Research on transparent cellulose biocomposites started with studies where thin cellulose nanopaper films from nanofibers were impregnated by acrylic monomers and cured [274]. The concept of transparent wood was then suggested for engineering applications [346, 347]. Wood substrates were delignified to remove the light-absorbing lignin component. In the next step, a monomer was impregnated into the substrate, followed by in situ polymerization to form a transparent wood biocomposite. Substrates other than wood have been used subsequently, for instance wood fibers [348] and bamboo [349]. From the point of view of cellulose biocomposites research, these materials are interesting because they combine structural properties (strength, stiffness) and the potential for making large structures with high optical transmittance. Optically transparent biocomposites opens a large field of materials research and opportunities for applications where semi-structural composites have photonic functions.

There are substantial challenges for transparent wood. Although processing is very similar to that of high-strength glass or carbon fiber composites (resin impregnation of a porous reinforcement), the added requirement of avoiding optical defects (voids, microcracks. cellulose agglomerates, etc.) makes it demanding. It is also difficult to achieve high optical transmittance in thick structures of high cellulose content, since light scattering inside the material is substantial. The research and materials development problems are illustrated in Fig. 13. When light reaches the transparent wood surface only a small fraction of it is reflected. Inside transparent wood, however, there is substantial light scattering, both forward- scattering and in other directions. Scattering takes place due to a mismatch in refractive index of phases, from optical defects (e.g., interfacial debonding) and possibly from Rayleigh scattering inside the cell wall. Very few ballistic photons can go through the material without scattering. Some light may also be absorbed by residual lignin. The detailed mechanisms of light propagation are still under investigation, and the results will support ongoing product development.

Yano and coworkers have prepared load-bearing biocomposites by impregnating delignified wood with polyphenol-formaldehyde thermoset precursors [351]. Li et al. [347] used NaClO₂ delignification for their transparent wood substrate to reduce light absorption from lignin whereas Zhu et al. used a kraft pulping approach [346]. Li et al. [352] further developed the possibilities by using a lignin-retaining method, where only lignin chromophores were removed.

Various polymer matrices have been used for transparent wood, where the best approaches involve impregnation of a monomer or thermoset precursor, including acrylic monomers [347, 353], epoxies [346], thiol-enes [354], and polyimide [355]. Although it is well known that the refractive index of the polymer needs to match the refractive index (RI) of the substrate to reduce scattering, the details of the scattering mechanisms are not fully understood. This understanding will contribute to materials development efforts. The need to determine the refractive indices (in two directions) for the wood substrate has led to an experimental determination of their values [356]. This method is important, since chemical modification of the wood substrate will change its RI. Recently, the scattering in real wood microstructures from RI mismatches between the wood substrate and a polymer matrix has been modeled using numerical methods and real microstructures [357]. This model can be used to predict haze, transmittance, etc., and also to analyze the effect of different wood substrate morphologies, wood content, and material thickness.

Transparent wood is of particular interest because of its contribution to sustainable development. For this reason, Montanari et al. [358] developed green procedures to modify wood substrates for improved polymer matrix compatibility and reduced moisture sensitivity. Subsequently, a new biobased acrylic polymer matrix was synthesized with a suitable RI, so that a fully biobased transparent wood biocomposite was obtained [359]. One of the challenges was to find chemical approaches which could be used in the chemically heterogeneous environment of a wood substrate.

For building materials applications, the mechanical behavior of transparent wood is important. Jungstedt et al. [360] measured a modulus of 19 GPa and a tensile strength of 263 MPa for birch/PMMA composites with 25 vol% wood substrate reinforcement. The optical transmittance of this composite was still as high as 70% at a thickness of 1.3 mm. Basic optical properties have also been investigated, including anisotropic scattering [361]. Vasileva et al. [362] reported on interesting polarization effects in transparent wood, where the effect depended on the wood species used for the reinforcing substrate. Haze was discussed by Li et al. [363], who pointed out favorable effects from high haze. Haze is defined as the proportion of forward scattered light to the total forwarded light. Broadband high haze (> 80%) can create a uniform and consistent light distribution for comfortable living environments [363].

The thickness of transparent wood will have strong effects on optical transmittance, which is important for building applications. Chen et al. [364] investigated this effect since the well-known Beer-Lambert law in principle is not applicable to scattering materials. A model was developed to predict thickness effects, as well as a procedure to determine parameters in this model for specific materials. In a previous study, Li et al. [365] demonstrated how acetylated wood substrates showed improved transmittance so that thicker structures could be prepared. Acetylation will reduce the extent of optical defects, such as wood/polymer interface debonding and cracks. Acetylation also facilitates the amount of acrylic monomer diffusing into the wood cell wall and may influence the refractive index of the wood substrate. It has been recently demonstrated how the cell wall in wood/PMMA composites prepared by solvent-assisted processing was impregnated by the MMA monomer so that PMMA becomes distributed at the nanoscale inside the wood cell wall [366].

The processing of very large structures is desirable and a requirement for many building applications. A key problem is that the permeability of liquid resins in wood substrates is often quite low. A practical consequence is therefore that thick structures are difficult to impregnate without the formation of significant optical defects. The main strategy suggested is to use lamination of veneer layers, as was demonstrated for plywood structures [367] and subsequently in several other studies [368].

Figure 14 suggests potential applications of transparent wood in smart buildings. High haze is a favorable feature, which allows uniform light distribution and can provide indoor privacy. Although high haze is beneficial for some applications, the possibility for low haze extends the range of product design possibilities. Li et al. [365] reported a smart window concept where indoor privacy was optically tuneable through tailoring of transmittance and haze in thick structures based on clear acetylated transparent wood. Transparent wood is an excellent base for further functionalization by the addition of functional particles and/or polymers. Montanari et al. [369] added phase change materials (PCM) to transparent wood, for heat storage purposes. Energy was adsorbed by the PCM melting during heating. Cooling resulted in heat release by PCM crystallization. Other examples include the addition of Cs_xWO3 particles for heat-shielding since transmission in the near-IR range was reduced [370], and Fe₃O₄ nanoparticles were used for magnetic transparent wood to provide electromagnetic interference shielding [371]. Recently, structural color has also been combined with optical transmittance. Höglund et al. [372] precipitated metal nanoparticles inside the nanoporous wood substrate. This material was then impregnated with a refractive index-matched thiol-ene resin, followed by curing. The resulting composite showed structural color from the plasmonic nanoparticle effect, with only a minor reduction in the optical transmittance. Mechanical properties were well preserved. Another smart window concept is based on the integration of optoelectronic devices. Lang et al. [373] thus prepared electro-chromic windows by building conjugated-polymer-based devices on transparent wood substrates. The device demonstrated vibrant magenta-to-clear switching (∆E* = 43.2) between -0.5 and 0.8 V, with a device contrast of 38 ∆%T at a 550 nm wavelength.

Another interesting modification possibility is to use stimuli-responsive particles. Li et al. [374] first reported transparent wood from luminescent particles in the form of quantum dots (QDs). Diffused luminescence was revealed under UV irradiation, where the luminescent color obviously depends on the QDs. Other stimuli-responsive properties have been developed, for instance by impregnation of thermo- and photochromic microcapsules [375], and photoluminescent properties through 1,3,3-trimethylindolino-60-nitrobenzopyrylospira-based photoresponsive molecules [376]. One area of applications is the detection of changes in environmental conditions. Liu et al. [374] reported tunable room-temperature phosphorescence through carbon dots doping for formaldehyde gas detection. The main research and development challenge is to obtain a uniform distribution of particles or active polymers despite the tendency for aggregation during processing. In situ particle synthesis [372] is one route to reduce the problems, compared with direct infiltration.

For solar cells, high haze is favorable due to increased length of the light path in the active layer, leading to improved efficiency. Therefore, transparent wood was used as light diffuser layer by direct attachment to a commercial solar cell [377]. Li et al. [378] instead fabricated a perovskite solar cell on a transparent wood substrate, for the purpose of energy positive buildings. Here, the main challenge was the introduction of the conductive layer since high optical transmittance, combined with good interlayer bonding, is required.

Durability and moisture stability are important for load-bearing building applications. One route is cell wall bulking of the wood substrate by acetylation [365] or other anhydrides [358]. This means that also the transparent wood biocomposites, based on such substrates [359, 379], will be durable in a moist environment, although the high cost of polyimides is a limitation. For long-term durability, the yellowing problem known for paper needs to be avoided. Attempts to address this potential problem have been reported [380].

Transparent wood is also attractive for wave-guiding photonics applications. Vasileva et al. [381] reported lasing from transparent wood impregnated by luminescent rhodamine 6G molecules. It was found that each fiber functioned as an optical resonator. The output signal is the collective contribution of the fiber-based resonators, which is broadly due to the fiber dimension variations and structural heterogeneity.

Optically transparent biocomposites based on cellulose or wood is a class of semi-structural materials which may combine load-bearing properties with eco-friendly characteristics, high optical transmittance, and photonics functionalities. The optical transmission criterion makes it necessary to improve micro- and nanostructural control during processing of cellulosic biocomposites. In addition, multifunctional biocomposites open new possibilities for applications of wood materials.

One of the most intriguing properties of nanocellulose is its intrinsic molecular chirality and how it can impact its behavior at much larger scales [382], which leads to the ability of CNCs to form functional materials with remarkable optical properties. The ability of CNCs to self-assemble into optically functional materials requires good dispersion in a solvent, usually water, and special care has to be taken to allow for the self-organization of the liquid crystalline order, followed by a removal of the solvent that preserves the acquired organization [383, 384]. While these conditions are usually easily met when casting a pure dispersion in a dish, modifying the formulation or the assembly conditions to gain further functionality without compromising on the assembly abilities represents the main challenge. The formation of colored films using CNCs has been introduced by Revol et al. [385, 386] and continues to be an active field of development, either to increase functionalities of such photonic films or to develop their potential as interference pigments by investigating scaling-up options. In this section, a few recent examples of optical functional materials are reviewed, leading to significant development in these two directions.

By tuning the assembly and type of CNMs it is possible to fully control light transport so to obtain from transparent to haze to highly scattering materials [419]. Since the first report of transparent nanocellulose films (also named ‘cellulose nanopaper’) [420], they have emerged as a promising flexible and green substrate candidate for solar cells. This is because of their better optical, mechanical, and barrier properties and surface smoothness compared with standard paper [421, 422]. Hu et al. [423] initially demonstrated the direct fabrication of organic solar cells (OSCs) on a CNF film substrate that showed a PCE of 0.4% (Fig. 18a), and the corresponding I-V curve is displayed in Fig. 18b. Despite the poor device performance, this work first indicated that a CNF film was an attractive substrate for mechanically supporting OSCs. Meanwhile, the same group proposed an optical haze for a transparent CNF film (Fig. 18c & d) and predicted its potential application in solar cells as a functional light management layer.

Previous work had focused on the uses of nanocellulose films as a substrate material for solar cells. A variety of strategies were adopted to enhance the PCE of solar cells on nanocellulose film substrates. These have included the use of CNCs [424] and a mixture with other materials (such as acrylic resin and cellulose derivatives) [425], surface modifications of nanocellulose film [424], water-free manufacturing techniques [426], the use of novel active materials [427], and exquisite design of the cell configurations [427]. However, at present, the PCE of solar cells on nanocellulose film is still less than 5%, which is much lower than that of devices fabricated on plastic films; these indicate a PCE of over 21% on a laboratory scale [428].

To expedite the practical applications of solar cells on nanocellulose film, constant improvement in their performance should be given high priority. Therefore, future endeavor should focus on rationally engineering barrier properties, water resistance, weather durability, and surface roughness of the order of only a few nanometers over surface areas in the millimeter or even centimeter range. For instance, the mechanical isolation of CNFs from the cell wall of natural cellulose fibers produced microscale fibril bundles and debris in an aqueous suspension, thus increasing the surface roughness of the substrate, significantly deteriorating the device’s performance, or even causing device failure.

Transparent nanocellulose film with high optical haze can go beyond the mechanically supporting substrate application and is becoming a functional light management layer for solar cells. Generally, high transparency and high transmission haze (light scattering) are mutually exclusive. However, nanocellulose films not only exhibit a ~ 90% transparency, but have also showed forward built-in light scattering behavior (Fig. 18a) [423]. These specific optical properties have allowed nanocellulose films to manipulate the propagation direction of transmitted light, which has extended applications toward solar cells as advanced light management layers to improve the efficiencies of light coupling into devices.

The interactions of visible light with nanocellulose plays a significant role in the optical properties of nanocellulose film. Theoretically, visible light can easily pass through nanocellulose films with negligible light scattering because the fibril diameters (3.5 ~ 30 nm) are much smaller than the wavelength (400-900 nm) of visible light. Therefore, nanocellulose films can display a transparent and clear appearance. However, optical haze is observed in most reported nanocellulose films (Fig. 18c&d). This abnormal phenomenon is primarily due to the existence of scattering particles (e.g., microscale fibril bundles and debris) derived from the incomplete mechanical homogenization of wood fibers.

The presence of micro-sized scattering elements gives rise to refractive index inhomogeneities and increased surface roughness of the as-prepared nanocellulose films, thereby increasing the scattering efficiency and finally indicating an improvement of optical haze. According to Mie theory, when the scattering particles have a diameter equal to or larger than the wavelength of the incident light, most of the transmitted light will be scattered along the incident direction.

Transparent nanocellulose films show forward scattering of light (Fig. 19a). As we can see from Fig. 19b, c and d, the original CNF suspension (right-most image in Fig. 19b) has a hazy appearance due to the occurrence of micro-sized fibril bundles and debris (Fig. 19d) serving as scattering particles to enhance the transmission haze (Fig. 19c). However, after removing these scattering materials by a centrifugation procedure, the nanocellulose suspension indicated a clear appearance and the green laser could propagate through the suspension with a narrow-angle scattering (middle vials shown in Fig. 19b and c) [429]. Purified nanocellulose showed a uniform distribution of fibril diameters at a nanoscale level, which contributed to the reduced inhomogeneities and surface roughness of the nanocellulose film that suppressed wide-angle light scattering.

Taking inspiration from previous works, Fang et al. [430] reported a highly transparent nanostructured paper with a high haze using micro-/nanocellulose, exhibiting a transparency of > 90% and a transmission haze of ~ 60% at 550 nm (Fig. 20a). Through 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidization of wood fibers, micro-/nanocellulose was obtained, where the microscale cellulose fibers functioned as a light scattering source to increase the optical haze while the nanocellulose worked as a filling matrix to improve the transparency. They also demonstrated the use of this nanostructured paper in organic solar cells as a light management layer and a 10% increase of PCE was achieved (Fig. 20b). Since then, microscale cellulose fibers have been the main raw material to prepare highly transparent cellulose film with high haze by bottom-up methods (e.g., filtration [431], impregnation [432, 433], and surface dissolution of fibers) due to their strong light scattering behavior [434].

Rechargeable batteries and supercapacitors are the two most popular types of electrochemical energy storage devices, and they have attracted increasing attention and enjoyed great success both in academic research and commercialization over the past few decades [416, 435‐437]. With a high aspect ratio, and abundant surface functional groups to interact with ions, nanocellulose is an ideal starting material for energy storage devices based on ion transport [438, 439]. It has been widely investigated as various important functional components in batteries and supercapacitors, such as current collectors, binders, electrolytes/separators, and electrodes [439‐442]. The diverse structural tunability of nanocellulose (e.g., pores, fibril orientation, fibril diameter/length, surface functional groups, surface charge, surface energy, and surface wettability, degree of crystallinity, crystal phase structure, etc.) is particularly attractive for the design of high-performance functional components of batteries and supercapacitors. A key design principle is to meet the specific parameters depending on the functions of each component of the device. For example, improving the electrical conductivity of nanocellulose is critical for current collector applications, while separators must be electrically insulating. The combination of a wide potential window, high ionic conductivity and good mechanical properties are critical for developing high-performance nanocellulose-based electrolytes and separators.

As a common component in batteries and supercapacitors, the properties of binders directly affect the properties of electrodes, particularly their mechanical and electrochemical performances [441‐443]. Polyvinylidene fluoride (PVDF) has been one of the most widely used binders in the most recent batteries and supercapacitors. This material, however, requires the use of volatile and toxic/hazardous solvents for processing. Alternatively, biomass-based binders such as nanocellulose, and its derivatives, are ‘greener’ and safer to use, and therefore are attracting increasing interest from the academic and industrial communities [444]. For example, carboxymethyl cellulose (CMC), as a derivative of the linear polymeric cellulose with the substitution of hydroxyl groups for anionic carboxymethyl groups, is widely used as a water-soluble binder in both the cathodes and anodes of batteries and supercapacitors [441].

Nanocellulose itself can also be used as binder for both cathode and anode electrodes. The high aspect ratio and tunable rheological properties of nanocellulose are desired for electrode manufacturing. Electroactive electrode particles can be wrapped by the high-aspect-ratio nanocellulose fibrils, forming a mechanically robust network. Moreover, at a finer scale, the abundant hydroxyl groups of nanocellulose fibrils can interact with the electroactive electrode particles, providing an excellent binding effect. Kuang et al. [445] demonstrated the excellent binding effect of nanocellulose (or cellulose nanofiber, CNF) in their recent work (Fig. 21). They first prepared a conductive CNF by mixing with carbon black (CB), which was then used as a binder to ‘glue’ the electroactive electrode (lithium iron phosphate, LFP) particles, forming a 3D interconnected porous foam (Fig. 21a). This 3D interconnected porous foam was further pressed to increase the density of the nanopaper electrode (Fig. 21b). A TEM image of the nanopaper showed that the LFP particles were glued by the CNF/CB network, which provided pathways for ion and electron transport (Fig. 21c). The nanopaper electrode demonstrated not only improved areal capacity over a conventional LFP electrode with similar thickness, but also excellent flexibility (Fig. 21d and e). Alternatively, nanocellulose can also be used as a viscosifier to prepare printable inks for printed electrode fabrication owing to its unique characteristics such as rich hydroxyl groups, negative zeta potential, one-dimensional (1D) fibrous structure, and chemical functionalities [440, 446, 447]. In these printed electrodes, nanocellulose not only acts as a viscosifier during the printing process, but also as a binder to ‘glue’ the electroactive electrode particles in the dried state.

Nanocellulose-based separators have been widely studied and developed. For instance, mesoporous CNC membranes with high surface areas were prepared by Gonçalves et al. [448] as the separator for environmentally safer lithium-ion batteries (Fig. 22). The obtained nanocellulose-based separators exhibited outstanding wettability when soaked with conventional ester-based and ionic liquid-based electrolytes. Meanwhile, the three-dimensional porous structures formed by nanocellulose can shorten the ion diffusion pathway, thus obtaining a competitive ionic conductivity of 2.7 mS cm⁻¹. Good compatibility of the interface between the electrode and electrolyte can also be obtained, in comparison with commercial separators like glass fiber or polypropylene (PP)/polyethylene (PE) membranes. Coupled with the LiFePO₄ cathode, the assembled lithium-ion batteries can deliver a specific capacity of 122 mAh g^–1 at a 0.5 C and an excellent rate capacity of 85 mAh g^–1 at 2.0 C, indicating the important role of the three-dimensional porous structures formed by nanocellulose. Meanwhile, the mechanical properties of nanocellulose-based separators are also superior to those of commercial separators according to the literature. In addition, there have been some companies like the Nippon Kodoshi corporation who have fabricated battery separators made from 100% cellulose which exhibit good heat resistance, exceptional porosity, and high liquid retention rates [449]. Therefore, by replacing conventional materials with nanocellulose-based separators, the development of real eco-friendly energy storage devices could be achieved in the near future.

As well as separators, nanocellulose can also be used as flexible substrates. Combined with a coating of conductive nanomaterials (carbon nanotubes, graphene, etc.) on the nanocellulose, free-standing electrodes can be obtained for energy storage devices such as supercapacitors [450].

When employed as the precursors, nanocellulose can also be converted into sustainable carbon materials for next-generation energy storage devices as well. For example, Li et al. [451] prepared hollow hard carbon microtubes derived from renewable cotton as high-performance anode materials for the next-generation of sodium-ion batteries, where the resulting hard carbon anodes at 1300 °C delivered an outstanding sodium-ion capacity of 315 mAh g⁻¹ at 0.1 C and a good rate capability as a result of their unique turbostratic structures. In addition, Xu et al. [452] developed sustainable hard carbons from CNCs at a lower carbonization temperature of 1000 °C for use in sodium-ion batteries. The low-cost hard carbon anodes derived from nanocellulose displayed a reversible specific capacity of 256.9 mAh g⁻¹ at a current density of 0.1 A g⁻¹, which was superior to the hard carbons obtained from cellulose microfibers at the same carbonization temperature because the layer-by-layer self-assembly of CNCs during the drying process formed a relatively low surface area to suppress the formation of a solid electrolyte interphase (SEI). The sodium-ion capacity of the hard carbons can compete with commercial graphite anodes for lithium-ion storage by consuming less energy. Sodium-ion batteries have also widely aroused interest from the battery community as promising successors to lithium-ion batteries for grid energy storage based on abundant sodium resources [453]. Therefore, nanocellulose-derived sustainable hard carbons perfectly meet the environmental and electrochemical requirements of eco-friendly sodium-ion batteries.

Meanwhile, all-cellulose-based devices have also been achieved inspired by the hierarchical building blocks of natural cellulose (Fig. 23) [452]. By coupling with the cellulose microfiber-derived porous carbon cathodes using the hydrothermal process and subsequent chemical activation, an all-cellulose-based sodium-ion hybrid capacitor has been assembled [452]. This uses a modified cellulose-based gel electrolyte instead of using oil-based feedstocks, and its capacity reached 58.2 mAh g⁻¹ at 0.2 A g⁻¹ [452]. The energy density of this all-cellulose-based sodium-ion capacitor reached 139 Wh kg⁻¹ at a power density of 478 W kg⁻¹, which can bridge the gap between high-energy batteries and high-power supercapacitors [452].

As we can see, great efforts have been made to develop the next generation of sustainable energy storage devices based on nanocellulose. There are still some challenges that remain to be resolved. For the commercialization of energy storage devices based on nanocellulose, a large-scale production platform is desirable to be established with decreased energy consumption [454]. In the meantime, how to rationally control the chemical and physical properties of nanocellulose to further enhance the electrochemical performance of devices needs to be considered at a scaled-up level [454]. In addition, most of the development of nanocellulose research in batteries and supercapacitors in the past few decades has been limited to sizes no smaller than the elementary fibril level, while the fundamental science and technologies at the molecular level of nanocellulose deserve further exploration [455]. All in all, the emerging applications of nanocellulose are of obvious benefits to energy storage toward carbon neutrality, but more efforts are needed to overcome existing shortcomings of technologies enabled by nanocellulose.