**1. Introduction**

Nanocellulose/nanocarbon composites are hybrid materials containing cellulose and carbon nanoparticles. Integration of nanocarbon materials with nanocellulose provides functionality of nanocarbons, using an eco-friendly, low-cost, strong, dimension-stable, nonmelting, nontoxic and nonmetal matrix or carrier, which alone has versatile applications in industry, biotechnology and biomedicine (for a review, see [1,2]). In addition to its advantageous combination with nanocarbon materials, nanocellulose is an appealing material for biomedical applications due to its tunable chemical properties, nonanimal origin, and resemblance to biological molecules in dimension, chemistry and viscoelastic properties, etc. [3–6].

Cellulose nanomaterials include cellulose nanofibrils (CNFs) and cellulose nanocrystals (CNCs) [3]. CNFs are manufactured using either a bottom-up or a top-down approach. The bottom-up approach involves bacterial (*Gluconacetobacter*) biosynthesis to obtain bacterial cellulose (BC), while, in the top-down method, cellulosic biomass from plant fibers is disintegrated into smaller CNFs [7] that contain amorphous and crystalline regions [3]. The fibrillation of cellulose is achieved using mechanical forces, chemical treatments, enzymes or combinations of these. After fibrillation, the width of CNFs is typically between 3 and 100 nm, and the length can be several micrometers [8]. Separation of the crystalline parts from the amorphous regions of the fibers or fibrils to obtain CNCs typically requires acid hydrolysis, which destroys the amorphous regions [9]. Entangled CNFs are longer, while CNCs possess shorter needle- or rod-like morphology with a similar diameter and a more rigid molecule due to their higher crystallinity [3,9]. In general, the properties of nanocelluloses are variable and depend on their origin, type, processing, pretreatments and functionalization. Integration with other materials, as well as fabrication of the final product, further affects the properties of the resulting composite or hybrid structure.

Carbon nanoparticles include fullerenes (usually C60), graphene-based particles (graphene, graphene oxide, reduced graphene oxide, graphene quantum dots), nanotubes (single-walled, double-walled, few-walled or multi-walled) and nanodiamonds (for a review, see [10–20]). The most frequently used nanocellulose/nanocarbon composites contain graphene or carbon nanotubes, while composites of nanocellulose with nanodiamond, and particularly with fullerenes, are less frequently used. Other carbon nanostructures, which are less frequently used in nanocellulose/nanocarbon composites, at least for biomedical applications, include carbon nanofibers [21–25], carbon quantum dots [26–28] activated carbon [29,30] and carbon black [31–33].

Nanocellulose/nanocarbon composites can be prepared in one-dimensional (1D), two-dimensional (2D) or three-dimensional (3D) forms. 1D composites are represented, for example, by C60 fullerenes grafted onto cellulose nanocrystals that have undergone amination or oxidation [34,35]. 2D composites are represented by films, which can be self-standing or supported, i.e., in the form of free-standing membranes [29,36–41] or in the form of coatings deposited on bulk materials [33,42]. The films can be formed by depositing carbon nanoparticles on a nanocellulose layer [43,44]. More frequently, however, they are fabricated from aqueous dispersions of nanocellulose and carbon nanoparticles [39,42]. It should be pointed out that cellulose nanoparticles are excellent dispersive agents for carbon nanoparticles, as they prevent the aggregation of these nanoparticles and maintain them in long-term stable homogeneous suspensions without the need to subject them to chemical functionalization [45,46]. Suspensions of cellulose and carbon nanoparticles are also starting materials for the creation of 3D nanocellulose/nanocarbon composites in the form of aerogels, foams or sponges [45,47–50]. In addition, composite 3D scaffolds, especially for tissue engineering and for regenerative medicine, can be fabricated by 3D printing using bioinks based on cellulose and carbon nanoparticles [51,52]. Both 2D composites and 3D composites can also be created by adding carbon nanoparticles to cultures of cellulose-producing bacteria, such as *Gluconacetobacter xylinus*. These nanoparticles are then incorporated into bacterial nanocellulose in situ during its growth [53–57]. Another approach is via the electrospinning or wet spinning of solutions containing cellulose and carbon nanoparticles [58–60].

Nanocellulose/nanocarbon composites exhibit several more advantageous properties than materials containing only cellulose nanoparticles or only carbon nanoparticles. Adding carbon nanoparticles to nanocellulose materials can further increase their mechanical strength [59,61]. At the same time, the presence of nanocellulose promotes the flexibility and stretchability of the materials [52,62,63]; for a review, see [64]. Adding graphene, carbon nanotubes or boron-doped diamond nanoparticles endows nanocellulose materials with electrical conductivity [39,50,57,65,66]. Other advantageous properties of nanocellulose/nanocarbon composites include their thermal stability [67–69], tunable thermal conductivity and optical transparency [48,57,70], intrinsic fluorescence

and luminescence [26,71,72] photothermal activity [56], hydrolytic stability [61], nanoporous character and high adsorption capacity [49,61]. Nanocellulose/nanocarbon composites can therefore be used in a wide range of industrial and technological applications, such as water purification [22,29,43,49,54,56, 61,73–76], the isolation and separation of various molecules [22,74,77–79], energy generation, storage and conversion [21,23,44,47,64,80–85], biocatalysis [86], food packaging [67–69,87], construction of fire retardants [48], heat spreaders [70] and shape memory devices [38,88–90]. These composites are also used as fillers for various materials, usually polymers, in order to improve their mechanical, electrical and other physical and chemical properties [67–69,87,91].

In addition, nanocellulose/nanocarbon composites are promising for biomedical applications, though these applications are less frequent than industrial applications. Biomedical applications include radical scavenging [34,92], photothermal ablation of pathogenic bacteria [93], photodynamic and combined chemophotothermal therapy against cancer [35,94], drug delivery [16,28,65,72,95–97], biosensorics [31–33,63,66,71,91,98–104], and particularly tissue engineering and wound dressings. Hybrid materials containing nanocellulose and nanocarbons stimulated the growth and osteogenic differentiation of human bone marrow mesenchymal stem cells [37,59]. They provided good substrates for the attachment, growth and differentiation of SH-SHY5Y human neuroblastoma cells [51] and PC12 neural cells, particularly under electrical stimulation [105]. They enhanced the outgrowth of neurites from rat dorsal root ganglions in vitro and stimulated nerve regeneration in rats in vivo [106]. They also promoted the growth of vascular endothelial cells, enhanced angiogenesis and arteriogenesis in a chick chorioallantoic membrane model [107], and improved cardiac conduction when applied to surgically disrupted myocardium in dogs [52]. In addition, these materials supported the growth of human dermal fibroblasts [108] and mouse subcutaneous L929 fibroblasts [58,62], promoted wound healing in vivo in mice [109] and showed an antibacterial effect [30]. These materials are therefore promising for bone, neural and vascular tissue engineering, for creating cardiac patches and for advanced wound dressings. The biomedical applications of nanocellulose/nanocarbon composites are summarized in Table 1.


*Nanomaterials* **2020**, *10*, 196

**Wound** 

**dressing**/**healing**

C60 derivatives [118]

in vitro [108]; mouse

cells in vitro, wound dressings delivering

doxorubicin [97]

dressing [30]

model in vivo [109]

This review summarizes recent knowledge on the types, properties and applications of nanocellulose/nanocarbon-based hybrid materials, particularly in biotechnology, biomedicine and tissue engineering, and reports on the experience acquired by our group.
