*4.1. Characterization of Carbon Nanotubes*

Carbon nanotubes (CNTs; Figure 3a) are tubular structures formed by a single cylindrical graphene sheet (single-walled carbon nanotubes, referred to as SWCNTs or SWNTs) or several graphene sheets arranged concentrically (multiwalled carbon nanotubes, referred to as MWCNTs or MWNTs, which also include double-walled CNTs (DWCNTs [16]), and few-walled CNTs (FWCNTs [17]). Carbon nanotubes were discovered as a by-product of fullerene synthesis, and were first described by Iijima et al. (1991) [138]. CNTs have a high aspect ratio (i.e., length to diameter ratio) and thus a relatively large surface area. Their diameter is on the nanometer scale (e.g., from 0.4 nm to 2–3 nm in single-walled nanotubes), but their length can reach several micrometers or even centimeters. Due to these properties, CNTs are suitable candidates for hydrogen storage, for the removal of contaminants from water and air, and also for drug delivery. CNTs have excellent mechanical properties, mainly due to the sp2 bonds. The tensile strength of SWCNTs has been reported to be almost 100 times higher than that of steel, while their specific weight is about six times lower. CNTs can therefore be used for reinforcing various synthetic and natural polymers for industrial and biomedical applications, e.g., for hard tissue engineering. When added to a polymer matrix, CNTs can resemble inorganic mineral nanoparticles in the bone tissue, and they can form nanoscale irregularities on the surface of 2D materials and in the pores of 3D materials, which improve the cell adhesion and growth. CNTs are electrically conductive and enable electrical stimulation of cells, which further improves the adhesion, growth and differentiation of cells (for a review, see [10–14,18,139]). However, free CNTs can be cytotoxic, which is attributed to their ability to cause oxidative damage, and also to their contamination with transition metals (e.g., Fe, Ni, Y), which serve as catalysts during CNT preparation. Methods for producing metal-free CNTs have therefore been developed, e.g., arc-discharge evaporation of graphite rods [139].

**Figure 3.** Scheme of multi-walled and single-walled carbon nanotubes (**a**) and of the preparation and structure of nanocellulose/carbon nanotube composites (**b**).

CNTs also resemble CNFs from the point of view of their morphology and their mechanical properties. For example, highly crystalline, thick CNFs derived from tunicates exhibited mean strength of 3–6 GPa, which was comparable with commercially available MWCNTs. However, the mean strength of other types of CNFs is lower; for example, in wood-derived CNFs the mean strength ranged from 1.6 to 3 GPa [140]. CNTs therefore improve the mechanical strength of nanocellulose/CNT composites, and endow them with electrical conductivity, similarly as graphene. As a result, nanocellulose/CNT composites are used in similar industrial and biomedical applications as nanocellulose/graphene composites, e.g., water purification, energy generation, storage and conversion, filling polymeric materials, constructing sensors and biosensors, drug delivery, cancer treatment, electrical stimulation of tissues, and tissue engineering.
