# Nanomaterials in Bone Regeneration
## 4 Nanoparticles
### Carbon Nanoparticles  
Carbon-based nanomaterials appear to be very promising for their mechanical strength and biocompatibility. The carbon can occur in several structurally different modifications, such as graphene, carbon nanotubes (CNTs), fullerenes, carbon dots (CDs), carbon nanofibers, nanodiamonds, and their derivatives [179]. In bone tissue engineering, nanomaterials based on graphene and carbon nanotubes seem to be among the most prominent [180]. Nanodiamonds (NDs) are carbon-based nanoparticles typically 4-10 nm in diameter and sp\({}^{3}\) hybridization of crystal lattice shape with the possibility of introducing a number of different functional groups on their surface. Several in vitro studies have shown significantly lower toxicity to diverse cell lines than graphene or CNTs [181; 182]. NDs can be used in the bone-anchoring stems of permanent bone implants in order to improve their interaction with the surrounding bone tissue. This beneficial characteristic can be assumed not only due to their excellent mechanical and chemical resistance but also due to their surface nanoroughness and surface nanotopography. It was stated that these properties could stimulate the adhesion, growth, and maturation of osteoblasts, as well as the differentiation of bone progenitor cells towards the osteoblastic phenotype [183]. Thanks to the ability to bind proteins, nanodiamond surfaces can serve as carriers for bioactive molecules (e.g., BMP or FGF) which promote bone formation. NDs and angiopoeitin-1 were used for the functionalization of bone implants to improve vascularization and bone regeneration [184]. A prenylated flavonoid glycoside icarini functionalized NDs (ICA-NDs) promoted osteogenic capacity. In vitro cell studies showed that ICA-NDs treated osteoblastic cells MC3T3-E1 greatly increased the production of osteogenic markers, including ALP, RUNX2, Col1, and OPN, and even calcium content compared to only NDs treated cells [185]. Carbon nanotubes (CNTs) are cylindrical molecules of carbon atoms in a hexagonal arrangement. Two types of CNTs can be distinguished, single-walled (SW-CNTs) and multi-walled carbon nanotubes (MW-CNTs), which are one of the strongest materials. These nanotubes are thermally and chemically very stable, contain carbons in sp\({}^{2}\) hybridization, and are prepared by layering individual graphene sheets joined by rolling. The length, diameter, and surface modification of nanotubes are important for their interaction with cells [186]. Nanocomposite scaffolds in which CNTs are used as a filler of HAp show a significant improvement in fracture toughness and tensile strength. Because SW-CTNs have a diameter close to the size of the collagen triple helix, which is between 0.7 and 1.5 nm, they are preferably used to prepare scaffolds promoting HAp growth and nucleation. The study showed that SW-CNTs/HAp scaffolds with different functional groups have a higher solubility in water and significantly affect the formation of HAp [187]. By adding nanotubes, especially MW-CNTs, to the porous bioglass foam, a nanocomposite scaffold with a suitable roughness for cell attachment was created, which proved to be highly bioactive and suitable for use in bone tissue regeneration. Due to the presence of MW-CNTs, the interaction among cells during their adhesion was significantly improved, osteoclastogenesis was suppressed, and proliferation of osteoblasts was promoted [188]. Due to the fact that CNTs have a highly hydrophobic character and insufficient solubility in water, their dispersion into the CS matrix eliminated this disadvantage and improved the mechanical properties of the CS itself. This nanomaterial is suitable for use in bone tissue defect regeneration and other biomedical applications. The scaffold prepared in this way showed high biocompatibility and biodegradability. The formed pores were suitable for penetration, proliferation, and differentiation of MSCs [189].