# Nanomaterials in Bone Regeneration
## 3 Nanostructured Materials
### Titanium  
Titanium is a commonly used material for both dental and orthopedic implants. Due to excellent properties such as high tensile strength, durability, high corrosion resistance, resistance to body fluids, and high biocompatibility, titanium and titanium alloys remain among the most appealing and valuable materials. The strength of titanium is too low for implants to be used in load-bearing situations. The addition of alloying elements, such as aluminum and vanadium, allows a significant improvement of the mechanical properties of titanium. Despite the good static and fatigue strength of Ti-6 Al-4V alloy, elevated concentrations of potentially cytotoxic alloying ions were detected in the tissues, serum, and urine [70]. An alternative approach to solve the ion release is to enhance the mechanical properties of pure titanium by nanoscale grain refinement. The studies note that conversion of the titanium into a nanostructured state is accompanied not only by the improvement of its physical and mechanical characteristics but also by the improvement of cell response to the interface between the material and biological surroundings [71, 72]. Titanium nanostructures provide a large surface area and give enhanced antibacterial characteristics, osseointegration, and protein interaction, all of which are beneficial to medical implants. In 2015, [PERSON] et al. tested the biocompatibility of nanostructured titanium with different grain sizes to human fibroblast cell line HFL1 and human osteoblast cell line hFOB 1.19. The results showed that the material with the smallest grain size, in the case of osteoblasts, had the best viability and confirmed that the smaller the grain size, the better the viability [73]. [PERSON] et al. evaluated how certain parameters of material can influence the interaction of nanostructured titanium and two types of human tissue cells, human osteosarcoma SaOS-2 and adipose-derived mesenchymal stem cells (ADMSCs). They found enhanced viability of both cell lines on samples with grain refinement by the equal channel angular pressing method. A positive linear correlation between nanoscale surface roughness and cell viability was found in the 0 nm to 5 nm range of stochastically varying nanoscale roughness [74]. Additionally, they noted that subsequent surface treatment techniques of bulk nanostructured titanium could significantly reduce the positive effect of the refined structure. This is in concordance with our results on laser surface treatment of nanostructured titanium implants [75]. In the work of [PERSON] et al., ultrafine-grained (UFG) titanium samples were subjected to a micro-arc oxidation surface treatment. This surface treatment effectively reduced the hemolysis rate, extended the dynamic coagulation time, reduced the amount of platelet adhesion and the degree of deformation, and enhanced blood compatibility [76]. The use of highly hydrophilic solutions may positively change the surface, as estimated by [PERSON] et al., by etching the nanostructured titanium in acidic and basic Piranha solution. The variation of the etching medium and time leads to micro-, nano-, or hierarchical structures on the surface. A significant difference between surfaces etched with basic and acidic solutions was caused by a less distinct oxidation process in the basic solution compared to the acidic one [77]. According to [PERSON] et al., the behavior of cells adhered to titanium implants is influenced by different nanotopography of the surface. They used sonochemical treatment in alkaline solution to prepare disordered mesoporous titanium and anodic oxidation to prepare ordered nanotubes. The results showed that cell morphology is affected by surface nanotopography, where cells display a polygonal shape and spread on mesoporous surfaces and an elongated, polarized shape on tubular surfaces [78]. A relatively small number of studies deal with differences in osseointegration of nanostructured titanium implants in vivo. The work of [PERSON] et al.