## 4 Bone Grafts and Substitutes for Bone Repair
### Synthetic Bone Substitute Materials
#### 4.2.3 Composite Bone Substitute Materials
The reasons lie in its flexibility to select the adequate materials for the matrix and the reinforcing precursor, the reinforcing content and the particle size of both components. Another important aspect is the selection of the sintering processes and parameters, providing the physical chemical conditions for the precursor's reactions and leading to the optimal reinforcing effect of the matrix. Recent research regarding advanced materials for alloplastic bone substitutes highlighted the PM biocomposites based on submicronic HA particles reinforced by titanium hydride micrometric powders (100-150 um; 25% wt.) as the precursor. The TiH\({}_{2}\) dehydrogenation reaction [89] occurring during the two-step sintering (TSS) treatment leads to the biocompatible TiO\({}_{2}\)-rutile allotropic phase synthesis [90; 91], which acts as a reinforcement for the HA/TiO\({}_{2}\) sintered biocomposite (Figure 7). Figure 6: Schematic representation of the micromechanical interlock model of the interface between the HA and CNTs, providing a physical bonding at the interface level (red curves). Figure 7: SEM micrograph of HA/TiO\({}_{2}\) biocomposite processed by TSS technology (Courtesy of Prof.). [PERSON], University of Craiova, Faculty of Mechanics). On the other hand, the hydrogen releasing during dehydrogenation reaction determines a specific porosity; thus, alloplastic bone substitutes or trabecular bone tissues may be designed using this technological approaching corroborated with the reinforcing content. Moreover, the PM biocomposites' porosity could be increased (30-60%) using different foaming agents such as calcium carbonate (CaCO\({}_{3}\)) and ammonium hydrogen carbonate (NH\({}_{4}\)HCO\({}_{3}\)) in the initial powder mixture. The TSS technology allows for the monitoring of the foaming reactions; thus, each foaming agent contributes to creating the specific morphology of the pores [92]. For the cortical bone tissue, BS applications could be manufactured by PM biocomposites obtained through spark plasma sintering (SPS) technology. The same dehydrogenation reaction mentioned above for HA/TiO\({}_{2}\) composite is restricted by the compaction developing simultaneously with the sintering treatment, so the porosity is much lower (5-12%) than in the case of the same biocomposite made via TSS (Figure 8) [93]. For some clinical cases, the necessity for geometrical matching between the alloplastic bone substitutes product and the bone defect arises. Recent research proved that the biocomposite type HA/TiO\({}_{2}\) manufactured via TSS technology presents high potential to be processed by laser micromachining in order to fit the bone defect (Figure 9). Using a pulsed Nd:YAG solid state laser with 1064 nm wave length, 4 mm laser beam diameter, pulse energy of 15 J max., average power of 1000 W and specific cutting regimes (pulse length, 0.02-20 ms; pulse frequency, 0.1-1000 Hz; and voltage, 240-320 V), the outer surface of the graft presents a variable roughness (Ra = 4.6-18.2 \(\upmu\)m), assuring good physical interface with the adjacent natural bone without cracks/fractures in the bulk composite bone substitutes [94]. The bone substitutes' biocompatibility is significantly increased using submicronic HA powder particles as ceramic matrix. According to [PERSON] et al., different antibiotics (such as ciprofloxacin, gentamicin) may be adsorbed on the nanostructured HA/Ti biocomposites, and their releasing in a few days provides a significant osteointegration support under antibacterial protection. Furthermore, these effects are more intense in the case of calcium fructoborate (CaFB) adsorption on the HA/TiO\({}_{2}\) PM biocomposites [95; 96]. Figure 8: SEM micrograph of HA/TiO\({}_{2}\) biocomposite processed by SPS technology (Courtesy of Prof. [PERSON], University of Craiova, Faculty of Mechanics). The nanostructured HA matrix, which is TiO\({}_{2}\)-rutile reinforced by the TSS treatment, is responsible for other improved biocompatible performances.