**6. Biological components**

The dogma of molecular biology is the basis for producing most bone cell and ECM components.

Bone remodeling is divided into five stages: activation, resorption, reversal, formation, and, finally, mineralization (see **Figure 3**). It is a process in which the old bone is reabsorbed and there is new bone formation. The cells that are involved in bone remodeling are osteoblasts, osteoclasts, and osteocytes, which actively participate in osseointegration and repair. Osteoclasts activate bone resorption, while osteocytes regulate bone homeostasis and osteoblasts form bone [15, 110].

In addition to bone cells, there are other cell lines that can be used in bone regeneration, which are human embryonic stem cells (hESCs), induced pluripotent stem cells (iPSCs), stem cells, and fibroblasts. According to Kuhn [111] and other workers [112, 113], hESCS present a rapidly proliferating rate. However, their transplantation induces uncontrollable spontaneous differentiation and can the teratoma formation may occur. Another type of stem cell is iPSCs. They can differentiate into several cells. However, there are studies that show that these cells can also give rise to teratomas and, in studies that distinguish high-quality lines from the iPSCs, allowed the detection of large duplications of genes that could potentially affect the differentiation and pluripotency of these cells [28, 99]. For these reasons, these cells are not considered the best ones for bone regeneration. Compared to fibroblasts, stem cells have a greater ability to migrate, so these type of cells are the most suitable cells for bone regeneration. Kargozar [58] recently studied the osteogenic potential of different MSCs, such as those derived from human bone marrow, umbilical cord (UC-MSCs) and adipose (AD-MSCs). It concluded that BMMSCs, according to collected histological data, is the most appropriate.

The combination of scaffold, AM, and bioreactor culture shows great potential for creating automated production ecosystems that will enable the formation of commercially available products for BTE application. Efficient nutrient and oxygen transport are important for this type of applications. To this end, bioreactor systems have tried to overcome this difficulty. Rotating-wall vessels are limited to small scaffolds as they do not provide optimal mass transport to the center of the scaffold and are not efficient in osteogenic differentiation, due to shear stress values transmitted to cells.

**171**

**Figure 4.**

*Natural organization of long bones.*

*Comprehensive Review on Full Bone Regeneration through 3D Printing Approaches*

and, therefore, obtain a much better cell distribution [6, 39, 61, 115, 116].

On the other hand, agitated tanks have a major disadvantage regarding the circulating flow pattern that strikes cells against the bioreactor wall, which damages them and can lead to cellular apoptosis [114]. Finally, perfusion bioreactors are the best suited for BTE as they promote oxygenation throughout the whole scaffold, through improved mass transfer and shear stress, can expose cells to mechanical stimulation,

Scaffold architectures were designed in SolidWorks 2018 software. The design was bioinspired. This inspiration came from the natural organization of long bones, as represented in **Figure 4**. The diaphysis is composed of cortical bone (external region) which covers the trabecular bone (internal region). The trabecular bone has a larger surface area than the cortical bone and has a bone volume fraction ranging from 5% to a maximum of 60% [117]. It is known that the cortical zone corresponds to ~20% of the total diameter [118]. Bearing this in mind, it is expected that mimicking this type of organization, the mechanical behavior of the final scaffolds

The design considered has a height of 10 mm and diameter of 30 mm (see **Figure 5**). Thus, the cortical zone, the outer part of the scaffold, has a thickness of 6 mm and the trabecular zone, the inner part, has 18 mm. In the middle, there is a canal that corresponds to the medullar cavity. As happens in the native tissue, the region corresponding to the trabecular bone presents a higher porosity than the cortical one. So, the proposed scaffold has pores with different sizes between the different parts, bigger in the trabecular and smaller in the cortical

*DOI: http://dx.doi.org/10.5772/intechopen.90864*

**7. Optimized approach of a 3D scaffold**

would be better and closer to the natural tissue.

**Figure 3.** *Bone Remodeling Cycle.*

*Comprehensive Review on Full Bone Regeneration through 3D Printing Approaches DOI: http://dx.doi.org/10.5772/intechopen.90864*

On the other hand, agitated tanks have a major disadvantage regarding the circulating flow pattern that strikes cells against the bioreactor wall, which damages them and can lead to cellular apoptosis [114]. Finally, perfusion bioreactors are the best suited for BTE as they promote oxygenation throughout the whole scaffold, through improved mass transfer and shear stress, can expose cells to mechanical stimulation, and, therefore, obtain a much better cell distribution [6, 39, 61, 115, 116].
