**6. Osteoconductive bioresorbable implants, fabricated via 3D printing with nanoadditives**

In the process of fabrication of the source materials for the bone implants capable of providing a sufficiently reliable biological integration with the bone tissue, it is important to search for new phase composition of these materials, to improve their microstructure, to create new architectural types of a macroporous structure, to develop approaches for generating materials with a specified surface roughness of the material, and macropores surface of the material. The material science aspect of the problem is connected with the choice of the chemical and phase composition of the composite based on of the synthesized powders. The backbone of the composite should be a biopolymer degradable in the body environment. The biopolymer can also be with functional nanoadditives [34].

One more method of obtaining strong implants by means of the 3D printing is their hardening by ceramic nanoadditives (Al2O3, ZrO2, AZO, HA) [14, 18, 19, 35–38]. The crucial parameter that determines the suitability of this method for making real bone implants and tissue engineering scaffolds is the print resolution. The polymer matrices framing the macropores must be of a specific architecture that, under the given pores fraction, ensures the following: (a) maximizing of permeability, (b) maximizing of the mechanical characteristics such as strength, hardness (elastic modules), (c) obtaining of the surface where to the cells of osteogenic type could be attached, divided, and differentiate. The task of the powder synthesis of spatially ordered complex structures with a connected system of macropores (not smaller than 100 μm) is quite solvable. These structures determine the osteoconductive properties of the implant.

The best resorption characteristics are observed for tricalcium phosphate β-Ca3(PO4)2 (β-TCP, Ca/P = 1.5) [39–43]. The increase of the resorption speed and level, obtaining of a more available environment for the bone tissue development can be achieved by lowering of the Ca/P ratio values below 1.5, while the pH level of the implant environment is maintained close to neutral. A further increase of the resorption limit and speed is associated with the decrease of the Ca/ P ratio, i.e., in the transition to the materials, including calcium phosphates phases with condensed phosphate ions, i.e., calcium pyrophosphate and polyphosphate. The presence of condensed phosphates improves the surface hydrophily of the composite and promotes the adsorption of special signaling proteins from the interstitial fluid, resulting in the acceleration of the implant integration in the body.

The task of forming a surface roughness is solved by regulating the system composition, CAD structures and choice of the SLS/M regimes which ensures the fabrication of the specific microporous surface. It is known that smooth matrixes ensure a high proliferative osteoblast potential, while the osteogenous cytodifferentiation is hampered [18]. The increase in the 3D print resolution for matrix in the form of a filled polymer with a given architecture, and then with a given microstructure and phase composition in a porous ceramic material is an important problem [44]. The surface modification by nanoparticles is a perspective approach to the microstructure management for different types of functional implants and tissue engineering scaffolds.

In the paper [33], a model was considered allowing numerical determination of a resulting field velocity under the laser melting of aluminum in the external magnetic field. The model included heat-dependent characteristics of the material (surface tension and viscosity). A heterogeneous distribution of the magnetic flow density was determined by the experimental Hall data measured for the prototype. It was shown that a constant magnetic flow applied coaxially with the LI, exerts its influence upon the direction of the melted material flow and

**6. Osteoconductive bioresorbable implants, fabricated via 3D printing with**

In the process of fabrication of the source materials for the bone implants capable of providing a sufficiently reliable biological integration with the bone tissue, it is important to search for new phase composition of these materials, to improve their microstructure, to create new architectural types of a macroporous structure, to develop approaches for generating materials with a specified surface roughness of the material, and macropores surface of the material. The material science aspect of the problem is connected with the choice of the chemical and phase composition of the composite based on of the synthesized powders. The backbone of the composite should be a biopolymer degradable in the body environment. The biopolymer can

One more method of obtaining strong implants by means of the 3D printing is their hardening by ceramic nanoadditives (Al2O3, ZrO2, AZO, HA) [14, 18, 19, 35–38]. The crucial parameter that determines the suitability of this method for making real bone implants and tissue engineering scaffolds is the print resolution. The polymer matrices framing the macropores must be of a specific architecture that, under the given pores fraction, ensures the following: (a) maximizing of permeability, (b) maximizing of the mechanical characteristics such as strength, hardness (elastic modules), (c) obtaining of the surface where to the cells of osteogenic type could be attached, divided, and differentiate. The task of the powder synthesis of spatially ordered complex structures with a connected system of macropores (not smaller than 100 μm) is quite solvable. These structures determine the osteoconductive properties of the implant.

The best resorption characteristics are observed for tricalcium phosphate β-Ca3(PO4)2 (β-TCP, Ca/P = 1.5) [39–43]. The increase of the resorption speed and level, obtaining of a more available environment for the bone tissue development can be achieved by lowering of the Ca/P ratio values below 1.5, while the pH level of the implant environment is maintained close to neutral. A further increase of the resorption limit and speed is associated with the decrease of the Ca/ P ratio, i.e., in the transition to the materials, including calcium phosphates phases with condensed phosphate ions, i.e., calcium pyrophosphate and polyphosphate. The presence of condensed phosphates improves the surface hydrophily of the composite and promotes the adsorption of special signaling proteins from the interstitial fluid, resulting in the acceleration

can be explained as a heterogeneity of electromagnetic destruction.

**nanoadditives**

246 New Trends in 3D Printing

also be with functional nanoadditives [34].

of the implant integration in the body.

Permeability optimization with the conservation of a sufficient toughness can be realized by the directed obtainment of the given porosity architecture of a 3D part. Topological structures (3D minimum surfaces) occurring in nature ensure the achievement of the maximum perme‐ ability under the maximum toughness obtainable for the porous samples. Under the compa‐ rable porosity (50%) the most permeable models are cubic, tetrahedral and gyroidal cell models [45, 46]. The gyroidal model has a reasonable compromise between permeability and tough‐ ness. Roughness can be introduced as a term with higher angular frequency. This will change the curvature locally, as required for the optimal cell adhesion and growth. A porosity gradient can be easily modeled by adding a linear term. Algebraic form for the function describing the gyroid surface is not complicated and can be represented as trigonometric function, thus allowing the generation and scaling of computer models for such architectures.

Osteoconductivity is the ability of the material to provide the possibility of biological flows, intergrowth of blood vessels into the implant (vascularization), adhesion and binding of osteogenic cells [47]. These characteristics are correlated with the physical permeability of a porous body (bone implants or tissue engineering scaffolds) and are provided by means of the inherent bimodal porosity.

Permeable pores of a large size provide permeability for the flow of necessary biological substances, while the pores of a small size are accountable for the roughness of the surface, giving the signal to spreading and proliferation of bone cells [35, 48, 49]. In order to fabricate a chaotic macroporous structure, different approaches could be used, but only the regular spatial architecture of porous-structured materials allows to increase the permeability and strength of the product to the desired values. The task of designing a regular architecture of the porous space can be only solved with the use of additive technologies, the 3D printing in particular.

Tissue engineering scaffolds possess a certain structured organization, capable to form a framework of life space for the bone-cells predecessors and thus stimulate their functional activity. The scaffolds themselves or in combination with other components exert a direct regulatory effect on the cells predecessors and hereby induce the osteogenesis within the implantation zone. The design of 3D scaffolds must stabilize mechanical loads in the place of contact and prevent the formation of a fibrous capsule around the implant [50].
