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Additively Manufactured NiTi Implants
A B

C

Figure 15. (A) The mechanical properties of NiTi with cubic and spherical pores. (B and C) The SEM images of fractured samples
[91]
(Reprinted from Materials Letters, 154, I. H. Abidi, F. A. Khalid, M. U. Farooq, et al., tailoring the pore morphology of porous nitinol with
suitable mechanical properties for biomedical applications, 17–20, Copyright (2015), with permission from Elsevier).
tissue can also coordinate the migration and differentiation A
of bone cells by delivering functional factors .
[97]
According to Oers et al. , the contact energy
[98]
with the bone (h ) is a function of the osteocyte signal
b
(S) of bone cells in adjacent bone elements, as shown
in Figure 16. Two signal thresholds S and S are used.
0
1
If the osteoclast signal is lower than S , the osteoclast
0
bone adhesion is strong. Between S and S , the adhesion
0
1
weakens, and above S , there is no adhesion. The
1
Haversian system is a necessary structural unit to support B
long bones, and its pore size is 50–250 μm. For bone
formation in the pores, the pore size must be larger than
the Haversian system. The length of osteoblasts is 20 μm
and the capillary diameter is 10–15 μm .
[98]
Simultaneously, the size of the interconnected pores
is crucial for the migration of cells and blood vessels .
[99]
In theory, for cells and blood vessels to pass through the
pore smoothly, the pore size must be larger than 35 μm.
It is impossible to have capillaries and osteoblasts in the Figure 16. (A) h is a function of S. (B) Configurations of minimal
b
connected pores due to the need for space in the process surface energy for different h b [98] (Reprinted from Bone, 43(3), R. F.
of cell migration and vascular growth . Furthermore, it M. V. Oers, R. Ruimerman, B. V. Rietbergen, et al., relating osteon
[65]
confirms that the porous structure with 50–300 μm pore diameter to strain, 476–482, Copyright (2008), with permission
size and good connectivity is conducive to bone tissue from Elsevier) .
formation .
[88]
In summary, good interconnectivity is an essential mechanical properties and pore parameters of porous
condition for bone ingrowth after implantation. In samples. In Figure 17A, the pore size of NiTi is 400 μm,
summary, good interconnectivity and proper pore and the porosity is 63.5%. The pore size of Ti-6Al-4V is
parameters jointly lead to the optimization of osteogenesis 500 μm and the porosity is 68.37% (triangular), 42.34%
effects. (hexagonal), and 66.65% (rectangular), respectively. In
Figure 17B, the porosity of CP-Ti is 70%, the porosity of
3.6. Relationship between mechanical properties
and pore parameters NiTi is 63.8%, and the porosity of Ti-6Al-4V is 42.34%.
In Figure 17C, the pore size of Ti-6Al-4V is 282 μm
As mentioned above, porous biomaterials must provide (29%) and 596 μm (51%), respectively. The pore size of
mechanical performance requirements in the design step. CP-Ti and Ti-TiB is both 400 μm, the pore size of Ti-25Ta
Figure 17 shows a summary of the relationship between is 606 μm, and the pore size of NiTi is 400 μm.

26 International Journal of Bioprinting (2021)–Volume 7, Issue 2