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Zhang, et al.
A B C

Figure 5. Laser absorptivity and heat conductivity. (A) P25. (B) P40. (C) P60 (Shape Memory and Superelasticity, Fabrication of SLM
[52]
NiTi Shape Memory Alloy through Repetitive Laser Scanning, 4, 2018, 112–120, Z. X. Khoo, Y. Liu, Z. H. Low, et al. with permission
from Springer).

parameters on NiTi transformation temperature
[48]
and the compactness of parts . Through improved
[50]
processes, such as repetitive laser scanning, they
obtained performance comparable to conventional NiTi
parts . Yang et al. found that when preparing thinner
[54]
[52]
struts (thickness <0.6 mm), a lower scanning speed
performs better. Besides, these researchers investigated
porous SLM-NiTi performance and found that as the
porosity increases, the strain recovery rate decreases
slightly, putting forward a requirement for the upper
limit of the porosity . Fu et al. used micro-SLM
[53]
[55]
technology (with a laser beam of 25 μm) to explore the
defects, microstructure, and thermomechanical behavior
of micro-SLM-NiTi parts. The authors comprehensively
analyzed the formation mechanism of defects, including
pores, unmelted areas, and cracks. At the same time, they
creatively proposed the process window for scanning
speed and hatch distance to produce micro-SLM-NiTi
with few gas pores and no cracks, as shown in Figure 6. Figure 6. Scanning speed-Hatch distance window (Reprinted
[55]
All the above studies have provided technical guidance from Optics & Laser Technology, 131, J. Fu, Z. Hu, X. Song, et al.,
for the production of dense and defect-free SLMed NiTi. Micro selective laser melting of NiTi shape memory alloy: Defects,
microstructures, and thermal/mechanical properties, 106374,
3. Design of porous structures Copyright [2020], with permission from Elsevier).

In orthopedic surgery, NiTi applications include natural bone, providing higher mechanical stability at the
compression bone stabilizers for osteotomy and fracture interface . Kuboki et al. demonstrated the necessity
[61]
[62]
fixation , rods for correcting scoliosis , and shape of pores in bone regeneration, they found no new bone
[57]
[56]
memory for cervical surgery expansion clip , small formed on the solid particles, but direct osteogenesis
[58]
bone clip surgery , and tissue suture fixation system for occurred in the porous scaffold.
[59]
minimally invasive surgery . Traditional production technologies used to form
[60]
Utilization of a porous structure in NiTi as an pores in biomedical materials include salt immersion
orthopedic implant is very beneficial. Although NiTi method, gas foaming method , phase separation method,
[63]
has a low Young’s modulus (40–60 GPa), it is still high freeze casting method , and sintering method . The
[64]
[65]
compared to natural human bone . It is necessary to most significant disadvantage of these technologies is that
[18]
further reduce Young’s modulus by increasing porosity. they cannot accurately control parameters such as pore
On the other hand, pores are essential for forming shape, pore size, and porosity. Now that AM technology
osteoblasts and the proliferation of mesenchymal cells . has matured, it is a general trend to directly design and
[61]
The porous surface also improves the mechanical manufacture porous parts. The biggest advantage of
interlocking between the implant and the surrounding AM technology in porous structure design is that it can
International Journal of Bioprinting (2021)–Volume 7, Issue 2 19