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Additively Manufactured NiTi Implants
phase (low-temperature phase) . The transformation alloy manufacturing processes such as casting cannot
[9]
between the two phases is not through atomic diffusion produce complex porous structures economically and
but due to the sheer lattice distortion, called martensitic efficiently. NiTi is sensitive to composition and difficult
transformation . There are two forms of martensite to machine due to its poor machinability [25] . Therefore,
[8]
variant assembly: Twinned martensite (M) composed of most conventionally produced NiTi parts have simple
t
self-accommodated martensite variants, and detwinned geometric shapes, such as wire, plate, strip, and tube,
martensite (M ) consisting of detwinned or reoriented which severely limits the full potential applicability of
d
martensite variants . When the M at low temperature is NiTi [25] . For overcoming the manufacturing problems
[8]
t
loaded, the martensite can be detwinned by reorienting a of complex structures, the current better solutions are
part of the deformation. When the load on the M is released, powder metallurgy (PM) and additive manufacturing
d
the deformed shape retains, and then the SMA is heated to a (AM). Most of the PM-NiTi components have high
temperature higher than A (from M to austenite), resulting impurities like intermetallic phases [26] , which may
f
d
in complete shape recovery, as Figure 1 shows. This specific significantly reduce the mechanical properties of
phenomenon is called the SME. When SMAs is above the NiTi. In most cases, the formation of these second
austenite transformation temperature, the enormous strain phases is inevitable because their formation is more
generated during the loading process will gradually recover thermodynamically stable than NiTi [27] . Usually,
with unloading, called superelasticity or pseudoelasticity . these inclusions are carbides TiC, intermetallic oxide
[10]
Due to its high ductility, low corrosion rate, and Ti Ni O , or intermetallic phases, such as NiTi ,
2
2
x
4
good biocompatibility , Wang et al. [12-16] have carried Ni Ti, and Ni Ti [28] . Although Ni Ti precipitates are
[11]
3
4
3
3
4
out a series of researches on the application of NiTi-based highly needed, carbides and oxides are not favorable
SMAs in biomedical fields. Moreover, the mechanical because they are not beneficial to corrosion resistance,
hysteresis of NiTi is very similar to natural bone, making biocompatibility, and transformation temperature, so PM
it an ideal choice for orthopedic implants . Commonly technology unfortunately will bring many disadvantages
[17]
used biomedical alloys, such as titanium alloy, cobalt- and limitations during the fabrication of porous NiTi [29] .
based alloy, and stainless steel have Young’s modulus of
110 GPa, 190 Gpa, and 210 GPa, respectively , which Table 1. Mechanical properties of biomedical metallic materials
[16]
is much higher than human cancellous bone (<3 GPa) or and natural human bone
cortical bone (12–20 GPa) . When loading, the implant Material Yield Modulus Reference
[18]
with much higher Young’s modulus withstands most of the strength of elasticity
stress, and the stress level of the bone is considerably low, (MPa) (GPa)
which is called the stress shielding effect . If the loading Stainless steel 760 ~190 [10,20]
[19]
force on the bone is too small for a long time, it will cause Co-based alloy - ~210 [21]
bone resorption and loosening of the implant and ultimately
cause implantation failure . The Young’s modulus of CP-Ti 240–550 100 [18]
[20]
NiTi is much lower (40–60 GPa), but it is still necessary to Ti-6Al-4V 950 112 [10,16]
further reduce it. Table 1 shows the mechanical properties Ti-35Nb-7Zr-5Ta 596 55 [16,21]
of natural human bone and biomedical metallic materials. NiTi 1050 48 [10,16]
Producing a porous structure is the most common Cortical bone 188–222 15–35 [22]
method to reduce Young’s modulus [24] . Traditional Trabecular bone 2–70 0.01–3 [23]
A B

Figure 1. (A) Schematic of shape memory effect in NiTi. (B) A typical Shape Memory Alloys pseudoelastic loading cycle (Shape Memory
[8]
Alloys, Introduction to Shape Memory Alloys, 2008, 1–51, P.K. Kumar, D.C. Lagoudas. with permission from Springer).
16 International Journal of Bioprinting (2021)–Volume 7, Issue 2