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3D Printing Technologies in Metallic Implants
SMA many applications, especially in medical and spine laser-energy leads to enhancement of component density
surgeries [134-136] . Moreover, the SMAs also have other up to the maximum value. After this peak, density
stimulating uses in actuators, sensors, the aerospace reduction is possible, especially in materials which are
industry, and even fashion products [137] . vulnerable to oxidation and evaporation materials. This
One of the most imperative classes of SMAs in issue is very crucial in NiTi alloys since slight changes
medical applications is NiTi alloys. In addition to its very in composition can alter the shape memory effect of the
interesting shape memory effect, NiTi has good ductility, material. Powder re-melting can potentially homogenize
outstanding corrosion and wear resistance, and terrific and eliminate the local compositional variations and
biocompatibility [137] . This section focuses only on NiTi or enhance the overall SMA effect. Reducing the SLM scan
nitinol since they are the most utilized SMAs. Manufacturing speed and increasing SLM energy density can increment
parameters can largely influence the functionality of SMAs, the transformation temperature and leads to a better
texture, microstructure, surface quality, precipitates, and temperature SMA temperature range [140,141] .
several defects [138,139] ; hence, the production process needs NiTi SMA has excellent corrosion and wears
to be precisely controlled. LAM methods, including LSM, resistance along with desirable biocompatibility which
are highly utilized in the production of 3D printed SMAs is attributed to the formation of the titanium oxide layer
since these methods ensure proper homogeneity, especially on its surface. This layer has a protective nature and
in complex and dense structures. Furthermore, these even prevents the possible toxic and allergic effects
methods are applied in the production of various-shaped of Ni release; therefore, the thicker TiO layers are
2
SMAs because they can produce graded porous structures preferred [142,143] . Laser irradiation can influence the TiO 2
accurately. Usually, SLM method and laser metal deposition formation and thickness which affects its biocompatibility.
(LMD) are commonly used in the production of 3D printed For instance, Nd-YAG laser irradiation has very beneficial
SMAs (Figure 18). Both methods utilize metal powders effect on corrosion improvement and prevention of Ni
and the sample is produced by cross-sectional slicing of the ion release. This Nd-YAG laser irradiation can produce a
CAD model and layer by layer deposition. In SLM, special thick oxide layer inhibiting the corrosion while retaining
regions in a metal powder bed are melted and solidified, other properties [144] . Moreover, to enhance the biological
while in LMD, several nozzles are used to feed successive response of the material and mesenchymal stem cells,
layers of powders onto the building substrate on which the the metal AM fiber lasers (optical fibers that doped with
powder is melted by laser exposure. Several key parameters rare-earth elements) can be a good option since they
that influence the final quality of 3D printed part should are associated with desirable surface parameters, such
be considered while using either one of both methods. as increased roughness and wettability, and improved
These parameters include material condition (powder size, surface chemistry [145] .
morphology, and composition), machine parameters (laser A review of research literature shows that 3D printed
type, atmosphere), processing variables (laser power, track NiTi alloys are potential candidates for implantation as
spacing, scanning rate and pattern, powder layer thickness, they act as a good host for living cells and tissues. Habijan
and beam spot diameter), and power feed rate in LDM et al. [146] showed that the surface of SLM-produced porous
method. NiTi scaffolds was entirely covered with live cells with a
Achieving the highest material density is of high very insignificant number of dead cells after 8 days of cell
importance in 3DP. In SLM, increasing the density of culture. Other similar studies on SLM-produced porous

A B

Figure 18. Schematic presentation of (A) selective laser melting method with a scanning electron microscope image of powder and (B) laser
metal deposition process with a metal deposition condition (Reproduced from Ref. [137] with permission from Cambridge University Press,
https://doi.org/10.1557/mrs.2016.209, Copyright 2016 Materials Research Society 2016).

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