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3D Printing Technologies in Metallic Implants
absorption capabilities . The high-quality trusses with
[98]
complex geometry and fine features can only be made by
AM methods, specifically the SLM and EBM techniques.
Some of the most popular truss lattices include simple
cubic (SC), body-centered cubic (BCC), and face-
centered cubic (FCC), as shown in Figure 13. These
structures are made from the unification of vertical and
inclined struts; for example, SC lattice includes vertical Figure 13. The most popular truss lattices: Simple cubic, body-
struts (and some tension in the horizontal struts) under centered cubic, and face-center-cubic (FCC) ((Reprinted from
compression; BCC consists of pure bended-inclined Composites Part A: Applied Science and Manufacturing, Volume
structure and FCC is a mixture of inclined struts with 135, Li X, Tan YH, Wang P, et al, Metallic microlattice and epoxy
both bending and tension . interpenetrating phase composites: Experimental and simulation
[98]
Structures can be categorized according to their studies on superior mechanical properties and their mechanisms,
[98]
geometrical features as shown in Figure 13 the well- Copyright (2020), with permission from Elsevier) .
known truss lattices include: SC, BCC, and FCC. The
structure with only some solid edges is called an open they have acceptable mechanical properties (strength,
cell structure, whereas the structure with both solid edges elastic modulus, and hardness). In fact, the Young elastic
and faces is known as a closed-cell structure. Foams have modulus is of great importance in bone applications since
random connectivity of unit cells, and lattices have regular a higher elastic modulus can lead to stress-shielding
or periodic connectivity . In this regard, other possible effect that could lead to implant failure; hence, the
[99]
lattice structures and topologies were shown in Figure 14 porous designs are preferred since they have a potential
[110]
and include (a) Kagome, (b) octet truss, (c) MS1 lattice, to control the stress-shielding effect . Furthermore,
(d) pillar textile, (e) square collinear/cubic, (f) re-entrant Ti-based alloys exhibit excellent corrosion resistance
auxetic, (g) octahedron, (h) honeycomb, (i) square, (j) in simulated body fluids [111,112] . Considering the above-
diamond, (k) triple periodic minimal surfaces (TPMS) mentioned superior features of Ti-based materials, 3DP
P-type, (l) TPMS gyroid, (m) TPMS D-type, and (n) TPMS of Ti is growing in importance for its application and
I-WP type . It was found that topological features can bound to attract much attention. Recently, a successful
[99]
affect the mechanical behavior of the material, and the octet case of 3DP of titanium was reported in a cancer patient;
truss design demonstrates superior mechanical properties, a 15-year-old boy received a Ti implant fabricated by
in addition to its ability to tolerate higher loads compared the EOS Technology in a process that takes only about 6
with other cellular structures. The main advantage of weeks, starting from the CAD model designing model to
[61]
using cellular structure design is its ability in systematic the final implanting .
and adroit utilization of material to reduce unnecessary
consumption of material, energy, and time and manufacture (1) Titanium-based porous structures
lightweight parts with favorable mechanical strength [100] . Recently, the AM manufactured porous structures attract
Furthermore, cellular structures have a potential to show much attention. Trabecular bone structure is one of the
excellent thermal energy absorption and acoustic insulation examples that can be designed by 3DP, and the obtained Ti
properties [101] . From the aspects of geometry, porosity, and porous structures can improve the bioactivity of implant,
pores size, TPMS can be used as a topology suitable for enhance cell adhesion, proliferation, and differentiation of
manufacturing trabecular bone scaffolds [102] . osteoblasts [113] . Li et al. performed a systematic investigation
about different aspects of 3D printed porous Ti-based
3.2. Metallic scaffold parts in medicine materials that were produced by the EBM technique [114] . The
At present, metallic porous scaffolds are becoming one highly porous and well-interconnected pore architecture
of the popular material choices in medical applications. shows good mechanical properties with enhancements in
One of the most important classes of materials in biological activity, osteoblast adhesion, cell morphology,
these applications is titanium (Ti) and its alloys due proliferation, and alkaline phosphatase (ALP) activity.
to its numerous mechanical advantages along with Moreover, to produce a Ti-based porous structure by the
biocompatibility with living tissues [103-105] . Compared to EBM technique, Zhang et al. designed a repeating array of
stainless steel and other metallic systems, the medical- titanium alloy unit-cells to mimic trabecular or cancellous
grade Ti-based alloys show enhanced performance, bone structure [115] . Toward this end, various kinds of unit
especially in bone tissue ingrowth capability since Ti cells mimicking the trabecular bone structure with different
has a 50% higher strength to weight ratio and less Young pore sizes and porosity were produced. The result shows
modulus in comparison to stainless steel [106-108] . Moreover, that the capacity of load-bearing is dependent on the
Ti-based materials are free of any toxic effects [109] , and porosity; a higher porosity value leads to a reduction of

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