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International Journal of Bioprinting 3D-printed biodegradable metals for bone regeneration

3.3. Three-dimensional printing of BMs by adhesives. 101,102 In addition, the wide selection of materials
material extrusion suitable for BJ enables the fabrication of components
Material extrusion is also commonly known as 3D plotting with appropriate mechanical strength by adjusting the
or 3D gel printing. When applied to metals, the “ink” materials used, such as magnesium–zinc alloy, magnesium
made by mixing a metal powder with a binder is extruded phosphate, and 316L stainless steel. 103-105 The microstructure
through a nozzle during printing and stacked layer by layer and mechanical properties of the components fabricated
to obtain the initial finished product. The preliminary using BJs change with respect to the sintering temperature.
product is then degreased and heat-treated to remove the By using the appropriate heat treatment temperature and
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binder, resulting in a metal component. The most obvious holding time, components with a density, compressive
benefit of ME over other approaches is that products can properties, and an elastic modulus similar to those of
be processed and printed at room temperature; this not human cortical bone and with appropriate porosity can be
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only reduces vaporization, sputtering, and oxidation of the obtained. Physical properties similar to those of human
metal but also allows the incorporation of materials that bone tissue can provide support while minimizing stress
are not resistant to high temperatures into the component. shielding for bone regeneration. 106-108
The equipment costs for ME are also lower than those for 3.5. Three-dimensional printing of BMs by
other printing methods. The disadvantages of ME include 3D weaving
the poor mechanical properties of the components, In addition to traditional 3D printing methods, several
rough surface of the products, possible distortion of the new printing techniques have achieved excellent results.
component structure during subsequent heat treatment, For example, 3D weaving magnesium wires, in which
possible presence of adhesive residues, and limited material magnesium alloy filaments are woven in a fixed sequence
selection. By choosing a lower extrusion temperature and and structure supplemented by filler wires, can create
layer thickness, it is possible to achieve a relatively smooth magnesium implants with porous structures. This
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component surface and improve the precision. 97 method does not require heat treatment, and the mechanical
Material extrusion can use high-purity magnesium properties of the components are significantly improved
powder as a raw material, while other printing methods, after being covered with a polylactic acid (PLA) coating
such as PBF and DED, cannot easily be applied to pure with appropriate mechanical strength. The structural
magnesium due to vaporization during the printing properties, including porosity, pore size, and connectivity,
process. The use of appropriate binders and methods of the components fabricated using 3D braiding are
such as short-term liquid-phase sintering can create more controllable than those of components fabricated
porous magnesium scaffolds with high porosity and high by any other printing method, but this method has the
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structural fidelity to support the regeneration of bone disadvantages of difficult fabrication and high cost. The
tissue. Furthermore, surface modification of porous excellent connectivity of 3D woven components provides
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magnesium scaffolds can reduce the degradation rate. 99,100 excellent support for cell attachment and spreading and
A high porosity provides a pathway for bone growth into provides pathways for neovascularization, which in turn
the scaffold, and a reduced degradation rate provides time promotes bone regeneration. 110
for bone regeneration, while magnesium has properties
such as osteogenic activity and pro-vascularization that 4. Selection of BM materials for 3D printing
can enhance the effect of bone regeneration. 23,30,31 The main BM materials that can be used as implants are
magnesium, zinc, iron, and their alloys. Table 2 and Figure 3
3.4. Three-dimensional printing of BMs by a show the advantages, disadvantages, mainstream 3D
binder jet printing technologies, common alloys, implants, and
The BJ manufacturing process is similar to that of PBF in applications of these three BMs.
that both use a powder bed for feeding; the difference is
that PBF uses a laser or electron beam to sinter the metal 4.1. Magnesium and its alloys
powder, whereas BJ binds the metal powder by directional The Young’s modulus of magnesium-based alloys is
spraying of a liquid binder or by relying on capillary forces. similar to that of bone cancellous mass, and the density
After the initial printing, the constructs are heat-treated of magnesium and its alloys is also close to that of human
to remove the adhesive and increase their mechanical bone, which is favorable for eliminating the stress masking
strength. Binder jetting has the following advantages: (i) of magnesium implants. 111,112 Moreover, the degradation
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printing can be performed at room temperature, (ii) the products of magnesium in vivo, magnesium ions, are
components are supported during the printing process, biologically active and can directly induce osteoblast
and (iii) the components can be printed without the use of proliferation while promoting CGRP release from

Volume 10 Issue 3 (2024) 44 doi: 10.36922/ijb.2460