Page 56 - IJB-10-3

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

enhance the precision of these components through 3D the in vivo degradation rates of various iron-based alloys.
170
printing while ensuring the uniform distribution of the Furthermore, iron oxides are difficult to metabolize in
alloy phase is yet to be studied. Zinc is an essential trace vivo and are prone to buildup, hindering bone defect
element for humans. The dietary allowance for zinc in healing. 46,47 Therefore, the main way to improve the rate of
the United States is 8–11 milligrams per day. In terms of iron degradation is to convert iron into porous structural
implants, the degradation products of zinc implants are alloy components. The main Fe-based alloys that have
171
mainly zinc oxide, zinc hydroxide, and zinc phosphate. been studied are Fe–Mn alloys and Fe–Cu alloys, such
The high biocompatibility of degradable zinc metal mainly as Fe-35Mn, Fe-30Mn, Fe-Mn-Pd, and Fe-xCu. 171-175 The
comes from zinc phosphate, which is a common dental mechanical properties of these alloys are weaker than or
adhesive. The mainstream 3D printing methods for comparable to those of 316L steel; additionally, these alloys
163
zinc and zinc alloys mainly include PBF, DED, and BJ. exhibit good biocompatibility, higher corrosion rates, and
Doping strontium (Sr) and zinc (Zn) into hydroxyapatite more homogeneous corrosion types but still fall short of
nanoparticle (HANPs) and incorporating them into 30 ideal biodegradation rates. 171,172,174-176 The addition of copper
wt.% polyether ether ketone (PEEK) can produce skull to the alloys led to antimicrobial activity, but the density
implants with good mechanical properties. Furthermore, and Young’s modulus decreased with increasing copper
164
3D-printed titanium-based implants modified with ZnO content. 50,177 Adding manganese, however, may adversely
coatings have efficient antibacterial properties and can play affect the cytocompatibility of the alloy. Another unique
178
a role in repairing severe bone defects. 165 alloying method is to form a powder with a copper shell
over an iron core by electroplating, where the powder
4.3. Iron and its alloys particles are alloyed on the outer periphery and pure iron
The promotion of bone regeneration by iron alloy implants exists on the inside. This powder can be used to generate
179
is mainly reflected in the provision of adequate physical macroscopically homogeneous and biodegradable sintered
support and connectivity as a result of the porous structure. parts composed of iron and copper alloys. However,
In addition, the good biocompatibility of iron leads to a this powder has the disadvantage of microscopic copper
stable immune microenvironment for the healing of bone precipitation during the sintering process, resulting in an
defects. A representative iron alloy commonly used in uneven distribution of copper in the components, a high
bone regeneration is 316L austenitic stainless steel. While content of the outer layer, and precipitated subnanometer
49
316L steel has excellent tensile and mechanical strengths, copper blockage in the pores of the components, which
making it the gold standard for orthopedic implants, its may adversely affect the degradation of the components.
austenitic morphology is MRI-compatible, thus facilitating In terms of in vivo implants, the degradation rate of iron
subsequent medical treatment; it is also biocompatible, implants is extremely low, which may cause negative effects
as no hydrogen is generated during degradation. The similar to permanent implants. The degradation products
high tensile and mechanical strength of 316L steel enable of iron mainly include Fe and Fe , which may catalyze
3+
2+
greater mechanical support and utilization for larger the generation of reactive oxygen species and free radicals.
bone defects. 166-168 Therefore, some scholars do not recognize iron as a safe and
180
The disadvantages of iron and iron alloy implants BM material. The mainstream 3D printing methods for
include their extremely low degradation rate and iron and iron alloys mainly include laser powder bed fusion
excessively high modulus of elasticity. The slow (L-PBF) and BJ. Common iron-based alloys include Fe,
degradation allows iron-based implants to remain in the Mn, and Ca/Mg alloys, which can be used for skull repair.
body after the patient has healed, and their modulus of A porous biodegradable Fe–Mn alloy, as a mesh scaffold, is
elasticity does not match that of the bone tissue, which can used for severe bone defect repair. The Fe–Si implant can
lead to stress interruptions similar to those experienced be used as a solid implant for bone transplantation and as a
by nondegradable metal implants. Additionally, due to temporary implant for fracture fixation. 181-183
the presence of certain amounts of chromium and nickel 4.4. Hybrid materials
in some ferrous alloys, these materials may cause allergic In addition to pure metal scaffolds, it is also possible
reactions in some patients. 169
to promote vascularization, enhance osteogenesis, and
The in vivo degradation process of ferrous alloys is strengthen the mechanical properties of new bone by
complex and slows the in vitro degradation process, with adding degradable metal components, such as magnesium
a dense layer of iron oxide degradation products being oxide nanoparticles, to other nonmetallic materials and
formed on the surface of the implant, which prevents releasing them through slow degradation, which can
oxygen transport and considerably slows the degradation significantly enhance the mechanical properties of some
of the implant, resulting in no significant difference among materials. Hybrid material scaffolds have the advantage
184

Volume 10 Issue 3 (2024) 48 doi: 10.36922/ijb.2460