A Review of Additive Manufacturing of Biodegradable Fe and Zn Alloys for Medical Implants Using Laser Powder Bed Fusion (LPBF)

Abstract

This review explores the advancements in additive manufacturing (AM) of biodegradable iron (Fe) and zinc (Zn) alloys, focusing on their potential for medical implants, particularly in vascular and bone applications. Fe alloys are noted for their superior mechanical properties and biocompatibility but exhibit a slow corrosion rate, limiting their biodegradability. Strategies such as alloying with manganese (Mn) and optimizing microstructure via laser powder bed fusion (LPBF) have been employed to increase Fe's corrosion rate and mechanical performance. Zn alloys, characterized by moderate biodegradation rates and biocompatible corrosion products, address the limitations of Fe, though their mechanical properties require improvement through alloying and microstructural refinement. LPBF has enabled the fabrication of dense and porous structures for both materials, with energy density optimization playing a critical role in achieving defect-free parts. Fe alloys exhibit higher strength and hardness, while Zn alloys offer better corrosion control and biocompatibility. In vitro and in vivo studies demonstrate promising outcomes for both materials, with Fe alloys excelling in load-bearing applications and Zn alloys in controlled degradation and vascular applications. Despite these advancements, challenges such as localized corrosion, cytotoxicity, and long-term performance require further investigation to fully harness the potential of AM-fabricated Fe and Zn biodegradable implants.

Keywords: additive manufacturing (AM); biodegradable metals; bioresorbable implants; iron alloys (Fe); laser powder bed fusion (LPBF); zinc alloys (Zn).

Figure 1

Applications of biodegradable metals in the medical devices industry. (a) Schematic diagram of cardiovascular diseases and stent [13]. (b) Common medical devices used for fracture internal fixation Adapted with permission from [14]. 2024 Elsevier. (c) Tissue engineering approach for reconstruction of large bone defects [15].

Figure 2

PRISMA flow diagram used for reporting systematic reviews [25].

Figure 3

Scheme from Li et al. showing the LPBF process at hatch distances of 120 μm (a) and 60 μm (b) Reprinted with permission from [35]. 2024 Elsevier.

Figure 4

Additive manufactured parts: (a) Adapted with permission from [51], 2024, Elsevier (b) [56], (c) Adapted with permission from [53] 2024, Elsevier, (d) Adapted with permission from [54] 2024, Elsevier and (e) [55].

Figure 5

Parts quality according to E_v and material. Green indicates optimum conditions, red excess of energy, and blue lack of energy [51,52,53,54,55,56].

Figure 6

Preliminary study of processing parameters for hatch distance of 90 µm. The number at the top left is the relative density (%), the top right is the number of the sample and the number at the bottom left is the energy density (J/mm³) [56].

Figure 7

(a) Processing windows for pure iron (Zone I—Deformation zone, Zone II—Formation zone, Zone III—Zone of poor formation, and Zone IV—Zone of non-forming) versus laser power and scanning speed; Red square: parts manufactured with 100 W of laser power, green asterisks 80 W and blue triangles at 60 W. (b) Density curves of iron parts as a function of the laser power and scanning speed Adapted with permission from [51]. 2024, Elsevier.

Figure 8

Pure Fe cross-sectional optical micrographs (a-1,b-1) and Fe35Mn SEM images (a-2,b-2) showing the microstructure with BD out-of-plane and in-plane. Adapted with permission from [54]. 2024, Elsevier.

Figure 9

Top view and longitudinal cross-section of the CAD models of functionally graded Fe scaffold. Adapted with permission from [60]. 2024, Elsevier.

Figure 10

Corrosion rate of Fe and its alloys. In orange electrochemical tests and in blue immersion tests. Filled symbols correspond to dense parts and empty symbols correspond to scaffolds [52,53,55,60,63,64].

Figure 11

Circuit for (a) pure iron. Adapted with permission from [52] (2024, Elsevier) and (b) Fe35Mn scaffold. Adapted with permission from [53] 2024, Elsevier.

Figure 12

Volumetric energy densities (E_v) used for fabricating Zn and Zn alloys parts by LPBF. Green indicates optimum conditions, red excess of energy, and blue lack of energy [33,77,78,79,80,81,82,83,84,85].

Figure 13

Appearance of LPBF-produced pure Zn parts showing the effect of fluence with (a) coarse particles and (b) fine particles Adapted with permission from [77]. 2024, Elsevier.

Figure 14

Picture of LPBF cross-sections of Zn-xWE43 bulk samples (a) Zn2WE43, (b) Zn5WE43 and (c) Zn8WE43 Adapted with permission from [80]. 2024, Elsevier.

Figure 15

Microhardness of high-density Zn and Zn alloy parts manufactured by LPBF [78,80,81,84,85].

Figure 16

Scanning electron microscopy top view for scaffolds with (a) diamond unit cell, (b) dodecahedron unit cell, (c) octet truss unit cell, (d) FCC unit cell, and (e) 3D Kagome unit cell Reprinted with permission from [86]. 2024, Elsevier.

Figure 17

Corrosion rate of Zn and its alloys. In orange electrochemical tests, in blue immersion tests. Filled symbols correspond to dense parts and empty symbols correspond to scaffolds [81,82,83,84,85,88,90,91].

Figure 18

Equivalent electrical circuit for ZnxCe samples Reprinted with permission from [82]. 2024, Elsevier.

Figure 19

Quantitative viability results of MG-63 cells in extracts of LPBF processed ZnxMg. Data were normalized to the control group. Values were mean ±SD, n = 3, * p < 0.05 between the test group and the pure Zn group Reprinted with permission from [83]. 2024, Elsevier.