Mechanical properties, in vitro biodegradable behavior, biocompatibility and osteogenic ability of additively manufactured Zn-0.8Li-0.1Mg alloy scaffolds

Magnesium-induced strengthening, degradation and osteogenesis for additively manufactured Zn-Mg orthopedic implants

Additively manufactured biodegradable metals demonstrate great potential in orthopedic implants, enabling patient-specific designs and eliminating the need for secondary surgeries through degradation. Zinc (Zn)-magnesium (Mg) alloys reconcile the dilemma between Zn's low biological activity and Mg's rapid degradation, and become promising bone-repair materials. However, Zn-Mg alloys currently fabricated via additive manufacturing exhibit high strength but low ductility, and the effects and mechanisms by which Mg influences their degradation and bone regeneration remain unclear. Here, Zn-xMg alloys (x = 0, 0.1, 0.2, 0.4, 1.0 wt%) are prepared using laser powder bed fusion (L-PBF). Adding Mg refines the microstructure and forms brittle secondary phases, improving strength but reducing ductility. Zn-0.4Mg shows superior balance, with an ultimate tensile strength of 289.40 MPa and elongation of 12.53 %. The addition of Mg slows degradation by forming protective Mg-containing products and accelerating the passivation. Furthermore, Mg alloying significantly enhances bone regeneration, as indicated by both in vitro and in vivo tests. This improvement is driven by the release of less Zn2+ and more Mg2+, which promotes cytocompatibility and osteogenic differentiation. This study offers critical insights for optimizing Zn-based biodegradable metals and advancing the development of next-generation orthopedic implants. STATEMENT OF SIGNIFICANCE: Laser powder bed fusion (L-PBF) enables the fabrication of customized implants. Zn-Mg alloys are among the most promising materials for bone repair. However, Zn-Mg alloys fabricated by l-PBF exhibit high strength but low ductility, while the effects and mechanisms of Mg alloying on degradation and osteogenesis remain unclear. This study first fabricated Zn-Mg bulk materials (0-1 wt% Mg) via l-PBF, clarifying their microstructure and degradation products. Mg enhanced mechanical strength, with Zn-0.4Mg achieving a balanced combination of strength and ductility. Mg slowed degradation by forming protective Mg-containing products and accelerating passivation, while promoting osteogenesis by releasing more Mg2+ and less Zn2+. These findings offer valuable insights for optimizing Zn-based metals and developing next-generation biodegradable implants.

Biodegradable Zn-Li-Mn alloy to achieve optimal strength and ductility for bone implants

Biodegradable zinc-based metals have received attention due to their strength, biodegradability, and desirable biocompatibility. However, the trade-off between strength and ductility has limited their use. Here, we designed a biodegradable Zn-Li-Mn ternary alloy with superior strength and ductility. The ultimate tensile strength (UTS) of Zn-0.4Li-xMn (x = 0.1, 0.4, and 0.8) alloys reached 438.74-469.96 MPa, similar to pure Ti, with elongation reaching 41.52%-54.91%, surpassing other Zn-Li-based alloys. We investigated the biodegradation behavior and osteogenic effects of the Zn-Li-Mn alloys both in vitro and in vivo. Immersion tests demonstrated that the alloys exhibited a more uniform degradation morphology with significantly less release of Zn2+ ion compared to pure Zn. Cytocompatibility, hemocompatibility, and histological analyses demonstrated their biosafety. In addition, Zn-Li-Mn alloy extracts significantly enhanced osteogenesis of human bone marrow-derived mesenchymal stem cells (hBMSCs), manifesting higher alkaline phosphatase activity, increased biomineralization, and elevated osteogenic gene expression. Zn-0.4Li-0.8Mn alloy showed the highest osteogenic activity in vitro. When implanted in rat femoral condyles, it demonstrated improved in vivo bone regeneration effects, exhibiting enhanced osteointegration. Transcriptomic analysis revealed that Zn2+, Mn2+, and Li+ ions released from Zn-Li-Mn alloy collectively activated the MAPK-ERK and Wnt/β-catenin signaling pathways, prompting osteogenic differentiation. These findings demonstrate the high potential of the Zn-0.4Li-0.8Mn alloy for bone implants. STATEMENT OF SIGNIFICANCE: 1. Biodegradable Zn-Li-Mn ternary alloy with superior mechanical strength and excellent ductility were designed. 2. Enhanced osteointegration were observed in Zn-0.4Li-0.8Mn implants in vivo. 3. Transcriptomic analysis revealed that the Zn2+, Mn2+, and Li+ released from Zn-0.4Li-0.8Mn collectively activated the MAPK-ERK and Wnt/β-catenin signaling pathways, enhancing osteogenesis.

Fabrication and Properties of Zn-Containing Intermetallic Compounds as Sacrificial Anodes of Zn-Based Implants

In the field of degradable metals, Zn-based implants have gradually gained more attention. However, the relatively slow degradation rate compared with the healing rate of the damaged bone tissue, along with the excessive Zn2+ release during the degradation process, limit the application of Zn-based implants. The use of intermetallic compounds with more negative electrode potentials as sacrificial anodes of Zn-based implants is likely to be a feasible approach to resolve this contradiction. In this work, three intermetallic compounds, MgZn2, CaZn13, and Ca2Mg6Zn3, were prepared. The phase structures, microstructures, and relevant properties, such as thermal stability, in vitro degradation properties, and cytotoxicity of the compounds, were investigated. The XRD patterns indicate that the MgZn2 and CaZn13 specimens contain single-phase MgZn2 and CaZn13, respectively, while the Ca2Mg6Zn3 specimen contains Mg2Ca and Ca2Mg6Zn3 phases. After purifying treatment in 0.9% NaCl solution, high purity Ca2Mg6Zn3 phase was obtained. Thermal stability tests suggest that the MgZn2 and CaZn13 specimens possess good thermal stability below 773 K. However, the Ca2Mg6Zn3 specimen melted at around 739.1 K. Polarization curve tests show that the corrosion potentials of MgZn2, CaZn13, and Ca2Mg6Zn3 in simulated body fluid (SBF) were -1.063 VSCE, -1.289 VSCE, and -1.432 VSCE, which were all more negative than that of the pure Zn specimen (-1.003 VSCE). Clearly, these compounds can act as sacrificial anodes in Zn-based implants. The immersion tests indicate that these compounds were degraded according to the atomic ratio of the elements in each compound. Besides that, the compounds can efficiently induce Ca-P deposition in SBF. Cytotoxicity tests demonstrate that the 10% extracts prepared from these compounds exhibit good cell activity on MC3T3-E1 cells.

Degradation Characteristics and Biocompatibility of Zinc Alloy in Advanced Biomedical Bone Implants

Biodegradable zinc-based alloys are regarded as a promising avenue of research for the development of bone fixation implants, offering potential solutions to clinical issues, such as stress shielding, secondary surgeries, and biocompatibility. In this study, a Zn-0.8Li-0.4Mg alloy was designed and fabricated and its potential for use as a clinical bone implant was evaluated. The alloy displays an ultimate tensile strength of 450 MPa and an elongation of 18%, thereby satisfying the requisite mechanical specifications for clinical bone implants. The results of the electrochemical and SBF in vitro corrosion tests indicate that the degradation mechanism evolves over time. The initial corrosion product layer is composed of a dense Li-containing corrosion product (LiOH/Li2CO3), which subsequently transforms into an Mg-containing corrosion product layer (MgO/Mg(OH)2) as corrosion progresses. Ultimately, due to the depletion of Li and the erosion by Cl-, it transitions to a corrosion product layer containing only the Zn and Ca/P layer. The overall degradation mechanism is jointly determined by the degree of local degradation and the corrosion resistance of the product layer. Cytotoxicity tests demonstrate that the Zn-0.8Li-0.4Mg alloy exhibits favorable biocompatibility.

Additively manufactured biodegradable Zn-Mn-based implants with an unprecedented balance of strength and ductility

Additively manufactured (AM) biodegradable zinc alloys hold huge potential as promising candidates for bone defect and fracture repair, thanks to their suitable biodegradation rates and acceptable biocompatibility. However, the mechanical properties of AM zinc alloys developed so far, ductility in particular, fall short of the requirements for bone substitution. Here, we present Zn-1Mn and Zn-1Mn-0.4Mg alloy implants with unique microstructures, fabricated using laser powder bed fusion (LPBF). Notably, the LPBF Zn-Mn-Mg alloy exhibited an extraordinary balance of strength and ductility, with an ultimate tensile strength of 289 MPa, yield strength of 213.5 MPa, and elongation over 20 %, outperforming all previously reported AM zinc alloys. The simultaneously enhanced strength and ductility of the ternary alloy were attributed to the strong grain-refining effect of the Mg2Zn11 second phase and the synthetic strengthening caused by the dispersion of the MnZn13 and Mg2Zn11 second phases inside the grains and at the grain boundaries. In addition, both alloys had similar rates of in vitro biodegradation (∼0.15 mm/year), properly aligned with the bone remodeling process, while also demonstrating favorable biocompatibility and upregulating multiple osteogenic markers. The Zn-Mn-Mg alloy showed even better osteogenic potential than the Zn-Mn alloy, owing to the addition of Mg. The combined attributes of the LPBF Zn-Mn-Mg ternary alloy indicated huge potential for its use as a bone repair material, especially for load-bearing bone fixation. STATEMENT OF SIGNIFICANCE: The mechanical properties of previously developed additively manufactured biodegradable zinc alloys, especially ductility, have not met the requirements for bone repair. Using laser powder bed fusion (LPBF), we fabricated Zn-1Mn and Zn-1Mn-0.4Mg alloy implants with unique microstructures. The LPBF Zn-Mn-Mg alloy demonstrated an exceptional balance of strength and ductility, achieving a tensile strength of 289 MPa, yield strength of 213.5 MPa, and elongation over 20 %, surpassing all reported AM zinc alloys. This study is the first to produce a directly printed biodegradable alloy meeting the mechanical requirements for bone fixation devices without post-processing. Additionally, the alloy exhibited moderate a biodegradation rate and excellent biocompatibility, underscoring its potential for load-bearing bone repair applications.

Clinical challenges in bone tissue engineering - A narrative review

Bone tissue engineering (BTE) has emerged as a promising approach to address large bone defects caused by trauma, infections, congenital malformations, and tumors. This review focuses on scaffold design, cell sources, growth factors, and vascularization strategies, highlighting their roles in developing effective treatments. We explore the complexities of balancing mechanical properties, porosity, and biocompatibility in scaffold materials, alongside optimizing mesenchymal stem cell delivery methods. The critical role of growth factors in bone regeneration and the need for controlled release systems are discussed. Vascularization remains a significant hurdle, with strategies such as angiogenic factors, co-culture systems, and bioprinting under investigation. Mechanical challenges, tissue responses, and inflammation management are examined, alongside gene therapy's potential for enhancing osteogenesis and angiogenesis via both viral and non-viral delivery methods. The review emphasizes the impact of patient-specific factors on bone healing outcomes and the importance of personalized approaches. Future directions are described, emphasizing the necessity of interdisciplinary cooperation to advance the field of BTE and convert laboratory results into clinically feasible solutions.

Precision pore structure optimization of additive manufacturing porous tantalum scaffolds for bone regeneration: A proof-of-concept study

Currently, the treatment of bone defects in arthroplasty is a challenge in clinical practice. Nonetheless, commercially available orthopaedic scaffolds have shown limited therapeutic effects for large bone defects, especially for massiveand irregular defects. Additively manufactured porous tantalum, in particular, has emerged as a promising material for such scaffolds and is widely used in orthopaedics for its exceptional biocompatibility, osteoinduction, and mechanical properties. Porous tantalum has also exhibited unique advantages in personalised rapid manufacturing, which allows for the creation of customised scaffolds with complex geometric shapes for clinical applications at a low cost and high efficiency. However, studies on the effect of the pore structure of additively manufactured porous tantalum on bone regeneration have been rare. In this study, our group designed and fabricated a batch of precision porous tantalum scaffolds via laser powder bed fusion (LPBF) with pore sizes of 250 μm (Ta 250), 450 μm (Ta 450), 650 μm (Ta 650), and 850 μm (Ta 850). We then performed a series of in vitro experiments and observed that all four groups showed good biocompatibility. In particular, Ta 450 demonstrated the best osteogenic performance. Afterwards, our team used a rat bone defect model to determine the in vivo osteogenic effects. Based on micro-computed tomography and histology, we identified that Ta 450 exhibited the best bone ingrowth performance. Subsequently, sheep femur and hip defect models were used to further confirm the osteogenic effects of Ta 450 scaffolds. Finally, we verified the aforementioned in vitro and in vivo results via clinical application (seven patients waiting for revision total hip arthroplasty) of the Ta 450 scaffold. The clinical results confirmed that Ta 450 had satisfactory clinical outcomes up to the 12-month follow-up. In summary, our findings indicate that 450 μm is the suitable pore size for porous tantalum scaffolds. This study may provide a new therapeutic strategy for the treatment of massive, irreparable, and protracted bone defects in arthroplasty.

Natural potential difference induced functional optimization mechanism for Zn-based multimetal bone implants

Zn-based biodegradable metals (BMs) are regarded as revolutionary biomaterials for bone implants. However, their clinical application is limited by insufficient mechanical properties, delayed in vivo degradation, and overdose-induced Zn2+ toxicity. Herein, innovative multi-material additive manufacturing (MMAM) is deployed to construct a Zn/titanium (Ti) hetero-structured composite. The biodegradation and biofunction of Zn exhibited intriguing characteristics in composites. A potential difference of about 300 mV naturally existed between Zn and Ti. This natural potential difference triggered galvanic coupling corrosion, resulting in 2.7 times accelerated degradation of Zn. The excess release of Zn2+ induced by accelerated degradation enhanced the antibacterial function. A voltage signal generated by the natural potential difference also promoted in vitro osteogenic differentiation through activating the PI3K-Akt signaling pathway, and inhibited the toxicity of overdose Zn2+ in vivo, significantly improving bone regeneration. Furthermore, MMAM technology allows for the specific region deployment of components. In the future, Ti and Zn could be respectively deployed in the primary and non-load-bearing regions of bone implants by structural designs, thereby achieving a functionally graded application to overcome the insufficient mechanical properties of Zn-based BMs. This work clarifies the functional optimization mechanism for multimetal bone implants, which possibly breaks the application dilemma of Zn-based BMs.

Corrosion behavior, antibacterial properties and in vitro and in vivo biocompatibility of biodegradable Zn-5Cu-xMg alloy for bone-implant applications

Reasonable optimization of degradation rate, antibacterial performance and biocompatibility is crucial for the development of biodegradable zinc alloy medical implant devices with antibacterial properties. In this study, various amounts of Mg elements were incorporated into Zn5Cu alloy to modulate the degradation rate, antibacterial properties and biocompatibility. The effects of Mg contents on the microstructure, corrosion behavior, antibacterial properties and biocompatibility of Zn-5Cu-xMg alloy were extensively investigated. The results revealed that with an increase of Mg content, the amount of Mg2Zn11 phase increased and its galvanic effect with the Zn matrix was enhanced, which accelerated the corrosion process and led to higher corrosion rate and high degradation rate of the alloy. Additionally, there was an increased release of Mg2+ and Zn2+ ions from the alloy which imparted excellent resistance against Escherichia coli and Staphylococcus aureus bacteria and improved biocompatibility, subcutaneous antibacterial and immune microenvironment regulation properties. Zn-5Cu-2 Mg exhibited superior antibacterial ability, cell compatibility, proliferation effect, subcutaneous antibacterial and immune microenvironment regulation performances, which can work as a promising candidate of biodegradable antibacterial medical implants.

Fabrication and performance of Zinc-based biodegradable metals: From conventional processes to laser powder bed fusion

Zinc (Zn)-based biodegradable metals (BMs) fabricated through conventional manufacturing methods exhibit adequate mechanical strength, moderate degradation behavior, acceptable biocompatibility, and bioactive functions. Consequently, they are recognized as a new generation of bioactive metals and show promise in several applications. However, conventional manufacturing processes face formidable limitations for the fabrication of customized implants, such as porous scaffolds for tissue engineering, which are future direction towards precise medicine. As a metal additive manufacturing technology, laser powder bed fusion (L-PBF) has the advantages of design freedom and formation precision by using fine powder particles to reliably fabricate metallic implants with customized structures according to patient-specific needs. The combination of Zn-based BMs and L-PBF has become a prominent research focus in the fields of biomaterials as well as biofabrication. Substantial progresses have been made in this interdisciplinary field recently. This work reviewed the current research status of Zn-based BMs manufactured by L-PBF, covering critical issues including powder particles, structure design, processing optimization, chemical compositions, surface modification, microstructure, mechanical properties, degradation behaviors, biocompatibility, and bioactive functions, and meanwhile clarified the influence mechanism of powder particle composition, structure design, and surface modification on the biodegradable performance of L-PBF Zn-based BM implants. Eventually, it was closed with the future perspectives of L-PBF of Zn-based BMs, putting forward based on state-of-the-art development and practical clinical needs.

Zinc based biodegradable metals for bone repair and regeneration: Bioactivity and molecular mechanisms

Bone fractures and critical-size bone defects are significant public health issues, and clinical treatment outcomes are closely related to the intrinsic properties of the utilized implant materials. Zinc (Zn)-based biodegradable metals (BMs) have emerged as promising bioactive materials because of their exceptional biocompatibility, appropriate mechanical properties, and controllable biodegradation. This review summarizes the state of the art in terms of Zn-based metals for bone repair and regeneration, focusing on bridging the gap between biological mechanism and required bioactivity. The molecular mechanism underlying the release of Zn ions from Zn-based BMs in the improvement of bone repair and regeneration is elucidated. By integrating clinical considerations and the specific bioactivity required for implant materials, this review summarizes the current research status of Zn-based internal fixation materials for promoting fracture healing, Zn-based scaffolds for regenerating critical-size bone defects, and Zn-based barrier membranes for reconstituting alveolar bone defects. Considering the significant progress made in the research on Zn-based BMs for potential clinical applications, the challenges and promising research directions are proposed and discussed.

In vitro and in vivo studies on biodegradable Zn porous scaffolds with a drug-loaded coating for the treatment of infected bone defect

Additively manufactured biodegradable zinc (Zn) scaffolds have great potential to repair infected bone defects due to their osteogenic and antibacterial properties. However, the enhancement of antibacterial properties depends on a high concentration of dissolved Zn2+, which in return deteriorates osteogenic activity. In this study, a vancomycin (Van)-loaded polydopamine (PDA) coating was prepared on pure Zn porous scaffolds to solve the above dilemma. Compared with pure Zn scaffolds according to comprehensive in vitro tests, the PDA coating resulted in a slow degradation and inhibited the excessive release of Zn2+ at the early stage, thus improving cytocompatibility and osteogenic activity. Meanwhile, the addition of Van drug substantially suppressed the attachment and proliferation of S. aureus and E. coli bacterial. Furthermore, in vivo implantation confirmed the simultaneously improved osteogenic and antibacterial functions by using the pure Zn scaffolds with Van-loaded PDA coating. Therefore, it is promising to employ biodegradable Zn porous scaffolds with the proposed drug-loaded coating for the treatment of infected bone defects.

Additively Manufactured Zn-2Mg Alloy Porous Scaffolds with Customizable Biodegradable Performance and Enhanced Osteogenic Ability

The combination of bioactive Zn-2Mg alloy and additively manufactured porous scaffold is expected to achieve customizable biodegradable performance and enhanced bone regeneration. Herein, Zn-2Mg alloy scaffolds with different porosities, including 40% (G-40-2), 60% (G-60-2), and 80% (G-80-2), and different unit sizes, including 1.5 mm (G-60-1.5), 2 mm (G-60-2), and 2.5 mm (G-60-2.5), are manufactured by a triply periodic minimal surface design and a reliable laser powder bed fusion process. With the same unit size, compressive strength (CS) and elastic modulus (EM) of scaffolds substantially decrease with increasing porosities. With the same porosity, CS and EM just slightly decrease with increasing unit sizes. The weight loss after degradation increases with increasing porosities and decreasing unit sizes. In vivo tests indicate that Zn-2Mg alloy scaffolds exhibit satisfactory biocompatibility and osteogenic ability. The osteogenic ability of scaffolds is mainly determined by their physical and chemical characteristics. Scaffolds with lower porosities and smaller unit sizes show better osteogenesis due to their suitable pore size and larger surface area. The results indicate that the biodegradable performance of scaffolds can be accurately regulated on a large scale by structure design and the additively manufactured Zn-2Mg alloy scaffolds have improved osteogenic ability for treating bone defects.

Fe-Zn alloy, a new biodegradable material capable of reducing ROS and inhibiting oxidative stress

Fe-based biodegradable materials have attracted significant attention due to their exceptional mechanical properties and favorable biocompatibility. Currently, research on Fe-based materials mainly focuses on regulating the degradation rate. However, excessive release of Fe ions during material degradation will induce the generation of reactive oxygen species (ROS), leading to oxidative stress and ferroptosis. Therefore, the control of ROS release and the improvement of biocompatibility for Fe-based materials are very important. In this study, new Fe-Zn alloys were prepared by electrodeposition with the intention of using Zn as an antioxidant to reduce oxidative damage during alloy degradation. Initially, the impact of three potential degradation ions (Fe2+, Fe3+, Zn2+) from the Fe-Zn alloy on human endothelial cell (EC) activity and migration ability was investigated. Subsequently, cell adhesion, cell activity, ROS production and DNA damage were assessed at various locations surrounding the alloy. Finally, the influence of different concentrations of Zn2+ in the medium on cell viability and ROS production was evaluated. High levels of ROS exhibited evident toxic effects on ECs and promoted DNA damage. As an antioxidant, Zn2+ effectively reduced ROS production around Fe and improved the cell viability on its surface at a concentration of 0.04 mmol/l. These findings demonstrate that Fe-Zn alloy can attenuate the ROS generated from Fe degradation thereby enhancing cytocompatibility.