Mg and Mg alloys: how comparable are in vitro and in vivo corrosion rates? A review

Treatment of osteoarthritis by implantation of Mg- and WE43-cylinders - A preclinical study on bone and cartilage changes and their influence on pain sensation in rabbits

With its main features of cartilage degeneration, subchondral bone sclerosis and osteophyte formation, osteoarthritis represents a multifactorial disease with no effective treatment options. As biomechanical shift in the trabecular network may be a driver for further cartilage degeneration, bone enhancement could possibly delay OA progression. Magnesium is known to be osteoconductive and already showed positive effects in OA models. We aimed to use magnesium cylinders to enhance subchondral bone quality, condition of cartilage and pain sensation compared to sole drilling in vivo. After eight weeks of implantation in rabbits, significant increase in subchondral bone volume and trabecular thickness with constant bone mineral density was found indicating favored biomechanics. As representative for pain, a higher number of CD271+ vessels were present in control samples without magnesium. However, this result could not be confirmed by sensitive, objective lameness evaluation using a pressure sensing mat and no positive effect could be shown on either cartilage degeneration evaluated by OARSI score nor the presence of regenerative cells in CD271-stained samples. The presented results show a relevant impact of implanted magnesium on key structures in OA pain with missing clinical relevance regarding pain. Further studies with shifted focus should examine additional structures as joint capsule or osteophytes.

An overview of magnesium-based implants in orthopaedics and a prospect of its application in spine fusion

Due to matching biomechanical properties and significant biological activity, Mg-based implants present great potential in orthopedic applications. In recent years, the biocompatibility and therapeutic effect of magnesium-based implants have been widely investigated in trauma repair. In contrast, the R&D work of Mg-based implants in spinal fusion is still limited. This review firstly introduced the general background for Mg-based implants. Secondly, the mechanical properties and degradation behaviors of Mg and its traditional and novel alloys were reviewed. Then, different surface modification techniques of Mg-based implants were described. Thirdly, this review comprehensively summarized the biological pathways of Mg degradation to promote bone formation in neuro-musculoskeletal circuit, angiogenesis with H-type vessel formation, osteogenesis with osteoblasts activation and chondrocyte ossification as an integrated system. Fourthly, this review followed the translation process of Mg-based implants via updating the preclinical studies in fracture fixation, sports trauma repair and reconstruction, and bone distraction for large bone defect. Furthermore, the pilot clinical studies were involved to demonstrate the reliable clinical safety and satisfactory bioactive effects of Mg-based implants in bone formation. Finally, this review introduced the background of spine fusion surgeryand the challenges of biological matching cage development. At last, this review prospected the translation potential of a hybrid Mg-PEEK spine fusion cage design.

Challenges and Pitfalls of Research Designs Involving Magnesium-Based Biomaterials: An Overview

Magnesium-based biomaterials hold remarkable promise for various clinical applications, offering advantages such as reduced stress-shielding and enhanced bone strengthening and vascular remodeling compared to traditional materials. However, ensuring the quality of preclinical research is crucial for the development of these implants. To achieve implant success, an understanding of the cellular responses post-implantation, proper model selection, and good study design are crucial. There are several challenges to reaching a safe and effective translation of laboratory findings into clinical practice. The utilization of Mg-based biomedical devices eliminates the need for biomaterial removal surgery post-healing and mitigates adverse effects associated with permanent biomaterial implantation. However, the high corrosion rate of Mg-based implants poses challenges such as unexpected degradation, structural failure, hydrogen evolution, alkalization, and cytotoxicity. The biocompatibility and degradability of materials based on magnesium have been studied by many researchers in vitro; however, evaluations addressing the impact of the material in vivo still need to be improved. Several animal models, including rats, rabbits, dogs, and pigs, have been explored to assess the potential of magnesium-based materials. Moreover, strategies such as alloying and coating have been identified to enhance the degradation rate of magnesium-based materials in vivo to transform these challenges into opportunities. This review aims to explore the utilization of Mg implants across various biomedical applications within cellular (in vitro) and animal (in vivo) models.

Finite element method-based simulation on bone fracture fixation configuration factors for biodegradable embossed locking compression plate

As an evolution, biodegradable implants need to maximize mechanical performance thereby may lead to confusion in selection of the biodegradable material and implant design to the fracture site. This requires selecting a unique fixation configuration to fit within the fractured bone, factors of which can be bone-plate clearance, interfragmentary gap, alteration in screw fixation position and variation in the number of screws whose configuration optimization can re-maximize the mechanical performance of the biodegradable implant. Therefore, these factors have been optimized based on the induced minimum stress using the finite element method-based simulation for which biodegradable embossed locking plates (BELCP) via screws made of Mg-alloy have been fitted over two fragments of femur body (as hollow cylindrical cortical bone). An average human weight of 62 kg is applied to one segment of the femur for all different configurations of each factor, where another segment is assumed to be fixed. By this simulation, the most optimal fixation configuration was found at a minimum induced stress value of 41.96 MPa which is approximately 85%, 18%, 6% and 48% respectively less than all maximum stress induced configurations in each of the factor. This optimized configuration was at the minimum clearance between bone and plate with a 3 mm interfragmentary gap using 8 screws where the locking screw begins to apply from the center of the BELCP. Overall, BELCP may be a better biodegradable implant plate for bone fracture fixation with these optimized fixation configurations as the improved mechanical performance after experimental validation.

In vitro calibration and in vivo validation of phenomenological corrosion models for resorbable magnesium-based orthopaedic implants

Metallic bioresorbable orthopaedic implants based on magnesium, iron and zinc-based alloys that provide rigid internal fixation without foreign-body complications associated with permanent implants have great potential as next-generation orthopaedic devices. Magnesium (Mg) based alloys exhibit excellent biocompatibility. However, the mechanical performance of such implants for orthopaedic applications is contingent on limiting the rate of corrosion in vivo throughout the bone healing process. Additionally, the surgical procedure for the implantation of internal bone fixation devices may impart plastic deformation to the device, potentially altering the corrosion rate of the device. The primary objective of this study was to develop a computer-based model for predicting the in vivo corrosion behaviour of implants manufactured from a Mg-1Zn-0.25Ca ternary alloy (ZX10). The proposed corrosion model was calibrated with an extensive range of mechanical and in vitro corrosion testing. Finally, the model was validated by comparing the in vivo corrosion performance of the implants during preliminary animal testing with the corrosion performance predicted by the model. The proposed model accurately predicts the in vitro corrosion rate, while overestimating the in vivo corrosion rate of ZX10 implants. Overall, the model provides a "first-line of design" for the development of new bioresorbable Mg-based orthopaedic devices. STATEMENT OF SIGNIFICANCE: Biodegradable metallic orthopaedic implant devices have emerged as a potential alternative to permanent implants, although successful adoption is contingent on achieving an acceptable degradation profile. A reliable computational method for accurately estimating the rate of biodegradation in vivo would greatly accelerate the development of resorbable orthopaedic implants by highlighting the potential risk of premature implant failure at an early stage of the device development. Phenomenological corrosion modelling approach is a promising computational tool for predicting the biodegradation of implants. However, the validity of the models for predicting the in vivo biodegradation of Mg alloys is yet to be determined. Present study investigates the validity of the phenomenological modelling approach for simulating the biodegradation of resorbable metallic orthopaedic implants by using a porcine model that targets craniofacial applications.

Magnesium-lithium thin films for neurological applications-An in vitro investigation of glial cytocompatibility and neuroinflammatory response

Lithium (Li), a widely used drug for bipolar disorder management, is associated with many side effects due to systemic exposure. The localized delivery of lithium through implants could be an approach to overcome this challenge, for which biodegradable magnesium (Mg)-based materials are a promising choice. In this study, we focus on Mg-Li thin film alloys as potential Li-releasing implants. Therefore, we investigated the in vitro short-term corrosion behavior and cytocompatibility of two alloys, Mg-1.6wt%Li and Mg-9.5wt%Li. As glial cells are the key players of foreign body responses to implants, we used human glial cell lines for cytocompatibility studies, and a murine brain slice model for a more holistic view at the neuroinflammatory response. We found that Mg-1.6wt%Li corrodes approximately six times slower than Mg-9.5wt%Li. Microscopic analysis showed that the material surface (Mg-1.6wt%Li) is suitable for cell adhesion. The cytocompatibility test with Mg-1.6wt%Li and Mg-9.5wt%Li alloy extracts revealed that both cell types proliferated well up to 10 mM Mg concentration, irrespective of the Li concentration. In the murine brain slice model, Mg-1.6wt%Li and Mg-9.5wt%Li alloy extracts did not provoke a significant upregulation of glial inflammatory/ reactivity markers (IL-1β, IL-6, FN1, TNC) after 24 h of exposure. Furthermore, the gene expression of IL-1β (up to 3-fold) and IL-6 (up to 16-fold) were significantly downregulated after 96 h, and IL-6 downregulation showed a Li concentration dependency. Together, these results indicate the acute cytocompatibility of two Mg-Li thin film alloys and provide basis for future studies to explore promising applications of the material. STATEMENT OF SIGNIFICANCE: We propose the idea of lithium delivery to the brain via biodegradable implants to reduce systemic side effects of lithium for bipolar disorder therapy and other neurological applications. This is the first in vitro study investigating Mg-xLi thin film degradation under physiological conditions and its influence on cellular responses such as proliferation, viability, morphology and inflammation. Utilizing human brain-derived cell lines, we showed that the material surface of such a thin film alloy is suitable for normal cell attachment. Using murine brain slices, which comprise a multicellular network, we demonstrated that the material extracts did not elicit a pro-inflammatory response. These results substantiate that degradable Mg-Li materials are biocompatible and support the further investigation of their potential as neurological implants.

Influence of Magnesium Degradation on Schwannoma Cell Responses to Nerve Injury Using an In Vitro Injury Model

Nerve guidance conduits for peripheral nerve injuries can be improved using bioactive materials such as magnesium (Mg) and its alloys, which could provide both structural and trophic support. Therefore, we investigated whether exposure to Mg and Mg-1.6wt%Li thin films (Mg/Mg-1.6Li) would alter acute Schwann cell responses to injury. Using the RT4-D6P2T Schwannoma cell line (SCs), we tested extracts from freeze-killed cells (FKC) and nerves (FKN) as in vitro injury stimulants. Both FKC and FKN induced SC release of the macrophage chemoattractant protein 1 (MCP-1), a marker of the repair SC phenotype after injury. Next, FKC-stimulated cells exposed to Mg/Mg-1.6Li reduced MCP-1 release by 30%, suggesting that these materials could have anti-inflammatory effects. Exposing FKC-treated cells to Mg/Mg-1.6Li reduced the gene expression of the nerve growth factor (NGF), glial cell line-derived neurotrophic factor (GDNF), and myelin protein zero (MPZ), but not the p75 neurotrophin receptor. In the absence of FKC, Mg/Mg-1.6Li treatment increased the expression of NGF, p75, and MPZ, which can be beneficial to nerve regeneration. Thus, the presence of Mg can differentially alter SCs, depending on the microenvironment. These results demonstrate the applicability of this in vitro nerve injury model, and that Mg has wide-ranging effects on the repair SC phenotype.

Anticorrosive properties of chitosan-derivatives coatings on Mg AZ31 alloy in Hank's Balanced Salt Solution

This study describes the preparation of chitosan-derivatives coatings on AZ31 Mg alloy for corrosion protection in Hank's Balanced Salt Solution (HBSS). The derivatives were prepared by reacting chitosan with natural aldehydes (vanillin, benzaldehyde and cinnamaldehyde) and the coatings were characterized by means of water contact angle, scanning electron microscopy and swelling essays. The corrosion behavior of the samples was investigated using potentiodynamic polarization, electrochemical impedance spectroscopy and hydrogen evolution essays. All derivatives present superior corrosion protection than neat chitosan and the best performance is observed for the vanillin derivative with the highest modification degree, which present hydrogen evolution rate of 0.05 mL cm-2 day-1, below the tolerance limit for biomedical application, and |Z|max in the order of 104.6 Ω cm2 even after 14 days of exposure to the corrosive solution.

How surface-to-volume ratio affects degradation of magnesium: in vitro and in vivo studies

Despite the many studies carried out over the past decade to determine the biodegradation performance of magnesium and its alloys, few studies focused on the effect of altered surface area to volume ratio on in vitro and in vivo degradation rate and osteogenesis. Here, high purity magnesium cylindrical rods with gradient of surface area to volume ratio were processed by excavating different numbers of grooves on the side surface. The immersion test in SBF solution and the rat femoral condylar bone defect model were used to evaluate the degradation of magnesium rods in vitro and in vivo, respectively. We demonstrated that, the increased number of grooves on the HP magnesium surface represented a decrease in the percentage of residual volume over time, not necessarily an increase in absolute degradation volume or a regular change in corrosion rate. Furthermore, there were strong linear correlations between the relative degradation volume and the initial surface-to-volume ratio of HP magnesium rods both in vitro and in vivo. The difference in the slope of this relationship in vitro and in vivo might help to determine the possible range of in vivo degradation rates via in vitro data. In addition, the corrosion rate is more suitable for evaluating bone formation surrounding the different HP magnesium rods. Our findings in this work may facilitate adjusting the in vivo degradation and osteogenesis of different kinds of orthopedic implants made of the same magnesium-based material, and thus, accelerate the clinical popularization and application.

In vivo chronic scaffolding force of a resorbable magnesium scaffold

The aim of this study is to qualitatively characterize the in vivo chronic scaffolding force of the Magmaris® Resorbable Magnesium Scaffold (RMS). This important parameter of scaffolds must be balanced between sufficient radial support during the healing period of the vessel and avoidance of long-term vessel caging. A finite element model was established using preclinical animal data and used to predict the device diameter and scaffolding force up to 90 days after implantation. To account for scaffold resorption, it included backbone degradation as well as formation of discontinuities as observed in vivo. The predictions of the model regarding acute recoil and chronic development of the device diameter were in good agreement with the preclinical data, supporting the validity of the model. It was found that after 28 and 90 days, the Magmaris® RMS retained 90 % and 47 % of its initial scaffolding force, respectively. The reduction in scaffolding force was mainly driven by discontinuities in the meandering segments. Finite element analysis combined with preclinical data is a reliable method to characterize the chronic scaffolding force.

In vitro and in vivo study on fine-grained Mg-Zn-RE-Zr alloy as a biodegradeable orthopedic implant produced by friction stir processing

Magnesium alloys containing biocompatible components show tremendous promise for applications as temporary biomedical devices. However, to ensure their safe use as biodegradeable implants, it is essential to control their corrosion rates. In concentrated Mg alloys, a microgalvanic coupling between the α-Mg matrix and secondary precipitates exists which results in increased corrosion rate. To address this challenge, we engineered the microstructure of a biodegradable Mg-Zn-RE-Zr alloy by friction stir processing (FSP), improving its corrosion resistance and mechanical properties simultaneously. The FS processed alloy with refined grains and broken and uniformly distributed secondary precipitates showed a relatively uniform corrosion morphology accompanied with the formation of a stable passive layer on the alloy surface. In vivo corrosion evaluation of the processed alloy in a small animal model showed that the material was well-tolerated with no signs of inflammation or harmful by-products. Remarkably, the processed alloy supported bone until it healed till eight weeks with a low in vivo corrosion rate of 0.7 mm/year. Moreover, we analyzed blood and histology of the critical organs such as liver and kidney, which showed normal functionality and consistent ion and enzyme levels, throughout the 12-week study period. These results demonstrate that the processed Mg-Zn-RE-Zr alloy offers promising potential for osseointegration in bone tissue healing while also exhibiting controlled biodegradability due to its engineered microstructure. The results from the present study will have profound benefit for bone fracture management, particularly in pediatric and elderly patients.

Physiomimetic biocompatibility evaluation of directly printed degradable porous iron implants using various cell types

Additively manufactured (AM) degradable porous metallic biomaterials offer unique opportunities for satisfying the design requirements of an ideal bone substitute. Among the currently available biodegradable metals, iron has the highest elastic modulus, meaning that it would benefit the most from porous design. Given the successful preclinical applications of such biomaterials for the treatment of cardiovascular diseases, the moderate compatibility of AM porous iron with osteoblast-like cells, reported in earlier studies, has been surprising. This may be because, as opposed to static in vitro conditions, the biodegradation products of iron in vivo are transported away and excreted. To better mimic the in situ situations of biodegradable biomaterials after implantation, we compared the biodegradation behavior and cytocompatibility of AM porous iron under static conditions to the conditions with dynamic in situ-like fluid flow perfusion in a bioreactor. Furthermore, the compatibility of these scaffolds with four different cell types was evaluated to better understand the implications of these implants for the complex process of natural wound healing. These included endothelial cells, L929 fibroblasts, RAW264.7 macrophage-like cells, and osteoblastic MG-63 cells. The biodegradation rate of the scaffolds was significantly increased in the perfusion bioreactor as compared to static immersion. Under either condition, the compatibility with L929 cells was the best. Moreover, the compatibility with all the cell types was much enhanced under physiomimetic dynamic flow conditions as compared to static biodegradation. Our study highlights the importance of physiomimetic culture conditions and cell type selection when evaluating the cytocompatibility of degradable biomaterials in vitro. STATEMENT OF SIGNIFICANCE: Additively manufactured (AM) degradable porous metals offer unique opportunities for the treatment of large bony defects. Despite the successful preclinical applications of biodegradable iron in the cardiovascular field, the moderate compatibility of AM porous iron with osteoblast-like cells was reported. To better mimic the in vivo condition, we compared the biodegradation behavior and cytocompatibility of AM porous iron under static condition to dynamic perfusion. Furthermore, the compatibility of these scaffolds with various cell types was evaluated to better simulate the process of natural wound healing. Our study suggests that AM porous iron holds great promise for orthopedic applications, while also highlighting the importance of physio-mimetic culture conditions and cell type selection when evaluating the cytocompatibility of degradable biomaterials in vitro.

Biodegradation and Cell Behavior of a Mg-Based Composite with Mesoporous Bioglass

Biodegradable magnesium (Mg) and its alloys show tremendous potential as orthopedic materials. Nevertheless, the fast degradation and insufficient osteogenic properties hinder their applications. In this study, mesoporous bioglass (MBG) with an ordered branch-like structure was synthesized via a modified sol-gel method and showed a high specific surface area of 656.45 m2/g. A Mg-based composite was prepared by introducing the MBG into a Mg matrix via powder metallurgy. Degradation tests showed that the introduction of MBG increased the adsorption sites for Ca and P ions, thus promoting the formation of a Ca-P protective layer on the Mg matrix. The Ca-P protective layer became thick and dense with an increase in the immersion time, improving the protection ability of the Mg matrix, as proven by electrochemical impedance spectroscopy measurements. Meanwhile, the Mg-based composite also exhibited excellent biocompatibility and osteogenic properties. This study demonstrated the advantages of MBG in the preparation of Mg-based bone implants and validated the feasibility of improving Mg matrix corrosion resistance and enhancing osteogenesis by introducing MBG.

Bioactivity Features of a Zn-1%Mg-0.1%Dy Alloy Strengthened by Equal-Channel Angular Pressing

The structure, phase composition, corrosion and mechanical properties, as well as aspects of biocompatibility in vitro and in vivo, of a Zn-1%Mg-0.1%Dy alloy after equal-channel angular pressing (ECAP) were studied. The structure refinement after ECAP leads to the formation of elongated α-Zn grains with a width of ~10 µm and of Mg- and Dy-containing phases. In addition, X-ray diffraction analysis demonstrated that ECAP resulted in the formation of the basal texture in the alloy. These changes in the microstructure and texture lead to an increase in ultimate tensile strength up to 262 ± 7 MPa and ductility up to 5.7 ± 0.2%. ECAP slows down the degradation process, apparently due to the formation of a more homogeneous microstructure. It was found that the alloy degradation rate in vivo after subcutaneous implantation in mice is significantly lower than in vitro ones. ECAP does not impair biocompatibility in vitro and in vivo of the Zn-1%Mg-0.1%Dy alloy. No signs of suppuration, allergic reactions, the formation of visible seals or skin ulcerations were observed after implantation of the alloy. This may indicate the absence of an acute reaction of the animal body to the Zn-1%Mg-0.1%Dy alloy in both states.

Benchtop proof of concept and comparison of iron- and magnesium-based bioresorbable flow diverters

OBJECTIVE

Bioresorbable flow diverters (BRFDs) could significantly improve the performance of next-generation flow diverter technology. In the current work, magnesium and iron alloy BRFDs were prototyped and compared in terms of porosity/pore density, radial strength, flow diversion functionality, and resorption kinetics to offer insights into selecting the best available bioresorbable metal candidate for the BRFD application.

METHODS

BRFDs were constructed with braided wires made from alloys of magnesium (MgBRFD) or iron (FeBRFD). Pore density and crush resistance force were measured using established methods. BRFDs were deployed in silicone aneurysm models attached to flow loops to investigate flow diversion functionality and resorption kinetics in a simulated physiological environment.

RESULTS

The FeBRFD exhibited higher pore density (9.9 vs 4.3 pores/mm2) and crush resistance force (0.69 ± 0.05 vs 0.53 ± 0.05 N/cm, p = 0.0765, n = 3 per group) than the MgBRFD, although both crush resistances were within the range previously reported for FDA-approved flow diverters. The FeBRFD demonstrated greater flow diversion functionality than the MgBRFD, with significantly higher values of established flow diversion metrics (mean transit time 159.6 ± 11.9 vs 110.9 ± 1.6, p = 0.015; inverse washout slope 192.5 ± 9.0 vs 116.5 ± 1.5, p = 0.001; n = 3 per group; both metrics expressed as a percentage of the control condition). Last, the FeBRFD was able to maintain its braided structure for > 12 weeks, whereas the MgBRFD was almost completely resorbed after 5 weeks.

CONCLUSIONS

The results of this study demonstrated the ability to manufacture BRFDs with magnesium and iron alloys. The data suggest that the iron alloy is the superior material candidate for the BRFD application due to its higher mechanical strength and lower resorption rate relative to the magnesium alloy.

In Vivo Analysis of a Biodegradable Magnesium Alloy Implant in an Animal Model Using Near-Infrared Spectroscopy

Biodegradable magnesium-based implants offer mechanical properties similar to natural bone, making them advantageous over nonbiodegradable metallic implants. However, monitoring the interaction between magnesium and tissue over time without interference is difficult. A noninvasive method, optical near-infrared spectroscopy, can be used to monitor tissue's functional and structural properties. In this paper, we collected optical data from an in vitro cell culture medium and in vivo studies using a specialized optical probe. Spectroscopic data were acquired over two weeks to study the combined effect of biodegradable Mg-based implant disks on the cell culture medium in vivo. Principal component analysis (PCA) was used for data analysis. In the in vivo study, we evaluated the feasibility of using the near-infrared (NIR) spectra to understand physiological events in response to magnesium alloy implantation at specific time points (Day 0, 3, 7, and 14) after surgery. Our results show that the optical probe can detect variations in vivo from biological tissues of rats with biodegradable magnesium alloy "WE43" implants, and the analysis identified a trend in the optical data over two weeks. The primary challenge of in vivo data analysis is the complexity of the implant interaction near the interface with the biological medium.

Recent research advances on corrosion mechanism and protection, and novel coating materials of magnesium alloys: a review

Magnesium alloys have achieved a good balance between biocompatibility and mechanical properties, and have great potential for clinical application, and their performance as implant materials has been continuously improved in recent years. However, a high degradation rate of Mg alloys in a physiological environment remains a major limitation before clinical application. In this review, according to the human body's intake of elements, the current mainstream implanted magnesium alloy system is classified and discussed, and the corrosion mechanism of magnesium alloy in vivo and in vitro is described, including general corrosion, localized corrosion, pitting corrosion, and degradation of body fluid environment impact etc. The introduction of methods to improve the mechanical properties and biocorrosion resistance of magnesium alloys is divided into two parts: the alloying part mainly discusses the strengthening mechanisms of alloying elements, including grain refinement strengthening, solid solution strengthening, dislocation strengthening and precipitation strengthening etc.; the surface modification part introduces the ideas and applications of novel materials with excellent properties such as graphene and biomimetic materials in the development of functional coatings. Finally, the existing problems are summarized, and the future development direction is prospected.

Zn doped CaP coatings used for controlling the degradation rate of MgCa1 alloy: In vitro anticorrosive properties, sterilization and bacteria/cell-material interactions

The purpose of this study was to investigate the effect of Zn doped CaP coatings prepared by micro-arc oxidation method, as a possible approach to control MgCa1 alloy degradation. All the prepared coatings comprised a calcium deficient CaP phase. The control in this evaluation was performed with undoped CaP coating in SBF solution at body temperature (37 ± 0.5⁰C). The investigation involved determination of microchemical, mechanical, morphological, properties along with anticorrosive, cytocompatibility and antibacterial efficacy. The effect of sterilization process on the properties of the surfaces was also investigated. The results showed that the addition of Zn into CaP increased the corrosion resistance of MgCa1 alloy. Moreover, the adhesion strength of the coatings to MgCa1 alloy was enhanced by Zn addition. In cytotoxicity testing of the samples, extracts of the samples in MEM were incubated with L929 cells and malformation, degeneration and lysis of the cells were examined microscopically after 72 h. The results showed that all samples were cytocompatible. The degradation of MgCa1 alloy in the simulated body fluids (SBF) or DMEM was decreased by coating with CaP. Moreover, the degradation rate of CaP was further decreased by adding a small amount of Zn into the CaP matrix. The samples having CaP coatings and Zn doped CaP coating demonstrated antibacterial efficacy against E.coli. As a result, coating of magnesium alloy with Zn-doped CaP decreased the degradation rate, increased the corrosion resistance, cytocompatibility and the antibacterial effects of the alloys.

Long-term in vivo observations show biocompatibility and performance of ZX00 magnesium screws surface-modified by plasma-electrolytic oxidation in Göttingen miniature pigs

Bioabsorbable magnesium implants for orthopedic fixation of bone have recently become available for different fields of indication. While general questions of biocompatibility have been answered, tailoring suitable degradation kinetics for specific applications as well as long-term tissue integration remain the focus of current research. The aim of this study was the evaluation of the long-term degradation behavior and osseointegration of Mg-Ca-Zn (ZX00MEO) based magnesium implants with plasma-electrolytic oxidation (PEO) surface modification (ZX00MEO-PEO) in comparison to non-surface modified implants in vivo and in vitro. Besides a general evaluation of the biological performance of the alloy over a prolonged period, the main hypothesis was that PEO surface modification significantly reduces implant degradation rate and improves tissue interaction. In vitro, the microstructure and surface of the bioabsorbable screws were characterized by SEM/EDS, cytocompatibility and degradation testing facilitating hydrogen gas evolution, carried out following ISO 10993-5/-12 and ASTM F3268-18a/ASTM G1-03 (E1:2017). In vivo, screws were implanted in the frontal bone of Minipigs for 6, 12, and 18 months, following radiological and histomorphometric analysis. A slower and more uniform degradation and improved cytocompatibility could be shown for the ZX00MEO-PEO group in vitro. A significant reduction of degradation rate and enhanced bone formation around the ZX00MEO-PEO screws in vivo was confirmed. Proficient biocompatibility and tissue integration could generally be shown in vivo regardless of surface state. The tested magnesium alloy shows generally beneficial properties as an implant material, while PEO-surface modification further improves the bioabsorption behavior both in vitro and in vivo. STATEMENT OF SIGNIFICANCE: Devices from bioabsorbable Magnesium have recently been introduced to orthopedic applications. However, the vast degradation of Magnesium within the human body still gives limitations. While reliable in-vivo data on most promising surface treatments such as Plasma-electrolytic-Oxidation is generally scarce, long-time results in large animals are to this date completely missing. To overcome this lack of evidence, we studied a Magnesium-Calzium-Zinc-alloy with surface enhancement by PEO for the first time ever over a period of 18 months in a large animal model. In-vitro, surface-modified screws showed significantly improved cytocompatibility and reduction of degradation confirmed by hydrogen gas evolution testing, while in-vivo radiological and histological evaluation generally showed good biocompatibility and bioabsorption as well as significantly enhanced reduction of degradation and faster bone regeneration in the PEO-surface-modified group.

Open-porous magnesium-based scaffolds withstand in vitro corrosion under cyclic loading: A mechanistic study

The successful application of magnesium (Mg) alloys as biodegradable bone substitutes for critical-sized defects may be comprised by their high degradation rate resulting in a loss of mechanical integrity. This study investigates the degradation pattern of an open-porous fluoride-coated Mg-based scaffold immersed in circulating Hanks' Balanced Salt Solution (HBSS) with and without in situ cyclic compression (30 N/1 Hz). The changes in morphological and mechanical properties have been studied by combining in situ high-resolution X-ray computed tomography mechanics and digital volume correlation. Although in situ cyclic compression induced acceleration of the corrosion rate, probably due to local disruption of the coating layer where fatigue microcracks were formed, no critical failures in the overall scaffold were observed, indicating that the mechanical integrity of the Mg scaffolds was preserved. Structural changes, due to the accumulation of corrosion debris between the scaffold fibres, resulted in a significant increase (p < 0.05) in the material volume fraction from 0.52 ± 0.07 to 0.47 ± 0.03 after 14 days of corrosion. However, despite an increase in fibre material loss, the accumulated corrosion products appear to have led to an increase in Young's modulus after 14 days as well as lower third principal strain (εp3) accumulation (−91000 ± 6361 με and −60093 ± 2414 με after 2 and 14 days, respectively). Therefore, this innovative Mg scaffold design and composition provide a bone replacement, capable of sustaining mechanical loads in situ during the postoperative phase allowing new bone formation to be initially supported as the scaffold resorbs.

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First report on in vitro cyclic loading of MgF₂ coated open-porous Mg scaffolds in HBSS simulating 2–3 months in humans.

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Fluoride-coating slows down corrosion under cyclic loading in vitro.

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Entangled scaffold structure accumulates local corrosion debris which keeps the mechanical integrity over 14 days in vitro.

Biomechanical Comparison of WE43-Based Magnesium vs. Titanium Miniplates in a Mandible Fracture Model in Sheep

In fractures of the mandible, osteosynthesis with titanium plates is considered the gold standard. Titanium is an established and reliable material, its main disadvantages being metal artefacts and the need for removal in case of osteosynthesis complications. Magnesium, as a resorbable material with an elastic modulus close to cortical bone, offers a resorbable alternative osteosynthesis material, yet mechanical studies in mandible fracture fixation are still missing. The hypothesis of this study was that magnesium miniplates show no significant difference in the mechanical integrity provided for fracture fixation in mandible fractures under load-sharing indications. In a non-inferiority test, a continuous load was applied to a sheep mandible fracture model with osteosynthesis using either titanium miniplates of 1.0 mm thickness (Ti1.0), magnesium plates of 1.75 mm (Mg1.75), or magnesium plates of 1.5 mm thickness (Mg1.5). No significant difference (p > 0.05) was found in the peak force at failure, stiffness, or force at vertical displacement of 1.0 mm between Mg1.75, Mg1.5, and Ti1.0. This study shows the non-inferiority of WE43 magnesium miniplates compared to the clinical gold standard titanium miniplates.

Performance of PEO/Polymer Coatings on the Biodegradability, Antibacterial Effect and Biocompatibility of Mg-Based Materials

Magnesium (Mg) alloys have recently attracted attention in biomedicine as biodegradable materials with non-toxic degradable products. Such compounds have become a frontier in the study of biodegradable materials because of their remarkable biomechanical compatibility and superior biocompatibility. The use of Mg-based implants reduces the negative consequences of permanent biological implants by eliminating the necessity for biomaterial surgery following the healing process. However, the quick deterioration, formation of considerable gas of hydrogen volumes and a rise in the body environment pH are obstacles in the application of Mg as an implant material. Hence, compelling advances for erosion resistance and biocompatibility of magnesium and its alloys are noteworthy. Surface modification may be a practical approach because it improves the erosion resistance compared with extensive preparation of a treated surface for progressed bone recovery and cell attachment. Coating produced by plasma electrolytic oxidation (PEO) seems a compelling method in order to enhance magnesium and the properties of its alloys. PEO-formed coatings cannot provide long-term protection in the physiological environment due to their porous nature. Thus, a polymer coating is applied on the porous PEO-formed coating, which is steadily applied on the surface. Polymer coatings improve the biocompatibility properties of Mg and its alloys and increase corrosion resistance. In this article, the most recent advancements in PEO/polymer composite coatings are reviewed, and the biocompatibility of such coatings is examined.

Effect of pH fluctuations on the biodegradability of nanocomposite Mg-alloy in simulated bodily fluids

According to the National Institute of Health, the biodegradability, non-toxic nature, and remarkable natural and mechanical properties of magnesium and its components make them desirable choices for use in the production of supplies for biomedical implantation. Simulated bodily fluid (SBF) is used as a standard electrolyte for in vitro corrosion research. Each SBF module's independent and synergistic corrosion effects are studied in this study. Artificial pH variations increase degradation, according to the results. This experiment examined the Mg corrosion submerged in a SBF solution. The effect of pH changes on the rate of corrosion of Mg immersed in standard SBF solution was investigated. According to the previously published study, the corrosion process of Mg has been confirmed by scanning electron microscopy observations of damaged surface morphology. Because of these investigations, pH 7 was selected as the pH for bodily fluids since it is neutral.

Degradation behavior and osseointegration of Mg-Zn-Ca screws in different bone regions of growing sheep: a pilot study

Magnesium (Mg)-based implants are highly attractive for the orthopedic field and may replace titanium (Ti) as support for fracture healing. To determine the implant-bone interaction in different bony regions, we implanted Mg-based alloy ZX00 (Mg < 0.5 Zn < 0.5 Ca, in wt%) and Ti-screws into the distal epiphysis and distal metaphysis of sheep tibiae. The implant degradation and osseointegration were assessed in vivo and ex vivo after 4, 6 and 12 weeks, using a combination of clinical computed tomography, medium-resolution micro computed tomography (µCT) and high-resolution synchrotron radiation µCT (SRµCT). Implant volume loss, gas formation and bone growth were evaluated for both implantation sites and each bone region independently. Additionally, histological analysis of bone growth was performed on embedded hard-tissue samples. We demonstrate that in all cases, the degradation rate of ZX00-implants ranges between 0.23 and 0.75 mm/year. The highest degradation rates were found in the epiphysis. Bone-to-implant contact varied between the time points and bone types for both materials. Mostly, bone-volume-to-total-volume was higher around Ti-implants. However, we found an increased cortical thickness around the ZX00-screws when compared with the Ti-screws. Our results showed the suitability of ZX00-screws for implantation into the distal meta- and epiphysis.

Modulation of Osteogenesis and Angiogenesis Activities Based on Ionic Release from Zn-Mg Alloys

The enhancement of osteogenesis and angiogenesis remains a great challenge for the successful regeneration of engineered tissue. Biodegradable Mg and Zn alloys have received increasing interest as potential biodegradable metallic materials, partially due to the biological functions of Mg²⁺ and Zn²⁺ with regard to osteogenesis and angiogenesis, respectively. In the present study, novel biodegradable Zn-xMg (x = 0.2, 0.5, 1.0 wt.%) alloys were designed and fabricated, and the effects of adding different amounts of Mg to the Zn matrix were investigated. The osteogenesis and angiogenesis beneficial effects of Zn²⁺ and Mg²⁺ release during the biodegradation were characterized, demonstrating coordination with the bone regeneration process in a dose-dependent manner. The results show that increased Mg content leads to a higher amount of released Mg²⁺ while decreasing the Zn²⁺ concentration in the extract. The osteogenesis of pre-osteoblasts was promoted in Zn-0.5Mg and Zn-1Mg due to the higher concentration of Mg²⁺. Moreover, pure Zn extract presented the highest activity in angiogenesis, owing to the highest concentration of Zn²⁺ release (6.415 μg/mL); the proliferation of osteoblast cells was, however, inhibited under such a high Zn²⁺ concentration. Although the concentration of Zn ion was decreased in Zn-0.5Mg and Zn-1Mg compared with pure Zn, the angiogenesis was not influenced when the concentration of Mg in the extract was sufficiently increased. Hence, Mg²⁺ and Zn²⁺ in Zn-Mg alloys show a dual modulation effect. The Zn-0.5Mg alloy was indicated to be a promising implant candidate due to demonstrating the appropriate activity in regulating osteogenesis and angiogenesis. The present work evaluates the effect of the Mg content in Zn-based alloys on biological activities, and the results provide guidance regarding the Zn-Mg composition in designs for orthopedic application.

Experimental study on novel biodegradable Zn-Fe-Si alloys

Bioabsorbable metals are increasingly attracting attention for their potential use as materials for degradable implant devices. Zinc (Zn) alloys have shown great promises due to their good biocompatibility and favorable degradation rate. However, it has been difficult to maintain an appropriate balance among strength, ductility, biocompatibility, and corrosion rate for Zn alloys historically. In this study, the microstructure, chemical composition, mechanical properties, biocompatibility, and corrosion rate of a new ternary zinc-iron-silicon (Zn-Fe-Si) alloy system was studied as a novel material for potential biodegradable implant applications. The results demonstrated that the in situ formed Fe-Si intermetallic phases enhanced the mechanical strength of the material while maintaining a favorable ductility. With Fe-Si reinforcements, the microhardness of the Zn alloys was enhanced by up to 43%. The tensile strength was increased by up to 76% while elongation to failure remained above 30%. Indirect cytotoxicity testing showed the Zn-Fe-Si system had good biocompatibility. Immersion testing revealed the corrosion rate of Zn-Fe-Si system was not statistically different from pure Zn. To understand the underlying phase formation mechanism, the reaction process in this ternary system during the processing was also studied via phase evolution and Gibbs free energy analysis. The results suggest the Zn-Fe-Si ternary system is a promising new material for bioabsorbable metallic medical devices.

Anticorrosion behaviour and tribological properties of AZ31 magnesium alloy coated with Nb2O5/Nb2O5-Mg/Mg layer by magnetron sputtering

Magnesium alloys are attracting increasing attention for the fabrication of temporary implants because of their superior biodegradability and biocompatibility. However, their high degradation rate under physiological conditions limits their clinical applications. In this work, a Nb₂O₅/Nb₂O₅-Mg/Mg multilayer coating was prepared on the surface of AZ31 magnesium alloy by magnetron sputtering in order to improve its corrosion resistance. The microstructure and performance of the layers were studied by SEM, AFM, EDS, and XPS, and a scratch tester, nanoindenter, friction tester, and electrochemical workstation, using Nb₂O₅ monolayer coating as a control. The results show that these two coatings significantly improved the mechanical, tribological, and anticorrosion performance of AZ31 magnesium alloy. Compared with a Nb₂O₅ monolayer coating, the multilayer coating exhibits an increased adhesion by about 10.6 times, and a decreased wear rate and corrosion current density by one order of magnitude, meaning higher damage resistance. This study provides a feasible strategy for enhancing the properties of ceramic layers on magnesium alloys for medical applications.

Biodegradable magnesium fixation screw for barrier membranes used in guided bone regeneration

An ideal fixation system for guided bone (GBR) regeneration in oral surgery must fulfil several criteria that includes the provision of adequate mechanical fixation, complete resorption when no longer needed, complete replacement by bone, as well as be biocompatible and have a good clinical manageability. For the first time, a biodegradable magnesium fixation screw made of the magnesium alloy WZM211 with a MgF₂ coating has been designed and tested to fulfill these criteria. Adequate mechanical fixation was shown for the magnesium fixation screw in several benchtop tests that directly compared the magnesium fixation screw with an equivalent polymeric resorbable device. Results demonstrated slightly superior mechanical properties of the magnesium device in comparison to the polymeric device even after 4 weeks of degradation. Biocompatibility of the magnesium fixation screw was demonstrated in several in vitro and in vivo tests. Degradation of the magnesium screw was investigated in in vitro and in vivo tests, where it was found that the screw is resorbed slowly and completely after 52 weeks, providing adequate fixation in the early critical healing phase. Overall, the magnesium fixation screw demonstrates all of the key properties required for an ideal fixation screw of membranes used in guided bone regeneration (GBR) surgeries.

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The first comprehensive report on experimental data for a biodegradable metallic fixation pin for use in oral surgery is presented.

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Results demonstrated superior mechanical properties of the Mg fixation pin in comparison to the polymeric pin even after 4 weeks of degradation.

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The MgF₂ coated Mg pin made of alloy WZM211 has a slow corrosion rate with a service time of 4 weeks and is fully resorped at 52 weeks after implantation.

Cross-Disciplinary Application for Qualitative Magnesium Corrosion Assays

At the moment, unserviceable magnesium implants make a good case in point for further responsible study in this field. Whether we are willing to admit it or not, existing methods for corrosion monitoring are exposed to susceptibility and instability. Interdisciplinary theories and the existing corrosion experiments were combined based on their various merits for developing an accurate and precise corroding experiment for Mg/Mg alloys. We used the water-soluble tetrazolium-8 (WST-8) reagent to further complete the immersion experiment. The color change of the solution reflects the rationale of corrosion, followed by monitoring the degree of corrosion. The feasibility of this idea will be demonstrated.

Biodegradability and Cytocompatibility of 3D-Printed Mg-Ti Interpenetrating Phase Composites

Orthopedic hybrid implants combining both titanium (Ti) and magnesium (Mg) have gained wide attraction nowadays. However, it still remains a huge challenge in the fabrication of Mg-Ti composites because of the different temperatures of Ti melting point and pure Mg volatilization point. In this study, we successfully fabricated a new Mg-Ti composite with bi-continuous interpenetrating phase architecture by infiltrating Mg melt into Ti scaffolds, which were prepared by 3D printing and subsequent acid treatment. We attempted to understand the 7-day degradation process of the Mg-Ti composite and examine the different Mg2+ concentration composite impacts on the MC3T3-E1 cells, including toxicity, morphology, apoptosis, and osteogenic activity. CCK-8 results indicated cytotoxicity and absence of the Mg-Ti composite during 7-day degradation. Moreover, the composite significantly improved the morphology, reduced the apoptosis rate, and enhanced the osteogenic activity of MC3T3-E1 cells. The favorable impacts might be attributed to the appropriate Mg2+ concentration of the extracts. The results on varying Mg2+ concentration tests indicated that Mg2+ showed no cell adverse effect under 10-mM concentration. The 8-mM group exhibited the best cell morphology, minimum apoptosis rate, and maximum osteogenic activity. This work may open a new perspective on the development and biomedical applications for Mg-Ti composites.

Biomechanical evaluation on a novel design of biodegradable embossed locking compression plate for orthopaedic applications using finite element analysis

In orthopaedics, conventional implant plates such as locking compression plate (LCP) made from non-biodegradable materials play a vital role in the fixation to support bone fractures, but also create a complication such as stress shielding. These again require a painful surgery to remove/replace after they have healed as it does not degrade into the physiological environment (PE). Currently, there has already been enough discovery of biodegradable materials that, despite being mechanically inefficient compared to non-biodegradable materials, can completely be biodegraded in PE during and after healing to avoid such problems. While there has been insufficient research on the design of biodegradable implant plates, the implementation of which may help achieve the goal with an effort of high mechanical strength. A novel design of biodegradable embossed locking compression plate (BELCP) is designed for biodegradable materials to approach superior mechanical performance and complete degradation over time, considering all such parameters and factors. For biomechanical evaluation, four-point bending test (4PBT), axial compressive and tensile test (ACTT) and torsion test (TT) have been performed on LCP, BELCP and its continuously degraded forms made of biodegradable material (Mg-alloy) using finite element method. BELCP has found 50%, 100% and 100% higher mechanical performance and safer in 4PBT, ACTT and TT, respectively, than LCP. Moreover, BELCP has also observed safe during continuous degradation up to 6 months after implantation under these three tests, considering an approximate sustained degradation rate of about 4 mm/year. Even Mg-alloy made BELCP can be sufficient and safer to support fractured bone than SS-alloy made LCP, but not Ti-alloy made LCP. BELCP can be a successful biodegradable bone implant plate after human/animal trials in the future.

Biodegradable open-porous scaffolds made of sintered magnesium W4 and WZ21 short fibres show biocompatibility in vitro and in long-term in vivo evaluation

Open-porous scaffolds made of W4 and WZ21 fibres were evaluated to analyse their potential as an implant material. WZ21 scaffolds without any surface modification or coating, showed promising mechanical properties which were comparable to the W4 scaffolds tested in previous studies. Eudiometric testing results were dependent on the experimental setup, with corrosion rates differing by a factor of 3. Cytotoxicity testing of WZ21 showed sufficient cytocompatibility. The corrosion behavior of the WZ21 scaffolds in different cell culture media are indicating a selective dealloying of elements from the magnesium scaffold by different solutions. Long term in-vivo studies were using 24 W4 scaffolds and 12 WZ21 scaffolds, both implanted in rabbit femoral condyles. The condyles and important inner organs were explanted after 6, 12 and 24 weeks and analyzed. The in-vivo corrosion rate of the WZ21 scaffolds calculated by microCT-based volume loss was up to 49 times slower than the in-vitro corrosion rate based on weight loss. Intramembranous bone formation within the scaffolds of both alloys was revealed, however a low corrosion rate and formation of gas cavities at initial time points were also detected. No systemic or local toxicity could be observed. Investigations by μ-XRF did not reveal accumulation of yttrium in the neighboring tissue. In summary, the magnesium scaffold´s performance is biocompatible, but would benefit from a surface modification, such as a coating to obtain lower the initial corrosion rates, and hereby establish a promising open-porous implant material for load-bearing applications. STATEMENT OF SIGNIFICANCE: Magnesium is an ideal temporary implant material for non-load bearing applications like bigger bone defects, since it degrades in the body over time. Here we developed and tested in vitro and in a rabbit model in vivo degradable open porous scaffolds made of sintered magnesium W4 and WZ21 short fibres. These scaffolds allow the ingrowth of cells and blood vessels to promote bone healing and regeneration. Both fibre types showed in vitro sufficient cytocompatibility and proliferation rates and in vivo, no systemic toxicity could be detected. At the implantation site, intramembranous bone formation accompanied by ingrowth of supplying blood vessels within the scaffolds of both alloys could be detected.

A review on properties of magnesium-based alloys for biomedical applications

With changing lifestyles, the demand for bone implantation has been increasing day by day. The deficiency of nutritious elements within the human body results in certain diseases like osteoporosis, rickets, and other skeletal disorders; lack of physical activities; and the increasing number of accidents are the primary reasons for bone damage/fracture. Metallic implants made up of chrome steel, cobalt-based alloys, and titanium-based alloys are being majorly used worldwide owing to their high strength and high corrosion resistance which makes them permanent orthopedic bioimplant materials, however, they display a stress-shielding effect and it also requires an implant removal surgery. Thus, these problems can be addressed through the employment of biodegradable materials. Among the available biodegradable metallic materials, Mg alloys have been identified as a prospective orthopedic implant material. These alloys are biodegradable as well as biocompatible, however, they experience a relatively higher rate of degradation limiting their usability as implant material. This study attempts to comprehensively assess the effects of various alloying elements such as Ca, Zn, Sn, Mn, Sr and Rare earth elements (REEs) on the mechanical and degradation behavior (bothin vivoandin vitro) of Mg alloys. Since the microstructure, mechanical properties and degradation response of the Mg alloys are dependent on the processing route, hence detailed processing- property database of different Mg alloys is provided in this paper.

Degradation Behavior and Mechanical Integrity of a Mg-0.7Zn-0.6Ca (wt.%) Alloy: Effect of Grain Sizes and Crystallographic Texture

The microstructural characteristics of biodegradable Mg alloys determine their performance and appropriateness for orthopedic fixation applications. In this work, the effect of the annealing treatment of a Mg-0.7Zn-0.6Ca (ZX11) alloy on the mechanical integrity, corrosive behavior, and biocompatibility-osteoinduction was studied considering two annealing temperatures, 350 and 450 °C. The microstructure showed a recrystallized structure, with a lower number of precipitates, grain size, and stronger basal texture for the ZX11-350 condition than the ZX11-450. The characteristics mentioned above induce a higher long-term degradation rate for the ZX11-450 than the ZX11-350 on days 7th and 15th of immersion. In consequence, the mechanical integrity changes within this period. The increased degradation rate of the ZX11-450 condition reduces 40% the elongation at failure, in contrast with the 16% reduction for the ZX11-350 condition. After that period, the mechanical integrity remained unchanged. No cytotoxic effects were observed for both treatments and significant differentiation of mesenchymal stem cells into the osteoblast phenotype was observed.

Effectiveness of non-uniform thickness on a locking compression plate used as a biodegradable bone implant plate

Conventional locking compression plate (LCP) made of non-biodegradable materials are well-known bone implants for internal fracture fixation because of their proven experimental success. LCP, however, is mechanically underpowered when made up of biodegradable materials (even with Mg-alloy). The biodegradable implant plate should not only exhibit adequate mechanical performance during implantation but also perform well after fracture, at least until complete healing of the fractured bone. With the aim of achieving enhanced mechanical performance, the design of the LCP has been modified to the design of Biodegradable Locking Compression Plate (BLCP) by adding a suitable thickness in the middle (only 4.6% of the total volume of the LCP), which may help retain some additional strength during implantation and after degradation. Both BLCP and LCP have been comparatively analyzed via FEM with the aid of axial compression and four-point bending tests. BLCP has a better mechanical capability of withstanding loads in its degraded form than in its non-degradable form. Furthermore, BLCP is up to 15.83% mechanically better in the non-degraded form as compared to LCP, which again becomes up to 100% more mechanically adequate in the degraded forms of BLCP than in LCP. BLCP is found safe for degradation up to 2 mm or 6 months with an estimated degradation rate of 4 mm/year, which may allow it to support fractured bone for at least the standard healing time. BLCP can be considered as a superior biodegradable bone implant plate after experimental assurance with the physiological environment and may replace LCP.

3D-Printed Double-Helical Biodegradable Iron Suture Anchor: A Rabbit Rotator Cuff Tear Model

Suture anchors are extensively used in rotator cuff tear surgery. With the advancement of three-dimensional printing technology, biodegradable metal has been developed for orthopedic applications. This study adopted three-dimensional-printed biodegradable Fe suture anchors with double-helical threads and commercialized non-vented screw-type Ti suture anchors with a tapered tip in the experimental and control groups, respectively. The in vitro study showed that the Fe and Ti suture anchors exhibited a similar ultimate failure load in 20-pound-per-cubic-foot polyurethane foam blocks and rabbit bone. In static immersion tests, the corrosion rate of Fe suture anchors was 0.049 ± 0.002 mm/year. The in vivo study was performed on New Zealand white rabbits and SAs were employed to reattach the ruptured supraspinatus tendon. The in vivo ultimate failure load of the Fe suture anchors was superior to that of the Ti suture anchors at 6 weeks. Micro-computed tomography showed that the bone volume fraction and bone surface density in the Fe suture anchors group 2 and 6 weeks after surgery were superior, and the histology confirmed that the increased bone volume around the anchor was attributable to mineralized osteocytes. The three-dimensional-printed Fe suture anchors outperformed the currently used Ti suture anchors.

Electrochemical Short-Time Testing Method for Simulating the Degradation Behavior of Magnesium-Based Biomaterials

In regenerative medicine, degradable, magnesium-based biomaterials represent a promising material class. The low corrosion resistance typical for magnesium is advantageous for this application since the entire implant degrades in the presence of the aqueous body fluids after fulfilling the intended function, making a second operation for implant removal obsolete. To ensure sufficient stability within the functional phase, the degradation behavior must be known for months. In order to reduce time and costs for these long-time investigations, an electrochemical short-time testing method is developed and validated, accelerating the dissolution process of a magnesium alloy with and without surface modification based on galvanostatic anodic polarization, enabling a simulation of longer immersion times. During anodic polarization, the hydrogen gas formed by the corrosion process increases linearly. Moreover, the gas volume shows a linear relationship to the dissolving mass, enabling a defined dissolution of magnesium. As a starting point, corrosion rates of both variants from three-week immersion tests are used. A simplified relationship between the current density and the dissolution rate, determined experimentally, is used to design the experiments. Ex situ µ-computed tomography scans are performed to compare the degradation morphologies of both test strategies. The results demonstrate that a simulation of the degradation rates and, hence, considerable time saving based on galvanostatic anodic polarization is possible. Since the method is accompanied by a changed degradation morphology, it is suitable for a worst-case estimation allowing the exclusion of new, unsuitable magnesium systems before subsequent preclinical studies.

A review of current challenges and prospects of magnesium and its alloy for bone implant applications

Medical application materials must meet multiple requirements, and the designed implant must mimic the bone structure in shape and support the formation of bone tissue (osteogenesis). Magnesium (Mg) alloys, as a "smart" biodegradable material and as "the green engineering material in the twenty-first century", have become an outstanding bone implant material due to their natural degradability, smart biocompatibility, and desirable mechanical properties. Magnesium is recognised as the next generation of orthopaedic appliances and bioresorbable scaffolds. At the same time, improving the mechanical properties and corrosion resistance of magnesium alloys is an urgent challenge to promote the application of magnesium alloys. Nevertheless, the excessively quick deterioration rate generally results in premature mechanical integrity disintegration and local hydrogen build-up, resulting in restricted clinical bone restoration applicability. The condition of Mg bone implants is thoroughly examined in this study. The relevant approaches to boost the corrosion resistance, including purification, alloying treatment, surface coating, and Mg-based metal matrix composite, are comprehensively revealed. These characteristics are reviewed to assess the progress of contemporary Mg-based biocomposites and alloys for biomedical applications. The fabricating techniques for Mg bone implants also are thoroughly investigated. Notably, laser-based additive manufacturing fabricates customised forms and complicated porous structures based on its distinctive additive manufacturing conception. Because of its high laser energy density and strong controllability, it is capable of fast heating and cooling, allowing it to modify the microstructure and performance. This review paper aims to provide more insight on the present challenges and continued research on Mg bone implants, highlighting some of the most important characteristics, challenges, and strategies for improving Mg bone implants.

Enhanced Plasticity and Corrosion Resistance in Mg-Zn-Ca-Cu Amorphous Alloy Composite via Plasma Electrolytic Oxidation Treatment

In this study, a dendrite-reinforced Mg-based amorphous alloy composite was prepared through an in situ precipitation strategy. After plasma electrolytic oxidation (PEO) treatment, the Mg85.1Zn12.7Ca2Cu0.2 amorphous alloy composite exhibited enhanced plasticity and corrosion resistance in a simulated body fluid solution (SBF). The PEO-treated composite showed a significant plastic strain of 10.5 ± 1.1%, as well as outstanding strain-hardening behavior. The enhancement of plasticity may be attributed to the in-situ formed coating, which can not only serve as a propagation barrier for shear bands but can also introduce nucleation sites in the bands as a result of stress mismatch and compositional heterogeneity. The corrosion density in the SBF decreased by three orders compared with the composite substrate. The spontaneous formation of apatite on the porous layer demonstrated that the prepared PEO coating has high bioactivity. The current work may provide a fundamental basis for developing biomedical Mg-based alloys with excellent comprehensive mechanical properties and corrosion resistance.

High-Efficient Vacuum Ultraviolet-Ozone Assist-Deposited Polydopamine for Poly(lactic-co-glycolic acid)-Coated Pure Zn toward Biodegradable Cardiovascular Stent Applications

Zinc is a prospective metal for biodegradable cardiovascular stent applications, but the excessively released Zn²⁺ during degradation remains a huge challenge in biocompatibility. Considerable efforts have been made to develop a high-efficient surface modification method, while maintaining adhesion strength, mechanical support, and vascular compatibility. Biomimetic polydopamine (PDA) can adhere to Zn tightly, subsequently achieving robust chemical bonds with poly(lactic-co-glycolic acid) (PLGA) coating. However, the deposition of PDA on Zn depends on the controlled conditions such as a sensitive pH and a long period of time. Herein, we introduce vacuum ultraviolet-ozone (VUV/O₃) assist-deposition technology to accelerate the polymerization of PDA on pure Zn, which shortens the process to 40 min at a moderate pH of 8.5 and improves the deposition rate by 1-2 orders of magnitude under sufficient active oxygen species (ROS). Additionally, PLGA/PDA coating enhances the corrosion resistance, and their effective protection maintains the mechanical properties after long-term corrosion. Moreover, the controlled Zn²⁺ release contributes to the superior in vitro biocompatibility, which inhibits the hemolysis rate and smooth muscle cell (SMC) proliferation. The enhanced endothelial cell (EC) proliferation is promising to promote the re-endothelialization, avoiding in-stent restenosis and neointimal hyperplasia. Such modified Zn might be a viable candidate for the treatment of cardiovascular diseases.

Biodegradation behaviors of magnesium(Mg)-based alloy nails in autologous bone grafts: In vivo study in rabbit skulls

OBJECTIVE

In this study, autologous bone grafts using bone-fixing nails made of magnesium-zinc-calcium ternary alloys were performed using rabbit skulls.

MATERIAL AND METHODS

Two types of nails for bone fixation were prepared: 2.5 mm width, 3 mm length and 2.5 mm width, 2 mm length. A disk-shaped bone with a diameter of 5 mm was resected from the parietal bone and fixed with a 3 mm long nail. As a control group, a 2 mm long nail was driven into the existing bone. The rabbits were sacrificed at 1, 4, 12, and 24 weeks after surgery. The resected samples were observed with micro X-ray CT, and embedded in methyl methacrylate to prepare non-decalcified specimens. The in vivo localization of elements was examined using energy-dispersive X-ray spectroscopy (EDS).

RESULTS

Micro X-ray CT images of samples showed volume reduction due to degradation in both the bone graft and control groups. No significant difference in the amount of degradation between the two groups was observed, however characteristic degradation processes were observed in each group. The samples stained with alizarin red S showed amorphous areas around the nails, which were considered as corrosion products and contacted directly with the newly formed bones. EDS analysis showed that corrosion products were mainly composed of magnesium and oxygen at an early stage, while calcium and phosphorus were detected on the surface layer during the long-term observation.

CONCLUSIONS

The degradation speed of the magnesium alloy nails varied depending on the shapes of the nails and surrounding tissue conditions. A calcium phosphate layer was formed on the surface of magnesium alloy nails, suggesting that the degradation rate of the nail was slow.

Effectiveness of laddered embossed structure in a locking compression plate for biodegradable orthopaedic implants

Locking compression plate (LCP) has conventionally been the most extensively employed plate in internal fixation bone implants used in orthopaedic applications. LCP is usually made up of non-biodegradable materials that have a higher mechanical capability. Biodegradable materials, by and large, have less mechanical strength at the point of implantation and lose strength even more after a few months of continuous degradation in the physiological environment. To attain the adequate mechanical capability of a biodegradable bone implant plate, LCP has been modified by adding laddered - type semicircular filleted embossed structure. This improved design may be named as laddered embossed locking compression plate (LELCP). It is likely to provide additional mechanical strength with the most eligible biodegradable material, namely, Mg-alloy, even after continuous degradation that results in diminished thickness. For mechanical validation and comparison of LELCP made up of Mg-alloy, four-point bending test (4PBT) and axial compressive test (ACT) have been performed on LELCP, LCP and continuously degraded LELCP (CD-LELCP) with the aid of finite element method (FEM) for the assembly of bone segments, plate and screw segments. LELCP, when subjected to the above mentioned two tests, has been observed to provide 26% and 10.4% lower equivalent stress, respectively, than LCP without degradation. It is also observed mechanically safe and capable of up to 2 and 6 months of continuous degradation (uniform reduction in thickness) for 4PBT and ACT, respectively. These results have also been found reasonably accurate through real-time surgical simulations by approaching the most optimal mesh. According to these improved mechanical performance parameters, LELCP may be used or considered as a viable biodegradable implant plate option in the future after real life or in vivo validation.

Evaluation of Corrosion Behavior and In Vitro of Strontium-Doped Calcium Phosphate Coating on Magnesium

This study investigated the biocompatibility of strontium-doped calcium phosphate (Sr-CaP) coatings on pure magnesium (Mg) surfaces for bone applications. Sr-CaP coated specimens were obtained by chemical immersion method on biodegradable magnesium. In this study, Sr-CaP coated magnesium was obtained by immersing pure magnesium in a solution containing Sr-CaP at 80 °C for 3 h. The corrosion resistance and biocompatibility of magnesium according to the content of Sr-CaP coated on the magnesium surface were evaluated. As a result, the corrosion resistance of Sr-CaP coated magnesium was improved compared to pure magnesium. In addition, it was confirmed that the biocompatibility of the group containing Sr was increased. Thus, the Ca-SrP coating with a reduced degradation and improved biocompatibility could be used in Mg-based orthopedic implant applications.

Fatigue and dynamic biodegradation behavior of additively manufactured Mg scaffolds

Additive manufacturing (AM) has enabled the fabrication of biodegradable porous metals to satisfy the desired characteristics for orthopedic applications. The geometrical design on AM biodegradable metallic scaffolds has been found to offer a favorable opportunity to regulate their mechanical and degradation performance in previous studies, however mostly confined to static responses. In this study, we presented the effect of the geometrical design on the dynamic responses of AM Mg scaffolds for the first time. Three different types of porous structures, based on various unit cells (i.e., biomimetic, diamond, and sheet-based gyroid), were established and then subjected to selective laser melting (SLM) process using group-developed Mg-Nd-Zn-Zr alloy (JDBM) powders. The topology after dynamic electropolishing, dynamic compressive properties, and dynamic biodegradation behavior of the AM Mg scaffolds were comprehensively evaluated. It was found that dynamic electropolishing effectively removed the excessive adhered powders on the surfaces and resulted in similar geometrical deviations amongst the AM Mg scaffolds, independent of their porous structures. The geometrical design significantly affected the compressive fatigue properties of the AM Mg scaffolds, of which the sheeted-based gyroid structure demonstrated a superior fatigue endurance limit of 0.85 at 10⁶ cycles. Furthermore, in vitro dynamic immersion behaviors of the AM Mg scaffolds revealed a decent dependence on local architectures, where the sheeted-based gyroid scaffold experienced the lowest structural loss with a relatively uniform degradation mode. The obtained results indicate that the geometrical design could provide a promising strategy to develop desirable bone substitutes for the treatment of critical-size load-bearing defects. STATEMENT OF SIGNIFICANCE: Additive manufacturing (AM) has provided unprecedented opportunities to fabricate geometrically complex biodegradable scaffolds where the topological design becomes a key determinant on comprehensive performance. In this paper, we fabricate 3 AM biodegradable Mg scaffolds (i.e., biomimetic, diamond, and sheet-based gyroid) and report the effect of the geometrical design on the dynamic responses of AM Mg scaffolds for the first time. The results revealed that the sheeted-based gyroid scaffold exhibited the best combination of superior compressive fatigue properties and relatively uniform dynamic biodegradation mode, suggesting that the regulation of the porous structures could be an effective approach for the optimization of AM Mg scaffolds as to satisfy clinical requirements in orthopedic applications.

Biodegradable shape memory alloys: Progress and prospects

Shape memory alloys (SMAs) have a wide range of potential novel medical applications due to their superelastic properties and ability to restore and retain a 'memorised' shape. However, most SMAs are permanent and do not degrade in the body when used in implantable devices. The use of non-degrading metals may lead to the requirement for secondary removal surgery and this in turn may introduce both short and long-term health risks, or additional waste disposal requirements. Biodegradable SMAs can effectively eliminate these issues by gradually degrading inside the human body while providing the necessary support for healing purposes, therefore significantly alleviating patient discomfort and improving healing efficiency. This paper reviews the current progress in biodegradable SMAs from the perspective of biodegradability, mechanical properties, and biocompatibility. By providing insights into the status of SMAs and biodegradation mechanisms, the prospects for Mg- and Fe-based biodegradable SMAs to advance biodegradable SMA-based medical devices are explored. Finally, the remaining challenges and potential solutions in the biodegradable SMAs area are discussed, providing suggestions and research frameworks for future studies on this topic.

Automated ex-situ detection of pitting corrosion and its effect on the mechanical integrity of rare earth magnesium alloy - WE43

This study develops a three-dimensional automated detection framework (PitScan) that systematically evaluates the severity and phenomenology of pitting corrosion. This framework uses a python-based algorithm to analyse microcomputer-tomography scans (μCT) of cylindrical specimens undergoing corrosion. The approach systematically identifies several surface-based corrosion features, enabling full spatial characterisation of pitting parameters, including pit density, pit size, pit depth as well as pitting factor according to ASTM G46-94. Furthermore, it is used to evaluate pitting formation in tensile specimens of a Rare Earth Magnesium alloy undergoing corrosion, and relationships between key pitting parameters and mechanical performance are established. Results demonstrated that several of the parameters described in ASTM G46-94, including pit number, pit density and pitting factor, showed little correlation to mechanical performance. However, this study did identify that other parameters showed strong correlations with the ultimate tensile strength and these tended to be directly linked to the reduction of the cross-sectional area of the specimen. Specifically, our results indicate, that parameters directly linked to the loss of the cross-sectional area (e.g. minimum material width), are parameters that are most suited to provide an indication of a specimen's mechanical performance. The automated detection framework developed in this study has the potential to provide a basis to standardise measurements of pitting corrosion across a range of metals and future prediction of mechanical strength over degradation time.

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In-vitro immersion study of dog bones manufactured from a WE43 Magnesium alloy.

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Novel approach characterizing spatial pit formation using micro-CT scans.

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Comparison of mass loss by hydrogen gas measurement and volume loss by μCT scans.

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Correlation between mechanical strength and geometrical pit formation features.

Additively manufactured biodegradable porous magnesium implants for elimination of implant-related infections: An in vitro and in vivo study

Magnesium (Mg) alloys that have both antibacterial and osteogenic properties are suitable candidates for orthopedic implants. However, the fabrication of ideal Mg implants suitable for bone repair remains challenging because it requires implants with interconnected pore structures and personalized geometric shapes. In this study, we fabricated a porous 3D-printed Mg-Nd-Zn-Zr (denoted as JDBM) implant with suitable mechanical properties using selective laser melting technology. The 3D-printed JDBM implant exhibited cytocompatibility in MC3T3-E1 and RAW267.4 cells and excellent osteoinductivity in vitro. Furthermore, the implant demonstrated excellent antibacterial ratios of 90.0% and 92.1% for methicillin-resistant S. aureus (MRSA) and Escherichia coli, respectively. The 3D-printed JDBM implant prevented MRSA-induced implant-related infection in a rabbit model and showed good in vivo biocompatibility based on the results of histological evaluation, blood tests, and Mg²⁺ deposition detection. In addition, enhanced inflammatory response and TNF-α secretion were observed at the bone-implant interface of the 3D-printed JDBM implants during the early implantation stage. The high Mg²⁺ environment produced by the degradation of 3D-printed JDBM implants could promote M1 phenotype of macrophages (Tnf, iNOS, Ccl3, Ccl4, Ccl5, Cxcl10, and Cxcl2), and enhance the phagocytic ability of macrophages. The enhanced immunoregulatory effect generated by relatively fast Mg²⁺ release and implant degradation during the early implantation stage is a potential antibacterial mechanism of Mg-based implant. Our findings indicate that 3D-printed porous JDBM implants, having both antibacterial property and osteoinductivity, hold potential for future orthopedic applications.

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Porous JDBM implants promising mechanical properties was fabricated by selective laser melting.

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3D-printed JDBM implant exhibited excellent antibacterial property, osteoinductivity, and biocompatibility.

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Temporally enhanced immunoregulatory effect in early stage was a potential antibacterial mechanism of Mg-based implant.

Longitudinally centered embossed structure in the locking compression plate for biodegradable bone implant plate: a finite element analysis

In the current revolution of internal fixation implant in orthopaedics, a biodegradable implant is the most awaited and exceptional medical device where biodegradable material has paid more attention to the success of a biodegradable implant than the design of a biodegradable bone implant plate. By far, LCP is the most traditionally used implant plate (using non-biodegradable material) because of its experimental success, but not with qualified biodegradable material (Mg-alloy). This lack of mechanical performance is a major drawback that can be rectified by better structural design. This will help avoid few other problems as well. Therefore, with proper consideration, the LCP has been added to a semicircular filleted longitudinally centered embossed (LCE) structure to enhance overall mechanical performance that can help emphasize mechanical support even after continuous degradation when applied in a physiological environment. For mechanical verification of this advanced design of biodegradable bone implant plate, four-point bending test (4PBT) and axial compression test (ACT) have been performed using FEM on LCELCP, LCP, continuously degraded (CD)-LCELCP, and CD-LCP. LCELCP showed reduced stress of about 22% and 10% in 4PBT and ACT, respectively, compared to LCP. CD-LCELCP is safe during ACT over 6 months of continuous degradation when the degradation rate is assumed to be 4 mm/year. These results also ensured accuracy using mesh convergence and also mesh checked for quality assurance. Overall, LCELCP can be considered as a biodegradable bone implant plate because of its superior performance, if its ultimate validation is carried out through animal/human trials as future work.

In vitro and in vivo degradation assessment and preventive measures of biodegradable Mg alloys for biomedical applications

Magnesium (Mg) and its alloys have been widely explored as a potential biodegradable implant material. However, the fast degradation of Mg-based alloys under physiological environment has hindered their widespread use for implant applications till date. The present review focuses on in vitro and in vivo degradation of biodegradable Mg alloys, and preventive measures for biomedical applications. Initially, the corrosion assessment approaches to predict the degradation behavior of Mg alloys are discussed along with the measures to control rapid corrosion. Furthermore, this review attempts to explore the correlation between in vitro and in vivo corrosion behavior of different Mg alloys. It was found that the corrosion depends on experimental conditions, materials and the results of different assessment procedures hardly matches with each other. It has been demonstrated the corrosion rate of magnesium can be tailored by alloying elements, surface treatments and heat treatments. Various researches also studied different biocompatible coatings such as dicalcium phosphate dihydrate (DCPD), β-tricalcium phosphate (β-TCP), hydroxyapatite (HA), polycaprolactone (PCL), polylactic acid (PLA), and so on, on Mg alloys to suppress rapid degradation and examine their influence on new bone regeneration as well. This review shows the need for a standard method of corrosion assessment to predict the in vivo corrosion rate based on in vitro data, and thus reducing the in vivo experimentation.

Tissue responses after implantation of biodegradable Mg alloys evaluated by multimodality 3D micro-bioimaging in vivo

The local response of tissue triggered by implantation of degradable magnesium-based implant materials was investigated in vivo in a murine model. Pins (5.0 mm length by 0.5 mm diameter) made of Mg, Mg-10Gd, and Ti were implanted in the leg muscle tissue of C57Bl/6N mice (n = 6). Implantation was generally well tolerated as documented by only a mild short term increase in a multidimensional scoring index. Lack of difference between the groups indicated that the response was systemic and surgery related rather than material dependent. Longitudinal in vivo monitoring utilizing micro-computed tomography over 42 days demonstrated the highest and most heterogeneous degradation for Mg-10Gd. Elemental imaging of the explants by micro X-ray fluorescence spectrometry showed a dense calcium-phosphate-containing degradation layer. In order to monitor resulting surgery induced and/or implant material associated local cell stress, sphingomyelin based liposomes containing indocyanine green were administered. An initial increase in fluorescent signals (3-7 days after implantation) indicating cell stress at the site of the implantation was measured by in vivo fluorescent molecular tomography. The signal decreased until the 42nd day for all materials. These findings demonstrate that Mg based implants are well tolerated causing only mild and short term adverse reactions.