In vitro interactions of blood, platelet, and fibroblast with biodegradable magnesium-zinc-strontium alloys

Thrombogenicity of biodegradable metals

Biodegradable metals offer a promising means to ameliorate many of the long-term risks associated with vascular devices made of conventional biostable stent metals. While numerous biodegradable metal alloys have been developed and characterized in animal models, knowledge of their blood reactivity and thrombogenicity remains unknown. Metal hemocompatibility is particularly valuable because current generation drug-eluting stents pose a significant long-term thrombosis risk. In this study, four pure metals, widely used as degradable base materials (Fe, Zn, Mg, and Mo), and three alloys commonly used in cardiovascular devices [NiTi, CoCr, and stainless steel (SS)] were evaluated. This work examined how each of these metals activate platelets, coagulation factors, and inflammation using in vitro hemocompatibility assays and a clinically relevant ex vivo non-human primate arteriovenous shunt model. Testing found that while all metals promoted a downstream activation of platelets and coagulation in flowing whole blood, platelet and fibrin attachment to Mg was markedly reduced. Additionally, Fe and Mo trended toward higher platelet attachment and contact pathway activation. Overall, the results suggest that Mg may delay clot initiation, but not eliminate clot formation, indicating the importance of understanding thrombosis in Mg alloys that are currently being developed for clinical use as biodegradable stents.

Research on alginate-polyacrylamide enhanced amnion hydrogel, a potential vascular substitute material

Although traditional synthetic vascular grafts have good mechanical stability, stenosis and even thrombus can be easily caused at the beginning of transplantation due to the material's procoagulant and low cell adhesion rate. In order to address these problems, by combining acellular amnion gel and polyacrylamide-alginate gel, we gained a composite hydrogel with high elasticity, mechanical stability, high bioactivity and low swelling ratio. The results showed that the composite gel had excellent mechanical strength, resistance to enzymatic degradation and anti-calcification ability. Also, it could significantly inhibit the adhesion, aggregation and activation of platelet and hemolysis. What is more, this composite hydrogel could significantly promote the adhesion and proliferation of ECs, as well as inducing the migration of ECs to the surface of the hydrogel. It could also stimulate the secretion of NO and PGI₂ from seeded HUVECs, which were important factors involved in vascular remodelling and repair. All the results indicated that prepared AlgSr/PAM-AM hydrogel was an excellent biomaterial with properties for potential use in vascular repair.

Effect of Foreign Ion Substitution and Micropore Tuning in Robocasting Single-Phase Bioceramic Scaffolds on the Physicochemical Property and Vascularization

The inorganic powder slurry extrusion printing technique known as robocasting is an interesting method to fabricate complex porous architectures whereby feedstocks containing organic binders and powders are printed and the resulting scaffolds are subjected to sintering. A major limiting factor of this technique is the simultaneous tailoring of vascularization efficacy and osteogenic activity, usually done by adding the secondary phase in the organic slurry before the writing step. Mechanical mixing of biphasic powders is required to avoid compromising the biological performance and physical defects caused by significantly different physicochemical properties. This study addresses this issue by developing a selective ion doping and microstructure tuning for the production of bioceramic scaffolds with a binozzle robocasting process. Different metal ions (Sr²⁺, Mg²⁺) were doped into wollastonite (CaSiO₃; CSi) powders considering the mechanical stability and bioactive enhancement of the bioceramic scaffolds. Subsequently, the Mg-doped CSi slurries were used as shell-nozzle feedstocks added with 5, 10, and 15 μm diameter polystyrene microbeads that allowed shell-layer micropore production in pore struts during sintering. Finally, the most promising pore-strut microstructures and mechanical evolution of scaffolds were evaluated, and especially the enhanced fibrovascularization potential was confirmed in dorsal muscle embedding model in rabbits. This study may open an avenue to designing multiproperty-tuned macro- and microporous bioceramics for bone regenerative medicine, especially in challenging bone defect conditions.

Antimicrobial Bioresorbable Mg-Zn-Ca Alloy for Bone Repair in a Comparison Study with Mg-Zn-Sr Alloy and Pure Mg

Magnesium-zinc-calcium (Mg-Zn-Ca) alloys have attracted increasing attention for biomedical implant applications, especially for bone repair, because of their biocompatibility, biodegradability, and similar mechanical properties to human bone. The objectives of this study were to characterize Mg-2 wt % Zn-0.5 wt % Ca (named ZC21) alloy pins microstructurally and mechanically, and determine their degradation and interactions with host cells and pathogenic bacteria in vitro and in vivo in comparison with the previously studied Mg-4 wt % Zn-1 wt % strontium (named ZSr41) alloy and Mg control. Specifically, the in vitro degradation and cytocompatibility of ZC21 pins with bone marrow derived mesenchymal stem cells (BMSCs) were investigated using both direct culture and direct exposure culture methods. The adhesion density of BMSCs on ZC21 pins (i.e., direct contact) was significantly higher than on pure Mg pins in both in vitro culture methods; the cell adhesion density around ZC21 pins (i.e., indirect contact) was similar to the cell-only positive control in both in vitro culture methods. Interestingly, ZC21 showed a higher daily degradation rate, crack width and crack area ratio in the direct exposure culture than in the direct culture, suggesting different culture methods did affect its in vitro degradation behaviors. When cultured with Gram-positive bacteria methicillin-resistant Staphylococcus aureus (MRSA), ZC21 reduced bacterial adhesion on the surface more significantly than that of ZSr41 and Mg. The in vivo degradation and biocompatibility of the ZC21 pins for bone regeneration were studied in a mouse femoral defect model. The in vivo degradation rate of ZC21 pins was much slower than that of ZSr41 alloy and Mg control pins. After 12 weeks of implantation in vivo, the ZC21 group showed the shortest gap at the femoral defect, indicating that ZC21 pins promoted osteogenesis and bone healing more than ZSr41 and Mg control pins. Overall, the ZC21 alloy is promising for bone repair, while providing antibacterial activities, and should be further studied toward clinical translation.

Degradable magnesium-based alloys for biomedical applications: The role of critical alloying elements

Magnesium-based alloys exhibit biodegradable, biocompatible and excellent mechanical properties which enable them to serve as ideal candidate biomedical materials. In particular, their biodegradable ability helps patients to avoid a second surgery. The corrosion rate, however, is too rapid to sustain the healing process. Alloying is an effective method to slow down the corrosion rate. However, currently magnesium alloys used as biomaterials are mostly commercial alloys without considering cytotoxicity from the perspective of biosafety. This article comprehensively reviews the status of various existing and newly developed degradable magnesium-based alloys specially designed for biomedical application. The effects of critical alloying elements, compositions, heat treatment and processing technology on the microstructure, mechanical properties and corrosion resistance of magnesium alloys are discussed in detail. This article covers Mg-Ca based, Mg-Zn based, Mg-Sr based, Mg-RE based and Mg-Cu-based alloy systems. The novel methods of fabricating Mg-based biomaterials and surface treatment on Mg based alloys for potential biomedical applications are summarized.

Responses of human urothelial cells to magnesium-zinc-strontium alloys and associated insoluble degradation products for urological stent applications

Current urological devices such as ureteral stents and catheters still face serious problems, such as encrustation and biofilm formation. Magnesium (Mg) and its alloys showed great potentials as an alternative material for urological devices, due to their excellent biodegradability and antibacterial property. In this study, a serial of four promising Mg alloys which contain zinc (Zn) and strontium (Sr), i.e., Mg-4Zn-xSr (ZSr41) alloys, were investigated in vitro for potential ureteral stent application. Specifically, these four alloys have 4 wt% Zn in all and 0.15 wt% Sr in ZSr41_A, 0.5 wt% Sr in ZSr41_B, 1.0 wt% Sr in ZSr41_C and 1.5 wt% Sr in ZSr41_D. The cytocompatibility and degradation behaviors of Mg-4Zn-xSr alloys were studied by culturing with human urothelial cells (HUCs) for 24 h and 48 h using exposure culture method. ZSr41_B showed a better cytocompatibility with HUCs among all the Mg-4Zn-xSr alloys in both 24-hour and 48-hour cultures. Moreover, the cytocompatibility of insoluble degradation products of Mg, i.e., MgO and Mg(OH)₂, was also investigated by culturing different concentrations of MgO and Mg(OH)₂ nanoparticles with HUCs for 24 h and 48 h. The concentration of MgO and Mg(OH)₂ particles at 0.5 mg/mL and above, showed a significant decrease of cell density and cell size after 24-hour and 48-hour cultures. The concentration of MgO and Mg(OH)₂ at 1.0 mg/mL and above, showed no viable cells after 24-hour culture. Collectively, it is recommended to further reduce the degradation rates of Mg alloys in order to control possible side effects of the soluble and insoluble degradation products and to take the benefits of Mg-based biodegradable ureteral stents toward the future clinical translation.

Degradation of Bioresorbable Mg-4Zn-1Sr Intramedullary Pins and Associated Biological Responses in Vitro and in Vivo

This article reports the degradation and biological properties of as-drawn Mg-4Zn-1Sr (designated as ZSr41) and pure Mg (P-Mg) wires as bioresorbable intramedullary pins for bone repair. Specifically, their cytocompatibility with bone marrow derived mesenchymal stem cells (BMSCs) and degradation in vitro, and their biological effects on peri-implant tissues and in vivo degradation in rat tibiae were studied. The as-drawn ZSr41 pins showed a significantly faster degradation than P-Mg in vitro and in vivo. The in vivo average daily degradation rates of both ZSr41 and P-Mg intramedullary pins were significantly greater than their respective in vitro degradation rates, likely because the intramedullary site of implantation is highly vascularized for removal of degradation products. Importantly, the concentrations of Mg2+, Zn2+, and Sr2+ ions in the BMSC culture in vitro and their concentrations in rat blood in vivo were all lower than their respective therapeutic dosages, i.e., in a safe range. Despite of rapid degradation with a complete resorption time of 8 weeks in vivo, the ZSr41 intramedullary pins showed a significant net bone growth because of stimulatory effects of the metallic ions released. However, proportionally released OH- ions and hydrogen gas caused adverse effects on bone marrow cells and resulted in cavities in surrounding bone. Thus, properly engineering the degradation properties of Mg-based implants is critical for harvesting the bioactivities of beneficial metallic ions, while controlling adverse reactions associated with the release of OH- ions and hydrogen gas. It is necessary to further optimize the alloy processing conditions and/or modify the surfaces, for example, applying coatings onto the surface, to reduce the degradation rate of ZSr41 wires for skeletal implant applications.

Cytocompatibility and antibacterial activity of nanostructured H2Ti5O11·H2O outlayered Zn-doped TiO2 coatings on Ti for percutaneous implants

To improve skin-integration and antibacterial activity of percutaneous implants, the coatings comprising an outer layer of H2Ti5O11·H2O (HTO) nanoarrays and an inner layer of microporous Zn-doped TiO2 were fabricated on Ti by micro-arc oxidation (MAO) followed with hydrothermal treatment (HT). During HT process, a large proportion of Zn2+ migrated out from TiO2 layer. TiO2 reacted with OH- and H2O, resulting in the nucleation of HTO. The nuclei grew to nanoplates, nanorods and nanofibres with HT process prolonged. Simultaneously, the orientation of nanoarrays changed from quasi-vertical to parallel to substrate. Compared to Ti, adhesion and proliferation of fibroblasts were enhanced on as-MAOed TiO2 and HTed coatings. The phenotype, differentiation and extracellular collagen secretion were obviously accelerated on vertical nanorods with proper interspace (e.g. 63 nm). HTed coatings showed enhanced antibacterial activity, which should be ascribed to the nano-topography of HTO.

A Comparison Study on the Degradation and Cytocompatibility of Mg-4Zn-xSr Alloys in Direct Culture

This article reports the behaviors of bone-marrow-derived mesenchymal stem cells (BMSCs) in the direct culture with four Mg-4Zn-xSr alloys (x = 0.15, 0.5, 1.0, 1.5 wt %), designated as ZSr41A, B, C, and D, respectively; and a systematic comparison on the degradation of the ZSr41 alloys and their biological impact in the direct culture with different cell types in their respective media. The direct culture method, in which cells are seeded directly onto the surface of the sample, was used to investigate cellular responses at the cell-biomaterial interface in vitro. The results showed that BMSCs adhered and remained viable on the surfaces of all ZSr41 alloys, but the faster degrading ZSr41A and ZSr41B alloys showed a significantly lower amount of viable BMSCs adhered to their surfaces. Moreover, BMSCs adhered to the culture plate surrounding the samples were unaffected by the solubilized degradation products from the ZSr41 alloys. The results from the comparison study showed that the in vitro degradation rates of Mg-based biomaterials in different culture systems might be mostly affected by media buffer capacity (i.e., HCO₃^- concentration), and to a lesser extent, d-glucose concentration. The comparison study also indicated that BMSCs were more robust than H9 human embryonic stem cells and human umbilical vein endothelial cells for screening the cytocompatibility of Mg-based biomaterials. In general, the adhesion and viability of BMSCs at the cell-material interface were inversely proportional to the alloy degradation rates. This study presented a clinically relevant in vitro culture system for screening bioresorbable alloys in direct culture, and provided valuable guidelines for determining the degradation rates of Mg-based biomaterials.

Cytocompatibility and early inflammatory response of human endothelial cells in direct culture with Mg-Zn-Sr alloys

Crystalline Mg-Zinc (Zn)-Strontium (Sr) ternary alloys consist of elements naturally present in the human body and provide attractive mechanical and biodegradable properties for a variety of biomedical applications. The first objective of this study was to investigate the degradation and cytocompatibility of four Mg-4Zn-xSr alloys (x=0.15, 0.5, 1.0, 1.5wt%; designated as ZSr41A, B, C, and D respectively) in the direct culture with human umbilical vein endothelial cells (HUVEC) in vitro. The second objective was to investigate, for the first time, the early-stage inflammatory response in cultured HUVECs as indicated by the induction of vascular cellular adhesion molecule-1 (VCAM-1). The results showed that the 24-h in vitro degradation of the ZSr41 alloys containing a β-phase with a Zn/Sr at% ratio ∼1.5 was significantly faster than the ZSr41 alloys with Zn/Sr at% ∼1. Additionally, the adhesion density of HUVECs in the direct culture but not in direct contact with the ZSr41 alloys for up to 24h was not adversely affected by the degradation of the alloys. Importantly, neither culture media supplemented with up to 27.6mM Mg2+ ions nor media intentionally adjusted up to alkaline pH 9 induced any detectable adverse effects on HUVEC responses. In contrast, the significantly higher, yet non-cytotoxic, Zn2+ ion concentration from the degradation of ZSr41D alloy was likely the cause for the initially higher VCAM-1 expression on cultured HUVECs. Lastly, analysis of the HUVEC-ZSr41 interface showed near-complete absence of cell adhesion directly on the sample surface, most likely caused by either a high local alkalinity, change in surface topography, and/or surface composition. The direct culture method used in this study was proposed as a valuable tool for studying the design aspects of Zn-containing Mg-based biomaterials in vitro, in order to engineer solutions to address current shortcomings of Mg alloys for vascular device applications.

STATEMENT OF SIGNIFICANCE

Magnesium (Mg) alloys specifically designed for biodegradable implant applications have been the focus of biomedical research since the early 2000s. Physicochemical properties of Mg alloys make these metallic biomaterials excellent candidates for temporary biodegradable implants in orthopedic and cardiovascular applications. As Mg alloys continue to be investigated for biomedical applications, it is necessary to understand whether Mg-based materials or the alloying elements have the intrinsic ability to direct an immune response to improve implant integration while avoiding cell-biomaterial interactions leading to chronic inflammation and/or foreign body reactions. The present study utilized the direct culture method to investigate for the first time the in vitro transient inflammatory activation of endothelial cells induced by the degradation products of Zn-containing Mg alloys.

Cytocompatibility of magnesium and AZ31 alloy with three types of cell lines using a direct in vitro method

Magnesium alloys have been investigated by many researchers as a new absorbable biomaterial owing to their excellent degradability with non-maleficence or low-maleficence in living tissues. In the present work, the in vitro cytocompatibility of an Magnesium alloy was investigated by culturing cells directly on it. Investigations were carried out in terms of the cell viability along with the use of scanning electron microscopy to observe its morphology. The cell lines used were derived from fibroblast, endothelial, and smooth muscle cells. Pure magnesium and AZ31 alloy composed of magnesium (96 %), aluminum (3 %), and zinc (1 %) were adopted as models. The viability of cells on the metal samples and on the margin area of a multi-well plate was investigated. For direct culturing on metal, a depression in the viability and morphologically stressed cells were observed. In addition, the cell viability was also depressed for the margin area. To clarify the factors causing the negative effects, the amount of eluted metal ions and pH changes in the medium because of the erosion of the Magnesium samples were investigated, together with the cytotoxicity of sole metal ions corresponding to the composition of the metals. It was found that Mg(2+), Zn(2+), and Al(3+) ions were less toxic at the investigated concentrations, and that these factors will not produce negative effects on cells. Consequently, these factors cannot fully explain the results.

Microstructure-modified biodegradable magnesium alloy for promoting cytocompatibility and wound healing in vitro

The microstructure of biomedical magnesium alloys has great influence on anti-corrosion performance and biocompatibility. In practical application and for the purpose of microstructure modification, heat treatments were chosen to provide widely varying microstructures. The aim of the present work was to investigate the influence of the microstructural parameters of an Al-free Mg-Zn-Zr alloy (ZK60), and the corresponding heat-treatment-modified microstructures on the resultant corrosion resistance and biological performance. Significant enhancement in corrosion resistance was obtained in Al-free Mg-Zn-Zr alloy (ZK60) through 400 °C solid-solution heat treatment. It was found that the optimal condition of solid-solution treatment homogenized the matrix and eliminated internal defects; after which, the problem of unfavorable corrosion behavior was improved. Further, it was also found that the Mg ion-release concentration from the modified ZK60 significantly induced the cellular activity of fibroblast cells, revealing in high viability value and migration ability. The experimental evidence suggests that this system can further accelerate wound healing. From the perspective of specific biomedical applications, this research result suggests that the heat treatment should be applied in order to improve the biological performance.