Fabrication of biodegradable Zn-Al-Mg alloy: Mechanical properties, corrosion behavior, cytotoxicity and antibacterial activities

Rod-like Eutectic Structure in Biodegradable Zn-Al-Sn Alloy Exhibiting Enhanced Mechanical Strength

Zn alloy is recognized as a promising biodegradable metal for bone implant applications because of its good biocompatibility and moderate degradation rate. Nevertheless, the insufficient strength limits its applications. In this study, a rod-like eutectic structure was fabricated in Zn-Al-Sn alloy with the addition of Sn via selective laser melting. It was found that the Al-enriched phase nucleated primarily during cooling and caused the rapid precipitation of Zn. This inevitably consumed the liquid Zn and increased the ratio of Sn to Zn in the liquid phase, resulting in the formation of the eutectic, which was composed of the Sn-enriched phase and the Zn-enriched phase. More importantly, the coupled growth of the Sn-enriched and Zn-enriched phases and their volume differences together led to a rod-like morphology of the eutectic according to the volume fraction theory. Consequently, the yield and ultimate compressive strengths were enhanced to 180 ± 18.8 and 325 ± 29.6 MPa for the Zn-Al-2Sn alloy, respectively. This could be attributed to the pinning effect of the rod-like eutectic, which could block dislocation motion and result in dislocation pile-up, thereby conducing to the mechanical reinforcement. In addition, the Zn-Al-Sn alloy also exhibited good biocompatibility and increased degradation rate because of the enhanced galvanic corrosion. This study showed the potential of rod-like eutectic for the mechanical enhancement of the biodegradable Zn alloy.

UV-irradiation induced biological activity and antibacterial activity of ZnO coated magnesium alloy

In order to improve the biological activity and antibacterial activity of magnesium alloy, the single zinc oxide (ZnO) coating was prepared on magnesium alloys using microwave aqueous synthesis method and followed heat treatment. Then, the coated magnesium alloys were irradiated with ultraviolet (UV) light for different time and subsequently immersed in simulated body fluids (SBF). The influences of UV-irradiated time on the morphology, composition, in vitro biological activity and antibacterial activity were investigated. The results indicated that the ability of the apatite formation on the ZnO coated magnesium alloys surface was significantly enhanced as UV irradiation time prolonged, and the bone-like apatite was formed after UV irradiation for 24 h and then immersing into SBF for 2 weeks, the newly formed apatite was dense and integrate, implying that UV irradiation could activate ZnO coating to improve the biological activity. Moreover, after immersing in SBF for 2 weeks, the antibacterial experiment results demonstrated that ZnO coated magnesium alloys with UV irradiation time of 24 h exhibited more effective antibacterial activity than those of naked magnesium alloys and ZnO coated magnesium alloys which were not irradiated by ultraviolet (UV) light. This work afforded a surface strategy for designing magnesium alloy implant with desirable osseointegration ability and antibacterial property simultaneously for orthopedic and dental applications.

Extrusion of the biodegradable ZnMg0.8Ca0.2 alloy - The influence of extrusion parameters on microstructure and mechanical characteristics

The Zn-based alloys, alloyed with the elements of the 2nd group of the periodic table, are considered as potential biodegradable materials suitable for the fabrication of small orthopaedic implants or cardiovascular stents. Unfortunately, the as-cast Zn-based alloys do not fulfil the requirements for mechanical properties for such applications. Extrusion is a thermomechanical process which is very powerful for breaking the cast microstructure and enhancing mechanical characteristics of metallic materials. In this study, we focused on the influence of extrusion parameters, such as temperature and extrusion ratio, on microstructural and mechanical characteristics of a ZnMg0.8Ca0.2 (wt.%) alloy. The extrusion led to a significant grain refinement and the formation of a crystallographic texture. Extrusion temperature played a more significant role in the mean grain size compared to the extrusion ratio (ER). At lower extrusion temperatures, the texture was less intensive and the subsequent mechanical anisotropy was weaker. Constants for the prediction of the grain size based on the Zener-Hollomon parameter were obtained. Prediction of mechanical properties using the Hall-Petch relationship appeared to be difficult because of the dependence of the texture on the extrusion temperature. Extrusion at the temperatures of 200 °C (ER = 25:1) and 150 °C (ER = 11:1) led to mechanical performance fulfilling the requirements for implantology.

Co-incorporation of graphene oxide/silver nanoparticle into poly-L-lactic acid fibrous: A route toward the development of cytocompatible and antibacterial coating layer on magnesium implants

Magnesium (Mg) alloys present great potential for the development of orthopedic implants, whereas, their high degradation rate and poor antibacterial performance have restricted orthopedic applications. In this work, PLLA/GO-AgNP (poly-L-lactic acid/graphene oxide- silver nanoparticle) with different concentration of GO-AgNPs were deposited on Mg alloy via electrospinning method for enhancement of corrosion resistance and antibacterial performance. The result revealed that incorporation of GO into PLLA fibrous considerably slowed down the degradation rate of Mg alloy substrate and reduced the H₂ release rate from the substrate. Also, co-incorporation of GO and AgNPs into PLLA fibrous resulted in substantial escalate in compressive strength after immersion in simulated body fluid (SBF). Antibacterial activity test exhibited that Mg alloy and neat PLLA fibrous presented minimal inhibition area toward Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). In contrast, using PLLA/GO-AgNPs fibrous improved antibacterial performance against both bacteria. Cytocompatibility results indicated that PLLA/GO-AgNPs fibrous with a low amount of GO-AgNPs enhanced cell proliferation and growth while high co-incorporation of GO-AgNPs showed a negative effect on cell proliferation. Taken together, PLLA/1GO-AgNPs fibrous coating shows suitable corrosion resistance, cytocompatibility, and antibacterial function for use in orthopedic applications.

Prospects and strategies for magnesium alloys as biodegradable implants from crystalline to bulk metallic glasses and composites-A review

As a biodegradable metal (BM), alloys of magnesium (Mg) offer great potential as an alternative to the permanent metallic implants currently being used for fracture repairs and tissue-healing processes. These alloys exhibit superior biocompatibility and appropriate mechanical strength and dissolution behavior in the physiological environment, essential prerequisites for a BM. However, rapid and generally non-uniform corrosion has been the major drawback of Mg alloys. Abrupt deterioration in mechanical strength is experienced due to the inhomogeneous corrosion, which is also considered detrimental to the surface passivation process. This review has analyzed a variety of strategies that can be adopted to address the core challenges with Mg alloy biomaterials. In addition, the review provides fundamental understanding of the mechanisms associated with these challenging problems, including discussion of crystalline and bulk metallic glasses (BMGs) and composites. Comparison among the properties and mechanisms observed in other metal alloy systems, including zinc (Zn) and iron (Fe) alloys and prominent BMGs, are also presented for analysis in order to provide new approaches to resolving the critical issues of Mg alloys. STATEMENT OF SIGNIFICANCE: The effects of alloying elements, microstructure, heat treatment and deformation on the mechanical and corrosion properties of biodegradable metals such as Mg-based alloys and bulk metal glasses (BMGs) are identified. Theoretical models and experimental findings are comprehensively analyzed to corroborate the actual corrosion and deformation mechanisms observed in biodegradable metals (BMs). This work also provides an in-depth comparison of mechanical and corrosion properties among the prominent biodegradable metal alloy systems, illustrating a clear outlook on their potentials. The proposed strategies to address the current challenges in BMs are substantiated with fundamental theories and experimental evidence.

Additively manufactured biodegradable porous zinc

Additively manufacturing (AM) opens up the possibility for biodegradable metals to possess uniquely combined characteristics that are desired for bone substitution, including bone-mimicking mechanical properties, topologically ordered porous structure, pore interconnectivity and biodegradability. Zinc is considered to be one of the promising biomaterials with respect to biodegradation rate and biocompatibility. However, no information regarding the biodegradability and biocompatibility of topologically ordered AM porous zinc is yet available. Here, we applied powder bed fusion to fabricate porous zinc with a topologically ordered diamond structure. An integrative study was conducted on the static and dynamic biodegradation behavior (in vitro, up to 4 weeks), evolution of mechanical properties with increasing immersion time, electrochemical performance, and biocompatibility of the AM porous zinc. The specimens lost 7.8% of their weight after 4 weeks of dynamic immersion in a revised simulated body fluid. The mechanisms of biodegradation were site-dependent and differed from the top of the specimens to the bottom. During the whole in vitro immersion time of 4 weeks, the elastic modulus values of the AM porous zinc (E = 700-1000 MPa) even increased and remained within the scope of those of cancellous bone. Indirect cytotoxicity revealed good cellular activity up to 72 h according to ISO 10,993-5 and -12. Live-dead staining confirmed good viability of MG-63 cells cultured on the surface of the AM porous zinc. These important findings could open up unprecedented opportunities for the development of multifunctional bone substituting materials that will enable reconstruction and regeneration of critical-size load-bearing bone defects. STATEMENT OF SIGNIFICANCE: No information regarding the biodegradability and biocompatibility of topologically ordered AM porous zinc is available. We applied selective laser melting to fabricate topologically ordered porous zinc and conducted a comprehensive study on the biodegradation behavior, electrochemical performance, time-dependent mechanical properties, and biocompatibility of the scaffolds. The specimens lost 7.8% of their weight after4 weeks dynamic biodegradation while their mechanical properties surprisingly increased after 4 weeks. Indirect cytotoxicity revealed good cellular activity up to 72 h. Intimate contact between MG-63 cells and the scaffolds was also observed. These important findings could open up unprecedented opportunities for the development of multifunctional bone substituting materials that mimic bone properties and enable full regeneration of critical-size load-bearing bony defects.

Evaluation of a Zn-2Ag-1.8Au-0.2V Alloy for Absorbable Biocompatible Materials

Zinc (Zn) and Zn-based alloys have been proposed as a new generation of absorbable metals mainly owing to the moderate degradation behavior of zinc between magnesium and iron. Nonetheless, mechanical strength of pure Zn is relatively poor, making it insufficient for the majority of clinical applications. In this study, a novel Zn–2Ag–1.8Au–0.2V (wt.%) alloy (Zn–Ag–Au–V) was fabricated and investigated for use as a potential absorbable biocompatible material. Microstructural characterization indicated an effective grain-refining effect on the Zn alloy after a thermomechanical treatment. Compared to pure Zn, the Zn–Ag–Au–V alloy showed significantly enhanced mechanical properties, with a yield strength of 168 MPa, an ultimate tensile strength of 233 MPa, and an elongation of 17%. Immersion test indicated that the degradation rate of the Zn–Ag–Au–V alloy in Dulbecco’s phosphate buffered saline was approximately 7.34 ± 0.64 μm/year, thus being slightly lower than that of pure Zn. Biocompatibility tests with L929 and Saos-2 cells showed a moderate cytotoxicity, alloy extracts at 16.7%, and 10% concentration did not affect metabolic activity and cell proliferation. Plaque formation in vitro was reduced, the Zn–Ag–Au–V surface inhibited adhesion and biofilm formation by the early oral colonizer Streptococcus gordonii, indicating antibacterial properties of the alloy.

Selection of extraction medium influences cytotoxicity of zinc and its alloys

Zinc (Zn) alloys have been considered as promising absorbable metals, mainly due to their moderate degradation rates ranging between magnesium alloys and iron alloys. The degradation behavior depends on the specific physiological environment. Released metallic ions and corrosion products directly influence biocompatibility. The initial contact of orthopedic implants or vascular stents after implantation will be with blood. In this study, fetal bovine serum (FBS) was used as a model system of blood components. We investigated the influence of FBS on in vitro degradation behavior and cytotoxicity of pure Zn, and Zn-4Ag and Zn-2Ag-1.8Au-0.2 V (wt%) alloys. The initial degradation rates in FBS were assessed and compared with the degradation and toxicity in four other common physiological model systems: DMEM cell culture medium ± FBS and McCoy's 5A medium ± FBS. Test samples in pure FBS showed the highest initial degradation rates, and accordingly, FBS supplemented media accelerated the degradation process as well. Moreover, an extract test according to ISO 10993-5 and -12 with L929 and Saos-2 cells was performed to investigate the role of FBS in the extraction medium. The cytotoxic effects observed in the tests were correlated with FBS-mediated Zn²⁺ release. These findings have significant implications regarding the selection of appropriate media for in vitro degradation and cytotoxicity evaluation of Zn and its alloys. STATEMENT OF SIGNIFICANCE: Metallic zinc and its alloys have been considered as promising biodegradable metals, mainly due to their moderate degradation rates. However, in vitro cytotoxicity tests according to the current ISO 10993 standard series are not suitable to predict biocompatibility of Zn alloys due to the inconsistent correlation between in vitro and in vitro biocompatibility. In this study, we show that the outcomes of standardized in vitro cytotoxicity tests of Zn and Zn alloys are influenced by fetal bovine serum in the extraction vehicle because FBS promotes Zn²⁺ release during the extraction process. The results of the study provide significant information for selection of appropriate model systems to evaluate in vitro degradation behavior and cytotoxicity.

Challenges in the use of zinc and its alloys as biodegradable metals: Perspective from biomechanical compatibility

To date, more than fifty articles have been published on the feasibility studies of zinc and its alloys as biodegradable metals. These preliminary in vitro and in vivo studies showed acceptable biodegradability and reasonable biocompatibility in bone and blood microenvironments for the experimental Zn-based biodegradable metals and, for some alloy systems, superior mechanical performance over Mg-based biodegradable metals. For instance, the Zn-Li alloys exhibited higher UTS (UTS), and the Zn-Mn alloys exhibited higher elongation (more than 100%). On the one hand, similar to Mg-based biodegradable metals, insufficient strength and ductility, as well as relatively low fatigue strength, may lead to premature failure of medical devices. On the other hand, owing to the low melting point of the element Zn, several new uncertainties with regard to the mechanical properties of biomedical zinc alloys, including low creep resistance, high susceptibility to natural aging, and static recrystallization (SRX), may lead to device failure during storage at room temperature and usage at body temperature. This paper comprehensively reviews studies on these mechanical aspects of industrial Zn and Zn alloys in the last century and biomedical Zn and Zn alloys in this century. The challenges for the future design of biomedical zinc alloys as biodegradable metals to guarantee 100% mechanical compatibility are pointed out, and this will guide the mechanical property design of Zn-based biodegradable metals. STATEMENT OF SIGNIFICANCE: Previous studies on mechanical properties of industrial Zn and Zn alloys in the last century and biomedical Zn and Zn alloys in this century are comprehensively reviewed herein. The challenges for the future design of zinc-based biodegradable materials considering mechanical compatibility are pointed out. Common considerations such as strength, ductility, and fatigue behaviors are covered together with special attention on several new uncertainties including low creep resistance, high susceptibility to natural aging, and static recrystallization (SRX). These new uncertainties, which are not significantly observed in Mg-based and Fe-based materials, are largely due to the low melting point of the element Zn and may lead to device failure during storage at room temperature and clinical usage at body temperature. Future studies are urgently needed on these topics.

Current status and perspectives of zinc-based absorbable alloys for biomedical applications

Absorbable metals have the potential to serve as the next generation of temporary medical implant devices by safely dissolving in the human body upon vascular tissue healing and bone regeneration. Their implementation in the market could greatly reduce the need of costly and risky additional surgeries for either implant replacement or removal, often required in current permanent implants. Despite the extensive research done over the last two decades on magnesium (Mg) and iron (Fe) based alloys, they have not generally shown a satisfactory combination of mechanical properties, biocompatibility and controlled degradation rate in the physiological environment. Consequently, zinc (Zn) based alloys were introduced in the last few years as alternative materials to overcome the limitations of Fe and Mg-based alloys. The blend of different alloying elements and processing conditions have led to a wide variety of Zn-based alloys having tunable mechanical properties and corrosion rates. This review provides the most recent progress in the development of absorbable Zn-based alloys for biomedical implant applications, primarily for cardiovascular and orthopedic devices. Their biocompatibility, processability and metallurgical aspects, as well as their mechanical behavior and corrosion properties are presented and discussed, including their opportunities, limitations and future research directions. STATEMENT OF SIGNIFICANCE: Temporary orthopedic bioimplants have become increasingly popular as they offer an alternative to prevent complications, like infections or secondary surgeries, often related to the implantation of permanent devices. Iron and magnesium alloys were extensively studied as candidates for absorbable medical applications, but they generally failed to provide a desirable mechanical performance and corrosion characteristics in the physiological environment. Zinc was introduced in the last decade as a potential implant material after showing outstanding biocompatibility and biodegradability. This review summarizes the research advances to date and provides a thorough discussion of the future challenges of absorbable zinc alloys to satisfy the demanding clinical benchmarks for absorbable medical applications. Their biocompatibility, mechanical, and corrosion aspects, both in vitro and in vivo, are comprehensively reviewed and assessed accordingly.

The Prospects for Biodegradable Zinc in Wound Closure Applications

Zinc is identified as a promising biodegradable metal along with magnesium and iron. In the last 5 years, considerable progress is made on understanding the mechanical properties, biodegradability, and biocompatibility of zinc and its alloys. A majority of these studies have focused on using zinc for absorbable cardiovascular and orthopedic device applications. However, it is likely that zinc is also suitable for other biomedical applications. In this work, the prospects for zinc in the fabrication of wound closure devices such as absorbable sutures, staples, and surgical tacks are critically assessed, with the aim of inspiring future research on biodegradable Zn for this medical application.

In vivo study of the efficacy, biosafety, and degradation of a zinc alloy osteosynthesis system

In this study, a comprehensive analysis of a novel zinc alloy osteosynthesis system in a canine mandibular fracture model is presented. The efficacy of the system was compared for PLLA (poly-l-lactic acid) and titanium materials using X-ray radiography, micro-CT tomography, undecalcified bone histomorphometry, and a three-point bending test. Histology, blood normal, blood biochemical, and serum zinc concentration tests were also performed to assess the biosafety of the zinc alloy osteosynthesis system. The degradability of the zinc alloy was evaluated using a micro-CT and scanning electron microscope during the 24-week post operation period. The results showed that zinc alloy possesses good mechanical properties that support fracture healing. Its uniform and slow corrosion leads to adequate degradation behavior in 24 weeks. Additionally, the zinc alloy proved to be biocompatible, indicating that this novel osteosynthesis system is safe for use in the body. The results of the study demonstrate that this zinc alloy-based osteosynthesis system is a promising candidate for a new generation of osteosynthesis systems, with further improvements required in the future.

The influence of alloying and fabrication techniques on the mechanical properties, biodegradability and biocompatibility of zinc: A comprehensive review

Zinc has been identified as one of the most promising biodegradable metals along with magnesium and iron. Zinc appears to address some of the core engineering problems associated with magnesium and iron when applied to biomedical implant applications; hence the increase in the amount of research investigations on the metal in the last few years. In this review, the current state-of-the-art on biodegradable Zn, including recent developments, current opportunities and future directions of research are discussed. The discussions are presented with a specific focus on reviewing the relationships that exist between mechanical properties, biodegradability, and biocompatibility of zinc with alloying and fabrication techniques. This work hopes to guide future studies on biodegradable Zn that will help in advancing this field of research. STATEMENT OF SIGNIFICANCE: (i) The review offers an up-to-date and comprehensive review of the influence of alloying and fabrication technique on mechanical properties, biodegradability and biocompatibility of Zn; (ii) the work cites the most relevant biodegradable Zn fabrication processes including additive manufacturing techniques; (iii) the review includes a listing of research gap and future research directions for the field of biodegradable Zn.

Bactericidal and Biocompatible Properties of Plasma Chemical Oxidized Titanium (TiOB®) with Antimicrobial Surface Functionalization

Coating of plasma chemical oxidized titanium (TiOB^®) with gentamicin-tannic acid (TiOB^® gta) has proven to be efficient in preventing bacterial colonization of implants. However, in times of increasing antibiotic resistance, the development of alternative antimicrobial functionalization strategies is of major interest. Therefore, the aim of the present study is to evaluate the antibacterial and biocompatible properties of TiOB^® functionalized with silver nanoparticles (TiOB^® SiOx Ag) and ionic zinc (TiOB^® Zn). Antibacterial efficiency was determined by agar diffusion and proliferation test on Staphylocuccus aureus. Cytocompatibility was analyzed by direct cultivation of MC3T3-E1 cells on top of the functionalized surfaces for 2 and 4 d. All functionalized surfaces showed significant bactericidal effects expressed by extended lag phases (TiOB^® gta for 5 h, TiOB^® SiOx Ag for 8 h, TiOB^® Zn for 10 h). While TiOB^® gta (positive control) and TiOB^® Zn remained bactericidal for 48 h, TiOB^® SiOx Ag was active for only 4 h. After direct cultivation for 4 d, viable MC3T3-E1 cells were found on all surfaces tested with the highest biocompatibility recorded for TiOB^® SiOx Ag. The present study revealed that functionalization of TiOB^® with ionic zinc shows bactericidal properties that are comparable to those of a gentamicin-containing coating.

Biodegradable Metals

Over the last two decades, significant scientific efforts have been devoted to developingbiodegradable metal implants for orthopedic and cardiovascular applications, mainly due to theirimproved mechanical properties compared to those of biodegradable polymers [...]

Surface treatment strategies to combat implant-related infection from the beginning

Orthopaedic implants are recognised as important therapeutic devices in the successful clinical management of a wide range of orthopaedic conditions. However, implant-related infections remain a challenging and not uncommon issue in patients with implanted instrumentation or medical devices. Bacterial adhesion and formation of biofilm on the surface of the implant represent important processes towards progression of infection. Given the intimate association between infection and the implant surface, adequate treatment of the implant surface may help mitigate the risk of infection. This review summarises the current surface treatment technologies and their role in prevention of implant-related infection from the beginning.

Translational potential of this article

Despite great technological advancements, the prevalence of implant-related infections remains high. Four main challenges can be identified. (i) Insufficient mechanical stability can cause detachment of the implant surface coating, altering the antimicrobial ability of functionalized surfaces. (ii) Regarding drug-loaded coatings, a stable drug release profile is of vital importance for achieving effective bactericidal effect locally; however, burst release of the loaded antibacterial agents remains common. (iii) Although many coatings and modified surfaces provide superior antibacterial action, such functionalisation of surfaces sometimes has a detrimental effect on tissue biocompatibility, impairing the integration of the implants into the surrounding tissue. (iv) Biofilm eradication at the implant surface remains particularly challenging. This review summarised the recent progress made to address the aforementioned problems. By providing a perspective on state-of-the-art surface treatment strategies for medical implants, we hope to support the timely adoption of modern materials and techniques into clinical practice.

In vivo performances of pure Zn and Zn-Fe alloy as biodegradable implants

The disadvantage of current biodegradable metals such as Mg and Fe is the release of hydrogen gas in vivo that can cause gas embolism and the production of voluminous iron oxide that can cause inflammation, respectively. Such considerations have turned focus towards Zn as an alternative. This is based on the fact that Zn plays a crucial role in many physiological processes, as well as potentially being biocompatible and capable of with biodegradation. As such, the purpose of the present study was to evaluate the in vivo performance of pure Zinc and Zn-2%Fe implants. The use of iron as an alloying element was aimed at accelerating the corrosion rate of pure zinc by a micro-galvanic effect so as to maintain the post-implantation biodegradation characteristics of the implant. In vivo assessment was carried out using cylindrical disks implanted in the back midline of 16 male Wistar rats for up to 24 weeks. Post-implantation evaluation included monitoring the well-being of rats, weekly examination of hematological parameters: serum Zn levels, red and white blood cell counts and hemoglobin levels, X-ray radiography, histological analysis and corrosion rate assessment. The results obtained in terms of well-being, hematological tests and histological analysis of the rats indicate that the in vivo behavior of pure Zn and Zn-2%Fe implants was adequate and in line with the results obtained by the control group containing inert Ti-6Al-4V alloy implants. The corrosion rate of Zn-2%Fe alloy in in vivo conditions was relatively increased compared to pure Zn due to micro-galvanic corrosion.

Benefits of hardware removal after plating

Routine implant removal is frequently performed although evidence-based guidelines are lacking. But routinely planned implant removal has significant economic implications and shows considerable complication rates. In general, clinical outcome seems to improve but pain relief after operation is often unpredictable. Even in patients reporting implant-related pain, implant removal does not guarantee relief and may be associated with further complications. The intra- and postoperative complication rate remains very high. Implant removal demonstrates a significant learning curve and unsupervised junior surgeons tend to cause more complications. The need for implant removal may be questioned. Even with the implant in place, contact activities can be resumed. However, a new adequate trauma can create a new fracture independently if there is an implant in-situ or not. It is important to understand the complications and outcomes to be expected with hardware removal to carefully evaluate its indication.

Zinc-based alloys for degradable vascular stent applications

The search for biodegradable metals with mechanical properties equal or higher to those of currently used permanent biomaterials, such as stainless steels, cobalt chromium and titanium alloys, desirable in vivo degradation rate and uniform corrosion is still an open challenge. Magnesium (Mg), iron (Fe) and zinc (Zn)-based alloys have been proposed as biodegradable metals for medical applications. Over the last two decades, extensive research has been done on Mg and Fe. Fe-based alloys show appropriate mechanical properties, but their degradation rate is an order of magnitude below the benchmark value. In comparison, alongside the insufficient mechanical performance of most of its alloys, Mg degradation rate has proven to be too high in a physiological environment and corrosion is rarely uniform. During the last few years, Zn alloys have been explored by the biomedical community as potential materials for bioabsorbable vascular stents due to their tolerable corrosion rates and tunable mechanical properties. This review summarizes recent progress made in developing Zn alloys for vascular stenting application. Novel Zn alloys are discussed regarding their microstructural characteristics, mechanical properties, corrosion behavior and in vivo performance.

STATEMENT OF SIGNIFICANCE

Numerous studies on magnesium and iron materials have been reported to date, in an effort to formulate bioabsorbable stents with tailorable mechanical characteristics and corrosion behavior. Crucial concerns regarding poor ductility and remarkably rapid corrosion of magnesium, and very slow degradation of iron, seem to be still not desirably fulfilled. Zinc was introduced as a potential implant material in 2013 due to its promising biodegradability and biocompatibility. Since then, extensive investigations have been made toward development of zinc alloys that meet clinical benchmarks for vascular scaffolding. This review critically surveys the zinc alloys developed since 2013 from metallurgical and biodegradation points of view. Microstructural features, mechanical, corrosion and in vivo performances of these new alloys are thoroughly reviewed and evaluated.

Fabrication and characterization of biodegradable Mg-Zn-Y-Nd-Ag alloy: Microstructure, mechanical properties, corrosion behavior and antibacterial activities

Magnesium (Mg), a potential biodegradable material, has drawn wide attention in the bone reconstruction field. However, Mg alloys, served as the bone graft substitution, remain a clinical challenge, the antibacterial activity of which is required to be enhanced. Here, we prepared biodegradable magnesium Mg-Zn-Y-Nd-Ag and then had it been further densified by extruding. The microstructure evolution of the as-cast and as-extruded Mg- Zn-Y-Nd-Ag was characterized using optical microscope and X-ray diffraction analyses (XRD). The results showed that the microstructure of the as-cast alloys was mainly dendrites, between which, the second phase was mainly distributed; with the increase of Ag additions, grain structure was further refined as well as the increase of amount of the second phase. After the extrusion, the grains were further refined. Microhardness tests indicated that both of the increase of Ag content and the extrusion process improved the microhardness of the alloys, specially the later. A systematic investigation of the in vivo antibacterial capability of Staphylococcus aurous and Escherichia coli was performed, and the results of which indicated that all Mg-Zn-Y-N-xAg (x = 0.2, 0.4, 0.6, 0.8) alloys exhibited certain antibacterial property, which would increased with the increase of Ag content. Taken all together, the antimicrobial property of the as-extruded alloy containing 0.4 wt% Ag exhibited the relatively better antimicrobial properties and mechanical property with the relatively small loss in corrosion resistance, which suggested the potential utility of as-extruded Mg-Zn-Y-N-0.4Ag in treating orthopedic infections.

Mechanical Characteristics, In Vitro Degradation, Cytotoxicity, and Antibacterial Evaluation of Zn-4.0Ag Alloy as a Biodegradable Material

Zn-based biodegradable metallic materials have been regarded as new potential biomaterials for use as biodegradable implants, mainly because of the ideal degradation rate compared with those of Mg-based alloys and Fe-based alloys. In this study, we developed and investigated a novel Zn-4 wt % Ag alloy as a potential biodegradable metal. A thermomechanical treatment was applied to refine the microstructure and, consequently, to improve the mechanical properties, compared to pure Zn. The yield strength (YS), ultimate tensile strength (UTS) and elongation of the Zn-4Ag alloy are 157 MPa, 261 MPa, and 37%, respectively. The corrosion rate of Zn-4Ag calculated from released Zn ions in DMEM extracts is approximately 10.75 ± 0.16 μg cm^-2 day^-1, which is higher than that of pure Zn [corrected]. In vitro cytotoxicity tests showed that the Zn-4Ag alloy exhibits acceptable toxicity to L929 and Saos-2 cells, and could effectively inhibit initial bacteria adhesion. This study shows that the Zn-4Ag exhibits excellent mechanical properties, predictable degradation behavior, acceptable biocompatibility, and effective antibacterial properties, which make it a candidate biodegradable material.

In Vitro Studies on the Degradability, Bioactivity, and Cell Differentiation of PRP/AZ31B Mg Alloys Composite Scaffold

In recent years, more and more methods have been developed to improve the bioactivity of the biodegradable materials in bone tissue regeneration. In present study, we used rat mesenchymal stem cells (rMSCs) to evaluate the outcomes of Mg alloys (AZ31B, Magnesium, and Aluminum) and Platelet-rich plasma (PRP)/Mg alloys on rMSCs biocompatibility and osteogenic differentiation. Water absorption experiments indicated that both bare AZ31B and PRP/AZ31B were capable of absorbing large amounts of water. But the water absorption ratio for PRP/AZ31B was significantly higher than that for bare AZ31B. The degradability experiments implied that both samples degraded at same speed. rMSCs on the surface of AZ31B distributed more and better than those on the AZ31B scaffold. In ALP activity experiment, the activity of rMSCs on the PRP/AZ31B was markedly higher than that on the AZ31B scaffolds on the 7th day and 14th day. qRT-PCR also showed that OPN and OCN were expressed in both samples. OPN and OCN expression in PRP/AZ31B sample were higher than those in bare AZ31B samples. In summary, the in vitro study implied that AZ31B combined with PRP could remarkably improve cell seeding, attachment, proliferation, and differentiation.