Enhanced Osseointegration of Zn-Mg Composites by Tuning the Release of Zn Ions with Sacrificial Mg-Rich Anode Design

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

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

Imidazole functionalized photo-crosslinked aliphatic polycarbonate drug-eluting coatings on zinc alloys for osteogenesis, angiogenesis, and bacteriostasis in bone regeneration

Zinc (Zn) alloys have demonstrated significant potential in healing critical-sized bone defects. However, the clinical application of Zn alloys implants is still hindered by challenges including excessive release of zinc ions (Zn2+), particularly in the early stage of implantation, and absence of bio-functions related to complex bone repair processes. Herein, a biodegradable aliphatic polycarbonate drug-eluting coating was fabricated on zinc-lithium (Zn-Li) alloys to inhibit Zn2+ release and enhance the osteogenesis, angiogenesis, and bacteriostasis of Zn alloys. Specifically, the photo-curable aliphatic polycarbonates were co-assembled with simvastatin and deposited onto Zn alloys to produce a drug-loaded coating, which was crosslinked by subsequent UV light irradiation. During the 60 days long-term immersion test, the coating showed distinguished stable drug release and Zn2+ release inhibition properties. Benefiting from the regulated release of Zn2+ and simvastatin, the coating facilitated the adhesion, proliferation, and differentiation of MC3T3-E1 cells, as well as the migration and tube formation of EA.hy926 cells. Astonishingly, the coating also showed remarkable antibacterial properties against both S. aureus and E. coli. The in vivo rabbit critical-size femur bone defects model demonstrated that the drug-eluting coating could efficiently promote new bone formation and the expression of platelet endothelial cell adhesion molecule-1 (CD31) and osteocalcin (OCN). The enhancement of osteogenesis, angiogenesis, and bacteriostasis is achieved by precisely controlling of the released Zn2+ at an appropriate level, as well as the stable release profile of simvastatin. This tailored aliphatic polycarbonate drug-eluting coating provides significant potential for clinical applications of Zn alloys implants.

Advances in biodegradable materials: Degradation mechanisms, mechanical properties, and biocompatibility for orthopedic applications

Mg-based and Zn-based biodegradable materials have the potential to become the next-generation implant materials to treat bone diseases, because of their desired degradation and mechanical properties. This article reviews the status of these implant materials. The required properties of biodegradable materials such as biodegradability, mechanical properties, and biocompatibility for performance evaluation were briefly discussed. The influence of fabrication techniques, microstructure, alloying elements, and post-processing techniques on the properties of Mg and Zn-based materials was addressed. The degradation mechanism by dissolution, oxidation, and interaction with human body cells was discussed. The biocompatibility of Mg and Zn-based biodegradable materials was analyzed. The significance of in vitro and in vivo biocompatibility testing was highlighted, emphasizing the superiority of in vivo results over cell line studies. This article identifies the many Mg and Zn-based biodegradable materials and summarizes the key findings.

Feasibility Insights of the Green-Assisted Calcium-Phosphate Coating on Biodegradable Zinc Alloys for Biomedical Application: In Vitro and In Vivo Studies

In the field of bone tissue engineering, recently developed Zn alloy scaffolds are considered potential candidates for biodegradable implants for bone regeneration and defect reconstruction. However, the clinical success of these alloys is limited due to their insufficient surface bioactivities. Further, the higher concentration of Zn2+ produced during degradation promotes antibacterial activity, but deteriorates osteogenic properties. This study fabricated an Azadirachta indica (neem)-assisted brushite-hydroxyapatite (HAp) coating on the recently developed Zn-2Cu-0.5Mg alloy to tackle the above dilemma. The microstructure, degradation behavior, antibacterial activity, and hemocompatibility, along with in vitro and in vivo cytocompatibility of the coated alloys, are systematically investigated. Microstructural analysis reveals flower-like morphology with uniformly grown flakes for neem-assisted deposition. The neem-assisted deposition significantly improves the adhesion strength from 12.7 to 18.8 MPa, enhancing the mechanical integrity. The potentiodynamic polarization study shows that the neem-assisted deposition decreases the degradation rate, with the lowest degradation rate of 0.027 mm/yr for the ZHN2 sample. In addition, the biomineralization process shows the apatite formation on the deposited coating after 21 days of immersion. In vitro cytotoxicity assay exhibits the maximum cell viability of 117% for neem-assisted coated alloy in 30% extract after 5d and the improved cytocompatibility which is due to the controlled release of Zn2+ ions. Meanwhile, neem-assisted coated alloy increases the ZOI by 32 and 24% for Gram-positive and Gram-negative bacteria, respectively. Acceptable hemolysis (<5%) and anticoagulation parameters demonstrate a promising hemocompatibility of the coated alloy. In vivo implantation illustrates a slight inflammatory response and vascularization after 2 weeks of subcutaneous implantation, and neo-bone formation in the defect areas of the rat femur. Micro-CT and histology studies demonstrate better osseointegration with satisfactory biosafety response for the neem-assisted coated alloy as compared to that without neem-assisted deposition. Hence, this neem-assisted brushite-Hap coating strategy elucidates a new perspective on the surface modification of biodegradable implants for the treatment of bone defects.

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

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

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

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

Mechanical properties, corrosion behavior, and cytotoxicity of biodegradable Zn/Mg multilayered composites prepared by accumulative roll bonding process

Zinc (Zn), magnesium (Mg), and their respective alloys have attracted great attention as biodegradable bone-implant materials due to their excellent biocompatibility and biodegradability. However, the poor mechanical strength of Zn alloys and the rapid degradation rate of Mg alloys limit their clinical application. The manufacture of Zn and Mg bimetals may be a promising way to improve their mechanical and degradation properties. Here we report on Zn/Mg multilayered composites prepared via an accumulative roll bonding (ARB) process. With an increase in the number of ARB cycles, the thicknesses of the Zn layer and the Mg layer were reduced, while a large number of heterogeneous interfaces were introduced into the Zn/Mg multilayered composites. The composite samples after 14 ARB cycles showed the highest yield strength of 411±3 MPa and highest ultimate tensile strength of 501±3 MPa among all the ARB processed samples, significantly higher than those of the Zn/Zn and Mg/Mg multilayered samples. The Zn and Mg layers remained continuous in the Zn/Mg composite samples after annealing at 150 °C for 10 min, resulting in a decrease in yield strength from 411±3 MPa to 349±3 MPa but an increase in elongation from 8±1% to 28±1%. The degradation rate of the Zn/Mg multilayered composite samples in Hanks' solution was ranged from 127±18 µm/y to 6±1 µm/y. The Zn/Mg multilayered composites showed over 100% cell viability with their 25% and 12.5% extracts in relation to MG-63 cells after culturing for 3 d, indicating excellent cytocompatibility. STATEMENT OF SIGNIFICANCE: This work reports a biodegradable Zn/Mg multilayered composite prepared by accumulative roll bonding (ARB) process. The yield and ultimate tensile strength of the Zn/Mg multilayered composites were improved due to grain refinement and the introduction of a large number of heterogeneous interfaces. The composite samples after 14 ARB cycles showed the highest yield strength of 411±3 MPa and highest ultimate tensile strength of 501±3 MPa among all the ARB processed samples. The degradation rate of the Zn/Mg multilayered composite meets the required degradation rate for biodegradable bone-implant materials. The results demonstrated that it is a very promising approach to improve the strength and biocompatibility of biodegradable Zn-based alloys.

Corrosion-tailoring, osteogenic, anti-inflammatory, and antibacterial aspirin-loaded organometallic hydrogel composite coating on biodegradable Zn for orthopedic applications

Zn and its alloys are receiving increasing interest for biodegradable orthopedic implant applications owing to their moderate corrosion rate and the potential functionality of Zn2+. However, their non-uniform corrosion behavior and insufficient osteogenic, anti-inflammatory, and antibacterial properties do not meet the comprehensive requirements of orthopedic implants in clinical use. Herein, an aspirin (an acetylsalicylic acid, ASA, 10, 50, 100, and 500 mg/L)-loaded carboxymethyl chitosan (CMC)/gelatin (Gel)-Zn2+ organometallic hydrogel composite coating (CMC/Gel&Zn2+/ASA) was fabricated on a Zn surface via an alternating dip-coating method, aiming to obtain a material with these comprehensive properties improved. The organometallic hydrogel composite coatings, ca. 12-16 μm in thickness, showed compact, homogeneous, and micro-bulge structured surface morphology. The coatings protected well the Zn substrate from pitting/localized corrosion and contained the release of the bioactive components, Zn2+ and ASA, in a sustained and stable manner in long-term in vitro immersions in Hank's solution. The coated Zn showed greater ability to promote proliferation and osteogenic differentiation for MC3T3-E1 osteoblasts, and better anti-inflammatory capacity when compared with uncoated Zn. Additionally, this coating displayed excellent antibacterial activity against both Escherichia coli (>99 % antibacterial rate) and Staphylococcus aureus (>98 % antibacterial rate). Such appealing properties can be attributed to the compositional nature of the coating, namely the sustained release of Zn2+ and ASA, as well as the surface physiochemical properties because of its unique microstructure. This organometallic hydrogel composite coating can be considered a promising option for the surface modification of biodegradable Zn-based orthopedic implants among others.

Fabrication and Properties of a Biodegradable Zn-Ca Composite

In recent years, Zn and its alloys have become some of the most promising degradable metals as in vivo implants due to their acceptable biocompatibility and more suitable degradation rate compared with Mg-based and Fe-based alloys. However, the degradation rate of Zn-based materials after implantation in the body for orthopedic applications is relatively slow, leading to long-term retention of the implants after fulfilling their missions. Moreover, the excessive release of Zn2+ during the degradation process of Zn-based implants usually leads to high cytotoxicity and delayed osseointegration. To provide a feasible solution to the problem faced by Zn-based implants, a Zn-Ca composite was fabricated by an air pressure infiltration method in this work. The XRD pattern of the composite suggests that the composite is fully composed of Zn-Ca intermetallic compounds. The degradation tests in vitro show that the composite has a much higher degradation rate than pure Zn, and the high Ca content regions in the composite can preferentially degrade as sacrificial anodes. In addition, the composite can efficiently induce Ca-P deposition during immersion tests in Hank's solution. Cytotoxicity tests indicate that L-929 cells exhibit around 82% cell viability (Grade 1) even after being cultured in the 100% extract prepared from the Zn-Ca composite for 1 day and show excellent cell viability.

β-tricalcium phosphate synthesized in organic medium for controlled release drug delivery application in bio-scaffolds

β-tricalcium phosphate (β-TCP) was synthesized in an organic medium (acetone) to obtain a single-phase product while calcium carbonate (CaCO3) and ortho-phosphoric acid (H3PO4) were the sources of Ca, and P, respectively. The synthesized β-TCP was characterized by employing a number of sophisticated techniques vis. XRD, FTIR, FESEM, VSM and UV-Vis-NIR spectrometry. On the other hand, cytotoxicity, hemolysis, and antimicrobial activity for Gram-negative as well as Gram-positive (E. coli and S. aureus) bacteria were explored using this synthesized sample in powder format. However, to assess the drug loading and releasing profile, these powdered samples were first compressed into disks followed by sintering at 900 °C. Prior to loading the drug, porosity, density, and water absorbance characteristics of the scaffolds were examined in deionized water. Both loading and releasing profiles of the antibiotic (ciprofloxacin) were looked over at various selected time intervals which were continued up to 28 days. The observed results revealed that 2.87% of ciprofloxacin was loaded while 37% of this loaded drug was released within the selected time frame as set in this study. The scaffold was also immersed in SBF solution maintaining identical interim periods for the bioactivity evaluation. Furthermore, all three types of samples (e.g. drug-loaded, drug-released, and SBF-soaked) were characterized by FESEM and EDX while antimicrobial activity (against E. coli, S. typhi, and S. aureus) and efficacy to prevent hemolysis were also investigated. The drug-loaded scaffold presented a larger inhibition zone than the standard for all three types of microbes. Although powdered β-TCP was inactive in killing the Gram-negative bacteria, surprisingly the drug-released scaffold showed an inhibition zone.

A drug-loaded composite coating to improve osteogenic and antibacterial properties of Zn-1Mg porous scaffolds as biodegradable bone implants

Zinc (Zn) alloy porous scaffolds produced by additive manufacturing own customizable structures and biodegradable functions, having a great application potential for repairing bone defect. In this work, a hydroxyapatite (HA)/polydopamine (PDA) composite coating was constructed on the surface of Zn-1Mg porous scaffolds fabricated by laser powder bed fusion, and was loaded with a bioactive factor BMP2 and an antibacterial drug vancomycin. The microstructure, degradation behavior, biocompatibility, antibacterial performance and osteogenic activities were systematically investigated. Compared with as-built Zn-1Mg scaffolds, the rapid increase of Zn²⁺, which resulted to the deteriorated cell viability and osteogenic differentiation, was inhibited due to the physical barrier of the composite coating. In vitro cellular and bacterial assay indicated that the loaded BMP2 and vancomycin considerably enhanced the cytocompatibility and antibacterial performance. Significantly improved osteogenic and antibacterial functions were also observed according to in vivo implantation in the lateral femoral condyle of rats. The design, influence and mechanism of the composite coating were discussed accordingly. It was concluded that the additively manufactured Zn-1Mg porous scaffolds together with the composite coating could modulate biodegradable performance and contribute to effective promotion of bone recovery and antibacterial function.

A detailed mechanism of degradation behaviour of biodegradable as-ECAPed Zn-0.8Mg-0.2Sr with emphasis on localized corrosion attack

In this study, advanced techniques such as atom probe tomography, atomic force microscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy were used to determine the corrosion mechanism of the as-ECAPed Zn-0.8Mg-0.2Sr alloy. The influence of microstructural and surface features on the corrosion mechanism was investigated. Despite its significance, the surface composition before exposure is often neglected by the scientific community. The analyses revealed the formation of thin ZnO, MgO, and MgCO₃ layers on the surface of the material before exposure. These layers participated in the formation of corrosion products, leading to the predominant occurrence of hydrozincite. In addition, the layers possessed different resistance to the environment, resulting in localized corrosion attacks. The segregation of Mg on the Zn grain boundaries with lower potential compared with the Zn-matrix was revealed by atom probe tomography and atomic force microscopy. The degradation process was initiated by the activity of micro-galvanic cells, specifically Zn - Mg₂Zn₁₁/SrZn₁₃. This process led to the activity of the crevice corrosion mechanism and subsequent attack to a depth of 250 μm. The corrosion rate of the alloy determined by the weight loss method was 0.36 mm·a^-1. Based on this detailed study, the degradation mechanism of the Zn-0.8Mg-0.2Sr alloy is proposed.

Proteomics as a tool to study the osteoimmunomodulatory role of metallic ions in a sol-gel coating

The success of bone implants depends on the osteoimmunomodulatory (OIM) activity of the biomaterials in the interactions with the periimplantary tissues. Many in vitro tests have been conducted to evaluate the osteoimmunology effects of biomaterials. However, results of these tests have often been inconclusive. This study examines the properties of newly developed sol-gel coatings doped with two metal ions associated with bone regeneration, Ca and Zn. The study uses both proteomic methods and traditional in vitro assays. The results demonstrate that proteomics is an effective tool to scrutinize the OIM properties of the materials. Moreover, sol-gel coatings offer excellent base materials to evaluate the effects of metal ions on these properties. The obtained data highlight the highly tunable nature of sol-gel materials; studying the materials with different doping levels supplies valuable information on the interactions between the immune and bone-forming processes.

Metal-organic Zn-zoledronic acid and 1-hydroxyethylidene-1,1-diphosphonic acid nanostick-mediated zinc phosphate hybrid coating on biodegradable Zn for osteoporotic fracture healing implants

Zn and its alloys are increasingly under consideration for biodegradable bone fracture fixation implants owing to their attractive biodegradability and mechanical properties. However, their clinical application is a challenge for osteoporotic bone fracture healing, due to their uneven degradation mode, burst release of zinc ions, and insufficient osteo-promotion and osteo-resorption regulating properties. In this study, a type of Zn2+ coordinated zoledronic acid (ZA) and 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) metal-organic hybrid nanostick was synthesized, which was further mixed into zinc phosphate (ZnP) solution to mediate the deposition and growth of ZnP to form a well-integrated micro-patterned metal-organic/inorganic hybrid coating on Zn. The coating protected noticeably the Zn substrate from corrosion, in particular reducing its localized occurrence as well as suppressing its Zn2+ release. Moreover, the modified Zn was osteo-compatible and osteo-promotive and, more important, performed osteogenesis in vitro and in vivo of well-balanced pro-osteoblast and anti-osteoclast responses. Such favorable functionalities are related to the nature of its bioactive components, especially the bio-functional ZA and the Zn ions it contains, as well as its unique micro- and nano-scale structure. This strategy provides not only a new avenue for surface modification of biodegradable metals but also sheds light on advanced biomaterials for osteoporotic fracture and other applications. STATEMENT OF SIGNIFICANCE: Developing appropriate biodegradable metallic materials is of clinical relevance for osteoporosis fracture healing, whereas current strategies are short of good balance between the bone formation and resorption. Here, we designed a micropatterned metal-organic nanostick mediated zinc phosphate hybrid coating modified Zn biodegradable metal to fulfill such a balanced osteogenicity. The in vitro assays verified the coated Zn demonstrated outstanding pro-osteoblasts and anti-osteoclasts properties and the coated intramedullary nail promoted fracture healing well in an osteoporotic femur fracture rat model. Our strategy may offer not only a new avenue for surface modification of biodegradable metals but also shed light on better understanding of new advanced biomaterials for orthopedic application among others.

A Comprehensive Review of the Current Research Status of Biodegradable Zinc Alloys and Composites for Biomedical Applications

Zinc (Zn)-based biodegradable materials show moderate degradation rates in comparison with other biodegradable materials (Fe and Mg). Biocompatibility and non-toxicity also make them a viable option for implant applications. Furthermore, Pure Zn has poor mechanical behavior, with a tensile strength of around 100-150 MPa and an elongation of 0.3-2%, which is far from reaching the strength required as an orthopedic implant material (tensile strength is more than 300 MPa, elongation more than 15%). Alloy and composite fabrication have proven to be excellent ways to improve the mechanical performance of Zn. Therefore, their alloys and composites have emerged as an innovative category of biodegradable materials. This paper summarizes the most important recent research results on the mechanical and biological characteristics of biodegradable Zn-based implants for orthopedic applications and the most commonly added components in Zn alloys and composites.

Development, Processing and Aging of Novel Zn-Ag-Cu Based Biodegradable Alloys

The use of biodegradable materials for implants is a promising strategy to overcome known long-term clinical complications related to permanent implants. Ideally, biodegradable implants support the damaged tissue for a certain period and then degrade, while the physiological function of the surrounding tissue is restored. Although Mg-based alloys nearly ideally lend themselves to biodegradable implants, a few critical shortcomings promoted the development of alternative alloy systems. Due to their reasonably good biocompatibility, moderate corrosion rate without hydrogen evolution and adequate mechanical properties, increasing attention has been paid to Zn alloys. In this work, precipitation-hardening alloys in the system Zn-Ag-Cu were developed relying on thermodynamic calculations. After casting the alloys, their microstructures were refined by thermomechanical treatment. The processing was tracked and directed, respectively, by routine investigations of the microstructure, associated with hardness assessments. Although microstructure refinement increased the hardness, the material proved to be susceptible to aging as the homologous temperature of zinc is at 0.43 T_m. Besides mechanical performance and corrosion rate, long-term mechanical stability is another crucial factor that must be taken into consideration to ensure the safety of the implant and thus requires a profound understanding of the aging process.

In vitro cytocompatibility and antibacterial studies on biodegradable Zn alloys supplemented by a critical assessment of direct contact cytotoxicity assay

In vitro cytotoxicity assessment is indispensable in developing new biodegradable implant materials. Zn, which demonstrates an ideal corrosion rate between Mg- and Fe-based alloys, has been reported to have excellent in vivo biocompatibility. Therefore, modifications aimed at improving Zn's mechanical properties should not degrade its biological response. As sufficient strength, ductility and corrosion behavior required of load-bearing implants has been obtained in plastically deformed Zn-3Ag-0.5Mg, the effect of simultaneous Ag and Mg additions on in vitro cytocompatibility and antibacterial properties was studied, in relation to Zn and Zn-3Ag. Direct cell culture on samples and indirect extract-based tests showed almost no significant differences between the tested Zn-based materials. The diluted extracts of Zn, Zn-3Ag, and Zn-3Ag-0.5Mg showed no cytotoxicity toward MG-63 cells at a concentration of ≤12.5%. The cytotoxic effect was observed only at high Zn²⁺ ion concentrations and when in direct contact with metallic samples. The highest LD₅₀ (lethal dose killing 50% of cells) of 13.4 mg/L of Zn²⁺ ions were determined for the Zn-3Ag-0.5Mg. Similar antibacterial activity against Escherichia coli and Staphylococcus aureus was observed for Zn and Zn alloys, so the effect is attributed mainly to the released Zn²⁺ ions exhibiting bactericidal properties. Most importantly, our experiments indicated the limitations of water-soluble tetrazolium salt-based cytotoxicity assays for direct tests on Zn-based materials. The discrepancies between the WST-8 assay and SEM observations are attributed to the interference of Zn²⁺ ions with tetrazolium salt, therefore favoring its transformation into formazan, giving false cell viability quantitative results.

Influence of Enzymes on the In Vitro Degradation Behavior of Pure Zn in Simulated Gastric and Intestinal Fluids

Zinc (Zn) alloys are being developed as the degradable biomaterial. However, the corrosion mechanism of Zn in the gastrointestinal environment is seldom investigated and needs to be addressed. In this study, the impacts of enzymes on the degradation of pure Zn via electrochemical measurements and immersion were investigated. Pepsin and pancreatin affected the degradation of pure Zn. In contrast with the solutions without enzymes, the degradation rates declined with the addition of enzymes in solutions. However, localized corrosion was observed because the adsorption of pepsin was not a perfect barrier to prevent corrosion. The adsorbed pancreatin protected the samples from corrosion mainly at the initial stage of immersion. With immersion in the simulated intestinal fluid, adsorption and desorption of pancreatin occurred simultaneously on the sample surface. These findings allow the development of Zn alloy-implanted devices for the digestive tract as well as the understanding of the Zn corrosion mechanism in the gastrointestinal environment.

Recent Developments in Zn-Based Biodegradable Materials for Biomedical Applications

Zn-based biodegradable alloys or composites have the potential to be developed to next-generation orthopedic implants as alternatives to conventional implants to avoid revision surgeries and to reduce biocompatibility issues. This review summarizes the current research status on Zn-based biodegradable materials. The biological function of Zn, design criteria for orthopedic implants, and corrosion behavior of biodegradable materials are briefly discussed. The performance of many novel zinc-based biodegradable materials is evaluated in terms of biodegradation, biocompatibility, and mechanical properties. Zn-based materials perform a significant role in bone metabolism and the growth of new cells and show medium degradation without the release of excessive hydrogen. The addition of alloying elements such as Mg, Zr, Mn, Ca, and Li into pure Zn enhances the mechanical properties of Zn alloys. Grain refinement by the application of post-processing techniques is effective for the development of many suitable Zn-based biodegradable materials.

Structure, Biodegradation, and In Vitro Bioactivity of Zn-1%Mg Alloy Strengthened by High-Pressure Torsion

The effect of high-pressure torsion (HPT) on the microstructure, phase composition, mechanical characteristics, degradation rate, and bioactive properties of the Zn-1%Mg alloy is studied. An ultrafine-grained (UFG) structure with an average grain size of α-Zn equal to 890 ± 26 nm and grains and subgrains of the Mg₂Zn₁₁ and MgZn₂ phases with a size of 50-100 nm are formed after HPT. This UFG structure leads to an increase in the ultimate tensile strength of the alloy by ~3 times with an increase in elongation to 6.3 ± 3.3% due to the formation of a basal texture. The study of corrosion resistance did not show a significant effect of HPT on the degradation rate of the alloy. In addition, no significant changes in the bioactivity of the alloy after HPT: hemolysis, cellular colonization and Escherichia coli growth inhibition.

Modified Biodegradation Behavior Induced Beneficial Microenvironments for Bone Regeneration by Low Addition of Gadolinium in Zinc

Zinc (Zn) shows a great potential as a biodegradable material for bone implants after a decade of systematic research and development. However, uncontrollable biodegradation behavior and biphasic dose-response prevent Zn from fulfilling its essential role in facilitating bone regeneration. In this study, the low addition of gadolinium (Gd) modifies the intrinsic microstructure of Zn in terms of grain size distribution, grain boundary misorientation, and texture. Adding Gd refines grain size distribution and creates a stronger basal plane texture in Zn, consequently, changing the current density distribution and reducing the anode dissolution rate during corrosion. As a result, uniform degradation is more predominant in Zn-0.4Gd alloy implant, in comparison to localized degradation in pure Zn implant in bone environments. The modified biodegradation behavior of the Zn-0.4Gd alloy implant induces significantly better new bone formation and osseointegration compared to the pure Zn implant. Therefore, Gd with trace amounts is able to tune the degradation behavior and improve the performance of Zn-based implants in promoting bone regeneration.

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.

In Situ Growth of a Metal-Organic Framework on Graphene Oxide for the Chemo-Photothermal Therapy of Bacterial Infection in Bone Repair

Bacterial infection with high morbidity (>30%) seriously affects the defect's healing after bone transplantation. To this end, chemotherapy and photothermal therapy have been utilized for antibacterial treatment owing to their high selectivity and minimal toxicity. However, they also face several dilemmas. For example, bacterial biofilms prevented the penetration of antibacterial agents and local temperatures (over 70 °C) caused by the photothermal therapy damaged normal tissue. Herein, a co-dispersion nanosystem with chemo-photothermal function was constructed via the in situ growth of zeolitic imidazolate framework-8 (ZIF-8) on graphene oxide (GO) nanosheets. In this nanosystem, GO generates a local temperature (∼50 °C) to increase the permeability of a bacterial biofilm under near-infrared laser irradiation. Then, Zn ions released by ZIF-8 seized this chance to react with the bacterial membrane and inactivate it, thus realizing efficient sterilization in a low-temperature environment. This antibacterial system was incorporated into a poly-l-lactic acid scaffold for bone repair. Results showed that the scaffold showed a high antibacterial rate of 85% against both Escherichia coli and Staphylococcus aureus. In vitro cell tests showed that the scaffold promoted cell proliferation.

Osteogenic and angiogenic bioactive collagen entrapped calcium/zinc phosphates coating on biodegradable Zn for orthopedic implant applications

Zinc is becoming one of the leading candidate materials for biodegradable orthopedic implants owing to its attractive properties in terms of degradation behavior and mechanical properties. However, the insufficient surface bio-activities postpone its clinical application. In this study, an organic-inorganic collagen entrapped calcium/zinc phosphates coating was constructed on Zn surface to lessen Zn²⁺ releasing rate and to leverage the surface osteogenic and angiogenic properties. Collagen molecules were immobilized onto Zn substrate and subsequently coordinated with calcium and zinc ions to promote the CaZnP inorganic phase growth, ensuing an intertwined collagen-CaZnP hybrid system. Consequently, the hybrid coating was highly coalesced and compact. Such high quality warranted the contained Zn²⁺ releasing in a tolerable rate favorable for cells viability. The collagen-CaZnP coated Zn showed remarkedly stronger osteogenicity as compared to the untreated Zn, ascertained by the MC3T3-E1 osteoblast cell proliferation and differentiation assays, such as alkaline phosphatase expression and calcium nodule formation results. In addition, this hybrid coating supported human umbilical vein endothelial cells (HUVECs) migration and tube formation. The enhanced osteogenic and angiogenic properties could be ascribed to the nature of collagen and calcium/zinc phosphate components, the hybrid micro/nano-structure as well as the ability of controlling the Zn²⁺ release of Zn substrate into a suitable concentration range. Our strategy provides a new avenue to surface modification of biodegradable metals for bone regenerative perspective.

Fabrication and Properties of Zn-3Mg-1Ti Alloy as a Potential Biodegradable Implant Material

A Zn-3Mg-1Ti alloy was fabricated by ultrasonic treatment of Zn-Mg alloy melt using a Ti ultrasonic radiation rod. The microstructure, phase structure, mechanical properties, degradation property, and in vitro cytotoxicity were investigated systematically. The obtained Zn-3Mg-1Ti alloy is composed of the Zn, Mg2Zn11, and TiZn16. Owing to the grain refinement and second phase reinforcement, the mechanical properties of Zn-3Mg-1Ti alloy is improved. In addition, the Zn-3Mg-1Ti alloy exhibits minimal cytotoxicity compared to pure Zn and Zn-1Ti alloy. Electrochemical tests show that the Zn-3Mg-1Ti alloy has an appropriate degradation rate in Hank's solution.

Feasibility evaluation of a Cu-38 Zn alloy for intrauterine devices: In vitro and in vivo studies

The existing adverse effects of copper in copper-containing intrauterine devices (Cu-IUDs) have raised concerns regarding their use. These adverse effects include burst release of cupric ions (Cu²⁺) at the initial stage and an increasingly rough surface of the Cu-IUDs. In this study, we investigated the use of two copper alloys, Cu-38 Zn and H62 as the new upgrading or alternative material for IUDs. Their corrosive properties were studied in simulated uterine fluid (SUF) by using electrochemical methods, with pure Cu as a control. We studied the in vitro long-term corrosion behaviors in SUF, cytotoxicity to uterine cells (human endometrial epithelial cells and human endometrial stromal cells), in vivo biocompatibility and contraceptive efficacy of pure Cu, H62, and Cu-38 Zn. In the first month, the burst release rate of Cu²⁺ in the Cu-38 Zn group was significantly lower than those in the pure Cu and H62 groups. The in vitro cytocompatibility Cu-38 Zn was better than that of pure Cu and H62. Moreover, Cu-38 Zn showed improved tissue biocompatibility in vivo experiments. Therefore, the contraceptive efficacy of the Cu-38 Zn is still maintained as high as the pure Cu while the adverse effects are significantly eased, suggesting that Cu-38 Zn can be a suitable potential candidate material for IUDs. STATEMENT OF SIGNIFICANCE: The existing adverse effects associated with the intrinsic properties of copper materials for copper-containing intrauterine devices (Cu-IUD) are of concern in their employment. Such as, burst release of cupric ions (Cu²⁺) at the initial stage and an increasingly rough surface of the Cu-IUD. In this work, Cu alloyed with a high amount of bioactive Zn was used for a Cu-IUD. The Cu-38 Zn alloy exhibited reduced burst release of Cu²⁺ within the first month compared with the pure Cu and H62. Furthermore, the Cu-38 Zn alloy displayed significantly improved biocompatibility and a much smoother surface. Therefore, high antifertility efficacy of the Cu-38 Zn alloy was well maintained, while the adverse effects are significantly eased, suggesting that the Cu-38 Zn alloy is promising for a Cu-IUD.

A review on current research status of the surface modification of Zn-based biodegradable metals

Recently, zinc and its alloys have been proposed as promising candidates for biodegradable metals (BMs), owning to their preferable corrosion behavior and acceptable biocompatibility in cardiovascular, bone and gastrointestinal environments, together with Mg-based and Fe-based BMs. However, there is the desire for surface treatment for Zn-based BMs to better control their biodegradation behavior. Firstly, the implantation of some Zn-based BMs in cardiovascular environment exhibited intimal activation with mild inflammation. Secondly, for orthopedic applications, the biodegradation rates of Zn-based BMs are relatively slow, resulting in a long-term retention after fulfilling their mission. Meanwhile, excessive Zn2+ release during degradation will cause in vitro cytotoxicity and in vivo delayed osseointegration. In this review, we firstly summarized the current surface modification methods of Zn-based alloys for the industrial applications. Then we comprehensively summarized the recent progress of biomedical bulk Zn-based BMs as well as the corresponding surface modification strategies. Last but not least, the future perspectives towards the design of surface bio-functionalized coatings on Zn-based BMs for orthopedic and cardiovascular applications were also briefly proposed.

Impact of sterilization treatments on biodegradability and cytocompatibility of zinc-based implant materials

Biodegradable zinc (Zn) and Zn-based alloys have been recognized as promising biomaterials for biomedical implants. Sterilization is an essential step in handling Zn-based implants before their use in clinical practice and there are various sterilization methods are available. However, how these treatments influence the Zn-based biomaterials remains unknown and is of critical relevance. In this study, three commonly-applied standard sterilization methods, namely gamma irradiation, hydrogen peroxide gas plasma and steam autoclave, were used on pure Zn and Zn3Cu (wt%) alloy. The treated Zn and ZnCu alloy were investigated to compare the different influences of sterilizations on surface characteristics, transient and long-term degradation behavior and cytotoxicity of Zn and Zn alloy. Our results indicate that autoclaving brought about apparently a formation of inhomogeneous zinc oxide film whereas the other two methods produced no apparent alterations on the material surfaces. Consequently, the samples after autoclaving showed significantly faster degradation rates and more severe localized corrosion, especially for the ZnCu alloy, owing to the incomplete covering and unstable zinc oxide layer. Moreover, the autoclave-treated Zn and ZnCu alloy exhibited apparent cytotoxic effects towards fibroblasts, which may be due to the excessive Zn ion releasing and its local concentration exceeds the cellular tolerance capacity. In contrast, gamma irradiation and hydrogen peroxide gas plasma had no apparent adverse effects on the biodegradability and cytocompatibility of Zn and ZnCu alloy. Our findings may have significant implications regarding the selection of suitable sterilization methods for Zn-based implant materials among others.

Fabrication and characterization of biodegradable zinc matrix composites reinforced by uniformly dispersed beta-tricalcium phosphate via graphene oxide-assisted hetero-agglomeration

The development of biodegradable Zn matrix composites has been considered a promising approach to achieving enhanced mechanical properties, controllable degradation rate, good biocompatibility, and good osseointegration as orthopedic implants. However, scant literature regarding Zn matrix composites has been reported because of the great difficulty in dispersing the nano-sized bioactive reinforcements uniformly within the Zn matrix. In the present study, a novel and effective method were employed to obtain Zn matrix composites reinforced by uniformly dispersed beta-tricalcium phosphate (β-TCP) via graphene oxide (GO)-assisted hetero-agglomeration and subsequent spark plasma sintering process. A very low-content (0.04 vol%) few-layered GO was used as a coupling reagent to connect the Zn matrix and nano-sized TCP particles. In an appropriate polarity solvent, the negatively charged GO sheets could combine with both the positively charged Zn powder and TCP particles by electrostatic attraction and charge neutralization. Due to the nature of hetero-agglomeration, the flexible GO sheet could adhere to the large Zn powder and attracted a certain amount of TCP particles to form a Zn/GO/TCP sandwich structure by charge neutralization thereby forming a uniform dispersion of TCP particles within Zn matrix. After the spark plasma sintering (SPS) process, the TCP particles incorporated with very thin ZnO layers (thickness of a few dozen nanometers) formed a homogeneous and unique 3D network-like distribution in as-sintered TCP/Zn composites. A unique "snap pea"-like structure was confirmed at the grain boundary of α-Zn grains, which consisted of the TCP particles as "pea" and thin ZnO layer as "pod". Due to the uniform dispersion of bioactive TCP particles and unique structure of the TCP incorporating grain boundary, as-sintered 3TCP/Zn matrix composites possessed yield strength (YS) of 140.8 ± 7.7 MPa, failure strain of 36.0 ± 2.8%, the moderate degradation rate of 19.1 ± 3.3 μm·y^-1 and good cytocompatibility to MC3T3-E1 cells. Moreover, osteogenic differentiation activity evaluation revealed that the addition of TCP could significantly improve the expressions of the osteogenic differentiation-related gene (ALP) in MC3T3-E1 cells, thereby resulting in improved osteogenic capability. Therefore, biodegradable 3TCP/Zn matrix composites fabricated by GO-assisted hetero-agglomeration and subsequent SPS process could be a promising material as orthopedic implants.

Pre-oxidation induced in situ interface strengthening in biodegradable Zn/nano-SiC composites prepared by selective laser melting

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Zn/nano-SiC biocomposites were prepared via pre-oxidation and selective laser melting.

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In situ reaction improved the interface bonding between nano-SiC and the Zn matrix.

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The improved interfacial bonding enhanced the mechanical properties of the biocomposite.

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The biocomposite exhibited favorable biocompatibility on cell proliferation and adhesion.

Introduction

Nano-SiC has attracted great attention as ceramic reinforcement in metal matrix composites, but the weak interface bonding between them remains a bottleneck for efficient strengthening.

Objective

In this study, pre-oxidation treatments and selective laser melting (SLM) were employed to prepare Zn/nano-SiC biocomposites with strengthened interface bonding via in situ reaction.

Methods

Nano-SiC and Zn powders were pre-oxidized respectively, and then used to prepare Zn/nano-SiC biocomposites via SLM. The powder microstructure, and the interface characteristics and mechanical properties of the biocomposites were investigated. The degradation properties and cell response were analyzed to evaluate their feasibility for orthopedic applications.

Results

The results indicated that the pre-oxidation treatments generated a uniform oxide layer on the surface of both nano-SiC and Zn particles and the thickness of the oxide layer increased with pre-oxidation temperature. During the SLM process, the oxide layers not only improved the metal-ceramic wettability by reducing interface energy, but also induced in situ reaction to form chemical bonding between the Zn matrix and nano-SiC, thereby improving the interface bonding. Consequently, the Zn biocomposite reinforced by nano-SiC with a pre-oxidation temperature of 1000 °C (ZS1000 biocomposite) exhibited more transgranular fracture and significantly enhanced compressive yield strength of 171.5 MPa, which was 31.6% higher than that of the Zn biocomposite reinforced by nano-SiC without pre-oxidation. Moreover, the ZS1000 biocomposite presented slightly accelerated degradation which might be ascribed to the facilitated electron transfer by the interface product (Zn₂SiO₄). In addition, the ZS1000 biocomposite also showed appropriate biocompatibility for MG-63 cell adhesion, growth, and proliferation.

Conclusion

This study shows the potential practical applicability for the preparation of Zn-based biocomposites with strong interface bonding and mechanical properties for orthopedic applications.

In vivo biocompatibility and degradability of a Zn-Mg-Fe alloy osteosynthesis system

Zinc is generally considered to be one of the most promising materials to be used in biodegradable implants, and many zinc alloys have been optimized to improve implant biocompatibility, degradation, and mechanical properties. However, long-term degradation leads to the prolonged presence of degradation products, which risks foreign body reactions. Herein, we investigated the in vivo biocompatibility and degradation of a biodegradable Zn–Mg–Fe alloy osteosynthesis system in the frontal bone, mandible, and femur in beagles for 1 year. Results of the routine blood, biochemical, trace element, and histological analyses of multiple organs, peripheral blood CD4/CD8a levels, and serum interleukin 2 and 4 levels showed good biocompatibility of the Zn–Mg–Fe alloy. Zinc content analysis revealed zinc accumulation in adjacent bone tissue, but not in the liver, kidney, and spleen, which was related to the degradation of the Zn–Mg–Fe alloy. The alloy demonstrated a uniform slowing degradation rate in vivo. No degradation differences in the frontal bone, mandible, and femur were observed. The degradation products included zinc oxide [ZnO], zinc hydroxide [Zn(OH)₂], hydrozincite [Zn₅(OH)₆(CO₃)₂], and hopeite [Zn₃(PO₄)₂·4H₂O]. The good biocompatibility and degradation properties of the Zn–Mg–Fe alloy render it a very attractive osteosynthesis system for clinical applications.

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Zn–Mg–Fe alloy possesses good biocompatibility.

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Zn–Mg–Fe alloy demonstrates a uniform slowing-down degradation rate in vivo.

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No degradation differences are observed in different sites, which supports the design of degradable osteosynthesis systems.

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The degradation products are zinc oxide, zinc hydroxide, hydrozincite, and hopeite.

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Zn–Mg–Fe alloy is a promising candidate for an osteosynthesis system.

Biodegradable Zn-Sr alloy for bone regeneration in rat femoral condyle defect model: In vitro and in vivo studies

Bone defects are commonly caused by severe trauma, malignant tumors, or congenital diseases and remain among the toughest clinical problems faced by orthopedic surgeons, especially when of critical size. Biodegradable zinc-based metals have recently gained popularity for their desirable biocompatibility, suitable degradation rate, and favorable osteogenesis-promoting properties. The biphasic activity of Sr promotes osteogenesis and inhibits osteoclastogenesis, which imparts Zn-Sr alloys with the ideal theoretical osteogenic properties. Herein, a biodegradable Zn-Sr binary alloy system was fabricated. The cytocompatibility and osteogenesis of the Zn-Sr alloys were significantly better than those of pure Zn in MC3T3-E1 cells. RNA-sequencing illustrated that the Zn-0.8Sr alloy promoted osteogenesis by activating the wnt/β-catenin, PI3K/Akt, and MAPK/Erk signaling pathways. Furthermore, rat femoral condyle defects were repaired using Zn-0.8Sr alloy scaffolds, with pure Ti as a control. The scaffold-bone integration and bone ingrowth confirmed the favorable in vivo repair properties of the Zn-Sr alloy, which was verified to offer satisfactory biosafety based on the hematoxylin-eosin (H&E) staining and ion concentration testing of important organs. The Zn-0.8Sr alloy was identified as an ideal bone repair material candidate, especially for application in critical-sized defects on load-bearing sites due to its favorable biocompatibility and osteogenic properties in vitro and in vivo.

Degradation-triggered release from biodegradable metallic surfaces

This work is dedicated to the investigation of drug-release control by a direct effect of degradation from biodegradable metallic surfaces. Degradation behaviors characterized by surface morphology, immersion, and electrochemical techniques demonstrated that curcumin-coated zinc (c-Zn) had a higher degradation rate compared to curcumin-coated Fe (c-Fe). High anodic dissolution rate due to the higher degradation rate and widely extended groove-like degradation structure of c-Zn propelled a higher curcumin release. On the other hand, a slower curcumin release rate shown by c-Fe scaffolds is ascribed to its lower anodic dissolution and to its pitting degradation regime with relatively smaller pits. These findings illuminate the remarkable advantage of different degradation behaviors of degradable metallic surfaces in directly controlling the drug release without the need for external electrical stimulus.

Biodegradable ZnLiCa ternary alloys for critical-sized bone defect regeneration at load-bearing sites: In vitro and in vivo studies

A novel biodegradable metal system, ZnLiCa ternary alloys, were systematically investigated both in vitro and in vivo. The ultimate tensile strength (UTS) of Zn0.8Li0.1Ca alloy reached 567.60 ± 9.56 MPa, which is comparable to pure Ti, one of the most common material used in orthopedics. The elongation of Zn0.8Li0.1Ca is 27.82 ± 18.35%, which is the highest among the ZnLiCa alloys. The in vitro degradation rate of Zn0.8Li0.1Ca alloy in simulated body fluid (SBF) showed significant acceleration than that of pure Zn. CCK-8 tests and hemocompatibility tests manifested that ZnLiCa alloys exhibit good biocompatibility. Real-time PCR showed that Zn0.8Li0.1Ca alloy successfully stimulated the expressions of osteogenesis-related genes (ALP, COL-1, OCN and Runx-2), especially the OCN. An in vivo implantation was conducted in the radius of New Zealand rabbits for 24 weeks, aiming to treat the bone defects. The Micro-CT and histological evaluations proved that the regeneration of bone defect was faster within the Zn0.8Li0.1Ca alloy scaffold than the pure Ti scaffold. Zn0.8Li0.1Ca alloy showed great potential to be applied in orthopedics, especially in the load-bearing sites.

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The first research work of ZnLiCa alloys to be used as biodegradable metals.

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The ultimate tensile strength (UTS) of Zn0.8Li0.1Ca alloy reached 567.60 ± 9.56 MPa, which is comparable to pure Ti, one of the most common material used in orthopedics.

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Porous scaffolds made of Zn0.8Li0.1Ca showed superior bone-defect-treating effects to pure Ti scaffolds in New Zealand rabbits.

In vitro and in vivo studies to evaluate the feasibility of Zn-0.1Li and Zn-0.8Mg application in the uterine cavity microenvironment compared to pure zinc

Significant advances have been achieved in the research evaluating Zn and its alloys as degradable metallic biomaterials mainly for application in bone and blood vessels. In the present study, the degradation behaviors of Zn-0.1Li and Zn-0.8Mg alloys in simulated uterine fluid (SUF) were systematically investigated for 300 days. In vitro viability assays were conducted in different uterine cells (HUSMCs, HEECs, and HESCs), and histological examination after the in vivo implantation into the uterine cavity was performed using pure Zn as control. The immersion test results indicated that both Zn-0.1Li and Zn-0.8Mg alloys exhibited better corrosion resistance than pure Zn, with Zn₃(PO₄)₂⋅4H₂O and CaZn₂(PO₄)₂⋅2H₂O being the main corrosion products detected in the SUF in addition to ZnO. The cell cytotoxicity assays revealed that Zn-0.1Li and Zn-0.8Mg exhibited better cytocompatibility than Zn. Moreover, the in vivo experiments demonstrated that the Zn-0.1Li and Zn-0.8Mg alloys induced less inflammation in the uterine tissue than pure Zn, with CaCO₃ and Zn(HPO₄)⋅3H₂O being the major biocorrosion products in addition to ZnO. According to these results, zinc alloys appear to be suitable potential candidate materials for future intrauterine biomedical devices.

Recent research and progress of biodegradable zinc alloys and composites for biomedical applications: Biomechanical and biocorrosion perspectives

Biodegradable metals (BMs) gradually degrade in vivo by releasing corrosion products once exposed to the physiological environment in the body. Complete dissolution of biodegradable implants assists tissue healing, with no implant residues in the surrounding tissues. In recent years, three classes of BMs have been extensively investigated, including magnesium (Mg)-based, iron (Fe)-based, and zinc (Zn)-based BMs. Among these three BMs, Mg-based materials have undergone the most clinical trials. However, Mg-based BMs generally exhibit faster degradation rates, which may not match the healing periods for bone tissue, whereas Fe-based BMs exhibit slower and less complete in vivo degradation. Zn-based BMs are now considered a new class of BMs due to their intermediate degradation rates, which fall between those of Mg-based BMs and Fe-based BMs, thus requiring extensive research to validate their suitability for biomedical applications. In the present study, recent research and development on Zn-based BMs are reviewed in conjunction with discussion of their advantages and limitations in relation to existing BMs. The underlying roles of alloy composition, microstructure, and processing technique on the mechanical and corrosion properties of Zn-based BMs are also discussed.

Cancellous bone-like porous Fe@Zn scaffolds with core-shell-structured skeletons for biodegradable bone implants

Three-dimensional (3D) porous zinc (Zn) with a moderate degradation rate is a promising candidate for biodegradable bone scaffolds. However, fabrication of such scaffolds with adequate mechanical properties remains a challenge. Moreover, the composition, crystallography and microstructure of the in vivo degradation products formed at or near the implant-bone interface are still not precisely known. Here, we have fabricated porous Fe@Zn scaffolds with skeletons consisting of an inner core layer of Fe and an outer shell layer of Zn using template-assisted electrodeposition technique, and systematically evaluated their porous structure, mechanical properties, degradation mechanism, antibacterial ability and in vitro and in vivo biocompatibility. In situ site-specific focused ion beam micromilling and transmission electron microscopy were used to identify the in vivo degradation products at the nanometer scale. The 3D porous Fe@Zn scaffolds show similar structure and comparable mechanical properties to human cancellous bone. The degradation rates can be adjusted by varying the layer thickness of Zn and Fe. The antibacterial rates reach over 95% against S. aureus and almost 100% against E. coli. A threshold of released Zn ion concentration (~ 0.3 mM) was found to determine the in vitro biocompatibility. Intense new bone formation and ingrowth were observed despite with a slight inflammatory response. The in vivo degradation products were identified to be equiaxed nanocrystalline zinc oxide with dispersed zinc carbonate. This study not only demonstrates the feasibility of porous Fe@Zn for biodegradable bone implants, but also provides significant insight into the degradation mechanism of porous Zn in physiological environment.

Bioactive zinc-doped sol-gel coating modulates protein adsorption patterns and in vitro cell responses

Zinc is an essential element with an important role in stimulating the osteogenesis and mineralization and suppressing osteoclast differentiation. In this study, new bioactive ZnCl₂-doped sol-gel materials were designed to be applied as coatings onto titanium. The biomaterials were physicochemically characterized and the cellular responses evaluated in vitro using MC3T3-E1 osteoblasts and RAW264.7 macrophages. The effect of Zn on the adsorption of human serum proteins onto the material surface was evaluated through nLC-MS/MS. The incorporation of Zn did not affect the crosslinking of the sol-gel network. A controlled Zn²⁺ release was obtained, reaching values below 10 ppm after 21 days. The materials were no cytotoxic and lead to increased gene expression of ALP, TGF-β, and RUNX2 in the osteoblasts. In macrophages, an increase of IL-1β, TGF-β, and IL-4 gene expression was accompanied by a reduced TNF-α liberation. Proteomic results showed changes in the adsorption patterns of proteins associated with immunological, coagulative, and regenerative functions, in a Zn dose-dependent manner. The variations in protein adsorption might lead to the downregulation of the NF-κB pathway, thus explain the observed biological effects of Zn incorporation into biomaterials. Overall, these coatings demonstrated their potential to promote bone tissue regeneration.

Microstructure evolution and texture tailoring of reduced graphene oxide reinforced Zn scaffold

Zinc (Zn) possesses desirable degradability and favorable biocompatibility, thus being recognized as a promising bone implant material. Nevertheless, the insufficient mechanical performance limits its further clinical application. In this study, reduced graphene oxide (RGO) was used as reinforcement in Zn scaffold fabricated via laser additive manufacturing. Results showed that the homogeneously dispersed RGO simultaneously enhanced the strength and ductility of Zn scaffold. On one hand, the enhanced strength was ascribed to (i) the grain refinement caused by the pinning effect of RGO, (ii) the efficient load shift due to the huge specific surface area of RGO and the favorable interface bonding between RGO and Zn matrix, and (iii) the Orowan strengthening by the homogeneously distributed RGO. On the other hand, the improved ductility was owing to the RGO-induced random orientation of grain with texture index reducing from 20.5 to 7.3, which activated more slip systems and provided more space to accommodate dislocation. Furthermore, the cell test confirmed that RGO promoted cell growth and differentiation. This study demonstrated the great potential of RGO in tailoring the mechanical performance and cell behavior of Zn scaffold for bone repair.

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Zn/RGO composite scaffold was successfully fabricated by laser additive manufacturing.

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RGO refined the grains and significantly weakened the texture with random grain orientation.

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The uniformly distributed RGO simultaneously enhanced the strength and ductility of scaffold.

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The incorporated RGO exerted a positive effect on cell growth and differentiation.

Biodegradable Zn-Cu alloys show antibacterial activity against MRSA bone infection by inhibiting pathogen adhesion and biofilm formation

Bone and joint-related infections remain the primary and most critical complications of orthopedic surgery. We have innovatively prepared Zn-Cu alloys to achieve outstanding material and antibacterial properties. In this study, we systematically assessed the material properties and antibacterial activity of these Zn-Cu alloys. Our results showed that the Zn-2Cu alloy had the best mechanical properties, biocompatibility, and osteogenic properties. Findings of microbial cultures, CLSM, SEM, and TEM indicated that Zn-2Cu alloy can inhibit both coagulase-positive and coagulase-negative staphylococci, as well as antibiotic-resistant strains (MRSA and MRSE), by preventing the bacteria adhesion and the biofilm formation. Zn-2Cu alloy could broadly affect the expression of MRSA genes associated with adhesion, autolysis, biofilm formation, virulence, and drug resistance. A rat femur intramedullary nail infection-prevention model was established and the Zn-2Cu alloy-treated group showed significant antibacterial activity against MRSA and reduced the inflammatory toxic side-effects and infection-related bone loss. Collectively, our results indicate the potential utility of Zn-Cu alloy implants with 2 wt% Cu in treating orthopedic infections. Statement of significance: Osteomyelitis is a serious complication of orthopedic surgeries. Wide use of antibiotics contributes to the appearance of multi-drug resistant strains like methicillin-resistant staphylococcus aureus (MRSA). Alternatively, anti-osteomyelitis implants with broad-spectrum antibacterial properties can be favorable. Here, the antibacterial performance of biodegradable Zn-Cu alloys was evaluated with four different bacteria strains including antibiotic-resistant strains (MRSA and MRSE). Zn-Cu alloys exert excellent bacterial killing capability in all strains. In a rat femur infection model, the alloy showed significant antibacterial activity against MRSA and reduced inflammatory toxic side-effects as well as infection-related bone loss. The antibacterial property of Zn-2Cu alloy was associated with inhibition of gene expression related to wall synthesis, adhesion, colonization, biofilm formation, autolysis, and secretion of virulence factors in MRSA.

Chandler-Loop surveyed blood compatibility and dynamic blood triggered degradation behavior of Zn-4Cu alloy and Zn

Zinc (Zn) and its alloys have been considered promising absorbable metals for medical implants. However, the dynamic interaction between Zn-based materials and human blood after implantation remains unclear. In this study, a modified Chandler-Loop system was applied to assess the blood compatibility and initial degradation behavior of a Zn-4.0Cu (wt%) alloy (Zn-4Cu) and Zn with human peripheral blood under circulation conditions. In this dynamic in vitro model, the Zn-4Cu and Zn showed sufficient blood compatibility. The numbers of erythrocytes, platelets, and leukocytes were not significantly altered, and appropriate activations of the coagulation and complement system were observed. Concerning initial degradation behavior, the product layers formed on the surfaces comprise a mixture of organic and inorganic compounds while the inorganic constituents decrease toward the outer surface. Considering the corrosion morphology and electrochemical behaviors, Zn-4Cu exhibited milder and more uniform degradation than Zn. Additionally, long-term degradation tests of 28 days in human peripheral blood, human serum, and Dulbecco's phosphate-buffered saline (DPBS) demonstrated that the Zn-4Cu showed relatively uniform degradation in blood and serum. On the contrary, in DPBS, severe localized corrosion appeared along the grain boundary of the secondary phase, which was likely attributed to the acceleration of galvanic corrosion. The Zn was found with localized corrosion impeded in the blood albeit with apparently developed deep pitting holes in the serum and DPBS.

A study of degradation behaviour and biocompatibility of Zn-Fe alloy prepared by electrodeposition

Zinc is a biodegradable metal, which exhibits more moderate biodegradability than magnesium and iron, so that it has great application potential in the field of biomedical materials. Alloying of zinc and iron may lead to producing a new type of implant material Zn-Fe alloy, which might be able to meet the requirements for a moderate degradation rate. However, due to the huge difference in the melting point between zinc and iron, the preparation of Zn-Fe alloy is quite challenging and hence rarely reported. In this study, we show that Zn-Fe alloys can be successfully prepared by electrodeposition technology. The microstructures, composition, degradation properties and biocompatibility of the Zn-Fe alloys were systematically studied. The results showed that the content of iron in the alloys ranged from 0 to 8 wt%, depending on the concentration of Fe ions and the current density. In the alloys, the major's phases were η, δ and Г₁, and they were mainly affected by the ion concentration in the electrolyte. In the in vitro immersion tests, the Zn-Fe alloy ZF2-1 showed the highest immersion corrosion rate, while ZF3-1 showed the highest electrochemical corrosion rate. Moreover, we found that the corrosion rates of the alloys were significantly higher than that of the pure Fe. In the in vivo experiments, we confirmed that the Zn-Fe alloy possessed good biocompatibility. These results demonstrate that the electrodeposition technology is a good method to prepare Zn-Fe alloys, and the Zn-Fe alloys prepared by this method are potentially promising materials for biomedical applications.

In vitro and in vivo studies of Zn-Mn biodegradable metals designed for orthopedic applications

In recent years, Zn-based materials provide a new option as biodegradable metals for orthopedic applications. To improve the low strength and brittle nature of pure Zn, small amounts of alloying element Mn (0.1, 0.4 and 0.8 wt.%) were added into Zn to fabricate binary Zn-Mn alloys. An extremely high elongation (83.96 ± 2.36%) was achieved in the resulting Zn-0.8 wt.%Mn alloy. Moreover, Zn-Mn alloys displayed significantly improved cytocompatibility as compared to pure Zn, according to cell proliferation and morphology analyses. More importantly, a significantly improved osteogenic activity was verified after adding Mn regarding ALP activity and osteogenic expression. Furthermore, Zn-0.8 wt.%Mn alloy scaffolds were implanted into the rat femoral condyle for repairing bone defects with pure Ti as control. Enhanced osteogenic activities were confirmed for Zn-0.8Mn alloy in contrast to pure Ti based on Micro-CT and histological results, and favorable in vivo biosafety of Zn-0.8Mn alloy was verified by H&E staining and blood tests. The exceptional mechanical performance and favorable osteogenic capability render Zn-Mn alloy a promising candidate material in the treatment of bone defects or fracture repair. STATEMENT OF SIGNIFICANCE: The element Mn, on the one hand, as an essential trace element in the human body, promotes cell proliferation, adhesion, spreading, and regulates bone metabolism; on the other hand, it could significantly improve the ductility of Zn alloys. Here, we systematically reported the biocompatibility and biofunctionality of binary biodegradable Zn-Mn alloys in the bone environment. The Zn-Mn alloys promoted MC3T3-E1 cell proliferation, adhesion, spreading, and osteogenic differentiation in vitro. Furthermore, a rat femoral condyle defect model was established; porous Zn-Mn alloy scaffolds were manufactured to repair the bone defects. Significant bone regenerations, considerable bone ingrowth, and desirable biosafety were confirmed in vivo. Therefore, biodegradable Zn-Mn with promising osteogenic properties may become new options for orthopedic implant materials.

Controllable biodegradation and enhanced osseointegration of ZrO2-nanofilm coated Zn-Li alloy: In vitro and in vivo studies

Zinc and its alloys have emerged as a new research direction of biodegradable metals (BMs) due to the significant physiological functions of Zn²⁺ ions in human body. However, low inhibitory concentration threshold value to cause cytotoxicity by Zn²⁺ ions during in vitro study and delayed osseointegration in vivo are two key flaws for the bulk Zn-based BMs. To combat these issues, we constructed a barrier layer of ZrO₂ nanofilm on the surface of Zn-0.1(wt.%) Li alloy via atomic layer deposition (ALD). A decreased release of Zn²⁺ ions accompanied with accelerated release of Li⁺ ions was observed on account of galvanic coupling between the coating compositions and Zn-0.1Li alloy substrate. Cytocompatibility assay reflected that ZrO₂ nanofilm coated Zn-0.1Li alloy exhibited improved cell adhesion and viability. Histological analysis also demonstrated better in vivo osseointegration for the ZrO₂ nanofilm coated Zn-0.1Li alloy. Hence, the present study elucidated that the ALD of ZrO₂ nanofilm on Zn-based BMs can effectively promote osseointegration and control their biodegradation behavior. STATEMENT OF SIGNIFICANCE: Zn-Li binary alloy was reported recently to be the promising biodegradable metals with ultimate tensile strength over 500 MPa, yet the low inhibitory concentration threshold value to cause cytotoxicity by Zn²⁺ ions is the obstacle needed to be overcome. As a pilot study, a systematic investigation on the ZrO₂ nanofilm coated Zn-Li alloy, prepared by atomic layer deposition (ALD) technique, was conducted in the present study, which involved in the formation process, in vitro and in vivo degradation behavior as well as biocompatibility evaluation. We found a controllable corrosion rate and better in vivo osseointegration can be achieved by ZrO₂ nanofilm coating on Zn-Li alloy, which provides new insight into the surface modification on biodegradable Zn alloys for usage within bone.

Alloying design of biodegradable zinc as promising bone implants for load-bearing applications

Magnesium-based biodegradable metals (BMs) as bone implants have better mechanical properties than biodegradable polymers, yet their strength is roughly less than 350 MPa. In this work, binary Zn alloys with alloying elements Mg, Ca, Sr, Li, Mn, Fe, Cu, and Ag respectively, are screened systemically by in vitro and in vivo studies. Li exhibits the most effective strengthening role in Zn, followed by Mg. Alloying leads to accelerated degradation, but adequate mechanical integrity can be expected for Zn alloys when considering bone fracture healing. Adding elements Mg, Ca, Sr and Li into Zn can improve the cytocompatibility, osteogenesis, and osseointegration. Further optimization of the ternary Zn-Li alloy system results in Zn-0.8Li-0.4Mg alloy with the ultimate tensile strength 646.69 ± 12.79 MPa and Zn-0.8Li-0.8Mn alloy with elongation 103.27 ± 20%. In summary, biocompatible Zn-based BMs with strength close to pure Ti are promising candidates in orthopedics for load-bearing applications.

Zn-Mg Biodegradable Composite: Novel Material with Tailored Mechanical and Corrosion Properties

Zinc-based alloys represent one of the most highly developed areas regarding biodegradable materials. Despite this, some general deficiencies such as cytotoxicity and poor mechanical properties (especially elongation), are not properly solved. In this work, a Zn-5Mg (5 wt.% Mg) composite material with tailored mechanical and superior corrosion properties is prepared by powder metallurgy techniques. Pure Zn and Mg are mixed and subsequently compacted by extrusion at 200 °C and an extrusion ratio of 10. The final product possesses appropriate mechanical properties (tensile yield strength = 148 MPa, ultimate tensile strength = 183 MPa, and elongation = 16%) and decreased by four times the release of Zn in the initial stage of degradation compared to pure Zn, which can highly decrease cytotoxicity effects and therefore positively affect the initial stage of the healing process.

Response of human periosteal cells to degradation products of zinc and its alloy

Zinc (Zn) and its alloys are proposed as promising resorbable materials for osteosynthesis implants. Detailed studies should be undertaken to clarify their properties in terms of degradability, biocompatibility and osteoinductivity. Degradation products of Zn alloys might affect directly adjacent cellular and tissue responses. Periosteal stem cells are responsible for participating in intramembranous ossification during fracture healing. The present study aims at examining possible effects emanating from Zn or Zn-4Ag (wt%) alloy degradation products on cell viability and osteogenic differentiation of a human immortalized cranial periosteal cell line (TAg cells). Therefore, a modified extraction method was used to investigate the degradation behavior of Zn and Zn-4Ag alloys under cell culture conditions. Compared with pure Zn, Zn-4Ag alloy showed almost fourfold higher degradation rates under cell culture conditions, while the associated degradation products had no adverse effects on cell viability. Osteogenic induction of TAg cells revealed that high concentration extracts significantly reduced calcium deposition of TAg cells, while low concentration extracts enhanced calcium deposition, indicating a dose-dependent effect of Zn ions. Our results give evidence that the observed cytotoxicity effects were determined by the released degradation products of Zn and Zn-4Ag alloys, rather than by degradation rates calculated by weight loss. Extracellular Zn ion concentration was found to modulate osteogenic differentiation of TAg cells. These findings provide significant implications and guidance for the development of Zn-based alloys with an optimized degradation behavior for Zn-based osteosynthesis implants.