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Article

Biodegradable Mg-Zn-MgO Composites for Locking Compression Fixation Plates for Pediatric Orthopedics: Improved Mechanical Properties and Corrosion Resistance

by
Rajesh Jesudoss Hynes Navasingh
1,2,*,
Daniel Asirvatham Amos
1,
Manoj Kumar Gurunathan
1 and
Maria P. Nikolova
3
1
Department of Mechanical Engineering, Mepco Schlenk Engineering College, Sivakasi 626005, Tamilnadu, India
2
Faculty of Mechanical Engineering, Opole University of Technology, Proszkowska 76, 45-758 Opole, Poland
3
Department of Material Science and Technology, University of Ruse “Angel Kanchev”, 8 Studentska Str., 7017 Ruse, Bulgaria
*
Author to whom correspondence should be addressed.
Processes 2025, 13(7), 2077; https://doi.org/10.3390/pr13072077
Submission received: 23 May 2025 / Revised: 23 June 2025 / Accepted: 24 June 2025 / Published: 30 June 2025
(This article belongs to the Section Materials Processes)
Browse Figures
Versions Notes

Abstract

Biodegradable magnesium-based composites show potential application in orthopedic implants, with excellent biocompatibility, low density, and biodegradable characteristics inside the human body. In this study, the stir casting procedure was employed to produce magnesium–zinc MMCs (metal matrix composites) reinforced with MgO nanoparticles, and they were characterized intensively. The analyzed compositions were Mg/4Zn, Mg/4Zn/0.4MgO, and Mg/4Zn/0.6MgO. Their mechanical properties, corrosion resistance, and microstructure were then investigated employing tensile, impact, hardness, wear, and corrosion tests, supplemented with SEM analysis. The results indicate that the Mg-4Zn-0.6MgO composite exhibited the highest performance among the tested formulations, with a tensile strength of 150 MPa, a hardness of 65 HRE (Rockwell Hardness, E-scale), and enhanced corrosion resistance. These improvements are attributed to the uniform dispersion of MgO nanoparticles and the formation of a protective Mg(OH)2 layer, which together contribute to mechanical reinforcement and controlled degradation behavior. The combination of superior mechanical properties and customizable biodegradability verifies the engineered Mg/4Zn/0.6MgO composite as a promising candidate for a biodegradable orthopedic fixation plate without secondary surgery.

1. Introduction

Modern orthopaedic implants are made of metallic materials due to their superior characteristics compared to polymers and polymer–ceramic composites [1,2]. The demand for these materials is rising quickly in tandem with the aging population’s rapid growth. Based on their propensity to biodegrade, metallic implant materials are divided into two groups: (i) bio-inert and (ii) biodegradable. Co-Cr alloys, stainless steel, and Ti alloys are the most common materials used to make conventional metallic orthopedic implants with acceptable biocompatibility [3,4]. Because the human body cannot break them down, they are referred to as bio-inert materials. To remove the implant, a second surgery is required, which can place a financial burden on the patient and cause morbidity. As a result, biodegradable metallic implants that can safely dissolve in the human body have been developed [5,6].
In paediatric orthopaedics, implant material selection is particularly critical due to ongoing skeletal growth. Implants used in children must support healing while avoiding interference with bone development or causing long-term complications. This necessitates materials that are not only mechanically compatible but also capable of biodegradation, to eliminate the need for secondary removal procedures. Figure 1 presents a comparative assessment of Ti-6Al-4V, 316L stainless steel, and Mg-Zn-MgO in the context of paediatric orthopaedic use. The Mg-based composite shows enhanced bone integration and reduced implant weight, and it eliminates follow-up removal procedures. Its mechanical balance and cost-effectiveness make it a promising alternative to traditional metallic systems.
Magnesium (Mg) has emerged as a promising candidate for biodegradable implants due to its favorable degradation behavior in physiological environments. Following degradation, Mg can be metabolized and excreted by the human body. In contrast to biodegradable polymers, Mg offers superior mechanical properties, making it suitable for load-bearing applications [7]. Its density (1.74 to 2.0 g/cm3) is closely aligned with that of cancellous (1.4 to 1.8 g/cm3) and cortical bone (1.8 to 2.1 g/cm3), and its elastic modulus (~45 GPa) is significantly lower than that of traditional bio-inert metals, thereby reducing the risk of stress shielding [8,9,10]. Magnesium is a naturally occurring element in the body, with a daily recommended intake of 300–420 mg for adults, further supporting its biosafety profile [11]. Figure 2 shows modern magnesium-based orthopedic implants such as screws, plates, and pins, which are designed to gradually degrade after implantation. These biodegradable devices support bone healing while eliminating the need for secondary removal surgeries, making them well-suited for pediatric and trauma care.
Despite these advantages, the rapid degradation rate of magnesium in physiological media remains a critical limitation. Strategies such as purification, alloying, surface modification, and the incorporation of biocompatible reinforcements into magnesium matrix composites have been proposed to mitigate this issue. BMMCs (Biodegradable Magnesium Matrix Composites) offer tuneable mechanical properties, controlled biodegradability, and enhanced corrosion resistance [12,13,14]. These composites consist of a magnesium alloy matrix and a reinforcement phase, both of which must be biocompatible and biodegradable [15]. Reinforcement materials commonly used include MgO, calcium phosphate-based ceramics, and other bioactive ceramics [16].
Zinc is often introduced as an alloying element due to its role as an essential trace mineral that improves both mechanical and corrosion behavior at controlled concentrations [17,18]. However, excessive zinc content may accelerate corrosion and result in cytotoxicity through the formation of galvanic couples with MgZn intermetallics [18,19]. Similarly, trace elements such as copper, although potentially detrimental, can offer antibacterial benefits when appropriately utilized [20,21].
The performance of BMMCs depends on the careful selection of both matrix alloys and reinforcement materials. Previous studies on magnesium composites have noted challenges, including particle agglomeration, uneven distribution, and undesirable interfacial interactions, particularly with microscale hydroxyapatite [22,23,24]. Furthermore, the poor in vivo solubility of hydroxyapatite has limited the complete biodegradability of such composites [25]. Reinforcement with stable, biocompatible ceramics, such as MgO, ZrO2, and TiO2, has shown potential in overcoming these issues. Among these, MgO, a major component in bioglass, exhibits high thermal and mechanical stability, as well as antimicrobial behavior [26].
The composite system investigated in this study includes magnesium as the base matrix, zinc as an alloying element, and magnesium oxide (MgO) nanoparticles as reinforcement. Magnesium was chosen for its biodegradability, compatibility with bone density and modulus, and favorable mechanical performance. Zinc enhances strength and corrosion resistance at low concentrations while maintaining biocompatibility. MgO nanoparticles contribute excellent thermal stability, antibacterial behavior, and structural reinforcement. Their nanoscale dispersion improves interfacial bonding and grain refinement, enhancing the composite’s overall integrity. This composition strategy is designed to deliver a clinically relevant balance of strength, biodegradability, and biological compatibility—ideally suited for paediatric orthopaedic use [27,28].
Reinforcement of magnesium alloys with MgO nanoparticles has been reported to enhance mechanical and corrosion properties due to improved interfacial bonding and nanoscale dispersion. In situ synthesis routes also produce reinforcing phases such as MgO and MgZn, further strengthening the composite [29,30]. However, conventional methods like powder metallurgy and in situ reactions are often expensive, complex, and unsuitable for clinical translation due to their limited scalability and processing constraints [31]. Pure magnesium matrices may also fall short in achieving the mechanical performance required for orthopaedic load-bearing applications [32,33].
Although related works on the Mg-Zn-MgO system have been investigated before, most of them were at the material level and did not consider the clinical device or the system’s application. The present study aims to meet this requirement by introducing and testing Mg/4Zn/0.6MgO for paediatric orthopaedic LCP. The composite is prepared by the stir casting process and exhibits quite good mechanical performance (150 MPa of strength and 65 HRE of hardness) and corrosion resistance.
In the present work, magnesium was selected as a biodegradable base matrix because of its mechanical compatibility with bone and its acceptable degradation profile, and 4 wt% of zinc was used for mechanical strengthening to obtain a stable Mg-Zn binary alloy system with the desired mechanical performance and corrosion resistance. MgO nanoparticles were added to the composite to act as reinforcement based on their antibacterial properties, thermal stability, and improved tensile and corrosion behavior. According to the literature and the preliminary casting, 0.4 wt. % and 0.6 wt. % MgO were chosen to prevent agglomeration and retain the matrix structure. This composite is tailored to suit the mechanical and biological requirements of paediatric orthopaedic implants.
Apart from the development of the material, the present study includes the fabrication of the locking compression plate implants, as well as their assessment in Simulated Body Fluid (SBF). This translational effort merges alloy design, device-level fabrication, and physiologically relevant testing to link materials science with paediatric orthopaedic clinical needs. The aim is to develop biodegradable implants that meet the unique biomechanical and biological requirements of pediatric patients while minimizing the need for secondary surgical procedures. To this end, a novel Mg-Zn-MgO biodegradable composite was developed using a cost-effective stir casting method. Three different compositions were fabricated and evaluated for their mechanical, microstructural, and corrosion performance to identify the optimal formulation suitable for pediatric orthopedic trauma applications.

2. Materials and Methods

2.1. Constituents of Magnesium Matrix Composite

Magnesium is a biodegradable metal that gradually dissolves in the physiological environment. Its density ranges from 1.74 to 2.0 g/cm3, which is comparable to that of cortical bone (1.8–2.1 g/cm3) and cancellous bone (1.4–1.8 g/cm3). While biodegradable polymers are often limited in load-bearing applications due to inferior mechanical properties compared to magnesium, pure magnesium itself is also constrained by its relatively low yield strength and high degradation rate [34]. Increasing magnesium purity can reduce degradation rates, but residual impurities may lead to galvanic corrosion through the formation of local galvanic cells. Therefore, alloying and reinforcement are critical to achieving a balance between biodegradability and mechanical performance.
Zinc, an essential trace element, is readily available from ores such as sphalerite and calamine. It has been widely used as an alloying addition to magnesium due to its compatibility with biomedical applications and its capacity to enhance mechanical strength and corrosion behavior [35]. The combination of magnesium and zinc forms a reliable binary alloy system, serving as a suitable matrix for further reinforcement.
Magnesium oxide (MgO) is incorporated as a reinforcement phase owing to its antibacterial, thermal, and biocompatible properties. MgO also helps reduce galvanic corrosion and contributes to improved yield strength, ultimate tensile strength, and microhardness [36]. As an inorganic salt composed of magnesium and oxygen ions, MgO degrades in vitro to form Mg(OH)2, distinguishing it from other ceramic oxides that exhibit lower biodegradability. The MgO nanoparticles used had an average particle size of 40–60 nm, as per the supplier’s specifications. (Because the SEM images in this study were acquired at a scale of 100 µm, individual nanoparticles are not directly resolved; however, the overall microstructural trends confirm their effective incorporation.)
Magnesium, Zinc and Magnesium oxide were bought from Swam Equipment, Chennai, India. In this study, MgO nanoparticles were added at concentrations of 0.4 wt.% and 0.6 wt.% based on evidence from prior studies and preliminary casting trials. Literature reports indicate that MgO contents above 0.6 wt.% often result in nanoparticle agglomeration, increased melt viscosity, and impaired dispersion—particularly in stir casting processes [34,37]. These issues compromise matrix continuity and mechanical integrity. Preliminary experimental work further supported these findings: higher MgO concentrations (e.g., 0.8% and 1.0%) led to observable clustering, increased porosity, and uneven distribution, negatively affecting tensile performance. Therefore, the reinforcement levels of 0.4% and 0.6% MgO were selected to optimize mechanical and corrosion properties while ensuring uniform dispersion and practical castability [38,39].
These reinforcement percentages represent a balance between achieving improved performance and maintaining the processability and integrity of the stir-cast magnesium matrix composites.

2.2. Preparation of Magnesium Matrix Composite

For the manufacture of biodegradable metal matrix composites, materials were selected based on biomedical compatibility and mechanical performance requirements. Magnesium and zinc were used as the matrix, with magnesium oxide (MgO) nanoparticles serving as reinforcement. Table 1 shows the three different compositions developed using varying amounts of MgO (0%, 0.4%, and 0.6%), with input quantities precisely measured using a calibrated digital scale. The theoretical density was estimated using the rule of mixtures to establish a baseline for mechanical and corrosion testing. While MgO is biocompatible, reinforcement levels beyond 0.6 wt.% were avoided as they were found to increase melt viscosity and hinder uniform dispersion during stir casting, resulting in poor matrix continuity.
In this study, 4 wt.% zinc was selected to enhance mechanical properties via solid solution strengthening and intermetallic formation. Although zinc above 2 wt.% can increase corrosion due to Mg-Zn intermetallics, the addition of MgO nanoparticles helps mitigate this by promoting uniform microstructure and formation of a protective Mg(OH)2 layer. The selected composition thus balances strength and corrosion resistance, which is suitable for orthopaedic applications.
Material preparation involved several key steps. Pure magnesium metal was first loaded into a graphite crucible and heated to 650 °C in an electric furnace under a protective argon atmosphere to prevent oxidation. After complete melting, the pre-weighed amount of zinc was added and stirred until fully dissolved. MgO nanoparticles were preheated separately at 200 °C for 1 h to remove moisture and improve wettability before being added to the molten alloy. The melt was stirred at 500 rpm for 10 min using a mechanical stirrer to ensure uniform nanoparticle dispersion. Once mixing was complete, the molten composite was poured at approximately 720 °C into a preheated steel mold (at 250 °C) shaped like a cylindrical rod. The mold was allowed to cool naturally to room temperature under ambient conditions to reduce thermal stress and solidify the specimens. The cast samples were later used for machining and characterization. These detailed process parameters enhance reproducibility and allow for the consistent fabrication of Mg-based composites.

2.3. Locking Compression Plate

Figure 3 shows the 2D diagram of a bone plate, including a top view and a side view. The bone plate has 4 holes of radius 1.75 mm for screw, its length is 80 mm, and its breadth is 3.50 mm. The 3D model of the bone plate was designed using SolidWorks 2023 version, with exact dimensions and design considerations outlined in Figure 4. The casted samples are cut and then taken into machining processes such as milling, drilling, and grinding to make the final product, i.e., bone plate. Finally, the bone plate is made from MMC of composition Mg/4Zn/0.6MgO.

2.4. Mechanical Testing

At room temperature, the tensile test was carried out on a universal tensile test machine with a tensile speed of 0.5 mm per minute. The tensile direction is adjacent to the extrusion path. From the sample, tensile specimens were machined as per ASTM D638-22 [40] (Type I proportions), with a gauge length of 40 mm, width of 13 mm, and thickness of 6 mm. The specimen is held on the vertically spaced grippers. The amount of energy that is taken from the specimen during a fracture shall be determined by the impact test. In the impact tester, the needle is calibrated to zero. Then, the specimen is held in the clamp on the worktable. By releasing the lock of the pendulum, the pendulum is made to strike the required specimen on the worktable. The result is recorded for the required specimen by reading the scale value. The impact test specimen has a square cross-section of side 10 mm, length 55 mm, and a V notch at the center of 2x45o, which is used in the impact test machine. The Rockwell test was the kind of hardness test employed. When compared to the penetration caused by a preload, this test determines how deeply the indenter penetrates under a specific stress. Because it is utilized for magnesium alloys, the test is chosen from a range of Rockwell E scales. A 1/8-inch ball was used as the indenter, and a 100 N load was applied. A pin-on-disk apparatus was used for determining wear on the materials during a slide. The pin is aligned parallel to a flat, circular disk with a radius tip. The test equipment causes the disk to spin about the center of the disk.

2.5. Simulated Body Fluid

Simulated Body Fluid (SBF) was employed to evaluate the corrosion and wear behavior of the Mg-Zn-MgO composites, as these materials are intended for biomedical applications. SBF was prepared according to the protocol established in prior studies to replicate the ionic concentration and pH of human blood plasma [41,42]. The composition of the prepared SBF is detailed in Table 2. The pH of the solution was maintained at 7.40 to simulate physiological conditions.
During wear testing, SBF was continuously supplied over the specimen surface to mimic the dynamic nature of the in vivo environment. For corrosion evaluation, SBF was sprayed using a modified salt spray apparatus to simulate prolonged exposure to an ion-rich moist atmosphere. While this method is not a standard corrosion testing protocol for biomedical alloys, it was adopted here as a comparative tool to assess the relative corrosion behavior of different composite formulations under consistent environmental conditions. This approach enabled the observation of surface degradation trends across compositions [43]. Figure 5 shows the ion concentrations of Simulated Body Fluid (SBF) prepared according to Kokubo’s protocol [44]. The composition mimics human blood plasma, with carefully maintained pH and ionic content, and is used in this study to simulate physiological conditions during the corrosion and wear testing of the Mg-Zn-MgO composites [45,46].
The preparation of SBF involved several key steps: All containers were pre-cleaned using 1N HCl and thoroughly rinsed with ion-exchanged distilled water before drying [47]. A 1 L polyethylene bottle was filled with 500 mL of ion-exchanged distilled water. Reagents were added sequentially as per Kokubo’s formula, using a magnetic stirrer for continuous mixing. The solution was maintained at 36.5 °C using a water bath, and pH was adjusted to 7.40 using 1N HCl. After stabilization, the solution was transferred to a volumetric flask and diluted to 1 L using additional ion-exchanged distilled water. The prepared SBF was stored in a polyethylene bottle at 5–10 °C until use [48].
While the use of SBF allows for a first estimation of corrosion behavior inside a simulated physiological environment, it does not reproduce the full complexity of the biological environment. The determination of values and concentration of pH evolution, magnesium ion release, and cytotoxicity response were out of the scope of this article. These parameters are currently under investigation in additional studies and will be reported in future articles to enable a better overall assessment of the biocompatibility and degradation of the composite materials.

2.6. Statistical Analysis

All experimental tests—mechanical (tensile strength, hardness, impact), wear (mass loss and sliding wear rate), and corrosion (rate and surface degradation)—were performed using three independently prepared specimens per composition group: Mg-4Zn, Mg-4Zn-0.4MgO, and Mg-4Zn-0.6MgO. The mean values were reported along with their respective standard deviation (SD), standard error (SE), and 95% confidence interval (CI) to ensure transparency and assess variability in results.
The standard error was calculated by dividing the SD by the square root of the number of specimens (n = 3), and 95% confidence intervals were computed using Student’s t-distribution (t = 4.303 for df = 2). These statistical measures enable a reliable interpretation of differences between compositions and highlight the consistency of the observed trends.
Due to the limited sample size (n = 3), one-way ANOVA was not performed, as it would not yield statistically robust conclusions with such low degrees of freedom. Nevertheless, the calculated confidence intervals provide an appropriate comparative framework for evaluating material performance enhancements, particularly the effects of MgO nanoparticle reinforcement on the mechanical and corrosion behaviour of the composites.
The full statistical summary is presented in Table 3, and error bars indicating 95% CI are shown in Figure 6.

3. Results and Discussion

3.1. Tensile Test

The load is applied vertically to the specimen. After a due course of time, the specimen, due to the applied load, elongates and fractures at a specific point. The results were obtained as the average of three specimens per composition. Figure 7 shows the tensile test specimens.
The ultimate tensile strength (UTS) of the magnesium composites increased significantly with reinforcement. The UTS for Mg/4Zn was recorded as 101.5 MPa, while Mg/4Zn/0.4MgO reached 130.2 MPa, and Mg/4Zn/0.6MgO achieved 150 MPa. The increase in strength is attributed to the uniform dispersion of MgO nanoparticles and the presence of Mg/Zn intermetallic compounds in the matrix, which contributed to better load transfer and matrix reinforcement [49].
To define the fracture mode, the rupture surfaces were examined post-fracture. The elongation percentages for Mg/4Zn, Mg/4Zn/0.4MgO, and Mg/4Zn/0.6MgO were found to be 6.92%, 4.66%, and 3.58%, respectively. The results show that increasing MgO content enhances UTS but slightly reduces ductility, a common trade-off in nanoparticle-reinforced composites. The enhancement in UTS by 32.3% from 101.5 MPa to 150 MPa with just 0.6 wt.% MgO addition demonstrates the effectiveness of nanoscale reinforcement and good interfacial bonding, consistent with prior studies [38,39].
From a clinical materials perspective, the Mg/4Zn/0.6MgO composite’s UTS of 150 MPa compares favorably with commonly used bioresorbable materials. For instance, PLA-based polymers typically exhibit UTS values between 50 and 70 MPa, which may be inadequate for load-bearing orthopaedic applications. High-strength Mg-based alloys, such as WE43 and Mg-xGd, provide UTS values between 160 and 210 MPa but are associated with rapid degradation and more complex alloying processes [34,50]. Therefore, the Mg/4Zn/0.6MgO composite offers a competitive mechanical profile, combined with enhanced corrosion resistance and manufacturing simplicity, making it a promising candidate for pediatric orthopedic implants.

3.2. Impact Test

The Izod impact test was performed on each sample. Figure 8 displays the outcomes of the impact test performed on Samples A, B, and C using an Izod impact tester. Sample C, 0.6% reinforced, showed the highest impact strength value (8.0 J/mm) among the other samples. The impact strength of materials reduces with longer aging time, as eutectic phases dissolve. The impact strengths of the materials depend on the matrix type, the reinforcement, and the existence of any intermetallic phase. In addition, the impact strength depends on the heat treatment conditions and whether the materials reach the precipitate phase.

3.3. Hardness Test

The Rockwell scale E is used for magnesium alloys. The intender is pressed on the surface of each sample, and the hardness value is noted. On each sample, three indents were made. As shown in Figure 9, measurements and an average were made of the diameter of the indentations taken by the Rockwell test. Strengthening mechanisms during synthesis and subsequent processing also play a role in the hardness of materials. The distribution of MgO reinforcement mostly affects the hardness of the magnesium zinc matrix. In addition, the reduction of grain size has contributed to increased hardness. The chart indicates that in comparison with samples A and B, sample C, which was 0.6% reinforced by MgO, has the highest hardness value of 65 HRE. The presence of reinforcement in the composites may have contributed to a drop in hardness values for other samples, thus increasing ductility and decreasing hardness [51].

3.4. Wear Test

At a specified load, usually with the help of an arm or lever and attached weights, the pin specimen shall be pressed against the disk (Figure 10). The test of a selected load, time, and velocity value is usually used for the determination of wear. Table 4 outlines the fixed parameters used during the dry sliding wear test. The applied load, sliding velocity, distance, and time were selected based on ASTM G99 [52] guidelines to simulate moderate physiological wear conditions. These settings were consistently applied to all specimens to ensure the reliable comparison of wear resistance among the composite formulations.
It is possible to determine the wear level by measuring both specimens in advance and after the test. A fluid called Simulated Body Fluid is used here. Wear loss and wear rate for all the samples are obtained from the test, and the results are compared. It shows a notable increase in wear resistance as the percentage of MgO nanoparticles increases. The increase in reinforcement content has also been observed to lead to a reduction in the wearing losses. This indicates that the presence of MgO acts as a resistance to wear on the contact surface. In sample B, when reinforced with 0.4% MgO at a sliding distance of 400 m, the volumetric wear loss was 1.5 × 10−6 cm3/Nm, whereas in sample A, when reinforced with Zinc, the volumetric wear loss was 2.375 × 10−6 cm3/Nm. Due to the plastic deformation of the material on the contact surface, materials removed during the wear process are formed as debris and worn grooves [53]. Friction increases the temperature on the contact surface, causing some of the wear debris to become oxides. Some wear grooves and delamination, along with wear debris and oxides, were observed in the morphology of the worn surface. In the case of composite materials, some cracked MgO particles were observed on the worn surface. The wear characteristics of composites are dependent on the formation of an intermodal bond between the matrix and reinforcement. At higher magnification of the materials, wear grooves, delaminations, and oxide particles were clearly visible on the wear surface.

3.5. Corrosion Test

The setting of the salt spray test environment is shown in Figure 11. For specimens of composite subjected to the test chamber, it is used to provide relative corrosion resistance statistics.
Figure 11 shows the corrosion rate. This test determines the degradability rate for the samples. Degradability should be neither too high nor too low. In the salt spray corrosion apparatus, instead of spraying salt solution, the SBF (Simulated Body Fluid) is sprayed for 48 h to simulate body conditions [54]. After 48 h, a solution of SBF will be sprayed over the sample. Measurements are made, and the corrosion rate is determined by measuring the starting and final weights. The corrosion test specimen is the size of a square, and its dimensions are 20 × 20 × 10 mm3. The sample with MgO added showed an improvement in corrosion resistance compared to Mg/Zn. This improvement can be attributed to several factors. Fine grains, which are beneficial for corrosion resistance, have been shown in the composites. The corrosion occurred preferentially at grain edges where the MgO nanoparticles were concentrated; uniformly dispersed MgO particles reacted with the SBF solution to form a continuous and dense protective film of Mg(OH)2 that could withstand the degradation of the SBF solution [55,56]. The reaction forms a surface barrier layer of Mg(OH)2 and evolves hydrogen gas. This protective mechanism is facilitated by the presence of MgO nanoparticles that are dispersed within the matrix. The reaction forms a surface barrier layer of Mg(OH)2 and evolves hydrogen gas. This protective mechanism is facilitated by the presence of MgO nanoparticles that are dispersed within the matrix. Adding MgO particles allowed for the formation on the surface of a continuous layer of protection, increasing the number of nucleation sites in the Mg(OH)2 film. The presence of uniformly distributed MgO particles ensures a denser and more coherent Mg(OH)2 layer compared to composites without MgO reinforcement. If the protective layer is damaged, the remaining MgO particles in the matrix react with water to regenerate the Mg (OH)2 layer, as in Equation (2). This self-healing property further enhances the corrosion resistance of the composite.
Mg + 2H2O → Mg(OH)2 + H2
MgO + H2O → Mg(OH)2
The graph shows the mean loss and corrosion rates of all tested specimens after being exposed to SBF for 4 days. Within three days, the mass loss of all specimens began to increase rapidly and slowed down as a layer of passivation formed on the surface. The results of the Mg2+ ion concentration measurement and pH value were also confirmed by a trend in weight loss. The weight loss of the nontreated Mg/Zn alloy is consistently much higher than that of other samples. The corrosion rate of Mg/4Zn, Mg/4Zn0.4MgO, and Mg/4Zn0.6MgO was 4.04 mm, 2.77 mm, and 2.62 mm per year after 4 days in SBF solution. Further confirmation that the composite consisting of Mg/4Zn/0.6MgO displayed the best corrosion resistance could be obtained by the weight loss test [53,57].

3.6. Morphology Analysis

The surface of the magnesium metal matrix composite has been examined using a Phenom ProX SEM machine. When a sample is scanned across the surface of a scanning electron microscope, focused electrons are employed to generate an image of the sample.
Figure 12a shows the SEM image of Sample A (Mg-4Zn). The image reveals a relatively non-uniform surface with visible cracks, likely due to the heterogeneous microstructure and the absence of reinforcement. Dark precipitate regions are seen dispersed across the matrix, which may indicate elemental segregation or residual intermetallics. Some regions exhibit brittle fracture characteristics. A homogeneous matrix phase with grain contours and isolated second-phase regions is visible, although no clear reinforcement is present. These features are consistent with prior studies [58], and the absence of MgO reinforcement corresponds to lower tensile strength and corrosion resistance, as reported in similar unreinforced magnesium alloys [59,60].
Figure 12b displays the SEM image of Sample B (Mg-4Zn-0.4MgO). Arrows in the image highlight white regions identified as clusters of MgO particles. Although individual nanoparticles (40–60 nm) are below the resolution of the image (scale bar: 100 µm), the presence of these bright regions indicates successful reinforcement incorporation. The distribution appears reasonably uniform, with minimal visible agglomeration. The matrix shows improved surface continuity and fewer cracks compared to Sample A, indicating better interfacial bonding. These observations support the role of MgO in enhancing particle dispersion and overall matrix integrity [38,61].
Figure 12c shows the SEM image of Sample C (Mg-4Zn-0.6MgO). A greater number of MgO-rich regions are observed throughout the matrix and are marked with arrows. Compared to Sample B, the distribution appears denser and more widespread, with fewer microstructural defects. Although minor particle clustering is noted, the overall dispersion is improved. The fracture surface morphology suggests enhanced bonding and particle–matrix interaction. These features are in agreement with literature reports that associate increased MgO content with improved mechanical properties, provided dispersion remains controlled. The SEM findings are consistent with the highest tensile strength and hardness measured in this composition, making it suitable for load-bearing biodegradable orthopaedic applications.
The SEM images in this study provide meaningful insight into the microstructure of the composites, particularly with regard to the distribution of MgO nanoparticles and the grain morphology across different compositions. The Mg/4Zn/0.6MgO sample demonstrated finer microstructural features, uniform dispersion of reinforcement particles, and minimal clustering, which contribute to its enhanced mechanical and corrosion performance. These observations support the conclusion that effective reinforcement integration and matrix refinement were achieved during stir casting.

4. Conclusions

To address the clinical limitations of conventional bio-inert orthopedic implants, biodegradable magnesium–zinc composites reinforced with magnesium oxide (MgO) nanoparticles were developed using a stir casting process. Among the three compositions investigated—Mg/4Zn, Mg/4Zn/0.4MgO, and Mg/4Zn/0.6MgO—the Mg/4Zn/0.6MgO composite demonstrated the most favorable performance.
Reinforcement with MgO nanoparticles significantly improved tensile strength (up to 150 MPa), hardness (65 HRE), and wear resistance, primarily due to uniform nanoparticle distribution and effective load transfer within the matrix. Corrosion resistance was also enhanced, attributed to the formation of a Mg(OH)2 layer that reduced degradation rates. Microstructural analysis confirmed homogeneous reinforcement dispersion with minimal agglomeration. The optimized composite showed a strong balance between mechanical integrity, biodegradability, and processing feasibility.
Statistical evaluation using standard deviation, standard error, and 95% confidence intervals confirmed the consistency and reliability of the experimental results. Although a non-standard SBF spray method was used for corrosion comparison, it enabled consistent relative performance analysis across compositions. The method’s limitations are acknowledged. Given its properties and processing simplicity, the Mg/4Zn/0.6MgO composite presents strong potential for use in pediatric orthopedic fixation plates. Its mechanical compatibility with growing bone, along with the elimination of secondary surgeries due to biodegradability, underscores its clinical promise.
Future investigations will focus on long-term in vitro and in vivo assessments, including pH evolution, ion release, cytotoxicity, and biological response. Additionally, future studies will adopt standardized electrochemical and immersion testing protocols to further validate the degradation behavior under physiological conditions.

Author Contributions

Conceptualization, R.J.H.N. and M.K.G.; methodology, M.K.G. and D.A.A.; formal analysis, D.A.A. and M.K.G.; investigation, R.J.H.N.; writing—original draft preparation, R.J.H.N., D.A.A. and M.K.G.; writing—review and editing, M.P.N.; supervision, R.J.H.N.; Funding, M.P.N. All authors have read and agreed to the published version of the manuscript.

Funding

This study is financed by the European Union—Next Generation EU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, project BG-RRP-2.013-0001.

Data Availability Statement

The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors declare that they have no competing interests.

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Figure 1. Comparison of properties of choice materials for pediatric orthopedic applications.
Figure 1. Comparison of properties of choice materials for pediatric orthopedic applications.
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Figure 2. Modern magnesium implants for orthopedic applications.
Figure 2. Modern magnesium implants for orthopedic applications.
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Figure 3. Development of fixation plates made of magnesium matrix composite.
Figure 3. Development of fixation plates made of magnesium matrix composite.
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Figure 4. Design considerations of locking compression plate.
Figure 4. Design considerations of locking compression plate.
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Figure 5. SBF composition.
Figure 5. SBF composition.
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Figure 6. Comparison of key mechanical and corrosion properties with 95% confidence intervals.
Figure 6. Comparison of key mechanical and corrosion properties with 95% confidence intervals.
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Figure 7. Tensile strength testing of magnesium matrix composites.
Figure 7. Tensile strength testing of magnesium matrix composites.
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Figure 8. Impact strength testing of magnesium matrix composites.
Figure 8. Impact strength testing of magnesium matrix composites.
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Figure 9. Hardness testing of magnesium matrix composites.
Figure 9. Hardness testing of magnesium matrix composites.
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Figure 10. Wear testing of magnesium matrix composites.
Figure 10. Wear testing of magnesium matrix composites.
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Figure 11. Corrosion testing of magnesium matrix composites.
Figure 11. Corrosion testing of magnesium matrix composites.
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Figure 12. SEM micrographs of the tensile fracture surfaces, segmentation boundary images, and particle analysis; (a) Mg-4Zn composite; (b) Mg-4Zn-0.4MgO composite; (c) Mg-4Zn-0.6MgO composite.
Figure 12. SEM micrographs of the tensile fracture surfaces, segmentation boundary images, and particle analysis; (a) Mg-4Zn composite; (b) Mg-4Zn-0.4MgO composite; (c) Mg-4Zn-0.6MgO composite.
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Table 1. Stir-cast combinations.
Table 1. Stir-cast combinations.
SampleMg (%)Zn (%)MgO (%)Total Density
(g/cm3)		Total Mass (g)
A96401.997718.92
B95.640.42.004721.44
C95.440.62.008722.88
Table 2. Ion concentration of Simulated Body Fluid (SBF) based on Kokubo’s protocol.
Table 2. Ion concentration of Simulated Body Fluid (SBF) based on Kokubo’s protocol.
IonConcentration (mM)
Na+142
K+5
Mg2+1.5
Ca2+2.5
Cl−147.8
HCO3−4.2
HPO42−1
SO42−0.5
Table 3. Statistical summary of evaluated properties.
Table 3. Statistical summary of evaluated properties.
PropertyMean (%)Standard Deviation (SD)Standard Error (SE)95% Confidence Interval (±)
Ultimate Tensile Strength (MPa)137.7731.0117.9177.09
Elongation (%)5.051.690.974.17
Izod Impact (J/mm)7.570.640.371.61
Wear Loss (g)0.01730.00460.00260.0111
Weight Loss (g)0.47630.1110.06410.276
Corrosion Rate (mils/year)124.8329.2916.9172.84
Hardness (HRE)63.332.081.25.17
Table 4. Wear parameters.
Table 4. Wear parameters.
Applied Load (N)Sliding Velocity
(m/s)		Sliding Distance (mm)Sliding Diameter
(mm)		R.P.MTime
(s)
20140040478400
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Navasingh, R.J.H.; Amos, D.A.; Gurunathan, M.K.; Nikolova, M.P. Biodegradable Mg-Zn-MgO Composites for Locking Compression Fixation Plates for Pediatric Orthopedics: Improved Mechanical Properties and Corrosion Resistance. Processes 2025, 13, 2077. https://doi.org/10.3390/pr13072077

AMA Style

Navasingh RJH, Amos DA, Gurunathan MK, Nikolova MP. Biodegradable Mg-Zn-MgO Composites for Locking Compression Fixation Plates for Pediatric Orthopedics: Improved Mechanical Properties and Corrosion Resistance. Processes. 2025; 13(7):2077. https://doi.org/10.3390/pr13072077

Chicago/Turabian Style

Navasingh, Rajesh Jesudoss Hynes, Daniel Asirvatham Amos, Manoj Kumar Gurunathan, and Maria P. Nikolova. 2025. "Biodegradable Mg-Zn-MgO Composites for Locking Compression Fixation Plates for Pediatric Orthopedics: Improved Mechanical Properties and Corrosion Resistance" Processes 13, no. 7: 2077. https://doi.org/10.3390/pr13072077

APA Style

Navasingh, R. J. H., Amos, D. A., Gurunathan, M. K., & Nikolova, M. P. (2025). Biodegradable Mg-Zn-MgO Composites for Locking Compression Fixation Plates for Pediatric Orthopedics: Improved Mechanical Properties and Corrosion Resistance. Processes, 13(7), 2077. https://doi.org/10.3390/pr13072077

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