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Article

MWCNTs-TiO2 Incorporated-Mg Composites to Improve the Mechanical, Corrosion and Biological Characteristics for Use in Biomedical Fields

by
Mohammad Taher Amirzade-Iranaq
1,
Mahdi Omidi
1,*,
Hamid Reza Bakhsheshi-Rad
1,*,
Abbas Saberi
1,
Somayeh Abazari
2,
Nadia Teymouri
1,
Farid Naeimi
1,
Claudia Sergi
3,
Ahmad Fauzi Ismail
4,
Safian Sharif
5 and
Filippo Berto
3,*
1
Advanced Materials Research Center, Department of Materials Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran
2
Department of Materials and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iran
3
Department of Chemical Engineering Materials Environment, Sapienza University of Rome, Eudossiana 18, 00184 Roma, Italy
4
Advanced Membrane Technology Research Center (AMTEC), Universiti Teknologi Malaysia, Johor Bahru 81310, Johor, Malaysia
5
Advanced Manufacturing Research Group, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Johor Bahru 81310, Johor, Malaysia
*
Authors to whom correspondence should be addressed.
Materials 2023, 16(5), 1919; https://doi.org/10.3390/ma16051919
Submission received: 27 December 2022 / Revised: 18 February 2023 / Accepted: 19 February 2023 / Published: 25 February 2023
(This article belongs to the Special Issue Biomaterials and Implant Biocompatibility (Second Volume))
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Abstract

:
This study attempts to synthesize MgZn/TiO2-MWCNTs composites with varying TiO2-MWCNT concentrations using mechanical alloying and a semi-powder metallurgy process coupled with spark plasma sintering. It also aims to investigate the mechanical, corrosion, and antibacterial properties of these composites. When compared to the MgZn composite, the microhardness and compressive strength of the MgZn/TiO2-MWCNTs composites were enhanced to 79 HV and 269 MPa, respectively. The results of cell culture and viability experiments revealed that incorporating TiO2-MWCNTs increased osteoblast proliferation and attachment and enhanced the biocompatibility of the TiO2-MWCNTs nanocomposite. It was observed that the corrosion resistance of the Mg-based composite was improved and the corrosion rate was reduced to about 2.1 mm/y with the addition of 10 wt% TiO2-1 wt% MWCNTs. In vitro testing for up to 14 days revealed a reduced degradation rate following the incorporation of TiO2-MWCNTs reinforcement into a MgZn matrix alloy. Antibacterial evaluations revealed that the composite had antibacterial activity, with an inhibition zone of 3.7 mm against Staphylococcus aureus. The MgZn/TiO2-MWCNTs composite structure has great potential for use in orthopedic fracture fixation devices.

1. Introduction

Magnesium alloys have many advantages as lightweight structural materials, which include their low density, high stiffness and specific strength, and great recyclability and cytocompatibility [1,2,3]. In the body’s fluid surroundings, they can gradually degrade which means there is no requirement for further operation to eliminate implants [4,5]. Mg ions, the body’s fourth most common ions, have been shown to stimulate bone regeneration and reduce healing rates [6,7]. Furthermore, MgZn alloys are comparable to real bones in terms of density and Young’s modulus, which lowers stress-shielding influence and deters osteoporosis [8,9,10]. However, rapid degradation in body fluid, which causes local alkalinity and hydrogen evolution, has proven to be complicated and potentially increases the risk of infection and implant failure. This is characterized by a lack of mechanical integrity because of rapid deterioration, crack propagation, and failure when still in use [11,12,13]. The time it takes for an implant to dissolve completely is determined by the location of the implant, the age of patient, the bone regeneration rate, and the mechanical strength of the broken bone [14]. Prior to the mechanical and structural characteristics deteriorating in the body and losing their effectiveness [15,16], a safety period of at least three months is recommended to allow for cell adhesion and hard bone tissue regeneration [17,18,19]. Nevertheless, Mg-based composites have demonstrated poor antibacterial efficiency, resulting in implants being associated with infections and post-operative complications. Bone infection is primarily a destructive problem with significant economic and clinical consequences [17,18,19,20]. As a result, there is a pressing need to improve Mg alloys’ antibacterial performance [8,20,21].
Hence, the encapsulation of an antibacterial agent into Mg-based matrix materials is a favorable method to diminish the viability, attachment, and growth of microbes on the surface of implants [20,21]. Because of its limitations, pure Mg cannot be used in service conditions without the addition of alloying elements or additional reinforcements [19]. The most common technique for enhancing the degradation resistance and antimicrobial performance of Mg is alloying and composite fabrication with appropriate nanofillers. Carbon nanotubes (CNTs) have generated great interest as a nanofiller because of their Young’s modulus (1 TPa), strength (30 GPa) and ultra-high thermal conductivity as well as their antimicrobial performance toward a wide variety of micro-organisms [7,11]. Because of these properties, CNTs are one of the best additives for Mg-based nanocomposites [12,14,22,23,24,25,26,27]. For example, Nai et al. [28] developed an Mg composite reinforced with CNTs via powder metallurgy with an excellent tensile strength (237 MPa); a 39% improvement compared to monolithic Mg. According to Han et al. [29], the elastic modulus and compressive yield strength of AZ31 composite containing CNTs were both improved in comparison with the AZ31 alloy. In the same manner, it was exhibited [3,12] that the addition of CNTs to Mg/Al composites increases ductility and decreases overall tensile strength. Because CNTs are one-dimensional and easily coagulated, this decrease in strength could be attributed to agglomerate formation. As a result, researchers face a significant challenge in achieving the uniform dispersion of CNTs in the matrix. As a result, many researchers have attempted to solve this problem through traditional mechanical milling and melting procedures [7,10,11,12]. However, these techniques could not be used to create composites with a uniform distribution of CNTs. Furthermore, CNTs were damaged during the traditional milling and casting procedures, causing breakage. To address these two issues (agglomerate formation of CNTs and CNT breakage), the semi-powder metallurgy (SPM) method is used to prepare the composites. SPM was used to prepare composites with a uniform dispersion of CNTs within the Mg-based matrix while avoiding CNT agglomeration and breakage [3,12]. However, until now, research on CNTs as reinforcement has primarily concentrated on mechanical properties, with little emphasis on the biocompatibility of Mg alloys [1,30,31]. However, these additions may affect the biosafety of magnesium alloys used in biological applications [32]. TiO2-based biomaterials are widely used as bone substitutes because of their biological compatibility with the body, and nanoscale TiO2 improves implant integration with host tissue in orthopedic applications [32,33,34]. Furthermore, nano-TiO2 has been reported to be biocompatible due to hydroxyapatite (HAp) formation via Ti–OH groups. It is corrosion resistant, has a large specific surface area, is non-toxic, flexible, high in tensile strength, has a relatively lower cost and a stable colloidal suspension [32,33,34]. It is possible to improve in vitro bioactivity of bone-like apatite development ability in simulated body fluid (SBF) by incorporating TiO2 nanoparticles into the composite [33,34]. However, findings from a published study [34] show that there has been no attempt to investigate the influences of TiO2 nano addition on the compressive and tensile performance of monolithic Mg. Currently, little research has been conducted to develop a new strategy to increase the antibacterial and mechanical characteristics of Mg metal matrix composites prepared with SPM. The goal of this study is to explore the feasibility of boosting the mechanical and antibacterial characteristics of Mg matrix nanocomposites by incorporating a TiO2-CNTs nanosystem during the manufacturing process. The mechanical alloying and SPM process coupled with spark plasma sintering (SPS) was used to create Mg/TiO2-CNTs composites with various amounts of TiO2-CNTs fillers, and their mechanical, corrosion and biological responses were investigated. The microstructural and mechanical characteristics of nanocomposites were investigated in comparison to those of an Mg alloy without TiO2-CNTs fillers.

2. Experimental Procedures

2.1. Materials and Preparation

Mg (size < 100 µm with 99.5% purity), Zn (size < 3 µm with 99.9% purity), and TiO2 (size < 500 nm with 99.5% purity) powders were provided by Merck Co. (Darmstadt, Germany). The procedure was conducted by mechanical alloying using 120 mL steel containers which were rotated at 600 rpm under an Ar gas atmosphere. A mixture of balls (ball diameter = 10 and 20 mm, mass = 35 g) were loaded into the container. The powder for the Mg–6Zn samples (Zn powders, 6 wt%; and Mg powders, 94 wt%) weighed about 12 g, where the ball material was zirconia with an approximate 20:1 mass ratio of powder to balls. The powder was prepared for 25 h at room temperature.
To create Mg-6Zn/TiO2-MWCNTs composite via the SPM technique [32], TiO2 nanoparticles and CNTs as the reinforcement agents were mixed in a 10:1 weight ratio and were then denoted as TM0 (Mg-6Zn), TM1 (5TiO2-0.5MWCNTs), TM2 (10TiO2-1MWCNTs), and TM3 (15TiO2-1.5MWCNTs). In this regard, in order to achieve homogenous distribution, MgZn composite powder was stirred ultrasonically in 100% ethanol for 2 h. In this case, MgZn was used as a matrix and magnetic stirring was used to mix it for 1 h in ethanol at 600 rpm. At the same time, 0.5, 1, and 1.5 wt% MWCNTs and 5, 10, and 15 wt% TiO2 were ultrasonically dispersed for 3 h in ethanol. A solution containing magnesium composite and MWCNT was kept in the oven under controlled conditions for 1 day. The mixing procedure was carried for 3 h in order to obtain a homogenous mixture. To obtain the composite powder, the mechanically stirred mix was filtered and vacuum-dried at 70 °C overnight. The composite powders were sintered in the SPS chamber for 10 min at 570 °C and 40 MPa pressure to produce green billets with dimensions of 30 mm in diameter and 5 mm in thickness (Figure 1).

2.2. Microstructure Characterization and Mechanical Property Testing

The composition distribution was examined via transmission electron microscopy (TEM; Phillips 208 m, (Amsterdam, The Netherlands)), field emission scanning electron microscope (FESEM, Tescan, Mira 3 Czech Republic, Prague, Czech Republic), and energy dispersive spectroscopy (JSM-5910LV, JOEL Ltd., Tokyo, Japan). Using an X-ray diffractometer, the constituent phases were identified (XRD, D8 Advance, Karlsruhe, Germany). The experimental parameters for XRD were Cu–Kα of X-ray and scan speed of 8° min−1. In order to assess the wettability of samples, the sessile drop method was used with a video contact angle device (GBX Digidrop, Romans-sur-Isère, France) and a drop size of 10 mL at room temperature. The results of 3 specimens were used to calculate the average contact angle for each kind of material. According to the ASTM-E9 standard, the compressive strength (SANTAM universal) of cylindrical composite materials with various contents of TiO2-MWCNTs was determined by pressing them at a weight of 10 kN and a speed of 2 mm/min. The microhardness of the samples was assessed using the Vickers microhardness (Shimadzu, Kyoto, Japan) test method with a load of 300 g.

2.3. Electrochemical and Immersion Tests

A potentiometric polarization experiment was carried out in the SBF using an EC-Lab device at a voltage range of between −250 and +250 mVSCE and an open circuit potential at a rate of 0.5 mV/s in order to assess electrochemical behavior. The specimens were evaluated using a working electrode that had a surface area of 1 cm2 associated with exposure to the electrolyte, with a graphite electrode operating as the counter electrode and a saturated calomel electrode (SCE) acting as the reference electrode. The corrosion potential, corrosion current density, and slope of the anodic and cathode curves were also assessed using the EC-Lab express program. Electrochemical impedance spectrometry (EIS) was employed to establish potential stability following a 30 min sample immersion in the SBF. This test was performed in accordance with ASTM G106 standards utilizing a sign signal with a potential amplitude of 10 mV and an open circuit potential in the frequency band of 105 to 10−2 Hz. The specimens were submerged in 200 mL of Kokubo SBF medium, in accordance with ASTM G1–03, for the immersion experiment. Throughout the immersion test, the pH change in SBF media was measured every 24 h.
The composites conducted mass loss testing, in accordance with the ASTM standard G31-72. During processing, the pH of the solution was measured using a pH meter. The volume of H2 gas emitted as a result of the Mg degradation was also measured, according to [20]. After the funneling and submerging of composites in SBF, a burette filled with SBF was positioned directly above them to collect the released H2 gas evolution and to measure its volume.

2.4. Antibacterial Properties

The biological properties of each Mg/TiO2-MWCNTs composite (n = 3) was examined, and the antibacterial activity against Gram-negative and Gram-positive bacteria was measured using the disc diffusion method. To accomplish this, lawns were used as the cultivation environments and sterile swabs dipped in the microbial suspension were flushed (by pressing swabs against a pipe’s side). The specimens were maintained at 37 °C for 1 day in an incubator while being incubated in an agar medium. If the specimens contain antibacterial capabilities, it will be visible in the inhibition area (IA) around them [35,36].

2.5. In Vitro Biocompatibility

The in vitro cytotoxicity of the Mg/TiO2-MWCNT composite was assessed using an indirect 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyl-tetrazolium-bromide (MTT, Sigma, Saint Louis, MO, USA) test according to the extraction method. The culture media was combined with the nanocomposites (5 mg) and incubated for 1 to 3 days at 37 °C. After 104 cells/mL were placed on the 96-well plates for 24 h, the cell media were replenished with 1 and 3-day extracts. The medium was withdrawn after another 24 h, and 100 μL of MTT agents (0.5 mg/mL in PBS) were added to each well. The incubator was then left in place for 4 h. To dissolve formazan crystals, 100 μL of DMSO was then added into the well. The absorbance was then determined at 545 nm using an ELISA Reader (Stat Fax-2100, Miami, FL, USA) in comparison to a control group using free nanocomposites culture media. To investigate the alkaline phosphatase (ALP) activity of the samples, the composites were washed twice after culture for 3, and 7 days, according to [35]. The staining of ALP was performed using staining reagents. A microscope (TE2000U, Nikon, Tokyo, Japan) was then used to examine the stained cells [37].

2.6. Statistical Analysis

The test findings were provided as mean standard error (SE) and evaluated utilizing Sigmaplot software (version 12) with (*) p < 0.05, (**) p < 0.01, and (***) p < 0.001 to reveal a considerable difference between data.

3. Results and Discussion

3.1. Microstructure Characterization, XRD and Raman Analysis

In Figure 2a, the peaks observed at 2θ = 32, 34, and 36° related to the (100) prism, (002) basal, and (101) pyramidal planes of HCP Mg structure, respectively [26,27]. The X-ray pattern of MgZn/TiO2-MWCNTs composites investigated along the cross-sections of the specimens is shown in Figure 2b. Meanwhile, the peak intensities corresponding to Mg51Zn20 in the Mg composite, and the existence of TiO2 and MgTiO3 peak suggests the integration of TiO2 in the Mg-based composite [38,39]. These precipitates act as barriers to grain boundary sliding, which limits further grain growth and thus improves the mechanical strength of the composite. The crystallite size of the composite was in the range of 64–87 nm, according to the Williamson–Hall equation [17]. The incorporation of TiO2 into the Mg-based composite caused a rise in reactive interface surfaces between the TiO2 and Mg powders via ball milling, facilitating the presence of reaction (1) through the applied mechanical self-propagating reaction (MSR) [40]. It is important to note that a small content of the geikielite (MgTiO3) phase forms in all milled specimens as a result of the following exothermic reaction [1]. Sul et al. [41] reported the cytocompatibility and bioresorbable quality of magnesium titanate oxide (MgTiO3) as a biological implant.
Mg + 1.5TiO2 ⟶ MgTiO3 + 0.5Ti
Raman spectroscopy is commonly utilized to evaluate the structure of carbonaceous materials, crystallization, and imperfections. The Raman spectrum of the as-received MWCNTs revealed three different peaks: D (1340 cm−1), G (1585 cm−1) and 2D (2666 cm−1) (as shown in Figure 2c). The D peak resulted from structural defects and disorder in the carbon lattice, whereas the G peak resulted from sp2 bonding [42,43]. The two-phonon double resonance Raman phenomena were represented by the 2D peak. The ID/IG ratio of the MWCNTs were enhanced from 0.82 to 0.97 in the MgZn/TiO2-MWCNTs composite powders (Figure 2d). As a result of the chemical oxidization and powder mixing operation, the degree of graphitization of the MWCNTs reduced, and the defects in the MWCNTs structure increased [42,43]. In this regard, it was demonstrated that [42] the acid oxidation reaction caused some minor etching of the MWCNTs surface, as well as continuous ball-to-powder contact during the ball milling procedure, which may have led to plastic deformation or fracture of the MWCNTs. This causes some structural damage to the MWCNTs. However, as compared to the G peak of the as-received MWCNTs, the G peak of the MgZn/TiO2-MWCNTs composite (1591 cm−1) shifted in the shorter wavelength direction. This peak shift was caused by the powder mixture procedure, which distorted the sp2 bonding microstructure of the MWCNTs. As a result, the blending operations of the introduced MWCNTs significantly affected the MWCNTs’ structure.
SEM was used to examine the powder morphologies of pure Mg, nano-TiO2, MgZn/TiO2, and TM2 mixed powders, as shown in Figure 3. SEM images revealed that all Mg and TiO2 powders had spherical particles, but the nano-TiO2 powder had much finer particles (Figure 3a,b). Due to strong plastic deformation and the cold welding of Mg particles, milled Mg powders have a semi-spherical shape and a higher surface area. The efficient contact surfaces increase, making sintering densification easier. It is worth noting that it is preferable that the initial Mg and Zn powder particle sizes are quite different in order to achieve uniform Zn distribution in Mg. Because of the higher surface reactivity of Mg, it was complicated to utilize smaller sizes. On the other hand, the structure has Zn concentration of only 3 wt%. As a result, incorporating Zn particles of a similar size increases the risk of Zn concentration in specific areas of the composite structure. Furthermore, the homogeneous dispersion of Zn and TiO2 particles at the powder stage is aided by their smaller size and high surface area (Figure 3c). The aggregated morphology of MWCNTs is significantly tangled together. This microstructure might decrease the performance of MWCNTs as an additive. Fine particles of MgTiO3, TiO2 and MWCNTs are distributed and adhere homogeneously to the surface of Mg matrix alloy particles after the milling process, as shown in Figure 3d.
Details of the EDS elemental mapping data and SEM images show that the Zn element and nano-TiO2 reinforcements and MWCNTs were dispersed fairly on the Mg alloy surface powders, as shown in Figure 3e. SPM was utilized to fabricate Mg-based composites containing CNT and TiO2, where all powders were incorporated into the surface of a deformed Mg matrix during ball milling [44]. Different colors have been used to depict the cumulative and discrete dispersion of elements. Figure 3f depicts the elemental analysis (EDS) of the surface of Mg based alloy, revealing the existence of all the elements Zn, Ti, O, Mg and C in a homogeneous distribution [45]. The spectrum of the TM2 specimen contained Mg and Zn peaks, indicating that the powder was of high purity. The C peak in the EDX pattern can be verified by the incorporation of MWCNT on the synthesized Mg-based alloy specimen.
The morphological shape and size of the powder mixture of the MgZn, MgZn/TiO2, TiO2, MgZn/TiO2-MWCNTs (TM3) is exhibited by the TEM image in Figure 4. Milled MgZn powders have a spherical shape and increased surface area because of significant plastic deformation and the cold welding of Mg powders (Figure 4a), unlike the brittle components of MgZn/TiO2 that are attributed to a small particle size with various shapes (Figure 4b–d). The TM3 composite consisted of fine particles of MgTiO3, TiO2, and MWCNTs, where MWCNTs presented a diameter of 50 nm with homogeneous distribution and great adhesion within the matrix particle (Mg-based alloy), as shown in Figure 4e–i. Due to the fact that nearly all Mg particles were covered on the MWCNTs surface and only a few Mg powders remained outside the MWCNT support, the Mg-MWCNT/TiO2 interaction was observed to be strong.
Wettability tests were also carried out using a contact angle instrument to determine the ability of the surfaces of the specimen to allow protein adhesion, which aids in cell attachment. The contact angles of the TM0, TM1, TM2 and TM3 composites are shown in Figure 5. The data from the in vitro cell culture test demonstrated that there is a significant relationship between the hydrophilicity of the specimen surfaces and cell behavior. Among all the samples, the TM0 composite had the highest hydrophilicity, with a contact angle of 99 ± 2.9°. The contact angles of the TM1 and TM2 samples were 87 ± 2.1° and 78 ± 1.3°, respectively [45]. The incorporation of MWCNTs enhanced the hydrophilic property of TM0 [41]. These results may be attributed to the hydrophilic nature of MWCNTs that contain a high concentration of hydroxyl functional groups [46]. It is well recognized that by increasing wettability the cell viability enhances. The MgZn base contains fewer voids and pores than the TM composites, which indicates that the corrosive medium penetrates the matrix of these composites less when compared to the TM3 containing 15 wt% TiO2 with contact angles of 72 ± 1.1°. As a result, the corrosion performance of the TM samples is highly influenced by the composite structure defect, with higher defects resulting in more corrosive solution infiltration and thus a lower corrosion rate.

3.2. Mechanical Characteristics

Figure 6a illustrates the microhardness experiments of the TM0, TM1, TM2 and TM3 nanocomposites. Compared to TM0, the incorporating TiO2-MWCNTs enhanced the hardness result considerably, with a maximum boost of approximately 64% observed with the incorporation of 10TiO2-1MWCNTs nanoparticles [34]. The hardness of the TM0, TM1, TM2 and TM3 composites was 50 ± 2.8 HV, 71 ± 3.3 HV, 79 ± 3.4 HV, and 75 ± 3.6 HV, respectively. The decrease in composite hardness for great incorporation of addition fractions (15TiO2-1.5CNTs) is attributed to the existence of agglomeration and, in particular, MWCNT segregation [47]. The main reason for the increase in hardness after the addition of 10TiO2-1CNTs fillers could be: (a) the appearance of TiO2, MgTiO3, and MgZn phases and MWCNTs fillers, (b) the presence of nanofillers that restrict localized deformation throughout indentation, and (c) grain size refinement [34].
Figure 6b shows the compression characteristics of the developed TM nanocomposite at room temperature. The ultimate compressive strength (UCS) of the TM1, TM2 and TM3 samples was greater than that of the TM0 sample. A maximum UCS (269 MPa) was detected for the TM2 sample, and a minimum UCS (152 MPa) was observed for the TM0. The boost in UCS of the nanocomposites may be related to: (a) grain size refinement, (b) Orowan strengthening attributed to the existence of nanofillers, (c) difference in the coefficient of thermal expansion values resulting in the formation of dislocations, and (d) the efficient load transfer from the matrix to MgTiO3, TiO2 and CNTs nanofillers. The capacity of a high-energy absorption up to failure under compression load, relating to the region under the stress–strain curve of the TM0 sample, was discovered to improve with the incorporation of the nanofillers. TM3 has a lower UCS than TM2 due to the formation of large agglomerates. Large, aggregated filler particles in the MgZn matrix would induce dislocation accumulation, causing crack formation and a lower compressive failure strain.
Figure 6c–f displays the fracture surfaces of the TM nanocomposite samples under the compression load. The fractographic images show that shear bands exist in all of the MgZn and nanocomposite specimens [34]. Several factors influence the reinforcing effectiveness of nanoscale additives such as MWCNTs fillers in the Mg matrix, including (1) the inherent mechanical characteristics of the reinforcement material, which includes TiO2 and MWCNTs, (2) the load transfer efficiency at the interface of the reinforcement and matrix, (3) the distribution level of the nanosize reinforcements in the Mg-based matrix, and (4) the uniform dispersion of the secondary phases and fillers in the Mg matrix [34]. Consolidation causes the MWCNTs to be bent or integrated between the Mg-based grains as a result of the force exerted on the MWCNTs by the matrix grains surrounding them, or if they are dispersed in the grain boundary and inside grain [48]. Because of the close contact between the MWCNTs and the grains, more binding occurs between the MWCNTs and the matrix grains, resulting in enhanced contact surface and mechanical interlocking, which results in improved load transfer performance between the Mg matrix and the MWCNTs. This has a significant effect on the mechanical characteristics of specimens [47]. These results demonstrate that uniform low contents of MWCNTs have a major influence on the mechanical characteristics of the sample.
Regarding the enhancing mechanism, mechanical alloying significantly reduces the second phase particle size in the Mg-based matrix from micron to nano length scale. Crack branching and load transfer are the toughening mechanisms that have been identified in all of the composites studied. The interaction of the propagating crack prevention and the TiO2-MWCNTs nano additions of varying sizes is the source of this mechanism. A crack is arrested and deflected in-plane when it propagates and comes in contact with the MgTiO3, TiO2, and MWCNTs nanofillers. It is well-known that such a crack deflection mechanism causes an extra tortuous route for stress release, where it enhances fracture toughness and the load transfer at the matrix-filler interface, which helps enhance the ductility and strength of Mg-based composites [47,48,49,50].

3.3. Corrosion Behavior

To determine the corrosion resistance of the TM composites under test conditions, potentiodynamic polarization experiments were conducted. Figure 7a depicts the typical Tafel curves for potentiodynamic polarization tests on all composites. The addition of TiO2-MWCNTs reduces corrosion current density (Icorr) and shifts corrosion potential (Ecorr) to the positive side. The electrochemical potential enhanced to −1498.9 mVSCE, which was considerably greater than the electrochemical potentials of TM2 (−1315.3 mVSCE), TM1 (−1338.7 mVSCE), and TM3 (−1480.8 mVSCE). A great reduction in current density and a significant boost in the electrochemical potential of polarization plots imply that the corrosion behavior of the TM composite has improved. Figure 7a presents the Icorr and Ecorr values that correspond to the polarization curve. The low incorporation of TiO2-MWCNTs to Mg alloy noticeably reduced the Icorr of the composite. The addition of 15 TiO2-1.5 MWCNTs resulted in a slight decrease in the Icorr. In this respect, the TM2 composite showed lower corrosion current density than the other composites in the following order: TM2 (118.6 µA/cm2) < TM1 (148.1 µA/cm2) < TM3 (156.4 µA/cm2) < TM0 (174.7 µA/cm2). This could be attributed to higher-content aggregation of 15 TiO2-1.5 CNTs, which function as just a secondary phase and cause more Mg degradation [11,51]. The existence of more fillers, including 15 TiO2-1.5 MWCNTs in the TM3 composite in comparison to TM2 and TM1, resulted in an increase in filler agglomerates. Furthermore, by adding more fillers (TM3 sample), the number of pores/voids around and/or within the TiO2-MWCNTs aggregates might be enhanced, resulting in a higher corrosion rate.
Figure 7b–d depicts the electrochemical impedance measured for the TM0, TM1, TM2 and TM3 composites. The diameter of the semicircles for the composites with a low filler content (TM1 and TM2) is clearly greater than that of the TM3 specimen, implying an escalation in corrosion resistance with the incorporation of a low number of fillers. The decrease in corrosion resistance of the specimen with high filler content (TM3), on the other hand, can be seen in the smaller diameter of the semicircles and defines the composite’s lower capacitive nature [15]. The equivalent circuit was comprised of a constant phase element (CPE), polarization resistance (Rp), charge transfer resistance (Rct) and solution resistance (Rs). The Rct value of TM1 (117 Ω·cm2) and TM2 (163 Ω·cm2) was greater than that of the TM3 (101 Ω·cm2) and TM3 (78 Ω·cm2) samples. It was shown that a composite containing a low content of TM1 and TM2 could greatly increase the corrosion resistance of the samples [52]. Similarly, the Bode magnitude and Bode phase curves exhibited the highest impedance modulus at a low-frequency (|Z|) and maximum phase angle for the TM1 and TM2 composites (Figure 7c,d). There are also reports [15,52] that a greater concentration of ceramic-based fillers led to more localized pitting corrosion of Mg-based composites. TiO2 and MWCNTs do not interact with metals throughout sintering due to their ceramic nature and high melting points, resulting in inadequate sintering at high TiO2 and MWCNTs content. This could also cause increased intensity of the galvanic couple, as TiO2 and MWCNTs are not well diffused in the matrix, resulting in a high corrosion rate, as seen in composite TM3. Furthermore, the corrosion rate was decreased by boosting the content and dispersion of TiO2 and MWCNTs [45]. It is important to note that the increase in corrosion resistance of the Mg-based composite containing ceramic additions can be related to a reduction in the number of pores and voids near the ceramic aggregates, which is induced by decreasing the ceramic content (TiO2-MWCNTs) of the Mg-based composite.
Figure 8a–h shows SEM images of the TM0, TM1, TM2 and TM3 samples after 14 days of immersion. On the surface of the TM0 sample, cracks (with an arrow) can be seen, which led to further penetration of the corrosive media and subsequently an escalation of the corrosion rate of the specimens during corrosion. The TM1 and TM2 composites hae fewer surface cracks than the TM3 composite with high TiO2-MWCNTs content. The TM2 specimen has the fewest corrosion cracks, which proves that the composite has good corrosion resistance and agrees completely with the outcomes of the polarization and EIS tests. On the corroded surface of the TM1 and TM2 biocomposites, there were fewer corrosion cracks and corrosion products (mainly white particles were visible); whereas, the TM3 composite had a severely corroded surface and corrosion products appear to cover the surface layer.
EDS analysis of these particles reveals that the product layer in point A-C was comprised of Mg, Ti, Ca, O and P elements, implying the possible development of apatite and Mg(OH)2 (Figure 8i–k). The corrosion products have a Ca/P ratio of 1.50, that is smaller than that of pure HA (Ca/P = 1.67) but close to that of calcium-phosphate (Ca/P = 1.63), implying that TiO2 and CNTs can form apatite. As a result of the greater calcium ion concentration in the SBF medium, greater negative charge, and many more nucleation sites, a TiO2-MWCNTs sample can create an apatite layer almost covering the entire composite surface, whereas apatite segments appear on the surface of the TM0 sample. Regarding formation of bone-like apatite, the apatite layer tends to attract phosphate ions in SBF, causing the development of a bone-like apatite cluster [53]. Because MWCNT reinforcements may influence both HA crystal nucleation and growth, the incorporation of MWCNTs had an influence on HA morphological changes. More specifically, the negatively charged carboxyl groups of MWCNTs could also function as a nucleation agent, which can attract Ca2+ ions and cause the deposition of HPO42− or PO43−, finally resulting in the creation of HA [54].
Immersion experiments were carried out to determine their degradation properties further. The pH values observed in the experiment drastically enhanced during the immersion time in the first 48 h. On the other hand, pH showed a comparatively gentle, greatly increased trend (Figure 9a). More notably, the pH plots indicate that the TM2 biocomposite had the lowest pH value of 8.9, compared to 9.3 for TM1 and 9.4 for TM3 and 9.8 for TM0 biocomposites after 14 days of immersion in SBF.
Furthermore, the degradation rates from the weight loss investigation were efficiently calculated. The degradation rate of the TM0 sample (3.1 mm/y) was gradually reduced for TM2 (2.1 mm/y) and TM1 (2.4 mm/y), but greatly expanded for TM3 (2.9 mm/y) after 14 days of immersion in SBF (Figure 9b), indicating a relatively better degradation resistance [55]. The slope of the pH plot and the weight loss rate of the TM composite reduced because the immersion time was enhanced for all specimens. This result could be because of the decreased surface area of composite specimens, which allows the corrosion reaction to occur. Within the first few days of immersion, a larger surface area was exposed to solution leading to more degradation reactions and, as a result, an increase in the weight loss rate. The weight loss rate of the composite specimens lowered and then stabilized as the exposure time was extended to 7 and 14 days. This condition might be explained by the greater immersion time, which causes the creation of barrier layer on the surface of the composite specimens from corrosion products [49]. These films keep the samples from coming into direct contact with the solution, which reduces the weight loss rate of the composite samples. Figure 9c depicts the H2 gas evolution results for the TM composite, and the trend in the quantity of H2 gas evolved per unit surface area of the corrosion medium is nearly identical to that seen in the mass loss data. The TM1 specimen released almost 40.1 mL·cm2 in 14 days, compared to 49.2 mL·cm2 from the TM0 specimen. The amount of H2 gas evolved from the TM2 and TM3 specimens was 38.4 and 43.2 mL·cm2, respectively, after 14 days of immersion in SBF. The changes in mass loss and H2 evolution can be described using SEM images of the composites, which show that the protection provided by the TM1 and TM2 composites caused lower corrosion rates than TM0 [15]. These results suggest that the TM2 composite performs better than other composites in terms of degradation resistance. In this context, significant differences between corrosion rates of Mg-based composite without TiO2-MWCNTs (TM0) and with TiO2-MWCNTs (TM2) were observed (p < 0.05). However, there was no significant difference between the corrosion rate of TM1 and TM2 (p > 0.05) at a similar immersion time. Similarly, no significant differences (p > 0.05) were found between TM1 and TM2 for H2 evolution, but a significant difference between the H2 evolution of TM0 and TM2 (p < 0.05) was detected.
The XRD data of the specimens after 14 days of immersion is also shown in Figure 9d. The new peaks of the Mg(OH)2 and HAp precipitates could be seen for the TM2 composites when compared to the specimens without immersion, verifying the mineralization of the Mg matrix composites. The newly formed Mg(OH)2 and HAp precipitates sealed the pitting and cracks, allowing the Mg alloys to remain protected during immersion [39]. Biomineralized HAp precipitates a layer which is of great importance in influencing Mg corrosion due to its potentially dual role as a precipitate layer which involves: (1) influencing the kinetics of Mg resorption; and (2) affecting cell signaling for good tissue repair [39]. The FT-IR spectrum of the TM2 sample after 14 days of immersion in SBF is shown in Figure 9e. The triply devolved asymmetric stretching mode of the υ3PO43− bands at 1032 cm−1 and the υ4PO43− bands at 602 cm−1 and 565 cm−1 in the spectrum obviously demonstrated the presence of PO43− [55,56,57]. The spectrum exhibited CO32− absorption bands at approximately the same time, including the stretching mode of the υ1CO32− group in carbonated apatite at 1422 cm−1 and the characteristic bending mode of the υ4CO32− in carbonated apatite at 877 cm−1, indicating that the calcium phosphate development on the surface of the coated sample was comparable to the bone-like apatite. The band at 3704 cm−1 corresponds to Mg(OH)2 (brucite) in the FT-IR spectrum, and the OH group can be seen at 3416 cm−1 and 1624 cm−1 [56]. The adsorbed water was responsible for the broad absorption bands at 1635 cm−1.

3.4. Evaluation of Biocompatibility

An MTT assay is a typical technique used to determine cell viability through a chemical reagent response. Figure 10a–d shows the fluorescence microscope image of the cell. Cell nuclei dyed with DAPI can be noticed as blue spots under a fluorescent microscope. Each spot represents the core of a single cell. It is clear that composites with low TiO2-MWCNTs amounts have greater total cell numbers than TM3 composites. The MTT assay shows that when osteoblast cells were co-cultured with TiO2-MWCNTs, cell viability increased with increasing time, implying that TiO2-MWCNTs affected cell viability. The TM1 and TM2 surfaces have shown a 92% and 96% increase in osteoblast cell viability, respectively, which is a greater increase than in the TM0 composite (80%) after 3 days of cultivation, as shown in Figure 10e. This suggests that TM2 is more suitable for cell adhesion than TM0 and TM3. There are numerous agglomerates on the composite surface of TM3, which may decrease cell adhesion. Interestingly, there is a significant difference between TM0 and TM2 for cell viability (p < 0.05). The number of cells increases as the amount of TiO2-MWCNTs increase up to a certain value (10TiO2-1MWCNTs), revealing that the surface structure of the specimen containing a low content of MWCNTs (0.5–1 wt%) is better for cell adhesion, whereas a higher amount of MWCNTs shows a lower number of cells, indicating cytocompatibility of the TM specimens with lower concentrations of MWCNTs (up to 1 wt%). The lower pH and H2 release in the TM1 and TM2 biocomposite extracts may illustrate the higher cell viability of these composites compared to the TM0 and TM3. These findings showed that integrating a small content of TiO2-MWCNTs improved the biocompatibility of the TM0 biocomposite [39,45,47,55]. A lower content of CNT (≈5 vol%) has been recommended in the composite in order to control the amount entering the human body [11]. The biosafety of these particles is ensured by the immobilization of CNT in metallic biomaterials, which prevents their direct exposure to surrounding tissues [7,11]. However, further research is needed to determine their safe clinical use as novel biomaterials for use in tissue engineering as CNT is safe when applied topically but not in certain areas such as the lungs and the abdominal cavity.
The ALP assay was used to evaluate the effect of material extracts on osteogenesis differentiation. During the first three days of incubation, the TM2 composite provided better stimulation for cell differentiation. However, when compared to TM0, the ALP activity of the TM3 composite is higher. It should be noted that increasing the incubation period to 5 days increased ALP activity in all groups, particularly in the TM1 and TM2 groups. This could be attributed to bone mineralization and implies that Mg-containing 10TiO2-1MWCNTs increased the ALP activity of cells grown on the surface. This is because, following the incorporation of TiO2-MWCNTs, a bioactive apatite layer precipitates on the composite surface, causing the binding of proteins and other bio-organic substances used to stimulate cell attachment and proliferation [57]. Cell viability was found to be less for TM3 compared to TM1 and TM2. At higher concentrations, the addition of 15TiO2-1.5MWCNTs tends to aggregate and act as a secondary phase in the sample, which increases the corrosion rate and hence reduces cell viability [58]. Cell adhesion and proliferation are physically inhibited by the corroded surface. Thus, a low content of MWCNTs (0.5–1 wt%) is proposed as the optimal content for boosting the biocompatibility of the composite. Evidently, there was no significant difference between the cell viability of TM1 and TM2 (p > 0.05) at a similar cultivation time. Webster et al. [59] discovered a boost in the number of adherent osteoblasts after incorporating carbon nanofibres into polycarbonate urethane. This is most likely the result of increased material deficiencies and electron delocalization on the nanostructured surface. Furthermore, the nanoscale topography of the TiO2 surface affects osteoblast functions such as attachment and proliferation, with broad nanosheets (>100 nm) being ideal for osteoblast attachment [27].

3.5. Antibacterial Properties

The antibacterial performance of the TM0, TM1, TM2 and TM3 composites was evaluated functionally using inhibition area (IA) for 24 h (Figure 11a–d). Figure 11 shows that as the concentration of TiO2-MWCNTs increased, subsequently the antibacterial activity of the composite escalated. Antibacterial activity was determined after 24 h of incubation by measuring the inhibition area diameter and is shown in Figure 11e. The inhibition zone of TM1 and TM2 was 2.2 mm and 3.3 mm, respectively, against Staphylococcus aureus (S. aureus) indicating effective antibacterial performance against Gram-positive bacteria. It was also discovered that incorporating fillers (TiO2-MWCNTs) and the boost in filler concentration resulted in a higher diameter zone against Gram-negative and Gram-positive bacteria in all cases. The bacteriostatic activity was high in the composite loaded with TM3, with a high inhibition zone of 3.7 mm against S. aureus. The disc diffusion tests show that the inhabitation area (IA) around the Mg-based composite without TiO2-MWCNTs (TM0) is significantly smaller (p < 0.001) than that of the composite containing TiO2-MWCNTs fillers. These findings demonstrate that the TiO2-MWCNTs-loaded composites developed in this research have strong antimicrobial activity, with the majority of bacteria being inhibited from growing or even killed. The findings revealed that the TM2 and TM3 composites containing 10–15 wt% TiO2 and 1–1.5 wt% MWCNTs fillers had increased activity against the bacterial species, namely, S. aureus and Escherichia coli (E. coli). According to the results in Figure 11e, the diameter zone of S. aureus bacteria was slightly greater than that of E. coli bacteria, which implies that there was no significant difference between the IA values (p > 0.05) in E. coli or S. aureus for any sample.
The obtained inhibition zone is attributable to the influence of the MWCNTs and TiO2 nano additions. Furthermore, the titanium ion release mechanism can improve antibacterial performance [60]. Moreover, because of regulatory and signal dysfunction, phosphorous and iron motion, and heme groups along the cell wall, TiO2NPs have antimicrobial activity. The antibacterial performance of TiO2 nano additions may also be due to the generation of reactive oxygen species and Ti ions [61,62,63]. Furthermore, several prior studies [64,65] have suggested that the antibacterial performance of CNTs is associated with cell membrane destruction induced by direct contact between agglomerated CNT and bacteria. Individually dispersed CNTs can be illustrated as numerous moving “nano darts” in the medium, which are continuously trying to attack the bacteria, thus also degrading the integrity of bacterial cell and ultimately leading to cell death, as demonstrated by Liu et al. [64]. Previous research has linked CNT antibacterial activity to the physical puncture of agglomerated CNT, leading to physical damage to the cell’s outer membrane. As a result, the well-dispersed surface with identifiable active particles and fillers leads to the inhibition of bacteria. Taken together, the addition of various amounts of fillers, such as TiO2 and CNTs, into the composites has a significant influence on the biological response, corrosion behavior and mechanical properties of the Mg-based composite [66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83].

4. Conclusions

This study used mechanical alloying and semi-powder metallurgy processes combined with spark plasma sintering to fabricate high-density MgZn/TiO2-MWCNTs composites. The MgZn matrix co-encapsulated with TiO2 and MWCNT nano additives has considerably higher strength than that required to meet the mechanical characteristics of biomedical devices. The incorporation of TiO2-MWCNTs fillers into the MgZn matrix largely controlled the adjustable mechanical characteristics and proper corrosion rate. The corrosion rate of MgZn/TiO2-MWCNTs composite (~2.1 mm/y) is lower than that of MgZn (~3 mm/y) when protected by a more even Ca-P layer with fewer cracks and defects on the surface of the composite. The MgZn/TiO2-MWCNTs composite is biocompatible with MG-63 cells in vitro, and the cytocompatibility of TM composite with low TiO2-MWCNTs content is slightly better than that of the MgZn composite. The findings also revealed that co-incorporating MWCNTs and TiO2 into the Mg-based matrix had a considerable effect on antibacterial action by physical puncture of CNT aggregates and more penetration of TiO2 nanoparticles, leading to physical damage of the outer membrane of cells with subsequent degradation of bacterial cell integrity and cell death. The TM1 and TM2 composites containing 5–10 wt% TiO2 and 0.5–1 wt% MWCNTs fillers may be potential candidates for use in biodegradable implants due to their excellent mechanical and antibacterial characteristics, acceptable biocompatibility, and relatively low in vitro corrosion rate.

Author Contributions

Methodology, formal analysis, writing—original draft preparation, M.T.A.-I., N.T., S.A., A.S.; Conceptualization, supervision, writing-review and editing, M.O., F.N., C.S., A.F.I., S.S.; Supervision, visualization, writing—review and editing, funding acquisition, H.R.B.-R. and F.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data provided in the present manuscript are available to whom it may concern.

Conflicts of Interest

The authors declare no conflict of interest.

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Materials 16 01919 g001 550
Figure 1. The prepared nanostructure (a) TM0, (b) TM1 and (c) TM3 composites fabricated via SPS methods.
Figure 1. The prepared nanostructure (a) TM0, (b) TM1 and (c) TM3 composites fabricated via SPS methods.
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Figure 2. XRD pattern of (a) pure Mg and (b) TM0, TM1, TM2 and TM3 composites, (c) Raman spectra of MWCNTs and (d) TiO2-MWCNTs reinforced MgZn nanocomposite.
Figure 2. XRD pattern of (a) pure Mg and (b) TM0, TM1, TM2 and TM3 composites, (c) Raman spectra of MWCNTs and (d) TiO2-MWCNTs reinforced MgZn nanocomposite.
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Figure 3. SEM images of (a) Mg, (b) TiO2, (c) MgZn/TiO2, and (d) TM2 composite and (e) EDS elemental mapping and (f) EDS analysis of TM2 powder composite.
Figure 3. SEM images of (a) Mg, (b) TiO2, (c) MgZn/TiO2, and (d) TM2 composite and (e) EDS elemental mapping and (f) EDS analysis of TM2 powder composite.
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Figure 4. TEM images of (a) MgZn, (b–d) MgZn/TiO2, (e–i) MgZn/TiO2-MWCNTs (TM3) composite powders.
Figure 4. TEM images of (a) MgZn, (b–d) MgZn/TiO2, (e–i) MgZn/TiO2-MWCNTs (TM3) composite powders.
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Figure 5. Contact angle of (a) TM0, (b) TM1, (c) TM2 and (d) TM3 composites and (e) values of the contact angles of the TM composites with various TiO2-MWCNTs content.
Figure 5. Contact angle of (a) TM0, (b) TM1, (c) TM2 and (d) TM3 composites and (e) values of the contact angles of the TM composites with various TiO2-MWCNTs content.
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Figure 6. (a) The microhardness, (b) Ultimate compressive strength (UCS) of composites and SEM micrographs of fracture surfaces of (c) TM0, (d) TM1, (e) TM2 and (f) TM3 composites (* p < 0.05, and ** p < 0.01).
Figure 6. (a) The microhardness, (b) Ultimate compressive strength (UCS) of composites and SEM micrographs of fracture surfaces of (c) TM0, (d) TM1, (e) TM2 and (f) TM3 composites (* p < 0.05, and ** p < 0.01).
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Figure 7. (a) Potentiodynamic polarization, (b) Nyquist plots, (c) Bode magnitude plot and (d) Bode phase curves of TM0, TM1, TM2 and TM3 composites.
Figure 7. (a) Potentiodynamic polarization, (b) Nyquist plots, (c) Bode magnitude plot and (d) Bode phase curves of TM0, TM1, TM2 and TM3 composites.
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Figure 8. Surface morphology of the (a,b) TM0, (c,d) TM1, (e,f) TM2, (g,h) TM3 composites after 14 days of immersion in SBF and EDS analyses of (i) Area A, (j) Area B, and (k) Area C.
Figure 8. Surface morphology of the (a,b) TM0, (c,d) TM1, (e,f) TM2, (g,h) TM3 composites after 14 days of immersion in SBF and EDS analyses of (i) Area A, (j) Area B, and (k) Area C.
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Figure 9. (a) pH value, (b) corrosion rate obtained by weight loss and (c) the rate of H2 evolution of TM0, TM1, TM2, and TM3 composites, (d) XRD pattern and (e) FTIR spectra of TM2 sample after 14 days of immersion in SBF (* p < 0.05, and ** p < 0.01).
Figure 9. (a) pH value, (b) corrosion rate obtained by weight loss and (c) the rate of H2 evolution of TM0, TM1, TM2, and TM3 composites, (d) XRD pattern and (e) FTIR spectra of TM2 sample after 14 days of immersion in SBF (* p < 0.05, and ** p < 0.01).
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Figure 10. Fluorescent microscopic images of (a) TM0, (b) TM1, (c) TM2 and (d) TM3 (e) Cell viability and (f) ALP activity of TM0, TM1, TM2 and TM3 samples (* p < 0.05).
Figure 10. Fluorescent microscopic images of (a) TM0, (b) TM1, (c) TM2 and (d) TM3 (e) Cell viability and (f) ALP activity of TM0, TM1, TM2 and TM3 samples (* p < 0.05).
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Figure 11. (a) Antibacterial activities of the (a) TM0 and TM1, (b) TM2 and TM3, (c) TM0 and TM1, and (d) TM2 and TM3 composites by the disc diffusion test against both Gram-positive (S. aureus) and Gram-negative (E. coli) and (e) diameters of the inhibition zones for TM0, TM1, TM2 and TM3 samples (** p < 0.01 and *** p < 0.001).
Figure 11. (a) Antibacterial activities of the (a) TM0 and TM1, (b) TM2 and TM3, (c) TM0 and TM1, and (d) TM2 and TM3 composites by the disc diffusion test against both Gram-positive (S. aureus) and Gram-negative (E. coli) and (e) diameters of the inhibition zones for TM0, TM1, TM2 and TM3 samples (** p < 0.01 and *** p < 0.001).
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MDPI and ACS Style

Amirzade-Iranaq, M.T.; Omidi, M.; Bakhsheshi-Rad, H.R.; Saberi, A.; Abazari, S.; Teymouri, N.; Naeimi, F.; Sergi, C.; Ismail, A.F.; Sharif, S.; et al. MWCNTs-TiO2 Incorporated-Mg Composites to Improve the Mechanical, Corrosion and Biological Characteristics for Use in Biomedical Fields. Materials 2023, 16, 1919. https://doi.org/10.3390/ma16051919

AMA Style

Amirzade-Iranaq MT, Omidi M, Bakhsheshi-Rad HR, Saberi A, Abazari S, Teymouri N, Naeimi F, Sergi C, Ismail AF, Sharif S, et al. MWCNTs-TiO2 Incorporated-Mg Composites to Improve the Mechanical, Corrosion and Biological Characteristics for Use in Biomedical Fields. Materials. 2023; 16(5):1919. https://doi.org/10.3390/ma16051919

Chicago/Turabian Style

Amirzade-Iranaq, Mohammad Taher, Mahdi Omidi, Hamid Reza Bakhsheshi-Rad, Abbas Saberi, Somayeh Abazari, Nadia Teymouri, Farid Naeimi, Claudia Sergi, Ahmad Fauzi Ismail, Safian Sharif, and et al. 2023. "MWCNTs-TiO2 Incorporated-Mg Composites to Improve the Mechanical, Corrosion and Biological Characteristics for Use in Biomedical Fields" Materials 16, no. 5: 1919. https://doi.org/10.3390/ma16051919

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