Research Article Open Access
Spectrophotometric Evaluation of Urethane Acrylate Based Pigment Colour Stability
Grigale-Sorocina Z1*, Pilipavica R1 and Birks I1
1National University Centre for Oral Health, Singapore, 9 Lower Kent Ridge Road, Singapore 119085, Singapore
2Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore
*Corresponding author: Professor, Manoj Gupta, Department of Mechanical Engineering, National University of Singapore, E-mail: @
Received: June 02, 2020; Accepted: June 15, 2020; Published: June 27, 2020
Citation: Somasundaram Prasadh, Raymond Wong, Manoj Gupta (2020) Viability of Magnesium Based Composites as Temporary Implants. SOJ Mater Sci Eng 7(1): 1-13. DOI: http://dx.doi.org/10.15226/sojmse.2019.00159
AbstractTop
Magnesium and its alloys are researched widely for their use in biodegradable temporary implant applications. However, higher rate of degradation and hydrogen gas evolution limits the use of magnesium-based materials as temporary implants. Till date, various modifications like addition of alloying elements, processing conditions, surface modification and coatings are done to overcome these drawbacks. Another way to circumvent these limitations is to add reinforcement to magnesium and its alloys. Magnesium-based composites, as biomaterials, can exhibit excellent mechanical and microstructural properties like ultimate tensile strength, ultimate compressive strength, elastic modulus, grain refinement and corrosion resistance in the physiological conditions. Accordingly, this paper targets to investigate the influence of addition of different types of reinforcements to magnesium and it alloys on their microstructural, mechanical and corrosion responses.
Keywords: Magnesium; Metal matrix composites; Corrosion; Biomaterial; Temporary Implants
Introduction
Metallic materials used in temporary fixation procedures in the form of stents, screws, nails and bone plates need good degradation properties, corrosion resistance, mechanical integrity and should be biocompatible to the human tissues [1]. Various metallic materials like titanium alloys, stainless steel and cobaltchromium- molybdenum (Co-Cr-Mo) alloys are used extensively in various biomedical applications [2-5]. These materials exhibit superior load bearing capacities and corrosion resistance; however, they have various pitfalls like high elastic modulus leading to stress shielding effects, non-biodegradability, and most often than not, their fixation procedures require secondary surgery for the removal of these materials from the body owing to their non-biodegradable nature [2, 6-11]. Additionally, the stress shielding effects caused by these materials lead to change in the loading pattern on the implants causing screw loosening and hinders new bone formation [8]. In an ideal scenario, a material should naturally resorb into the human body over the time after the bone healing process is completed without creating local or systemic toxicity. Magnesium is one such bioresorbable metal having functional properties like elastic modulus (40–45 GPa) and density (1.738 g/cc) closely matching that of the natural bone (~15–30 GPa, 1.1-1.3 g/cc) thereby avoiding the stress shielding effects and eliminating the need for second surgical intervention [12]. Additionally, magnesium is osteoconductive and thereby facilitates bone cell growth and has also been shown to contribute to cell attachment [13]. However, the major limitation of magnesium is its poor corrosion resistance in physiological environments [14, 15]. Due to fast degradation rate, magnesium implants lose their mechanical integrity and lack the ability to stay inside the tissue until the new bone formation has occurred [16]. Alloying, composite and/or coating technology can be effectively used to alleviate these concerns. Alloying Mg with biocompatible elements like Zn, Ca, Si, amongst others can improve the corrosion resistance and biocompatibility [17- 19]. Further, coating magnesium substrates with bioceramics or fluorides is also known to enhance the functional properties of magnesium [20-22]. However, many of these processes are expensive and their inability to tailor the specific properties could be a potential concern. Another way to enhance the performance of magnesium-based materials in physiological environment is through the judicious addition of low-cost low-volume secondary reinforcements. Such reinforcements in magnesiumbased matrices can provide a suitable alternative to enhance the corrosion resistance and biocompatibility of magnesium. The addition of nanoscale particle to the magnesium is known to increase its mechanical integrity and decreases the corrosion rate. Literature shows that various nanoparticles like alumina (Al2O3) [23], zirconia (ZrO2), yttria (Y2O3) [10], graphene nanoplatelets (GNPs) [67], calcium phosphate ceramics (CPCs) [24-26] when in magnesium-based matrices can improve the mechanical properties and reduce the corrosion rate. Accordingly, the main aim of this short review is to focus on various reinforcements that can be added to magnesium and it alloys to influence their mechanical and corrosion responses to suit for biomedical applications.
Reinforcements Used in MagnesiumTop
Titanium Based Reinforcements
Titanium is a bioactive material widely used for the manufacture of orthopaedic and dental implants, and is a preferred reinforcement to improve bioactivity of composite materials owing to its high mechanical strength, excellent fatigue and tensile strength, chemical inertness, superior corrosion resistance, wear resistance, biocompatibility, low modulus of elasticity, good hardness and most importantly comes at low cost [27-33]. Similarly, titanium-based ceramics have also been used in multiple bio-applications qualifying them to be used a reinforcement [28]. Bordji et al. developed magnesium-based composites using Titanium dioxide (TiO2) particles reinforcement (45 μm) in various amounts (4 wt. %, 8 wt. %, 12 wt. %, 16 wt. %, and 20 wt. %) using powder metallurgy technique [27]. By increasing the amount from 4 wt. %, to 20 wt. % the microhardness increased from 37 HV to 50 HV. Similarly, the ultimate compressive strength (UCS) increased up to 330 MPa for 20 wt. % TiO2 reinforcement containing composite.Aydin et al. demonstrated that addition of TiB2particulates (40 μm) to magnesium matrix in the amount of 10, 20 and 30 wt.%, increased the hardness of the matrix to 65 HV for 30 wt.%, when compared to that of pure Mg of 40 HV [34]. Titanium dioxide (TiO2) nanoparticulates (∼21 nm) reinforced magnesium matrix composite was synthesised using powder metallurgy technique coupled with hot extrusion by Meenashisundaram et al.[29]. The addition of (TiO2) with different amounts (1.5, 2.5 and 5 wt. %) showed a 21% decrease in grain size. The grain size of pure Mg was 34 μm. With increasing the amount of TiO2 from 1.5% to 5%, grain size decreased from 32 to 27 μm. Well-defined grain boundaries and grain boundary pinning effect was reported by investigators [35]. Mg reinforced with 0.58, 0.97, 1.98 and 2.5 vol. % titanium nitride (TiN) nano particulates of ~20 nm size were successfully synthesized by disintegrated melt deposition (DMD) technique followed by hot extrusion by Meenashisundaram et al. [30]. Addition of TiN reinforcement progressively reduced the grain size with 2.5 vol. % reinforcement addition showing the minimum grain size accounting to 57 % reduction (Table 1)[36]. Meenashisundaram et al. synthesised Mg–Ti nanocomposites using DMD followed by hot extrusion with Ti (30–50 nm) amounts 0.58, 0.97 and 1.98 vol. %. The room temperature tensile properties of the synthesized nanocomposites revealed a significant increase in the 0.2%YS (∼79% to ∼112%) and UTS (∼ 46% to ∼81%) when compared to pure magnesium, with decrease in ductility. The addition of 0.97Ti increased the 0.2%CYS of Mg by ∼59% and with 0.58Ti, the UCS of Mg increased by ∼34% with an inappreciable decrease in the ductility [37]. Magnesium matrix (AZ91D) reinforced with TiC-Ti2-AlC-TiB2 (R-Mg) particulates (< 59 μm) was fabricated using an in-situ reactive infiltration technique by Gobara et al. [38]. Electrochemical Impedance Spectroscopy (EIS) and Potentio Dynamic Polarization (PDP) showed that the reinforcing particles greatly increased the strengthened alloy’s corrosion resistance in a 3.5% percent NaCl solution. Potentio Dynamic polarization showed that AZ91D is vulnerable to localized corrosion and a passivating activity was demonstrated by the composite alloy. Polarization resistance values obtained from the EIS study suggested that the composite alloy, R-Mg, showed high activity on the first day of immersion in NaCl due to the initial dissolution of alpha magnesium from the R-Mg surface leaving a more corrosion resistant enhanced matrix (titanium carbides and borides) [38]. Mg60Zn35Ca5 BMGC, 40 vol. % of Ti (75- 105 μm) particles showed a more stable and lower corrosion rate relative to that of the other composites suggesting that the Mg60Zn35Ca5 BMGC releases its Mg ions gradually and uniformly [39]. Meenashisundaram et al.[34] researched Mg 1 vol. % Ti and Mg 1 vol. % of TiB2 composites including Ti (30-50 nm) and TiB2 (~60 nm) nanoparticles which were synthesized using the technique of disintegrated melt deposition (DMD) followed by hot extrusion. Determination of corrosion rates by weight loss technique in Dulbecco’s Modified Eagle’s Medium (DMEM) + 10% Fetal Bovine Serum (FBS) solution revealed that after 28 days of immersion testing, Mg 1 vol.% Ti exhibited the best corrosion resistance followed by pure magnesium and finally by Mg 1 vol.% TiB2 composite. Mg ~ 1 vol. % Ti nanocomposite exhibited zero degradation whereas for pure Mg, after 24 h of immersion, the corrosion rate increased to a maximum of ~ 0.69 mm/year and gradually decreased. The average corrosion rate of Mg -1 vol.% Ti nanocomposite was found to be ~ 0.1150 mm/year for 28 days which is ~ 71% lesser than that of pure Mg [40].

The mechanical and corrosion results discussed above shows that Mg-Ti composite systems can be used in clinical scenario for orthopaedic and dental applications. However, a more systematic approach is required to equally balance the triangle of biocompatibility, mechanical integrity and corrosion resistance. (Table 1) provides a summarized view of different types of titanium-based reinforcements in different magnesium-based matrices.

Traditional Ceramic Reinforcements
Traditional ceramic reinforcements are used due to their wider availability, low cost and chemical inertness in physiological mediums. Accordingly, they are investigated by many researchers. Guo et al. [48] synthesised a (SiC/TiC)p/AZ91 hybrid nanocomposite using slow extrusion speed (1, 0.5 and 0.1 mm/s, ) and at low temperature (270 oC). The addition of 1 wt. % (SiC/TiC)p (40 and 50 nm) nanoparticles hindered the movement of grain boundaries leading to grain refinement. With the addition of nano reinforcements, they reported an enhancement in tensile strengths (YS 345.6 MPa and UTS of 397.2 MPa) of AZ91 alloy [48]. Deng etal.[49]used 1 wt.% SiCp (40 nm) as reinforcement in magnesium matrix and the composite exhibited high strength (∼441MPa) and high modulus (∼60 GPa). The research work conducted so far has established that the size or length scale of the reinforcement significantly influences the strength of the end composite. For example, within micron length scale, smaller the size of the micron particles, better the mechanical strength of the composites [49]. This was also supported by Chen et al. [50,51] who added SiCp particles of 1-2 μm size range to AZ91 alloy and observed a finer grain size and increased strength
Table 1: Mechanical Properties of Mg Composites with Ti based Reinforcements.

 

Material

 

Condition

 

UCS (MPa)

 

UTS (MPa)

 

Ref.

Pure Mg

 

326 ±1

130.3 ± 4.4

[35, 37]

Mg/0.58Ti
(30–50 nm)

DMD

431 ±8(↑32%)

190 ± 7(↑46%)

Mg/0.97Ti

 

413 ± 15(↑26%)

197 ± 8(↑51%)

Mg/1.98Ti

 

415 ± 4(↑27%)

231 ± 12(↑77%)

Pure Mg

 

142 ± 6

275 ± 4

 

 

[41]

Mg/1.98 TiO2 (∼ 21 nm)

(PM+MW)

132 ± 8 (↓5%)

245 ± 8 (↓9%)

Mg/2.5 TiO2

 

134 ± 7 (↓5%)

233 ± 6 (↓14%)

Mg-4% TiO2
(45 μm)

 

292

 

 

[42]

Mg-8% TiO2

 

307

 

Mg-12%TiO2

 

312

 

Mg-16%TiO2

 

321

 

Mg-20%TiO2

 

330

 

ACB (30.82 μm)

 

 

155

 

[43]

ACC

 

 

190

HACB

Hybrid Method

 

201

HACC

 

 

231

 

 

 

 

 

Mg60

 

580

 

 

[39]

Mg67
(75-105 μm)

As-cast

440

 

Mg60T40

 

800

 

Mg67T40

 

700

 

 

 

 

 

 

Mg-0.58(vol%)TiO2

 

285

128

 

 

[44]

Mg-0.97(vol%)TiO2
(20–40 nm)

DMD

278.4

154

Mg-1.98(vol%)TiO2

 

297

165

Mg-2.5(vol%)TiO2

 

305.5

170

AZ91

As-cast

 

165

 

[45]

TiB2/AZ91(30 nm)

As-cast

 

207

AZ91

Homogenized

 

201

TiB2/AZ91

Homogenized

 

237

Mg-0.58(vol%)TiO2 (20-30 nm)

 

285

128

 

 

[46]

 

Mg-0.97(vol%)TiO2

PM

278.4

154

Mg-1.98(vol%)TiO2

 

297

165

Mg-2.5(vol%)TiO2

 

305.5

170

Pure Mg

DMD

332±10

157±5

 

[40]

Mg - 1Ti
(30-50 nm)

 

413±15

197±8

Mg - 1TiB2(60 nm)

 

333±9

173±8

Pure Mg

 

347±4

161

 

[36]

Mg 0.58 TiN
(30–50 nm)

 

355±5

151

Mg 0.97 TiN

DMD

365±2

173

Mg 1.98 TiN

 

385±11

190

Mg 2.5 TiN

 

345±1

197

Pure Mg

 

332±10

157±5

 

[47]

Mg0.58TiB2
(~ 60 nm )

DMD

352±4

149±9

Mg0.97TiB2

 

333±9

173±8

Mg1.98TiB2

 

365±10

186±1

DMD- Disintegrated Melt Deposition, PM- Powder Metallurgy, MW-Microwave Sintering 
ACB- As-cast base, ACC - As-cast composite, HACB - Homogenized as-cast base,
HACC - Homogenized as-cast composite.

and ductility. This was attributed to the fact that smaller the particle size higher is the extent of interfacial bonding between the particles and matrix owing to decreased surface defects around the particles, leading to increased strength [50, 51]. Nie et al. [52], synthesised Mg-4Zn-0.5Ca alloy with addition of low mass fraction of TiC (0.2, 0.5, 1, 2%) nanoparticles (50 nm) using ultrasonic treatment and mechanical stirring. Increasing the amount of TiC decreased the grain size and reduced the secondary phase particles of MgZn and Ca2Mg6Zn. 1 wt. % of TiC composite showed YS of 369.8 MPa and UTS of 393.6 MPa [52]. To note that research activities in magnesium-based composites containing nanoreinforcements have increased in recent times due to the capability of nanoreinforcements to simultaneously increase the combination of mechanical properties such as strength and ductility and corrosion response of the base matrices.

Calcium Phosphate Ceramics (CPC)
For more than 20 years ceramics made from calcium phosphate salts are successfully used to replace and increase bone tissue growth. Hydroxyapatite (HAP) and β-tricalcium phosphate (β-TCP) are the most widely used bio ceramics based on calcium phosphate [53-55]. Hydroxyapatite (HAP) reinforced magnesium matrix has been used for various biomedical applications. HAP reinforced magnesium composites were studied by Gu et al. and the corrosion and mechanical properties of the composites were evaluated [56, 57]. They reported an increase in tensile strength and corrosion resistance with addition of 10 wt. % HAP (2–3 μm)when compared to composites with higher amounts of HAP (20 wt.% and 30 wt.%) [56, 57]. Cui et al. investigated Mg- 2.5 Zn/HAP composites and showed that the addition of 5 and 10 wt.% HAP (60 μm) assists in decreasing corrosion rate and increases the mechanical strength [58]. They reported that addition of 5 wt.% HAP retained the tensile integrity of the structures by 34% and compressive strength by 66 % after 14 days of immersion [59]. Jaiswal et al. reported that addition of 5 wt.% HAP (30 μm) into Mg-3 Zn matrix increased the CYS by 23% and reduced the corrosion rate by 42% [60]. The addition of nano HAP (30 nm) particles to Mg66Zn30Ca4 metallic glass led to marginal improvement in the mechanical properties and increased corrosion resistance of the composite [61]. From the microstructural perspective, addition of HAP nano reinforcement reduced the grain growth and showed grain refinement [26]. Khanra et al. [62, 63] evaluated the microstructure of Mg–HAP and ZM61–HAP composites and reported grain refinement. Xu etal.[64] showed that Mg–HAP (0, 5, 10 and 15 wt.%) exhibits high compressive strength, yield strength and reduced tensile strength when compared to the base matrix (Table 2). (Figure 1) shows typical micrographs illustrating the role of β -TCP in grain boundary pinning and grain refinement (~
Table 2: Mechanical and corrosion properties of calcium phosphates-based composites

Material

Results

Reference(s)

Mg + xHAP (2–3 μm)
x = 10; 20; 30 wt. %

Mg/10 HAP composite sample showed higher YS but reduced UTS.

Increase in amount decreased the strength and ductility.

[57]

 

Mg + xHAP(30 μm)
x = 5; 10; 15 wt. %

Reduced grain size and grain refinement.

Tensile strength of the Mg–HAP composites decreased with the addition of HAP.

Elongation and yield stress decreased with increasing HAP content.

 

[62, 63]

 

Mg + xHAP (25 µm)

x = 5; 10; 15 wt. %

Addition of HAP increased the microhardness and decreased the yield stress and ultimate compressive strength of the composites.

Mg–5HAP showed decreased corrosion rate.

 

[65-67]

 

Mg + xHAP
x = 1; 2; 3; 4; 5 wt. %

Addition of HAP increases the microhardness.

Decreased the corrosion rate.

 

[68]

 

Mg + xHAP(32 nm)
x = 0; 8; 10; 15 wt. %

Fracture toughness slightly increased for composites with addition of 8% HAP.

Mg + 10% HAP improved the corrosion resistance of magnesium in 3.5% NaCl solution.

 

 

[69]

 

Mg + 20%HAP
Mg-4Zn + 20%HAP
Mg-4Zn + 40%HAP

Addition of HAP improved the corrosion resistance.
HA induced deposition of Ca-P compound on composites surfaces.
Zn addition decreased the corrosion rate.

 

[70]

 

Mg + xHAP(60 nm)
ZM61 + xHAP
x = 0, 5, 10, 15 wt. %

HAP particles improved the grain refinement of the composites.

Composites containing Zn and Mn exhibited finer grains, higher hardness and higher tensile and compressive properties than Mg-HAP.

 

 

[62]

 

AZ91D + 20%HAP       (<44 μm)

Minor corrosion attack observed in composites when compared to alloy without HAP.

HAP particles decreased the corrosion rate of the alloy matrix.

 

[71]

 

 

 

AZ31–nHAP

Friction stir processing led to grain refinement.
Agglomerated HAP particles were found.

Composite exhibited highest surface energy indicating good wettability.

Deposition of Ca-P phase resulted in the excellent bioactivity of the composite.

Nano hydroxyapatite improved corrosion resistance in SBF solution.

 

 

 

[68]

WE43 + 20% HAP
(40 μm)

HAP addition weakened the alloy.

Unreinforced alloy exhibited higher corrosion rate than those with HAP addition.

 

[72]

 

 

Mg–2.9Zn–0.7Zr
+ 1% HAP(50 nm)

Zirconium improved the alloy properties due to grain refinement.

HAP addition improved the corrosion resistance. It induced deposition of Ca-P protective layer which prevented pitting corrosion.

Composites exhibited better cytocompatibility than alloy due to slower degradation and did not cause rapid increase of PH.

 

 

 

[73]

 

ZK60A+ xCPP (10 μm)

x = 10, 20, 30 wt. %

Addition of CPP increased mechanical strength.

Addition of CPP decreased corrosion rate in SBF solution.

 

[74]

 

 

Mg-3Zn-Ca/β-TCP

β-TCP particles were located along grain boundaries.

Decreased grain size.

Increased mechanical strength.

 

 

[75]

 

Mg-1Ca-2Zn + 1% β -TCP

Processing methods used enabled grain refinement and uniform distribution of ceramic particles.

Increased mechanical strength.

Decreased corrosion rate.

 

[76]

Mg-3Zn-0.8Zr + x β-TCP (100 nm ) x = 0.5; 1; 1.5%

Addition of β -TCP significantly refined microstructure, with increasing β TCP content led to reduction in grain size.

Decreased corrosion rate.

 

[77]

MgCaxHAP/ y β-TCP
x = 0, 35, 60, 85,
100 y = 100, 65, 40, 15, 0

Mechanical and corrosion properties of the composites were adjustable by the choice of HAP content.  Mechanical properties increased with an increase in HAP content.

 

[78]

Mg-Ca /TCP/HAP

Increased mechanical strength.

Superior corrosion resistance.

 

[79]

Mg-Ca/ β -TCP

Mechanical properties of the composite were lower than for Mg-Ca alloy.

Corrosion resistance of the composite was better than that of the Mg-Ca bulk alloy.

 

[80]

DMD- Disintegrated Melt Deposition, PM- Powder Metallurgy, MW-Microwave Sintering, HAP-Hydroxyapatite,
β -TCP- Tricalcium Phosphate

70%) leads to an increase in the mechanical strength of a Mg- β -TCP composite [81]. Zheng et al. [82] altered the surfaces of β-TCP nanoparticles to achieve a uniform dispersion in the Mg- Zn-Zr matrix by coating with magnesium oxide (MgO). Uniformly scattered β-TCP nanoparticles strengthened the composites’ mechanical properties by grain refining reinforcement mechanism. CPC particles react with body fluids and encourage the development of new bone tissues that lead to rapid bone growth and can be personalized in vitro and in vivo with Ca2 +, HPO42 and PO43- degradation products [82,83]. Feng et al. [74] studied the microstructures and mechanical integrity of ZK60A (6 wt.% Zn, 0.5 wt.% Zr, bal. Mg) reinforced with 2.5, 5, 7.5, and 10 wt.% CPC particles. The composites containing lower amounts of CPC particles (2.5-5 wt.%) exhibited minimum defects in the fabricated composites resulting in improved corrosion resistance whereas the composites containing 7.5 and 10 wt. % CPC showed voids and cracks upon loading [68].

The presence of β-TCP particles (0.7–4.6 μm) assisted in the corrosion protection of pure Mg. The pH values stabilized for the composites compared to pure Mg and displayed lower corrosion rate (~9 times) with a superior protection displayed by the Mg-1.5 TCP composite as compared to pure Mg [81]. To note here that calcium phosphate ceramics, similar to traditional ceramics, assist in grain refinement, enhance the mechanical properties typically the elastic modulus and enhances corrosion response. However, not all the systems with all reinforcements and length scales will do so and experimental validation will be required for each new system. (Table 2) provides a summarized view of balancing act of different types of reinforcements in different magnesium-based matrices.
Figure 1: (a) Grain boundary pinning mechanism of Mg-1.0 TCP composite; (b) β-TCP particle distribution within the Mg matrix in Mg-1.5 TCP composite [81].
Bioglass Reinforcement
Bioactive glass, consisting of a family of glass compositions and capable to bind in a few hours to hard and soft tissues, displays the best-known bioactive actions [84]. These bioactive glasses could gradually degrade after implantation accompanied by the release of ionic products that stimulate bone-related cell proliferation. These advantageous features make the bioactive glass a preferred choice as the reinforcement phase to be added to a Mg-based biomaterial for the enhancement of bioactivity [84, 85].

Nano-SiO2particles (~20 nm) were used as reinforcement in AZ31 magnesium alloy to form AZ31/ SiO2 composites by friction stir process. Addition of the SiO2 particles increased the hardness of the matrix to 90 HV when compared to AZ31 alloy hardness of 49HV [86]. Low volume fraction SiO2 (0.5, 1, and 2 vol. %) nano particle (15 nm) reinforced magnesium composites synthesized using powder metallurgy technique showed marginal decrease in the grain size. There was 32 % decrease in the grain size for 2 vol. % reinforcement (23±1.5) when compared to pure Mg (34±2). The ultimate tensile strength (UTS) for pure Mg progressively increased with the addition of SiO2 nano- particulates and 2 vol. % SiO2 showed the maximum UTS of 171 MPa which surpassed the UTS value of pure Mg by 34 % [87]. The ultimate compressive strength (UCS) was 207 ± 3 for 2 vol. % SiO2 composite when compared to pure Mg with 174 ± 7 MPa. The addition of SiO2 showed 18.9 % increase in the UCS of the composite [88]. The AM60 magnesium alloy reinforced with 1 and 2 wt. % of SiO2 nanoparticles (~35 nm) reduced the grain size and increased the hardness of samples from 34.8 HV in AM60 to 51.5 HV in 2 wt.% composite. Improvement of the mechanical properties was attributed to a combination of Orowan, Hall– Petch and load-bearing mechanisms [89]. Corrosion resistance of pure Mg reinforced with bioglass (BG, 45S5) particles (30– 75 μm ) amounts of 5, 10, and 15 wt.% in simulated body fluid (SBF) at 37 °C was studied by Wan et al.[90]. Increasing the amount of bioglass reduced the hydrogen gas evolution from the composite. Accordingly, compared with pure Mg, hydrogen evolution from BG/Mg composites as a result of degradation, was less. Moreover, the hydrogen evolution is dependent on bioglass content. Expectedly, BG-15/Mg with the highest bioglass content showed the lowest hydrogen evolution rate among all composite samples. Addition of bioglass into Mg significantly reduced the pH value from 9.3 for pure Mg to 8.3 for BG-15/Mg [90]. Huan et al. synthesised ZK30 magnesium alloy as the matrix with 5 and 10 wt. % (40 μm) bioactive glass (BG, 45S5) as the reinforcement. Immersion tests in the minimum essential medium (MEM) at 37°C showed that the composites with 5 and 10% BG had lower rates of degradation and hydrogen evolution than the ZK30 magnesium alloy [91]. These results validated that hydrogen evolution and corrosion of BG/Mg composites can be controlled by adjusting the content of bioglass such that various clinical needs may be met.

Nano-carbon Reinforced Magnesium Composites
Carbon Nanotubes (CNTs) are considered as one of the ideal reinforcements due to their high strength, lightweight properties and also due to favourable geometrical properties such as high aspect ratio, high specific surface area and excellent mechanical properties [92-95]. Due to their excellent mechanical strength and corrosion resistance to improve the performance of composite materials, CNTs reinforced materials are widely used in artificial joints and bone fixation materials [96]. CNTs are intended to be used in the orthopaedic field as a locally implanted

biomaterial such as artificial joints or fixation devices. Various studies conducted on the CNTs have reported that the material exhibits high biocompatibility [96-98]. Li et al. [99] synthesised magnesium metal matrix composites containing carbon nanotubes (CNTs) using powder metallurgy process. They reported that addition of 4.0 wt.% CNT increased the microhardness by 43.5% and UTS by 33.4% when compared to pure magnesium samples [99]. Xian et al. fabricated magnesium alloy ZK60 composites containing low amount of graphene nano plates (GNPs) by facile melt stirring and hot extrusion processes. The composite with only 0.05 wt. % GNPs showed enhancement in yield strength up to 256 MPa. The hardness of the graphene reinforced magnesium composites was evaluated and the results showed that the microhardness increases with the increased graphene content. Pure magnesium sample hardness was 59.3±3.0 HV, while magnesium-graphene composites reinforced with 0.05 wt. % graphene showed hardness of 89.9 ± 4.5 HV indicating 51 % increases over unreinforced magnesium. This is attributed to the fact that the graphene gets aligned along the grain boundaries leading to grain boundary pinning effect. The Tensile Yield Strength (TYS) increased by 62% with 0.05 wt. % GNPs, the compressive yield strength (CYS) increased by 98% with 0.05 wt.% GNPs [100, 101]. Compared to pure Mg, Mg–GNP composites containing 0.1wt. % G-15 (15μm) and G-5 (5μm) GNPs showed the best corrosion resistance in the same corrosion environment. In situ formation of a MgH2 phase in Mg–GNP composites containing lower concentrations (0.1wt.%) of G-15 GNPs in the Mg matrices resulted in slower H2 production rates and lower Icorr values [102]. Carbon nanotubes (CNTs) assist in grain refinement and thus assist in increasing the mechanical properties typically the Tensile Yield Strength (TYS) and Compressive Yield Strength (CYS) and an addition also increase the corrosion resistance. However, not all the systems with all reinforcements and length scales will do so and experimentation validation will be required for each new system.
ConclusionTop
Temporary implant applications need the implant material to be in a desired location in the body without showing rapid degradation and to maintain the mechanical integrity until the purpose of bone formation is fulfilled. Magnesium and its alloys containing different types reinforcements have gained significant attention in last decade as potential materials for temporary implants. Addition of reinforcement in many cases has shown to influence the mechanical and corrosion resistance properties of the magnesium matrix. The amount of addition of reinforcement depends on their length scale. Moreover, addition of many of these reinforcements in magnesium assists in enhancing corrosion resistance and in maintaining the structural integrity of the temporary implants. While the research activities in magnesium-based composites are actively being pursued, much work still needs to be done to understand their corrosion behaviour in-vivo.
Conflict of Interest
No conflict of interest.
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