Characteristics Investigation of the Initial Development of Miniplate Made from Composite of Magnesium/Carbonate Apatite Fabricated by
Powder Metallurgy Method for Biodegradable Implant Applications
Iwan Setyadi1,a, Pancar Muhammad Pribadi2,b, Aldo Fransiskus Marsetio3,c,Achmad Fauzi Kamal3,d, Rahyusalim3,e, Bambang Suharno1,f,
Sugeng Supriadi*2,g
1Department of Metallurgical and Materials Engineering, Faculty of Engineering, Universitas Indonesia, Depok, I 16424, Indonesia
2Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Depok, 16424, Indonesia
3Department Orthopedic and Traumatology, Faculty of Medicine, Universitas Indonesia, Cipto Mangunkusumo General Hospital, 10430, Indonesia
a[email protected] , b[email protected], c[email protected],
d[email protected], e[email protected], f[email protected],
*corresponding author: [email protected]
Keywords: magnesium composites, carbonate apatite, powder metallurgy, biodegradable implant, miniplate.
Abstract. The development of magnesium-based materials, applied for a biodegradable implant, attracted the attention of many researchers. In this research, the initial development of the Mg/carbonate apatite (CA) miniplate was carried out. The miniplate Mg/5CA is fabricated through powder metallurgy and is followed by a sintering process. Pure magnesium is also fabricated with the same process and is used as a reference. The visual form, microstructure (OM), bending test and corrosion test of miniplate were investigated. The results showed that the visual form of the Mg/5CA miniplate is still not perfect. Flexural stress, flexural strain, and elasticity modulus were obtained at 34.02 MPa, 0.9%, and 3.53 GPa, respectively. The corrosion rate is obtained at 12.64 mm/year. The compaction process of Mg/5CA powder followed by sintering is considered to be less appropriate. The addition of the extrusion process and/or the ECAP process in fabrication can be an option to improve its properties.
Introduction
The appearance of the idea to produce orthopedic biodegradable implants has prompted many researchers to study related supporting materials. No need for post-implantation surgery makes a special attraction in its research development [1].
The type of metal that is considered potential to be developed for a biodegradable implant is magnesium. Besides its light nature (ρ 1.74-2.0 g / cm3), magnesium also has biodegradable properties, modulus of elasticity similar to bone (41-45 GPa), better strength to weight than polymers, high toxicity limits in the body (700 mg/day), and also functions as an enzyme activator and protein regulator. [1, 2-4]. However, Mg weakness in terms of mechanical properties and degradation rates need to be improved.
One of the methods used to develop Mg is to make a magnesium composite structure [2].
Composite structures are considered to be more flexible in terms of control of the component design, the use of reinforcing powder, and biocompatibility [4, 5]. The optimal results in the study of Mg/hydroxyapatite (HA) composite reported by Campo [6] were with a content of 5% wt. HA.
Carbonate apatite (CO3Ap;Ca10-a(PO4)6-b(CO3)c(OH)2-d) is a calcium phosphate-based bioceramics. CA has bone mineral-like properties, biocompatibility, osteoconductivity, and it is
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more easily absorbed than HA[4, 7], so it is considered potential to be developed as reinforcement in Mg composites.
The powder metallurgical process (PM) is considered as a suitable process for making magnesium composites if compared to the casting process. The active properties of molten magnesium, the low wettability, and reactivity of some reinforcing powder against liquid Mg are the reasons for the use of the PM process [5].
One of the bone implant types that need to be developed with biodegradable material is a miniplate implant for the maxillofacial bone [8]. One of the researches on the development of biodegradable miniplate was reported by Supriadi [9] related to the fabrication of the ECAP magnesium miniplate
The focus of the study was to investigate the characteristics of the results of the initial development of Mg/5CA miniplate samples which were fabricated through powder metallurgy processes.
Material and Method
Commercial magnesium powder (purity: ≥99.9%, 60≤ size ≤ 300µm, Merck KGaA, Austria) and local carbonate apatite (CA) powder with a size of ± 100 μm used as starting raw material.
Magnesium composites were made with a content of 5% wt. CA (referring to Campo's research [8]). Mg and CA powders (weight 91.1 mg/sample) were mixed manually and dried using vacuum drying (LabTech) at 200°C for 12 hours (referring to Gu [10]) and ± - 0.096 MPa pressure. The sample was made in the blank miniplate form with a size of ± 23 x 1.9 x 0.85 mm which referred to the Supriadi research [9]. Composite powder was put into a mold, then heated in a muffle furnace (Carbolite RHF 1600) up to 320°C. The compacting was carried out at 300°C with the pressure of 265 MPa. The sintering process was carried out at 550°C in a vacuum furnace with a pressure of -0.1 MPa, heating rate of 5°C/min and held for 1 hour. The vacuum process is done in 2 ways before sintering and while sintering. As a comparison, pure magnesium material was prepared and processed by the same method. Characterization of miniplate samples includes visual forms, a microstructure with an optical microscope (OM) and mechanical properties. Mechanical properties test was carried out through a three-point bending test (ASTM E290) with test speed of 10mm/min.
Corrosion rate was carried out by testing the evolution of hydrogen (H2) gas. Samples were
immersed in Hank's Balanced Salt Solution (HBSS) at 37°C. The sample size was Ø10.5 x 7.77 mm. Volume changes in the burette were observed at the beginning and after sample
immersion 24 hours and 48 hours and referred to experiments carried out by Widyaratih [11].
Fig. 1 Compaction process with hydraulic press machines Result and Discussion
Conditions and visual images of miniplate sample fabrication result. Before the compacting process began (Fig. 1), all the mold fastening bolts had to be tightened to minimize the clearance between shoe dies and the upper dies. Fig. 2. shows the visual form of compacting miniplate (blank
form). It appeared that some pure Mg miniplate samples have flash (Fig. 2a.1). Whereas in some samples of Mg/5CA miniplate were found ruggedness of the outer walls (Fig.2b.1). The presence of flash could occur, presumably, because there was gap still found between shoe dies and upper dies.
If the powder size is smaller than the clearance, the formation of flash when compacting is very likely.
(a.1) (a.2) (b.1) (b.2) (c.1) (c.2)
Fig. 2 Images of miniplate specimens before and after the sintering process, each made of; (a1) pure Mg; (a2) sintered pure Mg (vacuum before sintering); (b1) Mg/5CA; (b2) sintered Mg/5CA
(vacuum before sintering); (c1) Mg/5CA; (c2) sintered Mg/5CA (vacuum while sintering) The unevenness of the Mg/5CA miniplate sample outer walls (Fig.2b.1) occurs due to a sticky side wall section on the mold wall. The presence of friction between the powder and the mold wall causes cold-welding. Giving lubricants to the wall may solve this, but because the samples were made for medical applications, lubricants were not provided in this fabrication.
The sintering results of pure Mg miniplate samples and Mg/5CA miniplate samples are shown in Fig. 2a.2 and Fig. 2b.2. The vacuum process carried out before sintering showed a different impact on both types of miniplate samples. On pure Mg miniplate samples no defects or changes in shape were found. However, on Mg/5CA miniplate samples, they were burned (Fig. 2b.2). Although the sintering process parameters are the same, it is suspected that the addition of carbonate apatite (CA) increased the possibility of burning Mg.
Doi [12] reported that at temperatures of 450-650°C there was a decarbonization process of CO2
in CA compounds, thus CO2 gas was produced and trapped between consolidated magnesium powder. At the same time during the heating process, expansion occurred and triggered the presence of internal pores [12]. Although the sintering process was carried out in a vacuum condition, the presence of oxygen in the furnace room was still possible. The flashpoint, heat energy from the furnace, and the Mg flashpoint temperature (473°C) had fulfilled the fire triangle which could give rise to fire. Even though the surrounding of the fire has been exposed to CO2 gas, the appearance of a small fire still caused Mg to burn into MgO [13]. The optimum vacuuming condition during sintering process is the continuous vacuuming that could eliminate CO2 gas and residual oxygen that was still present in the furnace, so that the burned Mg/5CA sample result could be minimized (Fig. 2c.2).
The microstructure of miniplate samples. Fig.3.b shows the appearance of pores on the microstructure of the Mg/5CA miniplate sample due to the presence of decarbonized CO2 gas after sintering. Whereas on pure Mg miniplate samples, there was no pore (Fig.3.a). The grain size of the sintered miniplate samples ranged from 150–451 µm, while the ECAPed-Mg ranged from 10 µm [9]. Besides, because the initial size of the large Mg powder (60-300µm), the sintering process also contributed to increasing the grain size. While the ECAPed Mg miniplate products were made from Mg material which has undergone superplastic deformation through the equal channel angular pressing (ECAP) process so that the grain size was smaller and denser.
(a) (b) Fig. 3 Optical microscope images of miniplate samples made from; (a) sintered-pure magnesium,
and (b) sintered-Mg / 5CA
Bending test results. On table 1, the flexural stress value of the Mg / 5CA miniplate sample of 34.02 MPa is lower than the pure Mg miniplate of 38.48 MPa. This shows that the consolidation process that occurs between Mg and carbonate apatite was not yet complete. These imperfections occurred because the sintered carbonate apatite and Mg melting points are at the same temperature at 650°C, as reported by Doi [12]. In its implementation, the sintering process was carried out at 550°C.
Table 1 Bending Test Results of on Pure Mg and Mg/ 5CA Miniplate Samples
Samples Average value
Flexural
Stress (MPa) Flexural
Strain (%) Modulus of Elasticity (Gpa)
Pure Mg 38.48 0.6 6.334
Mg/5CA 34.02 0.9 3.53
However, there are also other reasons found, because, in reality, the pure Mg miniplate bending test results are not much different compared to the Mg/5CA miniplate. The presence of a stable oxide layer on each surface of the Mg grain also inhibits the occurrence of perfect sintering and densification [14]. The cohesive energy between the grains became weak and caused brittleness so flexural strain and the elasticity modulus of the miniplate was low.
Test results for hydrogen evolution. Based on the results of experiments and calculations (table 2), the corrosion rate of Mg/5CA samples was converted to the equivalent of 90.256 g/m2/ day or 18.53 mm/year after first-day immersion and 61.562 g/m2/day or 12.64 mm/year after second-day immersion. The Mg/5CA corrosion rate is higher than the corrosion rate of ECAPed- Mg miniplate which is 0.61 ± 0.39 mm/year [15]. The weak bond between composite grains and the presence of pores that caused HBSS fluid can infiltrate. Corrosion that occurred was not only surface corrosion but also intergranular and/or pitting corrosion. This factor was believed to increase the corrosion rate.
Table 2 Hydrogen Evolution Test Results in Mg/5CA samples
24 hours 48 hours
intial end intial end Experimental
results
Volume in Burette (ml) 50 14.3 50 1.3
Volume changes of HBSS in
Burette (ml) -35.7 -48.7
The calculation
results Amount of H2 (mol) 0.00159 0.00217
Amount of degraded Mg (mg) 38.744 52.853
Conclusion
The initial development of the manufacture of Mg/5CA miniplate implants through the powder metallurgy process has been carried out. The compacting process followed by a conventional sintering process has not given good results to the Mg/5CA miniplate samples. The sintering process caused the decarbonization process of carbonate apatite (CA) to be CO2 gas so so that porosity can be formed. The possibility of remaining oxygen and the presence of CO2 gas accelerates the combustion of Mg in the Mg/5CA miniplate sample. The stable oxide on the Mg surface also inhibited perfect sintering and densification.
The bending test results also showed low properties of flexural stresses, flexural strain and modulus of elasticity respectively 34.02 MPa; 0.9%; and 3.53 GPa. This result is lower than the pure Mg miniplate. Besides the density factor, the grain size of the miniplate affected the bending test results. From the H2 gas evolution test, the corrosion rate of the Mg/5CA sample was equivalent to 61,56 g/m2/day or 12.64 mm/year.
For further development of Mg/5CA miniplate fabrication, an alternative to powder metallurgy process without sintering is needed. Addition of compaction processes with deformation processes such as extrusion and/or through equal channel angular pressing (ECAP) can be an option to improve its characteristics.
Acknowledgments
This research is supported by PIT 9, research grant of Universitas Indonesia, 2019.
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