DEVELOPMENT OF BIODEGRADABLE COMPOSITES THROUGH MICROWAVE SINTERING FOR ORTHOPEDIC APPLICATIONS

Abstract

Magnesium is one of the essential minerals required for a human body. Magnesium assists in the dissolution of other minerals, vitamins and proteins present in the human body. On the other hand, a deficiency of magnesium can be the cause of nausea, vomiting, fatigue, cramps, tremors, etc. Therefore, magnesium-based implants are beneficial for a healthy life. This research work is focused on the development of magnesium-based biodegradable metal matrix composites (BMMCs) reinforced with hydroxyapatite (HA). Hydroxyapatite is an artificial bioceramic with a similar structure to the human bone. The BMMCs were developed through the powder metallurgy route using conventional sintering and microwave-assisted sintering techniques. Conventional and microwave sintering was carried out on four different compositions varying from 0 wt.% to 20 wt.% of HA in the matrix. Process parameters like compaction pressure, sintering time and sintering temperature were optimised before deciding the sintering temperature of 500 and 550 °C from TGA and DSC analysis and sintering was explored in two different environments such as argon and forming gas. The BMMCs were termed as AZ31-0HA, AZ31-10HA, AZ31-15HA and AZ31-20HA and compacted at an optimised pressure of 450 MPa. The composites were then characterised for material and other properties, such as density, phase analysis, microstructures, elemental analysis, mechanical properties (microhardness, compressive strength and Young modulus), chemical properties, in-vitro biocorrosion kinetics, and in-vitro biocompatibility. It was found that the conventionally sintered BMMCs showed improved densification with the addition of HA in the matrix sintered at 500 °C in argon as well as in forming gas; while similar BMMCs sintered at 550 °C in forming gas exhibited the highest densification in AZ31-10HA composition as higher HA percentage degrade the dense structure by making the sample porous. The highest densification of 97.58% was achieved in the AZ31-10HA sintered at 550 °C in forming gas ambient. The BMMCs were also characterised for the phases formed; generally α-Mg and HA phases are observed in all BMMCs except in the AZ31-0HA sintered at 500 °C, along with oxides – MgO, ZnO and CaO. While in AZ31-10HA and AZ31-15HA, the HA phase got decomposed into Ca3(PO4)2, CaMg2 and CaO which enhanced the density of AZ31-10HA but reduced the density of AZ31-15HA. Microstructures observed through field emission scanning electron microscopy (FESEM) confirmed the presence of Mg grains as the largest grain size, while the nano-HA particles got settled at the Mg grain boundaries as well as interfacing space of the grains. A higher percentage of HA addition in the matrix exhibited agglomerated nano-HA particles and resulted in dropping properties of the BMMCs. The highest microhardness, compressive strength and elastic modulus were found in AZ31-10HA sintered in argon and forming gas; however, composites sintered in forming gas showed improved mechanical properties than the same sintered in argon. The AZ31-10HA composite sintered in forming gas possesses 10.4% higher microhardness than in argon gas, however, only marginal improvement was detected in compressive strength and elastic modulus. The electrochemical corrosion test and in-vitro biocorrosion immersion test results revealed that the AZ31-20HA composite sintered in forming gas exhibited the highest corrosion resistance. The pH of HBSS of the AZ31-20HA, after immersion test for 1 day, was found to be the highest indicating the release of Mg2+ and Ca2+ ions. In addition, it is seen that after 3 days of cell culture test, AZ31-20HA BMMCs showed the maximum cell viability and did not show any toxicity indicating that the material can be used as an artificial material. The characterisation of the microwave sintered BMMCs at 500 and 550 °C in argon and forming gas showed decreasing characteristics in densification– as HA content increases, the relative density was found decreasing. The highest relative density was observed in the BMMC sintered in forming gas at 500 °C. Further, the XRD spectrum identified that α-Mg and HA were the major phases, while the MgO and Ca3(PO4)2 were the minor phases in the BMMCs sintered in argon gas. On the other hand, it was found that the oxidation suppressed in the composites sintered in forming gas. The α-Mg, HA and MgO phases were observed in all BMMCs, except in AZ31-0HA. In the case of sintering at 550 °C, composites contained an extra phase of ZnO which could be the reason for the reduction in densification of BMMCs sintered at 550 °C. Additionally, microstructures also revealed that as the HA increases, micropores present at the surface increase, however, these pores were occupied by the nano-HA particles which reinforce the composites. Improved mechanical properties, compressive strength and elastic modulus were measured; even though, compressive strength and elastic modulus increased upto AZ31-15HA and then decreased. Microhardness and mechanical properties of BMMCs sintered in argon gas were larger than the same sintered in forming gas. The reason could be the existence of hard oxide phases and agglomeration of nano-HA particles in the matrix. The biocorrosion behavior showed improved corrosion resistance; the highest corrosion resistance was exhibited by the AZ31-15HA sintered in argon gas, while the AZ31-10HA sintered in forming gas showed the lowest corrosion rate. The in-vitro biocorrosion study revealed improved corrosion resistance in BMMCs than pure AZ31-0HA. The corroded samples were further analysed to find the causes of weight gain using XRD and EDX studies. It was found that layers of Mg(OH)2 and MgCl2 were formed, and these layers acted like corrosion protective shields. After analyzing the biocorrosion kinetics, the compatibility of BMMCs with human body tissues was analysed using MG-63 cells on MTT assay. The findings of the biocompatibility test displayed that the AZ31-15HA composite showed the highest cell viability and also depicted the best adhesion properties. The hydrophilic nature of the AZ31-15HA, observed through contact angle measurement, exhibited promising qualities for the highest protein adsorption. Measurement of dielectric properties showed that the green compact of BMMCs before sintering exhibits a higher dielectric constant value than the microwave sintered samples. The reason for the high dielectric constant in a green compact can be attributed to the presence of moisture and hydroxyl ions (OH)-, basically present in hydroxyapatite; these elements evaporated during sintering above 100 °C and resulted in a low dielectric constant. Overall, the results suggested that microwave sintered BMMCs exhibited better properties than conventionally sintered and required shorter sintering time. Indirectly, shorter sintering time usually requires less power for processing – say, for sintering. Therefore, the findings recommend that the methods for the development of the given BMMCs can be a preferred route and the developed BMMCs can be one of the promising candidates as an artificial biodegradable material orthopedic application.