The Pros And Cons Of Titanium

Introduction

In 1791, William Gregor discovered titanium in igneous rocks and its sediments and it is now the ninth most abundant element of the earth. However, the pure metal was only deduced in 1910, where Matthew Hunter heated TiCl4 with sodium at 700°C. Titanium is a strong and shiny metal with a melting point of 1670°C and boiling point of 3287°C1. Titanium displays a range of properties that allow it to be used in medical applications, where it is usually alloyed with metals such as Aluminium and Vanadium2. However, its properties can cause problems such as implant failure and lack of bone implant integration. Modifications to the surface of titanium can help to overcome this.

Properties Titanium Exhibits

Titanium exhibits a variety of mechanical properties that allows it to be used in different applications. Titanium is extremely resistant to corrosion especially when in close proximity with different media such as human bone, synovial fluid and plasma. This is achieved through the use of a stable and insoluble oxide film that strongly adheres to the surface of titanium. Research has shown that titaniums resistance is considerably better than

other metals used in the body, such as steel.3 Titanium carries out an appropriate host response depending on the situation it is placed in in the body, so it shows biocompatibility. The oxide film stops reactions from taking place between the metal and surrounding environment and therefore resists harsh bodily environments. Evidence shows that biocompatibility is better expressed when it is in direct contact with bone or bodily tissues3. Titanium has a very high tensile strength, reaching above 1400MPa for high strength alloys. It also has a high proof strength, with a maximum of 1100MPa. This allows it to withstand heavy loads imposed on it. With raised temperatures, titanium alloys are able to maintain a high tensile and proof strength, in comparison with pure titanium. Similarly, ductility increases with increasing temperatures and allows for a change in form without it breaking under tensile stress4.

Uses of Titanium
The vast number of properties that titanium exhibits allows it to be used in both orthopaedics and dentistry. Titanium is a highly favoured material by surgeons when rebuilding damaged or destroyed hip sockets, shoulder joints or bones. It can be found in the body being used as bone plates, screws and spinal fusion cages. The most common titanium alloys used in orthopaedics are HA-coated Ti-alloys, Ti6Al4V, NiTi and TNZT alloys
Image 1: Titanium being used in a hip replacement7 (Ti-Nb-Zr-Ta)5. Likewise, titanium has played an important role in the development of dentistry. It is used for orthodontic wires, crowns and partial/complete dentures. Dentistry requires the use of commercially pure titanium, which is available in four grades. The grades of titanium vary in oxygen (0.18 - 0.40 wt%) and iron (0.20 - 0.50 wt%) concentration, which increases its suitability for different applications6. Scientists are confident that the use of titanium will continue to expand in latter years and research is currently being taken to achieve this7.

Disadvantages of Titanium
Still though, titanium has its downfalls which may result in implant failure.

Titanium is prone to implant centred infections as a result of a biofilm being generated on its surface and its immune response being compromised at the implant/tissue interface. As previously mentioned, titanium is biocompatible and this is a result of the surface protein layer that is created under physiological conditions. However, the protein layer also creates an appropriate environment for bacteria to survive, colonise the implant and create a biofilm8. Bacteria commonly found on the surface of titanium include Staphylococcus aureus, Streptococcus mutans, Streptococcus sanguinis and Lactobacillus casei, all of which are facultative anaerobes9. As a result, the hosts defence system is impaired and this may result in an

infection8.
Another disadvantage of titanium is that it lacks bone implant integration which usually leads to implant failure, mechanical instability and discomfort. Titanium displays osseointegration and although the oxide layer allows it to integrate with bone tissue, the oxide layer prevents it from attaining true adhesion between the implant and the bone. Also, when titanium is placed into the body it is encapsulated by fibrous tissue. Fibroblasts secrete an extracellular matrix which is different from the bone matrix formation generated by osteoblast. This can then lead to wear particles being formed on the surface of titanium. This then leads to osteolysis and eventually aseptic loosening, which occurs between 60 - 70% of all occasions. This takes place when the bond between the implant and bone is destroyed whilst the body tries to digest wear particles. In response to this, revision surgery takes place to try and address the issue which leads to increased costs, complications and risk10.

Modifying Titanium
Evidence from previous studies shows that modifying the surface of titanium can reduce bacterial colonisation. Specifically, surface treatment to titanium creates a protective crystalline layer containing an abundance of anatase. It brings forth antibacterial properties to titanium and causes it to precipitate apatite once in stimulated body fluid. It results in decreased bacterial attachment whilst maintaining cell metabolic activity. Overall, it helps to increase the lifespan of different implants11.
Thermal oxidation is an ideal method which relies on the use of TiO2, high temperature and time, ideally for 60 minutes at 800°C. Thermal oxidation results in an oxide film being formed on the surface of titanium which has the ability to protect it from corrosion and causes oxygen dissolution. The higher the temperature, the thicker the oxide layer. It causes an alteration in the surface morphology where there are oxide scales throughout the surface without spallation. With
Image 2: a. Surface of Titanium treated with thermal oxidation at 500°C; c. Surface of Titanium treated with thermal oxidation at 800°C 12 increasing temperature, the thin adherent surface layer changes to a small grain structure that is covered with oxide islands and grains that are positioned perpendicular to the substrate, as seen in image 2. Titanium also increasingly hardens, about six fold compared to an untreated surface of titanium. Evidence shows that a hard coat negatively effects bacterial colonisation and adherence12.
Evidence of this can be yielded from a study conducted by Großner-Schreiber et al., 2000. Titanium discs were either left untreated or were treated via thermal oxidation and then were incubated with Streptococcus mutans and Streptococcus sanguinis. There was a significant increase in the number of bacteria colonies on the untreated titanium discs. Bacterial colonisation resulted in unevenly distributed microgrooves, scratching and pitting. However, the oxide layer formed on the treated discs displayed amphoteric qualities, which supported cationic and anionic exchange absorption, resulting in less bacterial adherence. The study also came to the conclusion that thermal oxidation is a cost effective method13.
The use of Ultraviolet (UV) light to modify titanium also helps to decrease bacteria colonisation. UV light induces photocatalytic activity through the use of titanium dioxide, which is a photocatalyst. This decomposes organic compounds by creating radicals under UV illumination, which helps to kill bacteria14.
A study conducted by Ahn et al., 2011 researched the photocatalytic effects on treated titanium incubated with Streptococcus sanguinis. The study came to the conclusion that a photocatalytic bactericidal effect is produced by photo-induced TiO2 films. The results show that UV illumination especially for long periods results in decreased bacterial activity and attachment. However the degree that this takes place at depends on the surface of titanium. There was a significant decrease in bacterial adhesion and bacterial survival rates when titanium that was prepared by being anodised and heat-treated was placed under UV illumination. The x-ray diffractometry revealed TiO2 peaks of anatase structure on the anodised surface, whilst heat-treated surfaces showed TiO2 peaks of rutile structure14.

Therefore, the aims of this project is to investigate how thermal oxidation and UV-light induced photocatalytic activity effect bacterial adhesion to titanium and address whether these methods can eliminate some of the problems that arise once titanium has been implant into the body.

Conclusion
In conclusion, titanium is a widely used metal that exhibits a variety of mechanical properties some of which include corrosion resistance, biocompatibility and high tensile strength. It can be found being used as bone plates, orthodontic wires and crowns. However titanium can result in implant centred infections and may lack bone implant integration. Modifying the surface of titanium via thermal oxidation and ultraviolet light which induces photocatalytic activity can help to eradicate this. These methods help to create a surface with a protective layer that provides a constant corrective level of antimicrobial concentration.

References
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