March 12, 2014.
Bone damage is a serious health condition that has a direct impact on the quality of life of sufferers. In recent years, metals and alloys are widely used for restoration of bone damage. However, the degradation of most metals implanted in human body has narrowed the choice of clinically usable metals and alloys to mainly stainless steel, cobalt-chromium and titanium and its alloys. Among all the metallic materials, the austenitic stainless steels type 316L (316L-SS) are the most popular materials because of their relatively low cost, ease of fabrication and reasonable corrosion resistance. However, clinical experience has shown that they are susceptible to localized attack in long-term application in the human body causing the release of metal ions, i.e., iron, chromium, nickel and molybdenum. Metal ions can be accumulated in the tissues surrounding the implant or be transported to distant parts of the body. It has been demonstrated that metallic ions resulting from an in vitro corrosion of austenitic stainless steels cause alteration of the expression of human lymphocyte-surface antigens and inhibit the immune response as assessed by lymphocyte proliferation. Moreover it is found that the tissues of the patient that come into contact with the materials cannot endure from any toxic, allergic, mutagenic, or carcinogenetic actions.
Titanium and its alloys are thought to be highly biocompatible materials, and their clinical applications are becoming increasingly often. The excellent biocompatibility of titanium and its alloys is associated with the properties of their protective surface oxides. However, the natural oxide film may not be protective enough in the aggressive biologic environment, and there have recently been some clinical papers reporting hypersensitivity and allergic reactions to titanium. One way to solve this problem is to coat titanium dioxide thin films directly. There are many methods for the preparation of titanium dioxide films such as sputtering, chemical vapor deposition, ion beam assisted deposition, reactive evaporation, laser-assisted evaporation, sol-gel process, and spray pyrolysis. However, the sputtering, especially magnetron sputtering technique seems to be the most favorable method. The material can be supplied to the growing surface layer in the correct proportions and with sufficient energy to ensure the formation of a dense structure. In addition, with this technique the deposition parameters can be controlled easily.
It was recently reported by Kokubo et al. that an in vitro chemically deposited bone-like apatite on cp Ti could be induced by an alkali and heat treatment processes followed by a simulated body fluid (SBF) soaking. These apatite layers formed in SBF have the chemical properties identical to the human bone. It is an essential requirement for an artificial material to bond to the living bone.
In this work, titanium dioxide (TiO2) thin films were deposited on 316L-SS by an unbalanced DC magnetron sputtering technique, followed by the alkali and thermal treatments. All samples were immersed in a SBF for demonstration of the bone-like apatite on the TiO2 films. The effect of the pretreatment on the formation of that apatite was then being investigated. It was found that TiO2 thin films of rutile phase have been deposited by an unbalance reactive magnetron sputtering (Fig.1) and the films were subjected to alkali or thermal treatment. Alkali-treated films do not show the crystallinity probably due to the formation of amorphous sodium titanate (Fig.2). After being soaked in SBF, the apatite was observed on all titanium dioxide films, but more uniform apatite layer was observed only on the non-treated TiO2 film (Fig. 3). The quantitative EDX analysis showed that Ca/P ratio of apatite layer was about 1.48, as show in Fig. 4, which nearly to Ca/P ratio of apatite or human bone (Ca/P =1.67).
Figure 1 XRD patterns of the TiO2 films coated on 316L-SS subjected to various treatments: (a) non-treatment, (b) thermal-treatment and (c) alkali-treatment.
Figure 2 XRD patterns of the TiO2 films as subjected to various pretreatment and soaking in SBF at 37°C for 7 days: (a) non-treatment, (b) thermal-treatment and (c) alkali-treatment.
Dr. Prasertsak Kasemanankul, Dr. Adisorn Buranawong and Asst. Prof. Dr. Nirun Witit-anun
Plasma for Surface Sciences Research Laboratory
Department of Physics, Faculty of Sciences, Burapha University