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Bioactive Nickel Titanium Shape Memory Alloys for Enhanced Osseo-integration

Final Activity Report Summary - BIOACTIVENITI (Bioactive Nickel Titanium Shape Memory Alloys for Enhanced Osseo-integration)

The primary aim of the research was to deliver surface modification techniques for enhance biological activity of NiTi surfaces in the osteological environment. The research program of the project was divided into the following main parts:
1. selection of the surface treatment;
2. evaluation of the bioacativity;
3. evaluation of the nanomechanical and corrosion properties;
4. evaluation of the cell and bacteria behaviour on modified surfaces.

At first, nickel titanium samples were prepared using a wide variety selection of methods: mechanical - grinding, chemical - etching and soaking, electrochemical - oxidation, plasma electrolysis, and thermal - heat treatment. For all of the samples, the following properties were evaluated: chemical composition of the surface (XPS), surface free energy (contact angle), topography and surface roughness (AFM and laser profilometry).

On the basis of the results six types of surface treatments were chosen:
1. alkali treatment followed by heat treatment;
2. spark oxidation (plasma electrolysis);
3. heat treatment at : 200, 400 and 600 degrees Celsius.

Criteria for the selection were mainly related to the chemical composition. Highly oxidised surface with high amounts of TiO2 and low concentration of Ni, and high wetting ability were desired. Roughness and surface topography was assessed but was not conclusive. However, these results were included in analysis of the roughness influence on bioactive properties of nickel titanium. In the research additionally, two types of reference samples were prepared: reference +ve - bioactive titanium (high bioactive properties were reported for this type of samples in the literature) and reference -ve - commercially pure titanium.

The primary aim of the next step of the study was to compare different surface treatments thermal, alkali treatment and spark oxidation, and assess their influence on bioactive properties of Ni-Ti alloys. Bioactive properties of materials can be interpreted in a different ways. In general, the term bioactivity related to the ability of the material to trigger a biological action. Authors link this term strictly with cells response or more commonly with the formation of apatite layer from simulated body fluids. To date in the literature, there is no agreement how to evaluate bioactivity and many authors follow Prof. Kokubo's approach immersing samples in SBF for 14 days and measuring a chemistry of the formed film using EDX and XRD. However, this evaluation does not involve considerations of immediate interactions of the surface with the fluids that were found to be the most important for cell response i.e. for cells to 'recognise' the surface, attach to it, and proliferate. Most of the cell studies are carried out in within few hours up to several days. For the same reason bioactivity evaluation should be done in a similar way. In the project new approach to evaluate bioactivity in medium not only containing inorganic but also organic components such as peptides, antibiotics, proteins and vitamins on the basis of XPS and AFM examinations was proposed.

The samples immersing time was also reduced up to 24 hours to evaluate kinetics of the calcium/phosphates formation. Supplementary DSC measurements were performed to find out start and finish temperatures of austenite and martensite transformation of the heat treated samples. It is believed that heat treatment alter transformation temperatures, which in turn can have an influence of structure and surface properties of the material.

These studies indicated that the native surface of Ni-Ti alloy is highly bioactive when assessing the precipitation of calcium phosphates from Hank's solution. Low temperature heat treatments also produced promising surfaces while high temperature treatment resulted in a very low rate of Ca and P precipitation. Alkali treatment and spark oxidation resulted in some bioactivity. The most interestingly, evaluation in the media that contained organic components (protein, vitamins, antibiotics and drugs) revealed that bioactivity for all the samples was at the same level (except of reference negative) without respect to the surface preparation.