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An international network on new strategies for processing calcium phosphates

Final Report Summary - REFINED STEP (An international network on new strategies for processing calcium phosphates)

The next generation calcium phosphates demand improved processing routes for capturing the full potential of this elite class of materials. Calcium phosphates are unique with their ability to accept replacement ions in the structure, be processed through intermediate phases, and modify their crystal structure.

The Refined Step project is about creating new calcium phosphates and investigating new processing routes that lead to cheaper production routes or materials with higher functionality.
The project is aimed at supporting international cooperation between European universities and universities abroad. Therefore, the project has assembled internationally competitive teams from a range of fields to work on the multidisciplinary field of biomaterials involving the design of high-temperature processes, materials processing, characterization and testing. The network includes eight partners with the hub in Europe (Latvia, Austria, United Kingdom, France), the next closest partners in Eastern Europe (Russia, Belarus), and the long distance partners (Australia and Canada). The partnership provides good synergy across the range of disciplines with a lasting effect.

The main tasks defined by the teams are to process calcium phosphates through metastable states to capture structural modifications for enhancing properties, and investigate high-temperature processing in a radio frequency inductively coupled plasma.

Refined Step has paved the way to a series of new pathways for producing calcium phosphates and also to new calcium phosphates to offer greater functionality. It has been found that an important aspect has been the use of transient phases - an amorphous calcium phosphate or oxyapatite. These metastable phases have been produced, their stability determined, chemical enrichment assessed and other crystalline calcium phosphates produced either with a higher purity or additional functionality from orientation of hydroxyl ion dipoles in the structure. This has been possible by understanding the range of structural order that it possible.

At one end representing disorder is the amorphous calcium phosphate, but the other end has hydroxyapatite with aligned hydroxyl ions. Amorphous calcium phosphate is the least dense, can accommodate changes in element size and provide the most freedom for the elements. Heat treatment provides an apatite, that has some point defects, most commonly hydroxyl ion defects. This is the most common product formed, when one refers to apatites. Disorder in apatite can be increased by introducing more defects in the structure; best done in the hydroxyl ion column for making oxyapatite. This phase is seldom produced. The more dense state is hydroxyapatite, with the full content of hydroxyl ions, that are all aligned, so that the dipole effect from hydroxyl ions provides a surface charge. Understanding the capabilities of the less dense phases, amorphous calcium phosphate and oxyapatite provides valuable pathways to produce the desired product with the least energy and the shortest time. So, peroxyapatite has been produced from an amorphous phase, tetracalcium calcium phosphate has been made starting with an amorphous phase and then passing through oxyapatite to produce tetracalcium phosphate, tricalcium phosphate has been made from an amorphous phase. The structural assembly and the bioactivity have been combined in a figure. The structural assembly is important for bioactivity of the final product, but the process pathway is important to offer the fastest production method with the least energy.

The goal of producing a more energy efficient processing route has been achieved, and paves the way for applications in the healthcare and medical industry. Refined Step has led to other projects that continue on the theme of metastable materials, and the momentum from the new network will lead to more developments.