Novel nanocomposite materials for artificial bone implants
The aim of this project is to develop a methodology to synthesise and process hydroxyapatite based biocompatible and biodegradable nanostructured composites for load-bearing biomedical applications.
The processing of nanostructured composites involves two steps. Firstly, a hybrid suspension containing polymers and nanosized hydroxyapatite is synthesised via an in situ polymer matrix mediated crystallisation. The method involves introducing a desired quantity of polymer solution to the reaction mixture containing amorphous calcium phosphate (ACP) nanoparticles (produced by a slow addition of orthophosphoric acid to calcium hydroxide suspension at 98°C) such that the crystallisation of hydroxyapatite (HA) takes place in presence of the polymer molecules. Secondly, the hybrid suspensions were concentrated by heating at 80°C to evaporate water so that composite films with adequate thickness could be produced by solvent casting methods. We used chitosan (a deactylated derivative of chitin, which is found in shells of crustaceans) and sodium alginate (alginic acid is extracted from seaweed and is commercially available as water soluble sodium salt, sodium alginate) to mediate the crystallisation of HA. Both the polymers are biocompatible and biodegradable.
In the case of chitosan, which is dissolved in aqueous acetic acid solution, its solution is treated with formaldehyde to enhance binding between the particles and chitosan. The hybrid suspensions prepared without formaldehyde treatment failed to yield a stable suspension (sedimentation of particles occurred) when concentrated. It is believed that the formaldehyde treatment promotes chemical binding between phosphate groups of HA and primary alcohol groups of chitosan, while protecting the amine groups from protonation. The structure and tensile properties of composite films are given below. Composite films containing up to 70 wt.% nHA have been produced. This is the first time such high inorganic loadings have been achieved within chitosan/ceramic nano-composites.
The excellent tensile properties displayed by the composite films in the dry conditions are completely lost in wet conditions due to swelling and/or dissolution of polymers. Therefore, currently our focus is on improving the mechanical properties under wet conditions, particularly, of chitosan-nHA composites.
Usually, cross-linking the polymer chains reduces swelling and therefore chitosan composite films were cross-linked using glutaraldehyde (CHO-(CH2)3-CHO). This dialdehyde reacts with amine groups in chitosan to form imines, which are then reduced using sodium formate.
However, the process induced an enormous amount of residual stress in the films and they broke under their own weight (without applying any load). Therefore, the cross-linking method was abandoned. Attempts were made to methylate the amine group using a mixture of formaldehyde, formic acid and sodium hydroxide solutions.
This process is effective in reducing the hydrophilicity of chitosan composites and resulted in an E value of 434 ± 7 MPa and a UTS values of 13 ± 2 MPa. Although the properties are low, this is still considered an improvement in properties because without the methylation the films are so weak that we are not able to test them.
By following this method, we believe the hydrophilicity of chitosan composites can further be reduced by increasing the carbon chain length at the amine group and the work is underway. We are also exploring the possibility of using the concentrated suspension to coat on bio-metals, such as titanium and its alloys and steel. Such composite coatings potentially render the metallic implants bioactive.
