GIC's are composed of glass powder and a polycarboxylic acid. When mixed wet, protons from the polymeric acid exchange with calcium and aluminium cations in the surface of the glass particles. The cations then form electrostatic bonds with carboxylate groups in the polymer chains, effectively cross-linking them to form a gel that sets the cement.

Above: A photomicrograph of a GIC, showing rough glass particles with an outer hydrolysed layer of silica gel. An area of the matrix has been magnified diagramatically to show its composition. This consists of poly-acrylic acid molecules, cross-linked with the calcium and aluminium ions liberated from the glass surface, to form a hydrogel.

Originally developed for dental repair, glass ionomer cements have shown excellent biocompatibility within the oral environment. They have reasonable strength and toughness, being effectively glass reinforced plastics. Their ease of preparation means that they are also ideal for use in general surgery. Since the 1990s considerable success has been achieved in cochlea implant surgery, where artificial ossicles can be attached to living bone using these cements, restoring hearing to patients.

At around the same time in Sheffield, however, model studies were indicating that there were potential problems with the material. Ions released from the glass during the setting reaction were being found in surrounding bone tissue, and were preventing this from mineralising. Unfortunately, if abused during surgery, unusually high levels of ion release can also cause neurotoxic effects. A number of operations repairing skull damage, and using larger volumes of the glass ceramic proved fatal to the patients involved.

Work at Sheffield has therefore focussed on developing safer glasses and ceramics specifically for surgical use. Considerable success has been achieved with a new material in which all of the aluminium has been replaced with iron. Iron is an essential element in the body, and mechanisms are available for sequestering excessive amounts and storing them safely. Ions leached during the setting reaction are thus far less likely to cause an adverse reaction. The new material is black, so it does offer good contrast with surrounding tissues, assisting the surgeon when it is being employed.

Above right: An electron micrograph of a biocompatible GIC developed at Sheffield showing dendritic magnatite inclusions, which appear dark against the glass matrix. The scale bar shown is 1 µm long.

Researchers engaged in this work are Prof. Ian M Brook, Dr Kathryn Hurrell-Gillingham, Prof. Paul Hatton, Naruporn Kanchanarat and Anthony Turner.

Acknowledgements: This work was supported by the UK Medical Devices Faraday Partnership (EPSRC grant GR/S78254/01) and Corinthian Surgical.