In situ remineralisation of surface softened human enamel studied with AFM nanoindentation

The early stage of enamel dissolution during an erosive process is characterised by a softening of the enamel surface. To date, research has been mainly concerned with the amount of surface softening caused by various demineralising solutions. Little attention has been paid to the investigation of whether and to what extent this early stage of surface softening is reversible by remineralisation. The primary purpose of this pilot study was to investigate the recovery process of surface softened enamel in the oral environment by observing the process of surface rehardening through investigation of the nanomechanical surface and near surface properties. Tapping mode AFM and AFM nanoindentation were used to study the morphology and nanomechanical properties of human enamel samples treated with various acidic demineralising solutions and subsequent prolonged exposure to human saliva. Surface softened enamel was not capable of rehardening under the chosen physiologically relevant conditions, within the limits of experimental error. The nanomechanical properties and the surface morphology of the softened enamel samples remained almost unchanged after the recovery phase in the oral environment. This was despite being exposed to human saliva which is considered to remineralise a structurally weakened enamel surface due to its high calcium and phosphate concentrations. These results are in agreement with the results of some previous research; however, further research employing natural (unpolished) enamel surfaces is necessary to understand fully the processes of demineralisation and remineralisation in the oral environment. (C) 2004 Elsevier B.V. All rights reserved.