Scanning Microradiographical Technique for Measuring Diffusion Coefficients in Carious Enamel

Objectives: Diffusion of acids and dissolution products within enamel is important in understanding progression of dental caries. Although mathematical theories of diffusion are well-developed, diffusion coefficients (DC) of species in aqueous solutions, particularly within permeable solids, are difficult to measure. Few values of DCs within enamel are published. We have developed a new X-ray absorptiometry method based on scanning microradiography (SMR). As part of this development, effective DCs in different permeable media were measured. The final objective is to measure DCs of solutions within enamel and dentine.

Methods: SMR is a point-by-point X-ray absorption technique. Samples are stepped past a 15 mm diameter X-ray beam under computer control, and the transmitted intensity is recorded. In this study, SMR was used to obtain time-series of concentration profiles as 1.0 mol L^-1 KI diffused into permeable media. All experiments were carried out at ambient temperature. Effective DCs were obtained by curve fitting of data plotted as concentration vs. distance.time^-1/2 to the solution of Fick's Second law.

Results: Concentration-distance profiles were obtained for KI diffusing into: alginate (Palgat Plus, ESPE Dental, Germany), agarose (4%, Sigma Chemical Co., USA), porous polyethylene (Small Parts Inc, USA), and compressed hydroxyapatite (HAP) powder (Plasma Biotal Ltd, UK). Alginate: KI reacted with alginate after first scan. Agarose: profiles obtained at 30 and 60 minute intervals. Porous polyethylene: profiles obtained at 30 minute intervals; HAP: profiles were obtained at 15 minute intervals. The measured effective DCs of KI were: alginate, 2.0-7.0x10^-5cm²s^-1; agarose gel, 5.0-9.0x10^-5 cm²s^-1; polyethylene, 1.0-5.0x10^-5cm²s^-1; HAP, 1.0-8.0x10^-5cm²s^-1 however, lower transmitted count-rate increased errors. The published value for the diffusion coefficient of KI in water is 2.0x10^-5 cm²s^-1.

Conclusion: SMR can be used to measure diffusion coefficients in permeable solids including enamel, dentine and porous HAPs.

Acknowledgments: MRC Programme No. G9824467 and QMUL Studentship