HUMAN DENTIN AS A HYDROGEL

The pores (tubules) of human dentine in 0.02-cm planoparallel sections of newly extracted permanent teeth were investigated. By the conventi onal scanning electron microscopy these pores appear empty, but by the newly developed scanning-probe microscopy the presence of a complex m atrix could be established. By measuring the transport of neutral myog lobin by diffusion alone and diffusion + bulk Row, the area of dentine occupied by the matrix was calculated to be 1.9 +/- 0.9% and 2.3 +/- 0.5%, respectively. The hydraulic conductivity was surprisingly small, 1.35 +/- 0.55 x 10(-7) ml/(s . cm(2) dentine) at a pressure differenc e of 0.1 kPa across a 1-cm thick section. This suggests a hydrogel wit h a relatively dense network, the width of meshes estimated at 2 x 30 nm. In line with this concept, enzymatic degradation of the organic ma tter increased the hydraulic conductivity 3000 times. By studying the transport of negatively charged myoglobin, the matrix was calculated t o carry 18 mEq/l of positive charges. Due to the consequent attraction of small, negative ions and thence of water, the pressure within the matrix would be about 1.33 kPa, a force which will act to immobilize t he water in the channels. The concept of a hydrogel in the dentine tub ules was also supported by the finding that shielding the charges with bathing media of high ionic strength reduced the hydraulic conductivi ty.