Lengthwise and oriented growth of octacalcium phosphate crystal in polyacrylamide gel in a model system of tooth enamel apatite formation

Abstract Crystal growth of octacalcium phosphate (OCP) in polyacrylamide gels was studied at 37°C and at pH 6.5–7.4 as part of a study of the growth mechanism of tooth enamel apatite crystals. Reaction was carried out in a model system of enamel formation, where calcium solution (10, 30, 100 mM) and phosphate solution (3.5, 5, 7 mM) were separated by a cation selective membrane with polyacrylamide gel on the PO 4 side. OCP crystallized in the gel under all the conditions studied. Crystal size decreased with an increase in gel concentration. OCP crystals grown in 5–20% gels were narrow and small ribbon-like ones with a large length to width ratio (aspect ratio). In 30% gel, growth of OCP in the c-axis direction was suppressed much, resulting in very small rectangular crystals. When the Ca-influx was increased by using 100 mM Ca solution, OCP became about two times longer than that grown under a small Ca influx. Lengthwise growth and orientation of OCP crystals were greatly enhanced, when the thickness of the gel layer was increased. Thus, OCP crystal changed in narrow and long ribbon-like morphology, when OCP was grown under influence of the gel and ionic influx.