Dimitra G. Kanellopoulou

Solubility of salts in water: Key issue for crystal growth and dissolution processes

The formation of sparingly soluble salts from aqueous solutions and their dissolution has attracted broad research interest. Of particular importance is the formation and transformation of minerals exhibiting polymorphism or encountered in more than one crystalline phase as, for example, in the case of calcium phosphates, formed in biological mineralization and in industrial-scale deposits. Understanding of these processes depends primarily on the equilibrium between the mineral phases considered and the aqueous medium in contact. Precipitation takes place in supersaturated solutions with rates depending on the solution supersaturation. The experimental investigation may reveal mechanistic details if done at sustained supersaturation. The kinetics of crystal growth depends either on surface diffusion or on bulk diffusion, which in turn is controlled by the medium fluid dynamics. In the case of magnesium ammonium phosphate (struvite), the presence of water-soluble organic compounds is responsible for the retardation both of the time needed for the onset of precipitation and for the kinetics of growth of the supercritical nuclei. Dissolution processes are controlled by the same mechanisms. In the case of calcitic marble, the dissolution in alkaline solutions is controlled by surface diffusion. Compounds active at the marble/water interface may in this case be used as protective agents.

Development of a new combined test setup for accelerated dynamic pH-controlled in vitro calcification of porcine heart valves.

Fifty years after their first implantation, bioprosthetic heart valves still suffer from tissue rupture and calcification. Since new bioprostheses exhibit a lower risk of calcification, fast and reliable in vitro methods need to be evaluated for testing the application of new anti-calcification techniques. This report describes a modification of the well-known in vitro dynamic calcification test method (Glasmacher et al, Leibniz University Hannover (LUH)), combined with the pH-controlled, constant solution supersaturation (CSS) method (University of Patras (UP)). The CSS method is based on monitoring the pH of the solution and the addition of calcium and phosphate ion solutions through the implementation of two syringe pumps. The pH and the activities of all ions in the solutions are thus kept constant, resulting in higher calcification rates compared to conventional in vitro methods in which solution supersaturation is allowed to decrease without any further control. To verify this hypothesis, five glutaraldehyde preserved porcine aortic valves were tested. Three of the valves were tested according to a free-drift methodology: the valves were immersed in a supersaturated calcification solution, with an initial total calcium times total phosphate product of (CaxP)=10.5 (mmol/L)2 renewed weekly. Two valves were tested by the new pH-controlled loop system, implementing the CSS methodology. All valves were tested for a 4-week period, loaded at 300 cycles per minute, resulting in a total of 12 million cycles at the end of the testing period. The degree of calcification was determined weekly by means of μx-ray, and by conventional, clinical and micro-computer tomography (CT, μCT). The results showed that the valves mineralizing at constant solution supersaturation in vitro yielded higher rates of calcification compared to the valves tested at conditions of decreasing solution supersaturation without any control, indicating the development of a new, accelerated, controllable in vitro calcification method.