Biomimetic Growth and Morphology Control of Calcium Oxalates
Annu Thomas, IMPRS for Dynamical Processes in Atoms, Molecules and Solids
Calcium oxalates (COM: calcium oxalate monohydrate; COD: -dihydrate; COT: -trihydrate) play a decisive role as pathogenic biomaterials in the form of kidney- or urinary-stones [1]. As a first step to unravel the complex pathology of uro/nephro lithiasis, we started to investigate the structure and morphology of calcium oxalates crystallized in the presence of organic additives. The present work is devoted to the morphological control of calcium oxalates in the presence of the sodium salt of polyacrylic acid (PAA) [2] as well as in agar gel.
It has been demonstrated that PAA not only inhibits the growth of COM, but also controls the morphology of COD [2]. By increasing the concentration of PAA, the shape of COD has been found to change from tetragonal bi-pyramids with dominant (101) pyramidal faces to tetragonal prisms with dominant (100) prism faces and finally to dumbbells. At still higher PAA concentrations, the morphology is reverted back to rod-like tetragonal prisms. The inner architecture of all the morphological varieties of COD was found to be dominated by an inner core consisting of thin elongated COD crystallites together with incorporated PAA and an outer shell formed as a consequence of secondary nucleation processes. As an example, figure 1 demonstrates the core-shell architecture of COD dumbbells consisting of approximately 3.5 wt.-% PAA (for further details refer to the caption of figure 1).
Figure 1. SEM images of a COD dumbbell (a) grown in the presence of 0.8 mM calcium oxalate and 96 µg/mL PAA with tetragonal cross-section of the central seed (b). (c) SEM image of a broken half dumbbell indicating the core made up of smaller crystallites and the shell constructed of tetragonal prismatic crystals. (d,e) TEM images of a [100] cross-section of a dumbbell after staining with uranyle acetate indicating the core front enriched with organic material. (f) SEM image of dumbbells after decalcification of the core with 0.25N EDTA and retaining the shell.
Apart from the experimental observations, the results of our investigations with the aid of atomistic simulations on the interaction of PAA with (100) and (101) faces of COD do confirm that during the development of the aggregates, strong interactions of PAA with the (100) faces take over control of the shape development.
It is widely accepted that in-vitro experimental models for the growth of calcium oxalates can give valuable information on the growth and aggregation of urinary stones. Therefore, the “double diffusion technique” [3,4] in organic gel matrices (agar, gelatine and carrageenan) has been used for the biomimetic growth of calcium oxalate (COM) stones. A great variety of morphological forms of COM are produced in agar gel matrices ranging from platy crystallites to dumbbells and spherulites (Fig. 2). The COM dumbbells and spherulites are assumed to be formed by the aggregation of smaller crystallites as a consequence of increased supersaturation inside the gel [5].
Figure 2. SEM images of COM aggregates grown in 2 wt.-% agar gel of pH 8.5 (0.033 M CaCl2·2H2O and 0.1 M Na2C2O4). Increase in supersaturation inside the gel matrix causes the aggregation of the crystallites resulting in morphological variations ranging from nearly rectangular platy crystallites (a) which then are piled up one over the other resulting in dumbbells (b). Aggregation of smaller crystallites at increased supersaturation results in COM spherulites (c,d).
Moreover, an increase of the pH value of the agar gel has been found to suppress the growth of COM and favours the growth of COD. The morphology of COD crystals grown in 2 wt.-% agar gel of pH 11.5 includes tetragonal prisms and dumbbells. All the other organic gels (gelatine and carrageenan) produced COM at all pH values whereas in agar it was possible to tune the hydration state of calcium oxalate with variation of the pH value.
On the aspect of biomineralization, it is of interest to study the crystallization of calcium oxalates under various experimental parameters.The system calcium oxalate/ PAA/ H2O is a suitable model system for the investigation of principles of composite growth (shape development) in general. The double diffusion method in agar gel is a convenient route to grow calcium oxalate aggregates showing close resemblance to biogenic calculi and to study their ontogeny.
Literature
[1] E.L. Prien, Annu. Rev. Med. 1975, 26, 173.
[2] T. Jung, W.-S. Kim, C.-K. Choi, J. Cryst. Growth. 2005, 279, 154.
[3] K.-Th.Wilke, J. Bohm, Kristallzüchtung, Verlag Harry Deutsch, Thun, Frankfurt/Main, 1988.
[4] R. Kniep, P. Simon, Topics in Current Chemistry. 2007, 270, 73.
[5] A. Thomas, Ph.D. Dissertation, in preparation.