Fluorapatite-gelatine nanocomposites serve as model system for mineralization steps of teeth and bone. The biomimetic composites show a hierarchical structural shape development starting from a hexagonal prismatic seed via dumbbell states and ending up with slightly notched spheres. This complex morphogenesis is caused by protein fibrils which are integrated into the nanocomposite superstructure. As evidenced by electron holography an intrinsic electric dipole field is generated by parallel alignment of nanocomposite subunits. The developing intrinsic electric field is then responsible for the formation and integration of the fibril pattern.

Teeth and bone are natural composites occurring in vertebrates and playing an essential role as endo skeleton or as grinding tool. They consist of hydroxyl apatite as inorganic component and collagen as bio-organic part. These natural systems are produced by cell activities in a complex biological environment in vivo. In order to mimic this biomineralization process in vitro, we reduced the level of complexity by excluding cells participation and replacing the highly cross-linked and therefore inactive biopolymer collagen by gelatine. Gelatine as denatured collagen exhibits a lot of free functional side groups. The most important feature is the presence of the monomeric unit of collagen the so-called triple-helix, which shows a well defined geometric and chemical unit with a typical length of about 280 nm and a width of about 1.4 nm consisting of three single α-chains. Main components are the amino acids such as glycine, proline and hydroxyproline. Further on, we replace hydroxyapatite by fluorapatite (FAP) (fig. 1, left) because of different reasons: fluorapatite forms a more simple crystal habit with strong resemblance to human dental enamel and it does not give rise to intrinsic piezoelectricity as is discussed for hydroxyl apatite. For the in vitro synthesis we create a 10 weight-% gelatine gel plug. This relatively high concentration of the protein may give rise to the formation of triple helical molecular arrangements and their subsequent aggregation to fibril bundles which serve as cross linking areas within the gel (fig.

1, right). This gel plug in a glass tube takes a centered position and from both sides inorganic salt solutions (CaCl₂ and Na₂HPO₄, NaF) are added. The set-up is known as the double diffusion technique and it assures a slow and diffusion controlled formation of the composite aggregates. ­After 3 weeks reaction time fluorapatite-gelatine composites are generated with a protein content of about 2.3 weight percent thus bearing strong resemblance to the chemical composition of human dental enamel [1]. After a cleaning process by washing with water a variety of morphologies of composite aggregates are found in the micron sized products.

The different morphologies correspond to different growth states of the composites starting with a hexagonal prismatic seed followed by growing dumbbell aggregates and finally resulting in slightly notched spheres (fig. 2). As the aggregates start develop at different times and different places all kinds of growth states are formed when growth procedure is stopped.

The formation of dumbbells is a widely occurring phenomenon in biology observed e.g. as precipitation product of bacteria, kidney stones or as biomimetic product of precipitation of inorganic compounds in the presence of organic molecules [2] ganic mineral phases also happens in the absence of organics explained by defect formation and supersaturation effects leading to nucleation of new crystals on the surface of the initial crystal [3]. This classical interpretation also holds for fluorapatite aggregates grown in a gelatine matrix where Calcium impregnation leads to rigid linear shaped dumbbells [4] and also for fluorapatite dumbbells grown in the absence of organics [5]. However, in this article we describe the case of phosphate impregnation of gelatine where highly symmetric and regular dumbbells are generated. This is a second kind of mechanism of dumbbell formation which is directed by intrinsic electric dipole fields.

The Initial State: Hexagonal Prismatic Seeds

The initial state is represented by a perfect hexagonal prismatic seed with a length of 3-7 μm and a width of 1-2 μm. As shown by synchroton X-ray analysis and electron diffraction the seeds are single crystalline concerning the inorganic component described as Calcium deficient fluorapatite with about 2.3 weight percent built-in gelatine [6]. We could show by transmission electron microscopy (TEM) on focussed ion beam (FIB) cuts that the inner structure of the young seeds already bears the potential for proceeding branching and to form out-growth areas at both ends [7].