Amorphous calcium phosphate (ACP) is a transient precursor phase in bone and teeth, transforming into crystalline carbonated hydroxyapatite. Some rare organisms like the Nemertean worm Amphiporus lactifloreus produce the biomineral ACP that is stable throughout the organism’s lifetime in its stylet [1]. The stylet is a 100-micron long nail-shaped “tooth” at the end of the proboscis and is used to capture and pierce prey. ACP, although transient under aqueous conditions, could be a valuable mineral phase for tissue-engineering of bone due to its similar elemental composition to carbonated apatite in bone.  We aim to study the worm's ACP stability mechanisms to develop methods to stabilize synthetic ACP. We study the 3D structure using x-ray nano computer tomography and focused ion beam electron microscopy (FIB/SEM), as well as the elemental composition and distribution of stylets extracted from A. lactifloreus.

The stylet consists of an inner core region, surrounded by an outer lamellar structured layer. In both regions, Ca and P are the main elements. Surprisingly, the concentrations of Ba and Sr are high with Ca/Ba 5:1 by atoms and Ca/Sr 10:1. The outermost layer, only a few tens of nanometre thick, exhibits elemental contrast of heavier elements in nano-CT and is enriched in S and Ba according to EDS. Raman detected SO$_4^{2-}$ . Nanoindentation results showed an elastic modulus of 10–28 GPa and a Vickers hardness of 30-160.

We carry out syntheses with different elemental compositions and monitor the ACP stability by FTIR. The addition of Ba at atomic ratio Ca/Ba = 9:10 is capable of stabilizing ACP and with Ca/Ba = 3:1 and 19:1 can delay crystallization. By substituting Ca in the ACP synthesis with different ratios of Ba and Sr, the crystalline transformation of ACP can be slowed down for weeks. This result shows that Ba can be used to modulate ACP stability. In the future this might lead to ACP that can be tailored to various industrial and medical applications in the field of bone repair and replacements.

[1] S. A. Stricker and S. Weiner, Experientia, 1985, vol. 41, no. 12, pp. 1557–1559