Biomineral Reactivity: The Kinetics of the Replacement Reaction of Biological Aragonite to Apatite

Martina Greiner,Andreas Ziegler,Sabino Veintemillas-Verdaguer,Wolfgang Schmahl,Moritz Zenkert,Lurdes Férnandez-Díaz,Erika Griesshaber,Xiaofei Yin

doi:10.3390/min8080315

Abstract

We present results of bioaragonite to apatite conversion in bivalve, coral and cuttlebone skeletons, biological hard materials distinguished by specific microstructures, skeletal densities, original porosities and biopolymer contents. The most profound conversion occurs in the cuttlebone of the cephalopod Sepia officinalis, the least effect is observed for the nacreous shell portion of the bivalve Hyriopsis cumingii. The shell of the bivalve Arctica islandica consists of cross-lamellar aragonite, is dense at its innermost and porous at the seaward pointing shell layers. Increased porosity facilitates infiltration of the reaction fluid and renders large surface areas for the dissolution of aragonite and conversion to apatite. Skeletal microstructures of the coral Porites sp. and prismatic H. cumingii allow considerable conversion to apatite. Even though the surface area in Porites sp. is significantly larger in comparison to that of prismatic H. cumingii, the coral skeleton consists of clusters of dense, acicular aragonite. Conversion in the latter is sluggish at first as most apatite precipitates only onto its surface area. However, the process is accelerated when, in addition, fluids enter the hard tissue at centers of calcification. The prismatic shell portion of H. cumingii is readily transformed to apatite as we find here an increased porosity between prisms as well as within the membranes encasing the prisms. In conclusion, we observe distinct differences in bioaragonite to apatite conversion rates and kinetics depending on the feasibility of the reaction fluid to access aragonite crystallites. The latter is dependent on the content of biopolymers within the hard tissue, their feasibility to be decomposed, the extent of newly formed mineral surface area and the specific biogenic ultra- and microstructures.

Highlights

Research of the last decades has shown that carbonate biological hard tissues of marine and terrestrial organisms are highly valuable for their use in medical applications (e.g., [1,2,3,4,5,6])e.g., as bone implant and bone graft materials
XRD measurements confirm that all investigated hard tissues consist of aragonite and evidence measurements confirm thatinvolve all investigated hard tissuescalcium consist ofphosphate
For all biological hard tissues that we investigated in this study, the highest organic content is present in Sepia officinalis cuttlebone

Summary

Introduction

Research of the last decades has shown that carbonate biological hard tissues of marine and terrestrial organisms are highly valuable for their use in medical applications (e.g., [1,2,3,4,5,6])e.g., as bone implant and bone graft materials. Many carbonate biological hard tissues were tested for their applicability for medical tasks (e.g., sponges, corals, coralline algae, cuttlefish bone, echinoderms, marine and terrestrial bivalves (e.g., [5]) Two of these emerged as being highly valuable: bivalve shell nacreous and coral skeleton aragonite [5,7,8,9,10,11,12,13,14,15]. The partial conversion of coral aragonite into AP ensures that the obtained bone graft material retains its porous structure and remains biocompatible, but has improved biodegradation properties that suit better bone remodeling and turnover

Methods

Results

Discussion

Conclusion