RESEARCH Morphology, characterization, and conversion of the corals Goniopora spp. and Porites cylindrica to hydroxyapatite S. Akyol 1,2 & B. Ben Nissan 2 & I. Karacan 2 & M. Yetmez 3 & H. Gokce 4 & D. J. Suggett 5 & F. N. Oktar 6,7 Received: 14 December 2018 /Accepted: 17 December 2018 # Australian Ceramic Society 2019 Abstract The aim of this study is to obtain pure natural hydroxyapatite (HAp) and tricalcium phosphate (TCP) from a Goniopora spp. and from hump coral (Porites cylindrica), both sourced from Australia. Due to the nature of the conversion process, commercial coralline HAp has retained coral or CaCO 3 , and the structure possesses both nano- and mesopores within the interpore trabeculae resulting in high dissolution rates. To overcome these limitations, a newly patented coral double-conversion technique has been developed. The current technique involves a two-stage application route where in the first-stage complete conversion of coral to pure HAp is achieved. In the second stage, a sol-gel-derived HAp nanocoating is directly applied to cover the meso- and nanopores within the intrapore material, while maintaining the large pores. Here, we specifically investigated the morphological changes and characterized these corals prior to and after conversion. For this purpose, four groups designated as C 0 ,C 1 ,C 2 , and C 3 were used. C 0 is Porites, Goniopora, and cylindrica; the original coral is calcium carbonate with aragonite structure that contains proteins and polysaccharides. C 1 is coral cleaned under ultrasound in bleach diluted with water. C 2 is coral converted to hydroxyapatite (HAp) by hydrothermal treatment method at 200 °C under pressure in the presence of ammonium biphosphate. C 3 is obtained by coating C 2 with sol-gel alkoxide-derived nanohydroxyapatite to obtain a more bioactive osteoconductive material and improve mechanical properties. All groups were characterized by XRD, EDAX, DTA/TGA, and SEM. The results showed that the biaxial strengths of the C 2 and C 3 were significantly higher than the original coral. The work also showed the advantages of the hydrothermal conversion method and the effect of the nanocoating which is expected to improve the final bioactivity through microstructural changes of the surfaces. Keywords Corals . Aragonite . Natural bioceramics . Ultrasonication . Hydrothermal treatment Introduction Synthetically derived materials have been used as bone grafts in an attempt to overcome the limitations of bone grafts such as autograft and allografts. During the last three decades, ma- rine sources have been introduced gradually for dental, max- illofacial, and orthopedic applications [1]. Other natural ma- terials such as ostrich eggs, chicken egg shells, and land and sea snails which have similar calcium carbonate structure have been also successfully converted [2–9]. Natural coral has shown considerable success due to its porous structure (which ranges from 100 to 500 μm) being similar to that of cancellous bone in morphology and is one of a limited number of mate- rials that will form chemical bonds with bone and soft tissues in vivo. Studies indicate that a favorable pore size and micro- structural composition are important factors facilitating in- growth of fibrovascular tissue or bone [10]. However, uncon- verted coral (calcium carbonate) is unsuitable for load-bearing * S. Akyol sibelakyol@gmail.com 1 Physiology Department, Cerrahpasa Medical Faculty, Istanbul University, Istanbul, Turkey 2 School of Life Sciences, University of Technology, Broadway, Sydney, NSW, Australia 3 Mechanical Engineering Department, Engineering Faculty, Zonguldak Bulent Ecevit University, Zonguldak, Turkey 4 Adnan Tekin Center, Istanbul Technical University, Istanbul, Turkey 5 Climate Change Cluster (C3), University of Technology, Broadway NSW, Sydney 2007, Australia 6 Bioengineering Department, Engineering Faculty, Marmara University, Istanbul, Turkey 7 Nanotechnology and Biomaterial Research and Implementation Centre, Marmara University, Istanbul, Turkey Journal of the Australian Ceramic Society https://doi.org/10.1007/s41779-018-00304-4