**Abstract**

Hydroxyapatite represents the natural inorganic component of the bone and may be considered an essential element required for the development of bone substitutes in the field of regenerative medicine. Hydroxyapatite bone substitutes own biomimetic, osteoconductive, and osteoinductive properties thanks to their chemical-physical properties and nanostructure that play a critical role for the reconstruction of calcified tissues. Controlling the structure of hydroxyapatite nanocrystals is vital for obtaining a sustained product, and it should be an advantage on the final materials suitable for bone replacement, in terms of adsorptive activity, drug delivery system, etc. Compared to other synthesis techniques, hydrothermal processing (refers to a synthesis in aqueous solution at elevated pressure and temperature, in a closed system) has the ability to precipitate the hydroxyapatite from overheated solution, regulating the rate and uniformity of nucleation, growth, and maturation, which affect size, morphology, and aggregation of the crystals. This chapter wants to provide an overview of realization of nanosized hydroxyapatite-based bioceramics (e.g., powder and 3D structures) with desired morphology of crystallites, by hydrothermal processing. In this way, some critical hydrothermal parameters fundamental on the control of the crystal shape and dimension (pH, temperature, starting precursors, etc.) are discussed.

**Keywords:** hydrothermal synthesis, nanostructured hydroxyapatite, crystal growth, morphology control, regenerative medicine

## **1. Introduction**

Calcium phosphates (CaP) are the main mineral constituent of human bones and teeth. For this reason, synthetically CaP-based materials nowadays are the most ubiquitous family of biomaterials for their use in biological applications and tissue engineering. These attractive biomedical materials possess excellent biocompatibility, osteoconductive properties, nontoxicity, and chemical similarity to the inorganic component of the natural bone [1].

The realization of CaP biomaterials reproducing the calcified tissue (dense and porous block, granules, and powders) is clinically needed as an alternative to autologous- and heterologous-derived scaffolds [2]. The majority of CaP biomaterials shall be applicable for bone reconstruction and replacement in tissue engineering when the bone has no self-regenerative capacity following severe illness or trauma, as well as other applications, like drug delivery agents, prosthetic coatings, and gene carriers [3, 4].

Among the CaP family compounds, hydroxyapatite Ca5(PO4)3OH (HA) is the most extensively used in medicine for implant fabrication as an alternative to the human bone; it is the thermodynamically most stable phase in physiological conditions and owns the most similarity in mineralogical phase and chemical composition to the mineral part of the bone tissue [5]. The in vivo formation of this mineral occurs through the biomineralization process, where nanometer-sized crystals of HA are precipitated on collagen fibrils into mineralized self-assembled hierarchical and calcified structure. The thickness of the HA crystals ranges from about 5 to 20 nm, while the length from 15 to 200 nm [6].

Biological HA is a nonstoichiometric, carbonated, and calcium-deficient form of apatite (Ca/P atomic ratio lower than 1.67), containing various amounts of positively charged ions (e.g., Mg2+, Na+ and K+ ) and negatively charged ions (e.g., CO3 <sup>2</sup><sup>−</sup>, Cl<sup>−</sup> and F<sup>−</sup>), in substitution of Ca2+ or PO4 <sup>3</sup><sup>−</sup>/OH<sup>−</sup> ions, respectively [7, 8].

Present-day researches concern new route or improving preexisting methods to accurately engineer HA-based materials with characteristics closer to the living bone, aiming more effective applications in the field of biomaterials. The in vivo and in vitro performance of HA biomaterials remarkably depends on the development of their properties during the manufacturing process, such as microscopic characteristic (e.g., grain size topography, particles size distribution, nanostructure), morphology (e.g., porosity, pore size, 3-D architecture), chemical compositions, crystallographic structure, etc.

It is well known that biological and mechanical properties of biomaterials are strongly affected by its nanostructural characteristics. Compared to conventional ceramic formulations, the nanophase of HA materials can significantly affect their mechanical strength and the solubility that has a substantial effect on resorption and bioactivity. Furthermore, nanostructures can enhance osteoblast adhesion and affect the surface wettability for the selective control of protein interactions [9]. The mechanical properties and microstructures of the resulting HA ceramics are mostly influenced by the microstructure of the produced powder, including crystallinity, agglomeration, stoichiometry, and substitutions and the processing conditions [10].

The control over HA crystallization through a precise control of crystal nucleation and growth is a major challenge in the synthesis of crystalline particles, with defined geometry, morphologies, orientations, sizes, and composition. These intrinsic features are closely related to their properties and may affect their applications.

 Many methods have been proposed in the literature to prepare nanostructured HA materials (e.g., nanoparticles or 3-D scaffolds with various shapes and sizes), such as coprecipitation, sol-gel synthesis, mechanical milling, hydrothermal reaction, etc. [11]. Between them, hydrothermal synthesis method allows more choice of variable factors that affect the morphology of the final material, such as nature of precursors and their concentration, saturation, temperature, pH, process time, and the presence of potential agents used for controlling the final morphological structures [12, 13].

This chapter focuses on the synthetic hydrothermal strategy employed in the preparation and design of different HA nanoparticles and nanostructured materials and reviews about the roles of important parameters on the HA nanostructured realization.
