Wojciech L. Suchanek

Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants

This paper reviews the past, present, and future of the hydroxyapatite (HAp)-based biomaterials from the point of view of preparation of hard tissue replacement implants. Properties of the hard tissues are also described. The mechanical reliability of the pure HAp ceramics is low, therefore it cannot be used as artificial teeth or bones. For these reasons, various HAp-based composites have been fabricated, but only the HAp-coated titanium alloys have found wide application. Among the others, the microstructurally controlled HAp ceramics such as fibers/whiskers-reinforced HAp, fibrous HAp-reinforced polymers, or biomimetically fabricated HAp/collagen composites seem to be the most suitable ceramic materials for the future hard tissue replacement implants.

Mechanochemical-hydrothermal synthesis of carbonated apatite powders at room temperature

Crystalline carbonate- and sodium-and-carbonate-substituted hydroxyapatite (CO3HAp and NaCO3HAp) powders were prepared at room temperature via a heterogeneous reaction between Ca(OH)2/CaCO3/Na2CO3 and (NH4)2HPO4 aqueous solution using the mechanochemical hydrothermal route. X-ray diffraction, infrared spectroscopy, thermogravimetry, and chemical analysis were performed. Room temperature products were phase-pure CO3HAp and NaCO3HAp containing 0.8-12 wt% of carbonate ions in the lattice. Dynamic light scattering revealed that the median agglomerate size of the room temperature CO3HAp and NaCO3HAp powders was in the range of 0.35-1.6 microm with a specific surface area between 82 and 121 m2/g. Scanning and transmission electron microscopy confirmed that the carbonated HAp powders consisted of mostly submicron aggregates of nanosized, approximately 20 nm crystals. The synthesized carbonated apatite powders exhibit chemical compositions and crystallinities similar to those of mineral constituents of hard tissues and therefore are promising for fabrication of bone-resembling implants.

Hydroxyapatite ceramics with selected sintering additives

Several sintering additives for hydroxyapatite (HA) have been tested in order to enhance its sinterability without decomposing the HA and/or decreasing bioactivity and biocompatibility, additionally providing a weak interface for HA ceramics. The ion species of sintering additives were selected from those in the mineral constituents of hard tissues and bioactive glasses. After investigation of phase diagrams in the CaO-P2O5-additive systems, and analysis of physiochemical properties of the additives, several sintering aids for HA have been chosen. Subsequently, densification, phase composition, grain growth and fracture behaviour of HA containing 5 wt% of each additive, sintered at 1000-1100 degrees C, have been studied. H3BO3, CaCl2, KCl, KH2PO4, (KPO3)n and Na2Si2O5 did not enhance densification of HA. K2CO2, Na2CO3, KF and sodium phosphates improved the densification significantly. Expect for KCl and some sodium phosphates, all the additives caused formation of large quantities of undesired beta-tricalcium phosphate or CaO; therefore, they are not appropriate for HA. In the case of sodium phosphate additives, it was possible to avoid formation of CaO or beta-tricalcium phosphate by control of the additive quantity and chemical composition. beta-NaCaPO4 has been found to be an effective sintering agent which causes neither decomposition of HA nor formation of other undesired phases.

Solution synthesis of hydroxyapatite designer particulates

Abstract This paper reviews our research program for intelligent synthesis of hydroxyapatite (HAp) designer particulates by low-temperature hydrothermal and mechanochemical–hydrothermal methods. Our common starting point for hydrothermal crystallization is the generation and validation of equilibrium diagrams to derive the relationship between initial reaction conditions and desired phase assemblage(s). Experimental conditions were planned based on calculated phase boundaries in the system CaO–P2O5–NH4NO3–H2O at 25–200 °C. HAp powders were then hydrothermally synthesized in stirred autoclaves at 50–200 °C and by the mechanochemical–hydrothermal method in a multi-ring media mill at room temperature. The synthesized powders were characterized using X-ray diffraction, infrared spectroscopy, thermogravimetry, chemical analysis and electron microscopy. Hydrothermally synthesized HAp particle morphologies and sizes were controlled through thermodynamic and non-thermodynamic processing variables, e.g. synthesis temperature, additives and stirring speed. Hydrothermal synthesis yielded well-crystallized needle-like HAp powders (size range 20–300 nm) with minimal levels of aggregation. Conversely, room-temperature mechanochemical–hydrothermal synthesis resulted in agglomerated, nanosized (∼20 nm), mostly equiaxed particles regardless of whether the HAp was stoichiometric, carbonate-substituted, or contained both sodium and carbonate. The thermodynamic model appears to be applicable for both stoichiometric and nonstoichiometric compositions. The mechanochemical–hydrothermal technique was particularly well suited for controlling carbonate substitution in HAp powders in the range of 0.8–12 wt.%. The use of organic surfactants, pH or nonaqueous solvents facilitated the preparation of stable colloidal dispersions of these mechanochemical–hydrothermal-derived HAp nanopowders.

Hydrothermal crystallization of ceramics

Abstract In broad terms, hydrothermal synthesis is a technology for crystallizing materials (chemical compounds) directly from aqueous solution by adept control of thermodynamic variables (temperature, pressure and composition). The objective of this chapter is to introduce the field of hydrothermal materials synthesis and slow how understanding solution thermodynamics of the aqueous medium can be used for engineering hydrothermal crystallization processes. In the first section, we will focus on hydrothermal synthesis as a materials synthesis technology by providing history, process definitions, technological merits and comments on its current implementation in industry. In the second section, we will describe how thermodynamic modeling is being developed as an engineering tool to predict equilibrium phase assemblages and use this predictive power as an engineering tool for development of hydrothermal technology for materials synthesis.

Processing and mechanical properties of hydroxyapatite reinforced with hydroxyapatite whiskers

Hydrothermally synthesized HAp fine crystals/HAp whiskers mixtures have been used for the preparation of HAp/0-30% (whiskers) composites. The composites have been fabricated by pressureless sintering and hot-pressing. The best mechanical properties and the highest densities have been achieved for composites hot pressed at 1000 degrees C (2 h, 30 MPa in flowing Ar). Their density was in the range of 90-97% of the theoretical density. Fracture toughness (Klc) of the composites reflected their microstructure and had the value of 1.4 MPa m1/2 (as compared with Klc = 1.0 MPa m1/2 for the non-reinforced HAp matrix). Compressive prestressing of the HAp matrix and crack deflection (both derived from the residual stress field) contributed to the increase of fracture toughness. Other toughening mechanisms have not been observed. HAp/HAp (whiskers) composites exhibited improved toughness without degradation of biocompatibility, because the HAp whiskers acted both as a reinforcement and as a biocompatible phase. Problems related to biocompatibility and mechanical properties of available HAp-based composites were also discussed.

Hydrothermal Synthesis of Advanced Ceramic Powders

This paper briefly reviews hydrothermal synthesis of ceramic powders and shows how understanding the underlying physico-chemical processes occurring in the aqueous solution can be used for engineering hydrothermal crystallization processes. Our overview covers the current status of hydrothermal technology for inorganic powders with respect to types of materials prepared, ability to control the process, and use in commercial manufacturing. General discussion is supported with specific examples derived from our own research (hydroxyapatite, PZT, -Al2O3, ZnO, carbon nanotubes). Hydrothermal crystallization processes afford excellent control of morphology (e.g., spherical, cubic, fibrous, and plate-like) size (from a couple of nanometers to tens of microns), and degree of agglomeration. These characteristics can be controlled in wide ranges using thermodynamic variables, such as reaction temperature, types and concentrations of the reactants, in addition to non-thermodynamic (kinetic) variables, such as stirring speed. Moreover, the chemical composition of the powders can be easily controlled from the perspective of stoichiometry and formation of solid solutions. Finally, hydrothermal technology affords the ability to achieve cost effective scale-up and commercial production.

In situ fabrication of morphology-controlled advanced ceramic materials by Soft Solution Processing

Abstract The basic features of Soft Solution Processing are discussed. Pioneering research carried out in our laboratory on in situ fabrication of morphology-controlled advanced ceramic materials by Soft Solution Processing (synthesis of double-oxide ABO 3 , ABO 4 films by hydrothermal, electrochemical, hydrothermal–electrochemical methods, coating of CaTiO 3 on TiAl, preparation of carbon films on SiC, hydroxyapatite whiskers and fine powders under hydrothermal conditions) is reviewed. Soft Solution Processing allows us to fabricate in aqueous solutions shaped/sized/oriented ceramics in only one step, without excess energies for firing/sintering or melting, and without expensive equipment, providing an environmentally friendly route for the preparation of advanced ceramic materials.

Biocompatible whiskers with controlled morphology and stoichiometry

Hydroxyapatite whiskers have been prepared by the hydrothermal method. The crystals had diameter, length, and aspect ratio in the range of 1–10 μm, 30–50 μm, and 5–20, respectively. Their Ca/P molar ratio varied from 1.59 to 1.62. The morphology of the crystals can easily be controlled by the concentrations of species in the starting solution, while the Ca/P ratio is almost independent of them. Through the reaction with calcite powder at 600 °C, the Ca/P ratio of the whiskers has been improved even to the stoichiometric value of 1.67. Taking into account morphology and chemical composition of the HAp whiskers, they should not be health hazardous and may find applications as substitutes for asbestos and other fibrous materials which presently have restricted use because of their carcinogenic natures.

Hydrothermal synthesis of monetite and hydroxyapatite from monocalcium phosphate monohydrate

Abstract Monetite and hydroxyapatite have been synthesized by hydrothermal treatment of monocalcium phosphate monohydrate (MCPM) suspension at 160°C and 200°C. The monetite formed whiskers typically having length, diameter and aspect ratio in the range of 26–40 μm, 1–2 μm and 20–26, respectively and the hydroxyapatite shaped needle like crystals typically being in the range of 0.2–0.5 μm in length and 0.02–0.04 μm in diameter. The morphology of the crystals seems to be controlled mainly by the solubility of the reactant species. The phase of the hydrothermal product is determined by the combination of the pH and the Ca/P ratio of the aqueous solution. The whiskers obtained can be used as a bioactive reinforcement for composites.