title: Textured and hierarchically structured calcium phosphate ceramic blocks through hydrothermal treatment.
abstract: Synthetic calcium phosphate bone graft substitutes are widely recognized for their biocompatibility and resorption characteristics in the treatment of large bone defects. However, due to their inherent brittleness, applications in load-bearing situations always require reinforcement by additional metallic implants. Improved mechanical stability would eliminate the need for non-resorbable metallic implants. In this context a new approach to obtain calcium phosphate scaffolds with improved mechanical stability by texturing the material in specific crystal orientations was evaluated. Texture and reduction of crystal size was achieved by recrystallizing α-TCP blocks into calcium deficient hydroxyapatite (CDHA) under hydrothermal conditions. SEM and XRD analysis revealed the formation of fine CDHA needles (diameter ≈ 0.1-0.5 μm), aligned over several hundreds of micrometers. The obtained microstructures were remarkably similar to the microstructures of the prismatic layer of mollusk shells or enamel, also showing organization at 5 hierarchical structure levels. Brazilian disc tests were used to determine the diametral tensile strength, σdts, and the work-of-fracture, WOF, of the textured materials. Hydrothermal incubation significantly increased σdts and WOF of the ceramic blocks as compared to sintered blocks. These improvements were attributed to the fine and entangled crystal structure obtained after incubation, which reduces the size of strength-determining critical defects and also leads to tortuous crack propagation. Rupture surfaces revealed intergranular tortuous crack paths, which dissipate much more energy than transgranular cracks as observed in the sintered samples. Hence, the refined and textured microstructure achieved through the proposed processing route is an effective way to improve the strength and particularly the toughness of calcium phosphate-based ceramics.
authors.affiliation: RMS Foundation, Bischmattstrasse 12, CH-2544 Bettlach, Switzerland; Technical University Bergakademie Freiberg, Institute for Ceramic, Glass- and Construction Materials, Agricolastraße 17, 09596 Freiberg, Germany.
authors.forename: Laetitia
authors.initials: L
authors.lastname: Galea
authors.affiliation: Complex Materials, Department of Materials, ETH Zürich, 8093 Zürich, Switzerland.
authors.forename: Dmitriy
authors.initials: D
authors.lastname: Alexeev
authors.affiliation: RMS Foundation, Bischmattstrasse 12, CH-2544 Bettlach, Switzerland. Electronic address: marc.bohner@rms-foundation.ch.
authors.forename: Marc
authors.initials: M
authors.lastname: Bohner
authors.affiliation: RMS Foundation, Bischmattstrasse 12, CH-2544 Bettlach, Switzerland.
authors.forename: Nicola
authors.initials: N
authors.lastname: Doebelin
authors.affiliation: Complex Materials, Department of Materials, ETH Zürich, 8093 Zürich, Switzerland.
authors.forename: André R
authors.initials: AR
authors.lastname: Studart
authors.affiliation: Technical University Bergakademie Freiberg, Institute for Ceramic, Glass- and Construction Materials, Agricolastraße 17, 09596 Freiberg, Germany.
authors.forename: Christos G
authors.initials: CG
authors.lastname: Aneziris
authors.affiliation: Technical University Bergakademie Freiberg, Institute for Ceramic, Glass- and Construction Materials, Agricolastraße 17, 09596 Freiberg, Germany; EMPA, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for High Performance Ceramics, Ueberlandstrasse 129, 8600 Dübendorf, Switzerland.
authors.forename: Thomas
authors.initials: T
authors.lastname: Graule
mesh_terms: D001843:Bone Cements / Q000737:chemistry*; D002130:Calcium Phosphates / Q000737:chemistry*; D003460:Crystallization; D008422:Materials Testing; D055595:Mechanical Phenomena; D008855:Microscopy, Electron, Scanning; D013696:Temperature; D014867:Water / Q000737:chemistry*
chemical_list: D001843:Bone Cements; D002130:Calcium Phosphates; D014867:Water; C020243:calcium phosphate
keywords: Calcium phosphate; Hierarchical structures; Hydrothermal treatment; Strength; Toughness; Work-of-fracture