David Grossin

1.111 – Bioactive Ceramics: Physical Chemistry

The chapter mainly discusses the physical–chemical properties of calcium phosphates (Ca-P), among the most important and most used bioactive ceramics. The main calcium phosphate compounds are presented with a brief description of their synthesis methods. Their characterization, using different techniques, including chemical analyses, X-ray diffraction, Fourier transform infrared (FTIR), Raman and solid-state nuclear magnetic resonance (NMR) spectrometries, and scanning and transmission electron microscopies (SEM and TEM), is reviewed and the different information obtained are discussed. The thermal stability and the relationships between different Ca-P phases are then described. The biological properties of Ca-P are related to their behavior in solution; their solubility, transformations and hydrolysis, nucleation ability, and surface properties and reactivity (ion exchange, adsorption) are presented especially in the case of apatites. The biological response regarding bioactivity, biodegradation, and simulated body fluid (SBF) testing is discussed from the point of view of the Ca-P physical chemistry. Several examples of applications are then proposed as ceramics, coatings, cements, and composite materials. A brief presentation of other bioactive mineral compounds follows (oxides and hydroxides, calcium carbonate, calcium sulfate).

Thermodynamic basis for evolution of apatite in calcified tissues

Abstract Bone remodeling and tooth enamel maturation are biological processes that alter the physicochemical features of biominerals with time. However, although the ubiquity of bone remodeling is clear, why is well-crystallized bone mineral systematically replaced by immature nanocrystalline inorganic material? In enamel, a clear evolution is also seen from the first mineral formed during the secretory stage and its mature well-crystalline form, which then changes little in the adult tooth. This contribution provides the thermodynamic basis underlying these biological phenomena. We determined, for the first time, the energetics of biomimetic apatites corresponding to an increasing degree of maturation. Our data point out the progressive evolution of the enthalpy (ΔHf°) and free energy (ΔGf°) of formation toward more negative values upon maturation. Entropy contributions to ΔGf ° values remained small compared to enthalpy contributions. ΔHf° varied from -12 058.9 ± 12.2 to -12 771.0 ± 21.4 kJ/mol for maturation times increasing from 20 min to 3 weeks, approaching the value for stoichiometric hydroxyapatite, -13 431.0 ± 22.7 kJ/mol. Apatite thermodynamic stability increased as its composition moved toward stoichiometry. These findings imply diminishing aqueous solubility of calcium and phosphate ions as well as decreased surface reactivity. Such thermodynamically driven maturation is favorable for enamel maturation since this biomineral is intended to resist external aggressions such as contact with acids. In contrast, maintaining a metastable highly reactive and soluble form of apatite is essential to the effective participation of bone as a source of calcium and phosphate for homeostasis. Therefore our data strongly suggest that, far from being trivial, the intrinsic thermodynamic properties of apatite mineral represent a critical driving force for continuous bone remodeling, in contrast to current views favoring a purely biologically driven cycle. These thermodynamic data may prove helpful in other domains relating, for example, to apatite-based biomaterials development or in the field of (geo)microbiology.

Characterization of Calcium Phosphates Using Vibrational Spectroscopies

Vibrational spectroscopies are extensively used for the characterization of calcium phosphates either as natural biological minerals (bone, teeth, ectopic calcifications) or as biomaterials (bioceramics, coatings, composites). The present review begins with a theoretical description of expected spectra for the main calcium phosphate phases (i.e., brushite, monetite, octacalcium phosphate, tricalcium phosphates, apatites, amorphous calcium phosphate) followed by the analysis of real spectra, line positions and assignments, and observed anomalies. In the second part, the spectra of complex well-crystallized ion-substituted apatites and other calcium phosphates, as well as solid solutions, are investigated, and the information gained regarding the substitution types and ion distributions are derived. Finally, we will examine and interpret the spectra of nanocrystalline apatites considering the ion substitution effects and the existence of a surface hydrated layer. Quantification processes and spectra treatments are briefly presented and discussed. Examples of the use of vibrational spectroscopies for biomaterials and biominerals characterization will be detailed for coating evaluations, including spectroscopic imaging, following up on mineral cement setting reactions, adsorption studies, near infrared investigations of surface water, residual strains determinations in bone, orientation of apatite crystals in biological tissues, and crystallinity and maturity of bone mineral.

Combinatorial MAPLE deposition of antimicrobial orthopedic maps fabricated from chitosan and biomimetic apatite powders

Chitosan/biomimetic apatite thin films were grown in mild conditions of temperature and pressure by Combinatorial Matrix-Assisted Pulsed Laser Evaporation on Ti, Si or glass substrates. Compositional gradients were obtained by simultaneous laser vaporization of the two distinct material targets. A KrF* excimer (λ=248nm, τFWHM=25ns) laser source was used in all experiments. The nature and surface composition of deposited materials and the spatial distribution of constituents were studied by SEM, EDS, AFM, GIXRD, FTIR, micro-Raman, and XPS. The antimicrobial efficiency of the chitosan/biomimetic apatite layers against Staphylococcus aureus and Escherichia coli strains was interrogated by viable cell count assay. The obtained thin films were XRD amorphous and exhibited a morphology characteristic to the laser deposited structures composed of nanometric round shaped grains. The surface roughness has progressively increased with chitosan concentration. FTIR, EDS and XPS analyses indicated that the composition of the BmAp-CHT C-MAPLE composite films gradually modified from pure apatite to chitosan. The bioevaluation tests indicated that S. aureus biofilm is more susceptible to the action of chitosan-rich areas of the films, whilst the E. coli biofilm proved more sensible to areas containing less chitosan. The best compromise should therefore go, in our opinion, to zones with intermediate-to-high chitosan concentration which can assure a large spectrum of antimicrobial protection concomitantly with a significant enhancement of osseointegration, favored by the presence of biomimetic hydroxyapatite.

Comparison of Physical-chemical and Mechanical Properties of Chlorapatite and Hydroxyapatite Plasma Sprayed Coatings

Chlorapatite can be considered a potential biomaterial for orthopaedic applications. Its use as plasma-sprayed coating could be of interest considering its thermal properties and particularly its ability to melt without decomposition unlike hydroxyapatite. Chlorapatite (ClA) was synthesized by a high-temperature ion exchange reaction starting from commercial stoichiometric hydroxyapatites (HA). The ClA powder showed similar characteristics as the original industrial HA powder, and was obtained in the monoclinic form. The HA and ClA powders were plasma-sprayed using a low-energy plasma spraying system with identical processing parameters. The coatings were characterized by physical-chemical methods, i.e. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy, including distribution mapping of the main phases detected such as amorphous calcium phosphate (ACP), oxyapatite (OA), and HA or ClA. The unexpected formation of oxyapatite in ClA coatings was assigned to a side reaction with contaminating oxygenated species (O2, H2O). ClA coatings exhibited characteristics different from HA, showing a lower content of oxyapatite and amorphous phase. Although their adhesion strength was found to be lower than that of HA coatings, their application could be an interesting alternative, offering, in particular, a larger range of spraying conditions without formation of massive impurities.

A review of the additive manufacturing (3DP) of bioceramics: alumina, zirconia (PSZ) and hydroxyapatite

The additive manufacturing of bioceramic parts has been investigated since the 1980s. This paper offers an overview of the present achievements in the production of alumina, zirconia and hydroxyapatite parts by means of selective laser sintering/melting of a powder bed or stereolithography. A focus is placed on these specific materials because of their widespread use in the biomedical field. It demonstrates that even though the manufacturing of parts with these processes is possible from pure bioceramics, the use of a binder (or another chemical adjuvant) is required in order to achieve good mechanical properties. Still, improvements in the raw material preparation and in the comprehension of the physical phenomena occurring during the processing remain necessary to be able to prevent the formation of cracks or to be able to control the porosity of the parts.

Biomineralization of a titanium-modified hydroxyapatite semiconductor on conductive wool fibers

Metal ions are frequently incorporated into crystalline materials to improve their electrochemical properties and to confer new physicochemical properties. Naturally-occurring phosphate apatite, which is formed geologically and in biomineralization processes, has extensive potential applications and is therefore an attractive functional material. In this study, we generate a novel building block for flexible optoelectronics using bio-inspired methods to deposit a layer of photoactive titanium-modified hydroxyapatite (TiHA) nanoparticles (NPs) on conductive polypyrrole(PPy)-coated wool yarns. The titanium concentration in the reaction solution was varied between 8-50 mol% with respect to the phosphorous, which led to titanate ions replacing phosphate in the hydroxyapatite lattice at levels up to 17 mol%. PPy was separately deposited on wool yarns by oxidative polymerization, using two dopants: (i) anthraquinone-2,6-disulfonic acid to increase the conductivity of the PPy layer and (ii) pyroglutamic acid, to reduce the resistivity of the wool yarns and to promote the heterogeneous nucleation of the TiHA NPs. A specific titanium concentration (25 mol% wrt P) was used to endow the TiHA NPs on the PPy-coated fibers with a desirable band gap value of 3.68 eV, and a specific surface area of 146 m2 g-1. This is the first time that a thin film of a wide-band gap semiconductor has been deposited on natural fibers to create a fiber-based building block that can be used to manufacture flexible electronic devices.

Electrodeposition of HAp coatings on Ti6Al4V alloy and its electrochemical behavior in simulated body fluid solution

Hydroxyapatite (HAp) coatings were prepared on Ti6Al4V substrate by electrodeposition method from electrolyte solution containing Ca(NO3)2, NH4H2PO4 and NaNO3. The results show that the HAp coatings were single phase crystals of HAp. Scanning electron microscope (SEM) images present that HAp/Ti6Al4V have flake shapes which arrange to form like-coral agglomerates. In vitro test of the Ti6Al4V and HAp/Ti6Al4V in simulated body fluid (SBF) solution was investigated with different immersion times. pH of SBF solution decreased and the mass of materials increased. SEM images prove the formation of apatite on the surface of Ti6Al4V and HAp/Ti6Al4V. The corrosion current density during immersion time of substrate is always higher than the one of HAp/Ti6Al4V because the deposited HAp can protect well for the substrate.

Formation of nanosized strontium substituted hydroxyapatites

Incorporation of specific elements into calcium phosphates offers the combination of a bioactive material and a therapeutic effect. This is important for improving the integration of implants as well as treating medical conditions. Strontium is a suitable candidate and displays the ability to stimulate bone growth and reducing bone resorption. This study investigated the formation of strontium carbonated hydroxyapatite nanoparticles from an amorphous phase. Crystallization of carbonated hydroxyapatite occurred at 585 °C, but samples with an intended 25% and 75% replacement of calcium with strontium crystallized at 624 °C. Heat treatment at the crystallization temperature revealed that strontium free apatite does not crystallize in 5 minutes, but an increasing strontium concentration leads to a higher rate of crystallization. X-ray diffraction patterns suggest that it may be difficult to include strontium, but higher strontium concentrations are possibly included with ease in the lattice. This work has produced a nanosized apatite accompanied by an amorphous phase after a short heat-treatment time. This offers a range of features that collectively show great promise for significantly enhancing the release of strontium for improved bone therapeutic effects.

1.11 Bioactive Calcium Phosphate Compounds: Physical Chemistry☆

The chapter discusses the physical–chemical properties of calcium phosphates (Ca-P), among the most used bioactive ceramics. The main calcium phosphate compounds are presented with a brief description of their synthesis methods, their characterization, using different techniques, including chemical analyzes, X-ray diffraction, Fourier transform infrared (FTIR) and Raman spectroscopies, solid-state nuclear magnetic resonance (NMR), scanning and transmission electron microscopies (SEM and TEM). Their thermochemical characteristics are given and the thermal stability and relationships between different Ca-P phases are then described. Their solubility, transformations and hydrolysis, nucleation ability, surface properties and reactivity (ion exchange, adsorption) are presented especially in the case of apatites. The biological response regarding bioactivity, biodegradation, and simulated body fluid (SBF) testing is discussed from the point of view of the Ca-P physical chemistry. Several examples of applications and processing techniques are then proposed as nanoparticles, bulk ceramics, coatings, cements, and composite materials.