Biomimetic apatite sintered at very low temperature by spark plasma sintering: Physico-chemistry and microstructure aspects

Nanocrystalline apatites analogous to bone mineral are very promising materials for the preparation of highly bioactive ceramics due to their unique intrinsic physico-chemical characteristics. Their surface reactivity is indeed linked to the presence of a metastable hydrated layer on the surface of the nanocrystals. Yet the sintering of such apatites by conventional techniques, at high temperature, strongly alters their physico-chemical characteristics and biological properties, which points out the need for "softer" sintering processes limiting such alterations. In the present work a non-conventional technique, spark plasma sintering, was used to consolidate such nanocrystalline apatites at non-conventional, very low temperatures (T° < 300 °C) so as to preserve the surface hydrated layer present on the nanocrystals. The bioceramics obtained were then thoroughly characterized by way of complementary techniques. In particular, microstructural, nanostructural and other major physico-chemical features were investigated and commented on. This work adds to the current international concern aiming at improving the capacities of present bioceramics, in view of elaborating a new generation of resorbable and highly bioactive ceramics for bone tissue engineering

PySINDy: A Python package for the sparse identification of nonlinear dynamical systems from data

Authors of papers retaincopyright and release the workunder a Creative CommonsAttribution 4.0 InternationalLicense (CC-BY)Scientists have long quantified empirical observations by developing mathematical models that characterize the observations, have some measure of interpretability, and are capable of making predictions. Dynamical systems models in particular have been widely used to study, explain, and predict system behavior in a wide range of application areas, with examples ranging from Newton’s laws of classical mechanics to the Michaelis-Menten kinetics for modeling enzyme kinetics. While governing laws and equations were traditionally derived by hand, the current growth of available measurement data and resulting emphasis on data-driven modeling motivates algorithmic approaches for model discovery. A number of such approaches have been developed in recent years and have generated widespread interest, including Eureqa (Schmidt & Lipson, 2009), sure independence screening and sparsifying operator (Ouyang, Curtarolo, Ahmetcik, Scheffler, & Ghiringhelli, 2018), and the sparse identification of nonlinear dynamics (SINDy) (Brunton, Proctor, & Kutz, 2016). Maximizing the impact of these model discovery methods requires tools to make them widely accessible to scientists across domains and at various levels of mathematical expertise.This project is a fork of sparsereg( Quade, 2018). SLB acknowledges funding supportfrom the Air Force Office of Scientific Research (AFOSR FA9550-18-1-0200) and the ArmyResearch Office (ARO W911NF-19-1-0045). JNK acknowledges support from the Air ForceOffice of Scientific Research (AFOSR FA9550-17-1-0329). This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under GrantNumber DGE-1256082

Influence of carbonation on the low-temperature consolidation by Spark Plasma Sintering of carbonated calcium phosphate bioceramics

Calcium phosphates (CaP) such as biomimetic nanocrystalline apatite or amorphous calcium phosphate are hydrated bioactive compounds particularly suitable for bone repair applications due to their similarity with bone mineral. However, their consolidation in ceramic parts deserves special attention as they are thermodynamically metastable and can decompose into less bioactive phases upon heating. Adapted strategies are needed to obtain bulk bioceramics. Spark Plasma Sintering (SPS) has been shown to allow cold sintering of such compounds at temperatures like 150 °C while preserving the hydrated character and nanosized dimensions of the precursor powders. To this date, however, the role of the degree of carbonation of these precursors on the densification of CO3-bearing CaP compounds via SPS has not been explored despite the natural carbonation of bone. In this work, several carbonated CaP hydrated compounds were prepared and consolidated by SPS and the characteristics of the obtained ceramics was scrutinized with respect to the starting powders. Two carbonation routes were carried out: via volume carbonation during powder synthesis or via subsequent surface ion exchange. All samples tested led to apatitic compounds after SPS, including amorphous CaP. We show that the degree of carbonation negatively affects the densification rate and propose possible hypotheses explaining this behavior. Evolution in the nature of the carbonate sites (apatitic A-, B-types and labile surface carbonates) before and after SPS is also noticed and commented. The consolidation of such compounds is however proven possible, and gives rise to bone-like apatitic compounds with great potential as bioactive resorbable ceramics for bone regeneration

Consolidation of bone-like apatite bioceramics by spark plasma sintering of amorphous carbonated calcium phosphate at very low temperature

Various carbonated calcium phosphate powders were synthesized by aqueous precipitation and ceramics consolidation by spark plasma sintering (SPS) at very low temperature was investigated. The objective was to preserve low crystallinity and avoid material decarbonation. SPS at low temperature only leads to little or no sintering when crystallized powders are used. Amorphous powders are required. In this case, consolidation occurs at temperatures below 150°C. It is accompanied by crystallization of the amorphous phase into calcium-deficient carbonated apatite Ca10-x-y(PO4)6-x-y(HPO4)x(CO3)y(OH)2-x-y-2z(CO3)z. The resulting ceramics are microporous and highly cohesive with good mechanical properties (flexural strength=18MPa). The sintering mechanism, called “crystal fusion”, is based on solid state diffusion of chemical species at the grain boundary and crystal growth within the amorphous particles. These bioceramics that mimic the composition of the bone mineral are expected to have a higher bioreactivity than well crystallized carbonated hydroxyapatite ceramics obtained by conventional sintering

Thermodynamic basis for evolution of apatite in calcified tissues

Bone remodeling and tooth enamel maturation are biological processes that alter the physico-chemical 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

Nanocrystalline apatites: The fundamental role of water

Bone is a natural nanocomposite. Its mineral component is nanocrystalline calcium phosphate apatite, whose synthetic biomimetic analogs can be prepared by wet chemistry. The initially formed crystals, whether biological or synthetic, exhibit very peculiar physicochemical features. In particular, they are nanocrystalline, nonstoichiometric, and hydrated. The surface of the nanocrystals is covered by a non-apatitic hydrated layer containing mobile ions, which may explain their exceptional surface reactivity. For their precipitation in vivo or in vitro, for their evolution in solution, for the 3D organization of the nanocrystals, and for their consolidation to obtain bulk ceramic materials, water appears to be a central component that has not received much attention. In this mini-review, we explore these key roles of water on the basis of physicochemical and thermodynamic data obtained by complementary tools including FTIR, XRD, ion titrations, oxide melt solution calorimetry, and cryo-FEG-SEM. We also report new data obtained by DSC, aiming to explore the types of water molecules associated with the nanocrystals. These data support the existence of two main types of water molecules associated with the nanocrystals, with different characteristics and probably different roles and functions. These findings improve our understanding of the behavior of bioinspired apatite-based systems for biomedi- cine and also of biomineralization processes taking place in vivo, at present and in the geologic past. This paper is thus intended to give an overview of the specificities of apatite nanocrystals and their close relationship with water