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- Herausgeber: Wiley-VCH
- Kategorie: Fachliteratur
- Sprache: Englisch
Reflecting the advances made in recent years, this is a comprehensive overview of calcium orthophosphates for bioceramics and biocomposites with a special focus on the detailed description of all those available, including their biological and geological occurrence, preparation, chemical composition, structure-property relationships and applications. In particular, the book discusses the suitability of these orthophosphates for biomedical applications and their use as bone grafts in surgery and medicine. The result is a useful reference for researchers with an academic, medical or commercial background.
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Seitenzahl: 943
Veröffentlichungsjahr: 2016
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Table of Contents
Cover
Related Titles
Title Page
Copyright
Preface
Part I: Calcium Orthophosphates (CaPO4): Occurrence, Properties, and Biomimetics
Chapter 1: Introduction
References
Chapter 2: Geological and Biological Occurrences
References
Chapter 3: The Members of CaPO4 Family
3.1 MCPM
3.2 MCPA (or MCP)
3.3 DCPD
3.4 DCPA (or DCP)
3.5 OCP
3.6 β-TCP
3.7 α-TCP
3.8 ACP
3.9 CDHA (or Ca-def HA, or CDHAp)
3.10 HA (or HAp, or OHAp)
3.11 FA (or FAp)
3.12 OA (or OAp, or OXA)
3.13 TTCP (or TetCP)
3.14 Biphasic, Triphasic, and Multiphasic CaPO
4
Formulations
3.15 Ion-Substituted CaPO
4
References
Chapter 4: Biological Hard Tissues of CaPO4
4.1 Bone
4.2 Teeth
4.3 Antlers
References
Chapter 5: Pathological Calcification of CaPO4
References
Chapter 6: Biomimetic Crystallization of CaPO4
References
Chapter 7: Conclusions and Outlook
References
Part II: Calcium Orthophosphate Bioceramics in Medicine
Chapter 8: Introduction
References
Chapter 9: General Knowledge and Definitions
References
Chapter 10: Bioceramics of CaPO4
10.1 History
10.2 Chemical Composition and Preparation
10.3 Forming and Shaping
10.4 Sintering and Firing
References
Chapter 11: The Major Properties
11.1 Mechanical Properties
11.2 Electric/Dielectric and Piezoelectric Properties
11.3 Possible Transparency
11.4 Porosity
References
Chapter 12: Biomedical Applications
12.1 Self-Setting (Self-Hardening) Formulations
12.2 Coatings, Films, and Layers
12.3 Functionally Graded Bioceramics
References
Chapter 13: Biological Properties and In Vivo Behavior
13.1 Interactions with Surrounding Tissues and the Host Responses
13.2 Osteoinduction
13.3 Biodegradation
13.4 Bioactivity
13.5 Cellular Response
References
Chapter 14: Nonbiomedical Applications of CaPO4
References
Chapter 15: CaPO4 Bioceramics in Tissue Engineering
15.1 Tissue Engineering
15.2 Scaffolds and Their Properties
15.3 Bioceramic Scaffolds from CaPO
4
15.4 A Clinical Experience
References
Chapter 16: Conclusions and Outlook
References
Part III: Biocomposites from Calcium Orthophosphates
Chapter 17: Introduction
References
Chapter 18: General Information and Knowledge
References
Chapter 19: The Major Constituents of Biocomposites and Hybrid Biomaterials for Bone Grafting
19.1 CaPO
4
19.2 Polymers
19.3 Inorganic Materials and Compounds
References
Chapter 20: Biocomposites and Hybrid Biomaterials Based on CaPO4
20.1 Biocomposites with Polymers
20.2 Self-Setting Formulations
20.3 Formulations Based on Nanodimensional CaPO
4
and Nanodimensional Biocomposites
20.4 Biocomposites with Collagen
20.5 Formulations with Other Bioorganic Compounds and/or Biological Macromolecules
20.6 Injectable Bone Substitutes (IBSs)
20.7 Biocomposites with Glasses, Inorganic Compounds, Carbon, and Metals
20.8 Functionally Graded Formulations
20.9 Biosensors
References
Chapter 21: Interaction among the Phases in CaPO4-Based Formulations
References
Chapter 22: Bioactivity and Biodegradation of CaPO4-Based Formulations
References
Chapter 23: Some Challenges and Critical Issues
References
Chapter 24: Conclusions
References
Index
End User License Agreement
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Guide
Cover
Table of Contents
Preface
Part I: Calcium Orthophosphates (CaPO4): Occurrence, Properties, and Biomimetics
Begin Reading
List of Illustrations
Chapter 2: Geological and Biological Occurrences
Figure 2.1 A simplified schematic of the phosphorus cycle from apatitic igneous rock to phosphorite sedimentary rock through chemical or physical weathering. Life forms accumulate soluble phosphorus species and can produce apatite through biomineralization.
Figure 2.2 Samples of natural FA: (a) polycrystalline, (b) single-crystalline, and (c) a gem. The colors are due to incorporated ions of transition metals.
Chapter 3: The Members of CaPO4 Family
Figure 3.1 Phase diagram of the system CaO–P
2
O
5
(C = CaO, P = P
2
O
5
) at elevated temperatures. Here, C
7
P
5
means 7CaO·5P
2
O
5
; other abbreviations should be written out in the same manner. (Reprinted from [4, 5] with permission.)
Figure 3.2 pH variation of ionic concentrations in triprotic equilibrium for orthophosphoric acid solutions. (Reprinted from [12] with permission.)
Figure 3.3 Various types of CaPO
4
obtained by neutralizing orthophosphoric acid by calcium hydroxide. Ca/P values of the known types of CaPO
4
(Table 1.1) are reported in the Figure The solubility of CaPO
4
in water decreases drastically from left to right, HA being the most insoluble and stable phase. (Reprinted from [13] with permission.)
Figure 3.4 (a) A 3D version of the classical solubility phase diagrams for the ternary system Ca(OH)
2
–H
3
PO
4
–H
2
O. (Reprinted from [14] with permission.) (b,c) Solubility phase diagrams in two-dimensional graphs, showing two logarithms of the concentrations of (i) calcium and (ii) orthophosphate ions as a function of the pH in solutions saturated with various salts. (Reprinted from [15] with permission.)
Figure 3.5 A model of ACP structure.
Figure 3.6 A biomimetically grown aggregate of FA that was crystallized in a gelatin matrix. Its shape can be explained and simulated by a fractal growth mechanism. Scale bar: 10 µm. (Reprinted from [263] with permission.)
Chapter 4: Biological Hard Tissues of CaPO4
Figure 4.1 (left) Crystal structure of a biological apatite. Powder X-ray diffraction patterns (center) and infrared spectra (right) of human enamel, dentine, and bone.
Figure 4.2 General structure of a mammalian bone. Other very good graphical sketches of the mammalian bone structure are available in [2, 12].
Figure 4.3 Classification of bones by shape.
Figure 4.4 A schematic illustration of the hierarchical organization of bones. Up to level V, the hierarchical levels can be divided into the ordered material (Dark grey) and the disordered material (light grey). At level VI, these two materials combine in lamellar bone and parallel fibered bone. Other members of the bone family still need to be investigated with respect to the presence of both material types; hence, they are depicted in a box without color. Level VII depicts the lamellar packets that make up trabecular bone material and the cylindrically shaped lamellar bone that makes up osteonal bone. The fibrolamellar unit comprises the primary hypercalcified layer, parallel fibered bone, and lamellar bone. Abbreviations: c-HAP – carbonated hydroxyapatite; GAGs – glycosaminoglycans; NCPs – non-collagenous proteins. Other good graphical sketches of the hierarchical structure of bones are available in [21, 54, 56, 57]. (Reprinted from [59] with permission.)
Figure 4.5 (a) A schematic view of the current model of the organic–inorganic composite nanostructure of bone. Polycrystalline particles of biological apatite, consisting of stacks of (single crystal) mineral platelets are sandwiched into the space between collagen fibrils. The large platelet (100) faces are parallel to each other and the platelet c axis is strongly ordered with the collagen fibril axis. (b) A schematic view of the structure of a single mineral platelet, with an atomically ordered core resembling the CDHA structure (with substitutions) surrounded by a surface layer of disordered, hydrated mineral ions. (c) A schematic view of the detailed structural model of bone mineral showing how citrate anions and water bind the mineral platelets together.
Figure 4.6 A schematic illustration of
in vivo
mineralization of a collagen fibril: (a,b) CaPO
4
clusters form complexes with biopolymers, forming stable mineral droplets; (c,d) mineral droplets bind to a distinct region on the collagen fibers and enter the fibril; (e,f) once inside the collagen, the mineral in a liquid state diffuses through the interior of the fibril and solidifies into a disordered phase of ACP; and (g,h) finally, directed by the collagen, ACP is transformed into oriented crystals of biological apatite.
Figure 4.7 A schematic drawing of a tooth. Other very good graphical sketches of the mammalian tooth structure, including the hierarchical levels, are available in [3, 57].
Figure 4.8 A scanning electron micrograph of the forming enamel of a continuously growing rat incisor showing ordered rods of CaPO
4
. Scale bar: 10 µm.
Figure 4.9 Red deer stag at velvet shedding. The bare bone of the hard antlers is exposed. A good cross-sectional image of a deer antler is available in [57].
Figure 4.10 Fallen antlers used to make a chandelier.
Chapter 5: Pathological Calcification of CaPO4
Figure 5.1 A schematic representation of the different stages of the surface-directed mineralization of CaPO
4
. In stage 1, aggregates of prenucleation clusters are in equilibrium with ions in solution. The clusters approach the surface with chemical functionality. In stage 2, prenucleation clusters aggregate near the surface, with loose aggregates still in solution. In stage 3, further aggregation causes densification near the surface. In stage 4, nucleation of spherical particles of ACP occurs at the surface only. In stage 5, crystallization occurs in the region of the ACP particles directed by the surface.
Chapter 8: Introduction
Figure 8.1 Several examples of the commercial CaPO
4
-based bioceramics.
Chapter 10: Bioceramics of CaPO4
Figure 10.1 Soft X-ray photographs of the operated portion of the rabbit femur: 4 weeks (a), 12 weeks (b), 24 weeks (c), and 72 weeks (d) after implantation of CDHA; 4 weeks (e), 12 weeks (f), 24 weeks (g), and 72 weeks (h) after implantation of sintered HA. (Reprinted from [73] with permission.)
Figure 10.2 A schematic of 3D printing and some 3D printed parts (fabricated at Washington State University) showing the versatility of 3D printing technology for ceramic scaffolds fabrication with complex architectural features. (Reprinted from [131] with permission.)
Figure 10.3 A schematic diagram representing the changes occurring with spherical particles under sintering. Shrinkage is noticeable.
Figure 10.4 Linear shrinkage of the compacted ACP powders that were converted into β-TCP, BCP (50% HA + 50% β-TCP), and HA upon heating. According to the authors, “At 1300 °C, the shrinkage reached a maximum of ∼25%, ∼30%, and ∼35% for the compacted ACP powders that converted into HA, BCP 50/50, and β-TCP, respectively” [214]. (Reprinted from [214] with permission.)
Chapter 11: The Major Properties
Figure 11.1 Transparent HA bioceramics prepared by spark plasma sintering at 900 °C from nano-sized HA single crystals.
Figure 11.2 Photographs of a commercially available porous CaPO
4
bioceramics with different porosity (a,b) and a method of their production (c). For photos, the horizontal field width is 20 mm.
Figure 11.3 Schematic drawings of various types of the ceramic porosity: (a) nonporous, (b) microporous, (c) macroporous (spherical), (d) macroporous (spherical) + micropores, (e) macroporous (3D-printing), and (f) macroporous (3D-printing) + micropores.
Figure 11.4 SEM pictures of HA bioceramics sintered at (a) 1050 °C and (b) 1200 °C. Note the presence of microporosity in (a) and not in (b).
Chapter 12: Biomedical Applications
Figure 12.1 Different types of biomedical applications of CaPO
4
bioceramics.
Figure 12.2 A typical microstructure of a CaPO
4
cement after hardening. The mechanical stability is provided by the physical entanglement of crystals.
Figure 12.3 Shows how a plasma-sprayed HA coating on a porous titanium (dark bars) dependent on the implantation time will improve the interfacial bond strength compared to uncoated porous titanium (light bars).
Figure 12.4 A schematic diagram showing the arrangement of the FA/β-TCP biocomposite layers. (a) A nonsymmetric functionally graded material (FGM) and (b) symmetric FGM.
Figure 12.5 Schematic illustrations of the fabrication of pore-graded bioceramics: (a) lamination of individual tapes, manufactured by tape casting and (b) a compression molding process.
Chapter 13: Biological Properties and In Vivo Behavior
Figure 13.1 Rounded β-TCP granules of 2.6–4.8 mm in size, providing no sharp edges for combination with bone cement. (Reprinted from [43] with permission.)
Figure 13.2 A sequence of interfacial reactions involved in forming a bond between tissue and bioactive ceramics. (Reprinted from [12, 13] with permission.)
Figure 13.3 A schematic diagram representing the events that take place at the interface between bioceramics and the surrounding biological environment: (1) dissolution of bioceramics; (2) precipitation from solution onto bioceramics; (3) ion exchange and structural rearrangement at the bioceramic/tissue interface; (4) interdiffusion from the surface boundary layer into the bioceramics; (5) solution-mediated effects on cellular activity; (6) deposition of either the mineral phase (a) or the organic phase (b) without integration into the bioceramic surface; (7) deposition of either the mineral phase (a) or the organic phase (b) with integration into the bioceramics; (8) chemotaxis to the bioceramic surface; (9) cell attachment and proliferation; (10) cell differentiation; and (11) extracellular matrix formation. All phenomena, collectively, lead to the gradual incorporation of a bioceramic implant into developing bone tissue. (Reprinted from [2] with permission.)
Figure 13.4 A schematic diagram representing the phenomena that occur on HA surface after implantation: (1) beginning of the implant procedure, where solubilization of the HA surface starts; (2) continuation of the solubilization of the HA surface; (3) the equilibrium between the physiological solutions and the modified surface of HA has been achieved (changes in the surface composition of HA does not mean that a new phase of DCPA or DCPD forms on the surface); (4) adsorption of proteins and/or other bioorganic compounds; (5) cell adhesion; (6) cell proliferation; (7) beginning of a new bone formation; and (8) new bone has been formed. (Reprinted from [119] with permission.)
Chapter 15: CaPO4 Bioceramics in Tissue Engineering
Figure 15.1 A schematic view of a third generation biomaterial, in which porous CaPO
4
bioceramics act as a scaffold or a template for cells, growth factors, and so on.
Figure 15.2 A schematic drawing presenting the potential usage of HA with various degrees of porosity.
Figure 15.3 A schematic drawing of the key scaffold properties affecting a cascade of biological processes occurring after CaPO
4
implantation.
Chapter 20: Biocomposites and Hybrid Biomaterials Based on CaPO4
Figure 20.1 Four types of mutual arrangements of nano-sized particles to a polymer chain: (a) inorganic particles embedded in an inorganic polymer, (b) incorporation of particles by bonding to the polymer backbone, (c) interpenetrating network with chemical bonds, and (d) inorganic–organic hybrid polymer.
Figure 20.2 SEM micrographs of (a) α-TCP compact and (b) α-TCP/PLGA biocomposite (bars = 5 µm).
Figure 20.3 Scanning electron microscopy image of reconstituted mineralized collagen I fibrils. An example of an organic–inorganic nanostructural composite, mimicking the extracellular matrix of bone tissue on the nanometer scale.
Figure 20.4 Schematic illustrations of collagen–CaPO
4
nanocomposites: (a) structure of bone: nanodimensional crystals of biological apatite aligned along with the collagen fibrils; linking occurs between the apatite and polar groups on collagen chains in bone, (b) nanodimensional crystals of CaPO
4
physically “trapped” within the collagen matrix, and (c) nanodimensional crystals of CaPO
4
deposited on and covered the collagen fibrils surface.
Figure 20.5 A proposed mechanism for the formation of ACP/amino acid biocomposites in aqueous solutions.
Figure 20.6 Expected properties of functionally graded biocomposite dental implant. For comparison, the upper drawing shows a functionally graded implant and the lower one shows a conventional uniform implant. The properties are shown in the middle. The implant with the composition changed from a biocompatible metal (Ti) at one end (left in the figure), increasing the concentration of bioceramics (HA) toward 100% HA at the other end (right in the figure), could control both mechanical properties and biocompatibility without an abrupt change due to the formation of discrete boundary. This FGM biocomposite was designed to provide more titanium for the upper part where occlusal force is directly applied and more HA for the lower part, which is implanted inside the jawbone.
Figure 20.7 A schematic diagram showing the arrangement of the FA/β-TCP biocomposite layers. (a) A nonsymmetric functionally graded material (FGM) and (b) symmetric FGM.
Chapter 21: Interaction among the Phases in CaPO4-Based Formulations
Figure 21.1 A schematic drawing of the relation between self-organization (directional deposition of HA on collagen) and interfacial interaction in biocomposites. Direction of interaction between HA and collagen is restricted by covalent bond between COO and Ca(2) to maintain regular coordination number of 7.
Figure 21.2 A schematic diagram of Ca
2+
ion binding with alginate chains.
Figure 21.3 Possible interactions between a chitosan–gelatin (CG) network and HA crystals in HA/CG biocomposites: (a) in the case of a nano-dimensional hydroxyapatite (nHA) and (b) in the case of a micro-dimensional hydroxyapatite (mHA). According to the authors, “When nHA formed on the surface of CG network via biomineralization, the corresponding ion interaction is the main drive force. However, as the mHA crystals depositing on the surface of CG network, the hydrogen bonds between COOH, OH, –NH
2
of CG films and OH groups of HA crystals take the important role” (p. 1215).
Figure 21.4 Surface modification of HA particles by grafting polymerization according to Lee
et al
. [93]: (A) surface thiol functionalized HA, (B) sulfur-centered radical on HA surface, and (C) surface grafting polymerization of ethylene glycol methacrylate phosphate.
List of Tables
Chapter 1: Introduction
Table 1.1 Existing calcium orthophosphates and their major properties [7, 8]
Chapter 2: Geological and Biological Occurrences
Table 2.1 The principal technological and scientific uses of apatites and other calcium orthophosphates [2]
Table 2.2 Comparative composition and structural parameters of inorganic phases of adult human calcified tissues
Chapter 3: The Members of CaPO4 Family
Table 3.1 Crystallographic data of calcium orthophosphates [6–8]
Chapter 5: Pathological Calcification of CaPO4
Table 5.1 Occurrence of calcium phosphates in biological systems (human) [66]
Chapter 11: The Major Properties
Table 11.1 The procedures used to manufacture porous CaPO
4
scaffolds for tissue engineering [206]
Chapter 12: Biomedical Applications
Table 12.1 Registered commercial trademarks (current and past) of CaPO
4
-based bioceramics and biomaterials
Chapter 15: CaPO4 Bioceramics in Tissue Engineering
Table 15.1 A hierarchical pore size distribution that an ideal scaffold should exhibit [72]
Chapter 17: Introduction
Table 17.1 The biochemical composition
a
of bones [22]
Chapter 18: General Information and Knowledge
Table 18.1 General respective properties from the bioorganic and inorganic domains, to be combined in various composites and hybrid materials [34]
Chapter 19: The Major Constituents of Biocomposites and Hybrid Biomaterials for Bone Grafting
Table 19.1 Major properties of several FDA approved biodegradable polymers [47]
Chapter 20: Biocomposites and Hybrid Biomaterials Based on CaPO4
Table 20.1 A list of some commercial nonsetting CaPO
4
-based IBS and pastes with indication of producer, product name, composition (when available), and form [706]
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Calcium Orthophosphate-based Bioceramics and Biocomposites
Author
Prof. Sergey V. Dorozhkin
Kudrinskaja sq. 1-155
123242 Moscow
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Preface
Calcium orthophosphates (abbreviated as CaPO4) have been of considerable interest to the mineralogists, chemists, material researchers, biologists, and clinicians for many decades. The reasons for this are clear: they form the mineral component of bones and teeth, and they are involved in biomineralization process in mammals. Furthermore, they are found in pathological calcifications. Therefore, CaPO4 appear to be biologically friendly inorganics and, thus, they are increasingly used as implantable biomaterials for various types of bone fillers and bone substitutes. As a final point, CaPO4 are widely distributed minerals in Nature, providing the world's supply of phosphorus, particularly phosphates for the production of fertilizers.
In view of the aforementioned, scientific databases reveal that the research on CaPO4 has a very long history. However, it exploded in the 1960s and since then the amount of publications on the subject is constantly increasing. Simultaneously, the variety of both investigations and biomedical applications of CaPO4 is greatly expanding. Namely, CaPO4-based bioceramics specific to in vivo applications have been designed, synthesized, investigated, and applied. Furthermore, new synthetic processes for fabrication of CaPO4 with the desired properties (such as, the Ca/P ratio, crystallinity, phase composition, particle shape and dimensions, ion-substitutions, etc.) have also been developed. Methods for their structural and surface analysis have also greatly progressed. For example, in early studies, the biological responses of living tissues to implanted materials were evaluated by optical microscopy. Nowadays, biological analysis is performed at the molecular level in combination with high-end physical techniques. In addition, long-term clinical data are now available. All these findings give important suggestions for designing new types of CaPO4-based formulations for biomedical applications.
Therefore, the aim in composing this monograph has been to provide an integrated account of the present knowledge on preparation, chemical composition, structure, properties, and applications of CaPO4, particularly in the biomedical context. Since the entire subject appears to be very broad (over 35 000 publications on the subject have been already published), a great number of references to the related publications detailing various specific aspects of the matter have been collected.
The monograph consists of three big parts. The division of the parts is generally based on the subject, with subdivisions on the major aspects, such as introduction, basic definitions and knowledge, structure, preparation, properties, biomedical application, future directions, ending with conclusions. This overall scheme is used to emphasize the mutual interrelationships among various types of CaPO4. The major principal has been to group the material in the most natural way and, if appropriate, to provide cross-references from other sections. This is sometimes done explicitly and sometimes by giving section references, where other aspects of the specific subject are discussed. Namely, Part I contains the general information on all available types of CaPO4 including their geological and biological occurrence, chemical composition, structure, solubility, as well as brief information on their location in calcified tissues of mammals (bones, teeth, and antlers), including the unwanted (pathological) calcifications. Furthermore, Part I also includes an important section on biomimetic crystallization, including artificial simulating solutions. Part II describes the available knowledge on CaPO4 bioceramics and their biomedical applications, while Part III is devoted to similar topics for CaPO4-containing biocomposites and hybrid biomaterials.
To conclude, this monograph represents my vision on the topic, which, by no means is ideal. Furthermore, since not every possible aspect of CaPO4 has been described, various imperfections are possible. Thus, any criticism, opinions, or suggestions are always welcome. However, it is worth remembering that the major goal is not only a further development of the subject itself but also making a possible contribution to the welfare of human beings, in particular those with diseases potentially treatable by CaPO4.
Finally, I would like to acknowledge the continuous encouragement of my mother Tamara, my wife Elena, and my son Denis. Hopefully, publication of this book will help me find a suitable position in science or industry to provide better financial support to the beloved members of my family.
Sergey V. Dorozhkin
June 2015Moscow, Russia
Part ICalcium Orthophosphates (CaPO4): Occurrence, Properties, and Biomimetics
Chapter 1Introduction
Owing to their abundance in nature (as phosphate ores) and presence in living organisms (as bones, teeth, deer antlers, and the majority of various pathological calcifications), calcium phosphates are inorganic compounds of special interest to human beings. They were discovered in 1769 and have been investigated since then [1, 2]. According to the databases in scientific literature (Web of knowledge, Scopus, Medline, etc.), the total amount of currently available publications on the subject exceeds 40 000 with an annual increase of, at least, 2000 papers. This is a clear confirmation of their importance.
By definition, all known calcium phosphates consist of three major chemical elements: calcium (oxidation state +2), phosphorus (oxidation state +5), and oxygen (reduction state −2), as a part of the phosphate anions. These three chemical elements are present in abundance on the surface of our planet: oxygen is the most widespread chemical element of the earth's surface (∼47 mass%), calcium occupies the fifth place (∼3.3 to 3.4 mass%), and phosphorus (∼0.08 to 0.12 mass%) is among the first 20 of the chemical elements most widespread on our planet [3]. In addition, the chemical composition of many calcium phosphates includes hydrogen, as an acidic orthophosphate anion (for example, HPO42− or H2PO4−), hydroxide (for example, Ca10(PO4)6(OH)2), and/or incorporated water (for example, CaHPO4·2H2O). Regarding their chemical composition, diverse combinations of CaO and P2O5 oxides (both in the presence of water and without it) provide a large variety of calcium phosphates, which are differentiated by the type of the phosphate anion. Namely, ortho-(PO43−), meta-(PO3−), pyro-(P2O74−), and poly-((PO3)nn−) phosphates are known. Furthermore, in the case of multicharged anions (valid for orthophosphates and pyrophosphates), the calcium phosphates are also differentiated by the number of hydrogen ions attached to the anion. Examples include mono- (Ca(H2PO4)2), di- (CaHPO4), tri- (Ca3(PO4)2), and tetra- (Ca2P2O7) calcium phosphates. Here, one must stress that prefixes “mono,” “di,” “tri,” and “tetra” are related to the amount of hydrogen ions replaced by calcium [4–6]. However, to narrow down the subject, only calcium orthophosphates (abbreviated as CaPO4) will be considered and discussed. Their names, standard abbreviations, chemical formulae, and solubility values are listed in Table 1.1 [7, 8]. Since all of them belong to CaPO4, strictly speaking, all abbreviations in Table 1.1 are incorrect; however, they have been extensively used in literature for decades and, to avoid confusion, there is no need to modify them.
Table 1.1 Existing calcium orthophosphates and their major properties [7, 8]
Ca/P molar ratio
Compound
Formula
Solubility at 25 °C, −log(
K
s
)
Solubility at 25 °C (g l
−1
)
pH stability range in aqueous solutions at 25 °C
0.5
Monocalcium phosphate monohydrate (MCPM)
Ca(H
2
PO
4
)
2
·H
2
O
1.14
∼18
0.0–2.0
0.5
Monocalcium phosphate anhydrous (MCPA or MCP)
Ca(H
2
PO
4
)
2
1.14
∼17
a
1.0
Dicalcium phosphate dihydrate (DCPD), mineral brushite
CaHPO
4
·2H
2
O
6.59
∼0.088
2.0–6.0
1.0
Dicalcium phosphate anhydrous (DCPA or DCP), mineral monetite
CaHPO
4
6.90
∼0.048
a
1.33
Octacalcium phosphate (OCP)
Ca
8
(HPO
4
)
2
(PO
4
)
4
·5H
2
O
96.6
∼0.0081
5.5–7.0
1.5
α-Tricalcium phosphate (α-TCP)
α-Ca
3
(PO
4
)
2
25.5
∼0.0025
b
1.5
β-Tricalcium phosphate (β-TCP)
β-Ca
3
(PO
4
)
2
28.9
∼0.0005
b
1.2–2.2
Amorphous calcium phosphates (ACP)
Ca
x
H
y
(PO
4
)
z
·
n
H
2
O,
n
= 3–4.5; 15–20% H
2
O
c
c
∼5–12
d
1.5–1.67
Calcium-deficient hydroxyapatite (CDHA or Ca-def HA)
e
Ca
10−
x
(HPO
4
)
x
(PO
4
)
6−
x
(OH)
2−
x
(0 <
x
< 1)
∼85
∼0.0094
6.5–9.5
1.67
Hydroxyapatite (HA, HAp, or OHAp)
Ca
10
(PO
4
)
6
(OH)
2
116.8
∼0.0003
9.5–12
1.67
Fluorapatite (FA or FAp)
Ca
10
(PO
4
)
6
F
2
120.0
∼0.0002
7–12
1.67
Oxyapatite (OA, OAp, or OXA)
f
, mineral voelckerite
Ca
10
(PO
4
)
6
O
∼69
∼0.087
b
2.0
Tetracalcium phosphate (TTCP or TetCP), mineral hilgenstockite
Ca
4
(PO
4
)
2
O
38–44
∼0.0007
b
a Stable at temperatures above 100 °C.
b These compounds cannot be precipitated from aqueous solutions.
c Cannot be measured precisely. However, the following values were found: 25.7 ± 0.1 (pH = 7.40), 29.9 ± 0.1 (pH = 6.00), and 32.7 ± 0.1 (pH = 5.28) [9]. The comparative extent of dissolution in acidic buffer is ACP ≫ α-TCP ≫ β-TCP ≫ CDHA ≫ HA > FA [10].
d Always metastable.
e Occasionally, it is called “precipitated hydroxyapatite (PHA).”
f Existence of OA remains questionable.
In general, the atomic arrangement of all CaPO4 is built around a network of orthophosphate (PO4) groups, which stabilize the entire structure. Therefore, the majority of CaPO4 are sparingly soluble in water (Table 1.1); however, all of them are easily soluble in acids but insoluble in alkaline solutions. In addition, all chemically pure CaPO4 are colorless transparent crystals of moderate hardness but, as powders, they are of white color. Nevertheless, natural minerals of CaPO4 are always colored because of the presence of impurities and dopants, such as ions of Fe, Mn, and rare earth elements [11, 12]. Biologically formed CaPO4 are the major component of all mammalian calcified tissues [13], while the geologically formed ones are the major raw material to produce phosphorus-containing agricultural fertilizers, chemicals, and detergents [14–16].
References
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The CRC Handbook of Chemistry and Physics
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