Calcium phosphate ceramics are bioactive, osteoconductive materials used as guides for bone regeneration. We were able to carry out a histological study of ceramics implanted in large mammals and humans. An identical sequence of events apparently occurred at the contact of the ceramics. The osteoblasts differentiated preferentially in close proximity to the ceramic surface in animals. The early stage lasted longer in humans and was characterized by the presence of a monolayer of multinucleated cells containing TRAP+ (tartrate resistant acid phosphatase activity) granules at the surface of the ceramic. The loose connective tissue invading the pores contained almost exclusively fibroblast-like cells with TRAP+ nucleus. Differentiation of the osteogenic cells around the ceramic is diffuse in humans and not always related to the material surface. Ceramics become embedded in immature bone both in humans and animals. The immature bone and the ceramics are then remodeled and progressively replaced by layered bone. The degradation of the ceramic occurs preferentially at the grain boundaries and the grains released are phagocytosed by the macrophages when below a critical size and degraded in the low pH compartments of the cell. In conclusion: the integration of calcium phosphate ceramics is comparable in animals and man. Although in the cases studied the ceramics were unable to induce osteogenic differentiation, they did favor the differentiation of pre-determined osteogenic cells.

Bioactive ceramics are known, which can bind bone tissue chemically. The authors tested bone-bonding strength of biomaterials using detaching test and observed the interface between bone and bioactive ceramics with transmission electron microscopy. An intervening apatite layer was observed at the interface of bone and bioactive ceramics. This layer was distinguished from bone apatite or ceramic. This apatite layer was formed within several days after implantation before bone was observed on the materials. Bisphosphonate is well known to inhibit apatite formation. The injection of bisphosphonate to rabbits concentrationdependently decreased bone-bonding strength of ceramics. The apatite layer was formed on bioactive ceramics in vitro by immersing them in simulated body fluid that contained similar concentrations of inorganic ions as plasma did. Using this apatite layer formed in vitro, it is possible to characterize the apatite layer. This apatite layer enhanced the differentiation of rat bone marrow cells to bone cells in vitro. When osteoclasts were cultured on this layer, they absorbed the apatite layer.

These results suggested this apatite layer not only plays a key role for bone bonding but also behaved as bone-like tissues.

This paper reviews recent studies on self-setting calcium phosphate cements (CPC). Discussions are focused on the cement setting reactions, the products formed, those properties of the cements that contribute to their clinical efficacy, and areas of future improvements that could make CPC useful in a wider range of applications. The strengths of CPC are considerably lower than ceramic calcium phosphate biomaterials and are also lower than some of the dental cements. On the other hand, the combination of self-setting capability and high biocompatibility makes CPC a unique biomaterial. Near perfect adaptation of the cement to the tissue surfaces in a defect, and a gradual resorption followed by new bone formation are some of the distinctive advantages of CPC. In its present state CPC appears to be suitable for a number of applications. Much remains to be done to further improve its properties to meet the requirements for different applications.

Poorly crystalline apatites (PCA) are characterized by a non-stoichiometric composition, a high specific surface area and the existence of labile, non-apatitic environments detected by spectroscopic techniques. These environments have been shown to belong mainly to an hydrated layer at the surface of the crystals and determine their surface and bulk reactivity. PCA can be easily synthesized in aqueous media and the amount of labile, non-apatitic environments can be modulated by using apatites at different maturation stage. PCA can adsorb many active proteins or drugs, Growth factors (FGF-2, for example), were strongly bound and remained mostly attached to the surface. Several mineral ions which have been shown to have a biological activity such as strontium, can also be trapped at the surface or into the lattice of poorly crystalline apatites. Due to their reactivity, PCA cannot be shaped easily into materials, however, they have the ability to agglomerate irreversibly into solid body at low temperature. These low temperature ceramics show a variable amount of pores (30 to 60 %) and pore size (5 to 25 nm) in which organic components can be trapped and are released only by the dissolution of the ceramic.

A new composite material consisting of hydroxyapatite (HA) and polysulfone (PSU) was produced and evaluated for potential medical applications. The HA/PSU composite containing up to 40vol% of particulate HA was manufactured via a standardised procedure which included drying, compounding, and injection or compression moulding. Defect-free composite samples (bars, discs and dumbbell specimens) could be obtained by injection moulding. Thick composite plates suitable for making fatigue specimens were compression moulded. Both compounded materials and moulded parts were assessed using various techniques. It was found that HA particles were well dispersed in the PSU matrix. Thermogravimetric analysis verified the amount of HA in the composite. Density close to the theoretical value could be achieved for the composite, indicating a void-free structure. Differential scanning calorimetry results indicated that the glass transition temperature of the polymer matrix was not significantly affected by the incorporation of HA. Rheological analysis revealed that PSU and the composite exhibited pseudoplastic flow behaviour at processing temperatures. It was shown that the hardness and modulus of the composite were increased with an increase in HA volume percentage.

Nanocomposites of hydroxyapatite (HAp) and collagen (Col) were prepared by a biomimetic coprecipitation method under controlled pH and temperature. Transmission electron micrographs showed that the c-axis of HAp nanocrystals aligned along collagen by a self- rganization mechanism; a very similar alignment to bone's structure was found in the sample prepared at nearbiological conditions, i.e. pH8–9 and 40'C, which consisted of fibers with length of 20μm. From the material scientific point of view, therefore, osteoblasts may control the chemical condition in a small area around the cells to pH 8–9 as well as supply raw materials of bone, hydroxyapaite and collagen, at the initial state of osteogenesis. The biocompatibility of the composites was examined by animal tests using beagles. The composites were absorbed and new bone was formed as rapidly as a bone remodeling process. The HAp/Col composites prepared by biomimetic process were expected to be applicable as filler materials changing to new bone instead of autograft.

The HA coatings were heated separately in vacuum, air and water vapor. The dissolution of the HA coatings was investigated by immersion in Tris buffer and SBF The dissolubility of HA coatings in the solutions decreased in this order: as-received, heated in vacuum, in air and in water vapor. The nucleation of bone-like apatite on the surfaces of HA coatings after immersing a period of 11 days in SBF was observed by SEM. The microenvironment with a sufficient supersaturation of Ca and P ions was crucial for the nucleation and growth of apatite in SBF. The dissolution of amorphous phase in coatings played an important part in establishing the supersaturation of Ca and P ions.

A process has been developed which permits the encapsulation of an established class of platinum anticancer drugs such as cis-Platin (cis-diammine-dichloroplatinum (II)) within synthetic biocompatible calcium phosphate films that are electrochemically-grown on porous Si/Si substrates. These platinum complex-doped hydroxyapatite / porous Si / Si materials have been characterized by scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (XEDS), and secondary ion mass spectrometry (SIMS). Upon immersion of these structures into aqueous media, the desired platinum species can be released into the surrounding environment. This study also focuses on the influence of initial platinum concentration for a given complex in the deposition process on the rate and resultant amount of platinum complex that can be delivered to the surroundings in vitro. Both inductively-coupled plasma (ICP) spectroscopy and uv-visible absorption spectrometry have been employed to monitor the release of the encapsulated drug from the calcium phosphate layers.

Titanium alloy (Ti-6AI-4V) and stainless steel (316L) are two of the most commonly used materials in the manufacture of orthopaedic implants. To achieve better biocompatibility with bone, metal implants made of 316L or Ti-6Al-4V are often coated with calcium hydroxyapatite (HA) bioceramics. This paper is to describe a new dipping solution recipe used for HA coating. Sample characterization was performed by SEM and XRD.

The aim of the present study is to obtain structural and chemical information about nucleation and growth of dental enamel as a function of location in secretory mouse foetus enamel. Then enamel tissues were obtained from 19-days-mouse foetus. TEM images show that enamel crystallites had the well-documented thin ribbons, or needles. They were found in the outermost zone in the vicinity of ameloblasts. Chemical analysis by EDS showed that, besides the elements of hydroxyapatite, Na, Cl, Al and Mg are presents from the beginning of enamel mineralization. All of these elements will play an important role on the physics and chemical properties in mature enamel.

We present a comparison between detailed structural parameters of a low temperature synthetic carbonate hydroxyapatite, a hydroxyapatite, and a natural carbonate fluorapatite from Rietveld refinements of neutron powder diffraction data. The observed modifications in the crystallographic parameters of the synthetic carbonate apatite are consistent with the carbonate substitution on the mirror plane of the tetrahedral site as in the case of the natural carbonate apatite.

The structure and mechanical properties of the dentin-enamel junction (DEJ) in human teeth play a critical role in transferring stress from hard enamel to soft dentin efficiently in order to preserve the longevity of this functionally gradient biocomposite. In this investigation, nano-hardness and elastic modulus of incisor teeth were studied across the dentin-enamel junction. It was found that, over a length scale of between 15 to 25 μm, there were decreasing trends in the values of both hardness and elastic modulus across the DEJ zone profiling from enamel to dentin. Images obtained, using atomic force and scanning electron microscopy techniques, from polished surfaces of cross-sectioned teeth samples showed an interpenetrating microstructure of enamel and dentin at the DEJ zone. These results suggest that the nano-mechanical property profiles across the DEJ were due to a continuous variation in the relative amount of enamel and dentin. These characteristics of the DEJ zone could be significant for describing the structural and mechanical coupling of the two structures. By increasing the interfacial contact area across the two mineralized tissues, stresses are dissipated into the softer dentin, thus reducing interfacial stress concentrations at the DEJ. This promotes effective load transfer from the hard enamel to soft dentin.

The systematic appearance of (000l), l=2n+1, forbidden reflections in the electron diffraction patterns of hydroxyapatite, both synthetic and natural (human tooth enamel), is discussed. Structural disorder, double diffraction, and modulated structures and are discussed as possible causes. Structural disorder and modulated structure could be responsible for the rupture of the reported symmetry, in which oxygen and hydrogen perform an important role.

The generation of minerals such as calcium phosphates on the surfaces of dental and joint replacement implants is beneficial since the facilitation of bone formation permits their fixation. In contrast, the prevention of crystallization is desired on other surfaces such as kidney and cardiac valve prostheses. A key to the development of successful biomaterials is therefore an understanding of the factors that control crystal nucleation, growth and dissolution in aqueous solution. The Constant Composition method was used to investigate the influence of factors such as solution composition, ionic strength, pH and temperature on the crystallization and dissolution of the calcium phosphates, brushite (DCPD), octacalcium phosphate (OCP), hydroxyapatite (HAP) and fluorapatite (FAP). In parallel with these studies, a contact angle method along with surface tension component theory was employed to investigate the roles of interfacial free energy in mineralization and demineralization. Values of the interfacial tensions, -4.2, 4.3, 10.4 and 18.5 mJm-2 obtained from contact angle measurements for DCPD, OCP, HAP and FAP, respectively, compare well with those calculated from dissolution kinetics experiments and provide information concerning the growth and dissolution mechanisms. The exploitation of these approaches is illustrated in studies of the coating of specific calcium phosphate phases on titanium metal and alloy surfaces and nucleation and growth of OCP on polymer surfaces modified by silanization to produce amine- and carboxylterminated end groups. In all these reactions involving the calcium phosphates, concomitant dissolution reactions are often involved. Constant Composition kinetic studies have shown that the rate of these reactions decrease markedly with time despite a sustained driving force, and eventually, the rates approach zero even though crystals remain in the undersaturated solutions. Dissolution can be reinitiated by exposing the crystals to the solutions of different undersaturations. These results suggest that dislocation sizes play a significant role in the dissolution kinetic processes.

In bone replacement surgery, replacement of joints (of hip, knee, finger and jaw) is a major sub discipline, requiring mechanically strong and biologically compatible, or biocompatible, implants. Since mechanical strength can only be achieved with metals that lack the required biocompatibility, surface treatments to improve that lack have been studied extensively. Since the mineral phase of bone consists of various calcium salts, the most relevant one being (carbonated) apatite, the surface treatments of choice are coatings of such salts on orthopedic implants have been successfully developed. (Bulk ceramics of CaP are too brittle to be used for load bearing implants). Although currently used coatings as obtained by plasma spraying have been highly successful, the high temperature and line-of-sight nature of the plasma spray process prevent (1) coatings to be deposited on implants made of polymer composites, (2) coatings to be deposited on complex surfaces and/or within porous implants and (3) biological molecules such as growth factors or antibiotics to be included in the coating. Therefore, we and several other research groups have focussed our attention to so called biomimetic coatings, that are produced at ambient temperature by a precipitation process from fluids resembling body fluids in their inorganic composition. In this presentation, we will give a review of some current developments on biomimetic coatings.

A method for applying HA coatings to metallic surfaces, without the use of high temperature plasma spraying, has been developed. The HA coating is precipitated in an aqueous solution under conditions similar to those occurring in the body during bone formation. This surface mineralization process results in a coating which is highly crystalline and 100% HA. Processing and characterization experimentation and results are included in the paper.

The coating method has the ability to penetrate macroporous (> 100 micron pores) metal structures and coat the undersides and interior surfaces of the areas designed for bone ingrowth on orthopedic devices used for joint replacements. This ability to coat all around a surface is the basis for the name Peri-Apatite HA. Electron microscopy is presented illustrating the coverage and morphology of the coating.

The Peri-Apatite HA coating is currently used on both knee joint replacement and hip joint replacement implants.

The present authors previously showed that titanium metal, which was exposed to 5.OMNaOH solution at 60°C for 24 h and heat-treated at 600°C for 1 h, spontaneously forms a bonelike apatite layer on its surface in the living body, and tightly bonds to the bone through the apatite layer. In the present study, mechanism of the apatite formation on the bioactive titanium metal was investigated in an acellular simulated body fluid (SBF). A thin sodium titanate layer was formed on the surface of the titanium metal by the NaOH and heat treatments. The sodium titanate layer released Na+ ions via exchange with H3O+ ions in SBF, to form a lot of Ti-OH groups on its surface. The Ti-OH groups first combined with Ca2+ ions in SBF, and then later with PO43- ions to form the apatite. Titania and Na2O-TiO2 gels prepared by a sol-gel method as model substances of the sodium titanate layer on the surface of the titanium metal showed that Ti-OH groups of anatase structure are effective for the apatite nucleation, whereas those of amorphous structure and Na2Ti5O11 crystal are not effective.

The aim of this study was to accelerate the formation of biomimetic Calcium-Phosphate (Ca-P) coatings on Ti6Al4V by using a 5 times more concentrated Simulated Body Fluid (SBFx5) than the regular SBF. The production of SBFx5 was possible by decreasing the pH of the solution to approximately 6 with CO2 gas. The release of this mildly acidic gas allowed the formation of a Ca-P film after 4h of immersion at pH=6.8. The structure of this coating was an amorphous carbonated Ca-P. In addition, our experiments showed that the presence of Mg2+ was absolutely necessary for the Ca-P coating formation on Ti6Al4V substrate. Mg2+ is a crystal growth inhibitor and favored the heterogeneous nucleation. Furthermore, depth profile X-Ray Photoelectron Spectroscopy showed that Ca-P nucleation on the passive Titanium oxide (TiO2) passive layer was initiated by Ca2+ and Mg2+. The attachment of this Ca-P coating resulted probably from chemical bonds such as P-O-Ti and Ca-O-Ti. Ca was more present at the coating/substrate interface than P. Thereby Ca-O-Ti seems to be favored.

Apatite formation on artificial materials in a body environment is the prerequisite condition for showing bioactivity i.e. bone-bonding ability. A specific hydrated silica or titania gel has the ability of apatite deposition in body environment. We electrochemically prepared such a bioactive titanium oxide layer on titanium(Ti) with a cell consisting of Ti as the working electrode, Pt as the counter one, Ag/AgCl as the reference one, and an aqueous solution of 0.1 mol/L Ca(NO3)2 as the electrolyte solution. Ti was kept at 9.5V for 1 hour for oxidation(denoted as Ca9.5). Ti was subject to cathodic polarization at −3.0V for 10 min(Ca-3.0).: calcium ions were expected to be adsorbed on its surface. On treatment Ca9.5–3.0 Ti was first oxidated at 9.5V for 1 hour and subsequently kept at −3.0V for 10 min. The specimens of Ca9.5–3.0 and Ca-3.0 were found so bioactive as to deposit apatite within 12 hours and 1 day, respectively, in a simulated body fluid(Kokubo solution) whereas those due to Ca9.5 could not deposit apatite within 7 days. Calcium hydroxide and calcium carbonate detected on the bioactive surface caused no harmful effects on spontaneous deposition of apatite in the fluid.

We have used Hartree-Fock molecular orbital calculations to model the interactions of Ca2+, H2PO41-, HPO42- and H2O with bioceramic reactive surface sites represented by [Si3O6H6] to determine the reaction sequence for apatite precipitation at bioceramic surfaces. By comparing predicted reaction energies, viberational frequencies, abd NMR shifts for 29Si, and 31P with experimental values we were able to identify stable intermediates during reaction progress.

At the pH of blood, our calculations predict that (i) the 3-ring is the active surface site, (ii) formation of [SiO-Ca-OPO3H] bonds is energetically preferred over direct Si-O-P bonds, and (iii) an acidic precursor with bidentate Ca>OPO3H bonds, [Si3O6H5CaHPO4(H2O)3]1-, nucleates rapidly (minutes to 1 hour) at the bioceramic surface. Predicted precursor complex, [Si3O6H5CaHPO4(H2O)3]1-, and young bone share unique IR bands at 631 and 1125–1145 cm-1. Going beyond the scope of our calculations, a literature survey suggests that subsequent slow (1–2 weeks) aggregation of oligomers by H-bonds between surface Si-OH and HPO4 groups results in apatite formation. Analogously, carboxyl and phosphoryl groups on sialoprotein and hydroxyl-terminations on collagen may provide the nucleation and H-bonding sites for natural bone formation in the absence of the bioceramic.