Development of hydroxyapatite/calcium silicate composites addressed to the design of load-bearing bone scaffolds.

This work deals with the preparation of bioactive ceramic composites to be employed for the development of load-bearing bone substitutes, made of hydroxyapatite (Ca(10)(PO(4))(6)(OH)(2), HA) and bioactive dicalcium silicate (Ca(2)SiO(4), C(2)S) as a reinforcing phase. The composite materials were prepared by Fast Hot-Pressing (FHP), which allowed the rapid sintering of monolithic ceramics at temperatures up to 1500 degrees C, well above the commonly adopted temperatures for the consolidation of hydroxyapatite (1200-1300 degrees C). The purpose was to achieve the grain coalescence of both HA and the strengthening phase, so that to obtain a homogeneous ceramic material characterized by controlled phase composition and improved mechanical strength; the dwell time was reduced as much as possible to prevent HA decomposition and excessive grain growth.

Development of Sr and CO3 co-substituted hydroxyapatites for biomedical applications.

Sr and CO3 co-substituted hydroxyapatite (SrCHA) nanopowder was synthesized by neutralization. The powder was characterized. The improved solubility in Hanks' balanced solution of SrCHA granules (400-600 microm of dimensional range), potentially usable as bone filler, was assessed and compared with that of an analogous carbonate free granulate.

Sr-substituted hydroxyapatites for osteoporotic bone replacement.

Porous apatites, which during resorption can release in situ Sr ions, were prepared to associate an anti-osteoporotic action with the peculiar features of the inorganic phase constituting the bone. Sr-substituted hydroxyapatite (SrHA) powder was directly synthesized using the classical neutralization route, but including Sr ions, and characterized. The higher solubility of SrHA granules of 400-600 microm size, potentially usable as a bone filler, was assessed compared with that of analogous stoichiometric HA granules.

Crystallinity in apatites: how can a truly disordered fraction be distinguished from nanosize crystalline domains?

In the last decade synthetic apatites mimicking the human natural one have been widely prepared and characterized from the physico-chemical point of view; however a shading zone is still remaining related to the evaluation and distinction of the less crystalline part, almost amorphous, and the crystallographically well ordered, nano-sized part, inside the apatite itself. Actually natural apatite forming bone tissue can include both types of crystals whose prevalence is dependent from the specific bone evolution stage and the specialized tissue performance. The quantitative description of such a combination usually represents a puzzling problem, but the result can also clarify the definition of "crystallinity in apatite" that appears still controversial.

HA/alginate hybrid composites prepared through bio-inspired nucleation.

Poorly crystalline apatite has been directly nucleated on self-assembling alginate chains by neutralization synthesis to obtain a biomimetic artificial bone-like composite. It has been observed that in preparing HA/alginate composites, Ca2+ ions present on the apatitic surface cross-link the alginate chains to produce a material with different morphology and thermal stability, both functions of the HA/alginate weight ratio. In vitro tests were performed on different samples in terms of both the HA/alginate ratio and synthesis temperature.

From biomimetic apatites to biologically inspired composites.

Hydroxyapatite is an elective material for bone substitution. In this outline of our recent activity the crucial role of nanostructured ceramics in the design and preparation of ceramic scaffolds will be described, focussing on our more recent interest in biomimetic apatites, in particular apatites containing HPO42- CO32- and Mg2+ which are similar to the mineral component of bone. The paper describes such nanostructured products and, in particular, innovative synthetic techniques capable of yielding powders with higher reactivity and bioactivity.

Nucleation of biomimetic apatite in synthetic body fluids: dense and porous scaffold development.

The effectiveness of synthetic body fluids (SBF) as biomimetic sources to synthesize carbonated hydroxyapatite (CHA) powder similar to the biological inorganic phase, in terms of composition and microstructure, was investigated. CHA apatite powders were prepared following two widely experimented routes: (1) calcium nitrate tetrahydrate and diammonium hydrogen phosphate and (2) calcium hydroxide and ortophosphoric acid, but using SBF as synthesis medium instead of pure water. The characteristics of the as-prepared powders were compared, also with the features of apatite powders synthesized via pure water-based classical methods.

Porous phosphate-gelatine composite as bone graft with drug delivery function.

The design and synthesis of porous phosphate-gelatine composite implant which mimicks the structure of natural bone and has drug delivery function is proposed. Gelatine reproducing the proteinaceous part of bone was cross-linked in order to modulate its solubility in the physiologic fluids. The kinetic of gelatine release from ceramic matrix was also evaluated as model of the release of any therapeutic compound which can be loaded into gelatine.

Influence of synthesis and sintering parameters on the characteristics of carbonate apatite.

A new method to synthesise carbonate-substituted hydroxyapatite (CHA) powder has been set up introducing a CO(2) flux, as a source of carbonate, in the HA synthesis process based on the neutralisation reaction. The reactants are abundant and inexpensive. The yield is good compared to other CHA powder synthesis.

Biologically inspired synthesis of bone-like composite: self-assembled collagen fibers/hydroxyapatite nanocrystals.

Replacement of bone tissue by graft materials and products of tissue engineering having composition, structure, and biological features that mimic natural tissue is a goal to be pursued. A biomimetic synthesis was performed to prepare new bone-like composites constituted of hydroxyapatite nanocrystals and self-assembled type I collagen fibers. We used a biological inspired approach that proved that the biological systems stored and processed information at the molecular level.

Porosity-graded hydroxyapatite ceramics to replace natural bone.

Porous hydroxyapatite HA bodies were prepared with an aim to simulate bone tissue morphology. By varying the characteristics of starting HA powders and the impregnation strategy of cellulosic sponges with rheologically optimized slurries, a wide range of physico-chemical and mechanical properties of the porous ceramics can be obtained. The samples were characterized microstructurally, by density and porosimetry and by mechanical strength.

Sintering and characterization of HA and TCP bioceramics with control of their strength and phase purity.

HA and beta-TCP-based ceramics were prepared using commercial powders. Powder characteristics were defined and the processing parameters studied, aimed at the production of samples with improved microstructural and mechanical properties. The behaviour of HA powder subjected to various thermal treatments was investigated in order to control the formation of secondary phases (alpha- and beta-TCP) during sintering.

Mineral evolution of bone.

A study on the evolution with age of the mineral composition of bones was performed on samples belonging to human and other common mammalian species (cattle, sheep, dog). The study was carried out on the ashes obtained by calcination of the bone samples (1 h at 900 degrees C). The calcined powders were carefully examined by X-ray diffraction, from which precise quantitative evaluation (also confirmed by chemical analysis) of the crystalline phases present was derived.

Hydroxyapatite-based porous aggregates: physico-chemical nature, structure, texture and architecture.

At the request of medical teams from the maxillofacial sector, a highly porous ceramic support based on hydroxyapatite of around 70-80% porosity was produced with a pore size distribution similar to bone texture (< 10 microns, approximately 3 vol%; 10-150 microns, approximately 110 vol%; > 150 microns, approximately 86 vol%). The ceramic substrates were conceived not only as a fillers for bone cavities, but also for use as drug dispensers and as supports to host cells to produce particular therapeutic agents. A method is suggested to obtain a substrate of high porosity, exploiting the impregnation of spongy substrate with hydroxyapatite ceramic particles.

Granulates based on calcium phosphate with controlled morphology and porosity for medical applications: physico-chemical parameters and production technique.

Since the pore size distribution of a material in contact with bone is decisive for its type of link with the tissue, many granules are commercially available as fillers and as bone reconstructing materials. We propose a new technological procedure. The optimum architectural design for obtaining the most suitable link in vivo is investigated.