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. However, so far the characteristics of artificial bone tissues have been shown to be very different from those of natural bone, mainly because of the absence of the peculiar self-organizing interaction between apatites and the protein component. This causes modification of the structure of apatites and of the features of the overall composite forming human bone tissue. Therefore, attempts to mimic the features and structure of natural bone tissue, leading toward so-called bio-inspired materials, will be speculated upon. New techniques used to reproduce a composite in which a nanosize blade-like crystal of hydroxyapatite (HA) grows in contact with self-assembling fibres of natural polymer will be presented. In this specific case, the amazing ability of biological systems to store and process information at the molecular level, nucleating nanosize apatites (bio-inspired material), is exploited.

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. All the samples were cultured for seven days with MG63 osteoblast-like cells and then underwent morphological and biochemical analyses (MTT and ALP tests). Scaffolds showed a different solubility into the culture media, which was related to the temperature of synthesis and to the HA/alginate ratio. All our data confirm the ability of the tested materials to favour cell growth and to maintain their osteoblastic functionality, at least during the examined period.

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.

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. Cylindrical specimens exhibiting a porosity gradient showed a promising behaviour after implantation in rabbits' femur: newly formed bone grew in tight contact with the ceramic in a very short time, no modified cells are induced and bone tissue fills even the inner pores.

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. The optimal thermal treatment required to prepare pure beta-TCP powder from the precursors (HA and DCP) was determined and the sintering method required to prepare fully dense beta-TCP completely free from alpha-form, was identified. Translucent hot-pressed beta-TCP ceramics with potential applications in aesthetic restorative prostheses were prepared and characterized. The interval of existence of alpha-TCP and alpha-TCP as secondary products was also defined. Crystallographic analysis was carried out on the imperfectly known low-temperature alpha-TCP phase, and a proper monoclinic unit cell determined.

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. These data were analysed as a function of the introduced fractional age phi, a new relative scale that allows even largely different lifespan species to be compared. An overall linear increase in (Ca + Mg)/P ratio with log phi was found and the other considerations on molecular constitution (especially as regards Mg2+ substituting for Ca2+ in very young subjects) of the various phases detected were formulated and relative implications evaluated. The results appear promising for an improvement of knowledge in the field of biomedical experimentation and clinical implantology.

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. X-ray and scanning electron microscopy analyses were carried out to evaluate the nature of the new ceramic support in comparison with the most common commercial product; pore size distribution and porosity were controlled to known hydroxyapatite ceramic architecture for the different possible uses.

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. Particular attention is attached to the granulate texture: micropores, macropores, total volume of pores, pore size distribution, and the morphology and shape of the pores. These characteristics are studied in order to obtain the best porosity for hydroxyapatite granulates now applied in vivo, with interesting results in mechanical and hard tissue linkage terms.