S. Padilla

Synthesis of porous hydroxyapatites by combination of gelcasting and foams burn out methods

The biocompatibility and the osteoconductive behavior of hydroxyapatite (OHAp) ceramics are well established. Bioceramics made of OHAp are available in dense and porous form. Recently it has been proved that the volume of bone ingrowth at early times is primarily interconnectivity dependent. A new method for the obtention of porous OHAp ceramics that combine the in situ polymerization (gel casting method) and the foams burn out is proposed. Four polyurethane foams with different cells/cm were used. The foams were fully filled of an OHAp polymerizable suspension that after gelled produced very homogeneous and strong green bodies. After different thermal treatments the green bodies yield porous OHAp ceramics that were a replica of the foams used. Materials used in this work were studied by X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), N2 adsorption isotherm, particle size distribution, and Hg porosimetry. Porous pieces of OHAp obtained are constituted by polyhedral-like particles (0.45–1.0 μm) that are surrounded by an interconnected network of pores. A bimodal distribution of the pores size between 30.8–58.6 and 1.0–1.2 μm has been observed. The size of the interconnected pores (30.8–58.6 μm) was controled as a function of the cells/cm of the foam while the volume of the small pores was modified as a function of the sintering time. The presence of pores could promote the bone ingrowth and also could be used to insert different drugs, which makes these porous pieces a potential candidate to be used as non-load-bearing bone implants and as drug delivery systems.

Gentamicin release from hydroxyapatite/poly(ethyl methacrylate)/poly(methyl methacrylate)composites

In this work the release kinetics of gentamicin sulfate (GEN) in samples composed by hydroxyapatite, poly(methyl methacrylate), and poly(ethyl methacrylate) has been studied. The release study was performed by soaking three samples in simulated body fluid at 37 degrees C; the medium was periodically replaced during 70 days. The concentration of GEN was determined by the o-phtaldialdehyde method. The release profile shows three stages: the first stage, occurring during the first 10 h, corresponds to a fast release (nearly 30% of the drug is released in this period). The second stage is slower and includes from the first 10 h to 16 days, releasing 60% of the total amount of GEN. The final stage is the slowest and it takes from 16 to 70 days (10% of GEN is released). The fraction of released GEN versus square root of time can be fitted to a third order polynomial, corresponding with the model proposed by Cobby et al. (J Pharm Sci 1974;63:725-732). The characterization of the samples after the release study shows that a carbonate hydroxyapatite layer has grown on the whole surface of the composites.

In vitro behaviour of adult mesenchymal stem cells seeded on a bioactive glass ceramic in the SiO2–CaO–P2O5 system

This work describes the evaluation of a glass ceramic (55S41C4P-1300) as a potential substrate for bone tissue engineering. For that purpose, the capacity of mesenchymal stem cells (MSCs), isolated from rabbit bone marrow, to adhere, proliferate and differentiate into osteoblast (OBs) with or without 55S41C4P-1300 was investigated. Two types of culture medium, i.e. growth medium (GM) and osteogenic medium (OM), were evaluated. The bioactive 55S41C4P-1300, containing pseudowollastonite, wollastonite, tricalcium phosphate and crystoballite as crystalline phases, was obtained by heat treatment of a sol-gel glass (55SiO(2), 41CaO, 4P(2)O(5) (mol.%)) at 1300 degrees C. The results showed that the MSCs adhered, spread, proliferated and produced mineralized extracellular matrix on 55S41C4P-1300 regardless of the culture medium used. As the same time, they showed an osteoblastic phenotype, and this phenomenon was accompanied by the gradual diminution of the marker CD90 expression. The 55S41C4P-1300 was able to induce the differentiation of MSCs into OBs in the same way as OM without glass ceramic. This effect increased with the combination of 55S41C4P-1300 with OM. The glass ceramic evaluated in this work is bioactive, cytocompatible and capable of promoting the differentiation of MSCs into OBs. For that reason, it could be regarded as a suitable matrix in tissue engineering for bone tissue regeneration.

In vitro release of gentamicin from OHAp/PEMA/PMMA samples.

The influence of hydroxyapatite (OHAp) and gentamicin sulphate (GEN) contents on the release kinetics of GEN, in samples composed of OHAp, poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA) has been studied. For this purpose, samples with 30 and 40% of OHAp and 5 and 9% of GEN were prepared. The in vitro release study was carried out soaking the samples in simulated body fluid (SBF) at 37 degrees C for 70 days. The release profiles showed a faster release during the first 10 h, diminishing progressively until the end of the study. It was noticed that the percentage of released GEN increased with the OHAp content. For samples with 40% of OHAp, GEN release is nearly independent of the initial amount of such drug (in the range 5-9%), whereas for samples with 30% of OHAp, the release process is favoured by higher contents of GEN, which would favour a higher SBF uptake. GEN release is related to SBF uptake, which is in turn related, on the one hand, to the OHAp content (increase of the porosity and the hydrophilic character of the samples) and on the other hand, to content of GEN.

Bioactivity and mechanical properties of SiO2–CaO–P2O5 glass-ceramics

Glass-ceramics in the system SiO2–CaO–P2O5, with 55 mol% of silicon content, with and without phosphorus, were obtained by thermal treatment of sol–gel glasses as the precursors. The influence of the composition of the precursor glass on the crystallisation, bioactive behaviour and mechanical properties of these glass-ceramics was investigated. An interesting result is that when phosphorus was not present in the precursor glass, the biaxial flexural strength of the glass-ceramics increased by one order of magnitude. The crystalline phases obtained were similar in both glass-ceramics, although when phosphorus was present the crystallisation of tricalcium phosphate and the stabilisation of the Wollastonite phase at high temperature in the glass-ceramic took place Both glass-ceramics, without and with phosphorus, showed a fast in vitro bioactive response; a new apatite-like layer fully covered the surface of both materials after soaking in simulated body fluid for a short time. The rate of the apatite layer formation mainly depended on the sintering process, which took place at lower temperatures in the glass-ceramics without phosphorus. The glass-ceramics containing phosphorus showed a decrease in the values of the Weibull coefficient, an increase of porosity and pore size, and a heterogeneous microstructure, worsening their mechanical properties.

Concentrated Suspensions of Hydroxyapatite for Gelcasting Shaping

The aim of this work was to prepare suspensions of Hydroxyapatite ( HA) with a high solid content, necessary for gelcasting shaping. The HA was obtained by the precipitation method by reaction between Ca(OH) 2 and H3PO4 and it was calcined at temperatures between 900 and 1200oC for 1h. A monomeric solution containing methacrylamide and N,N’-methyl enebisacrylamide was used as dispersing vehicle and Darvan 811 was used as disper ant. The monomeric solution, dispersant and HA powder were mixed in a planetary ball mil l for 30 min. The influence of powder calcination temperature on phase composition, surface area, poros ity, optimum concentration of dispersant and rheological behaviour were studied. Concentra ted slurries (up to 60 vol.%) could be prepared using HA powder previously calcined at 1200oC. Introduction Janney and co-workers [1,2] developed the gelcasting method in 1990. This method allows obtaining pieces with complex shapes and high mechanical strength. In t e gelcasting method, slurries with high solid content are prepared using an aqueous solution of mon mers as dispersing vehicle. After polymerisation and drying, green bodies with high density and strength are obtained. Gelcasting requires slurries with good flow properties and high soli d c ntent (at least 50 vol.%). However, only solid contents lower than 50 vol.% have been achieved in most of the previous works dealing with preparation of HA slurries [3-6]. HA is a very important implantation material, because of its excellent biocompatibility and chemical composition, s imilar to the mineral component of the bone. However, its clinical applications have been limite d du to its low mechanical properties and to the difficulty to obtain pieces with com plex shapes similar to the bone defects. In this sense, the gelcasting method could be a good alternat ive for HA shaping. The aim of this work was to prepare suspensions of HA with a high solid content, nec essary for gelcasting shaping. For this purpose the influence of calcination temperature of HA on phase composition, surface area, porosity, optimum concentration of dispersant and rheological behaviour was studied. Materials and Methods HA was prepared by the precipitation method by reaction of Ca(OH) 2 and H3PO4 solution. The H3PO4 solution was slowly added to the Ca(OH) 2 suspension until pH neutralization, the reaction was carried out at 90oC. The obtained HA was calcined at 900, 1000, 1100 and 1200oC f or 1h. HA was characterized by FTIR, XRD, N 2 adsorption porosimetry and Hg intrusion porosimetry. The XRD patterns were register in a Philips X’Pert MDP diffra ctometer (Cu Kα radiation), the specific surface area was calculated using the BET method from N 2 adsorption isotherm obtained in a Micromeritics ASAP2010 analyser. The porosity was determined by Me rcury intrusion porosimetry (Micromeritics AutoPore III). The molar ratio Ca/P was deter mined quantitatively by XR fluorescence (S4 Explorer-Brucker AXS). The dispersing vehicle was an aqueous solution containing 15 wt% of methac rylamide (Aldrich) and N,N’-methylenebisacrylamide (Aldrich) monomers in a 6/1 ratio. D arvan 811 (D811, R.T. Vanderbilt Company, Inc.) was used as dispersant. The HA suspensions wer e prepared mixing the suspension vehicle, the dispersant and the HA powders in a planetary ball mill for 30 min. Key Engineering Materials Online: 2003-12-15 ISSN: 1662-9795, Vols. 254-256, pp 35-38 doi:10.4028/www.scientific.net/KEM.254-256.35

Bioactive Behaviour in Biphasic Mixtures of Hydroxyapatite-sol Gel Glasses in the System SiO2-CaO-P2O5

Abstract. Biphasic mixtures of hidroxyapatite (HA) and sol gel glasses with a ratio of 95/5 (wt %) were prepared. The composition of the studied glasses was 55S: 55-SiO2; 41-CaO; 4-P2O5 (mol %) and 70S: 70-SiO2; 30-CaO (mol %). The mixtures were conformed as disks by uniaxial and isostatic pressure and then heated at 1300oC for 24 h. After thermal treatment of HA, small diffraction maxima corresponding to Ca4P2O9 (TeCP) were observed, whereas in the biphasic materials TeCP was not observed and c and dCa3(PO4)2 (TCP) phases appeared, being the ratio between these two phases and HA slightly higher in the glass 70S. After soaked in SBF for 7 days the surface of biphasic mixtures was covered by an apatite like phase. In the sample HA/70S the surface was completely covered whereas in the HA/55S sample some zones of the surface were not covered.