Desenvolvimento de vidros e de vidros cerâmicos sílico-fosfatados com elevado teor de alcalino-terrosos para aplicações biomédicas

A presente tese dá seguimento ao trabalho de mestrado realizado pelo mesmo autor. Pretendeu-se estudar os processos de separação de fases amorfas e de cristalização em vidros do sistema base SiO2-CaO-P2O5-MgO obtidos por fusão. Avaliou-se também como o comportamento in vitro de vidros e de vidros cerâmicos desse sistema, em especial a influência da estrutura nos primeiros e de características morfológicas nos segundos. A tese é constituída por um conjunto de artigos científicos apresentados em revistas da especialidade e em congressos. Os artigos estão agrupados em 3 capítulos (capítulos 2, 3 e 4) sendo a primeira secção de cada um deles constituída por uma introdução ao tema. No primeiro capítulo faz-se uma breve referência à evolução histórica da investigação em biovidros e biovidros cerâmicos e apresentam-se os materiais que foram objecto de estudo nesta tese. O segundo capítulo respeita a estudos de separação de fases amorfas e de cristalização em vidros. Os vidros bifásicos apresentam uma fase dispersa constituída por sílica amorfa, a qual tem uma morfologia peculiar – formações do tipo mórula. O desenvolvimento destas formações depende de parâmetros como a temperatura, o tempo de fusão e a composição química da matriz. Concluiu-se que a formação das mórulas resulta de um processo rápido de coalescência a partir de um número de formações existentes no fundido, característico da temperatura de fusão mas independente do tempo, e que as suas dimensões médias finais são função da viscosidade da fase contínua. Estudos com TiO2 permitiram concluir que a sua adição a um vidro monofásico do sistema SiO2-3CaO.P2O5-MgO provoca separação de fases amorfas e facilita a sua posterior cristalização. Com base na técnica de análise térmica diferencial (ATD) foram realizados estudos cinéticos de cristalização a temperatura variável num vidro bifásico (V4) de composição (% ponderal) SiO2 30, CaO 28,61, P2O5 24,14 e MgO 17,25. Através da aplicação de modelos teóricos concluiu-se que o processo de cristalização é constituído por duas etapas: nucleação (a » 730 °C) e crescimento de cristais (para temperaturas superiores a 800 °C), sendo a energia de activação do processo de crescimento de cristais elevada (» 763 kJ/mol) mas inferior à energia de activação de fluxo viscoso (» 1000 kJ/mol). Concluiu-se ainda ser aconselhável adaptar os modelos cinéticos às condições experimentais usadas na ATD. No terceiro capítulo são apresentados estudos de natureza estrutural subjacentes à compreensão do comportamento dos vidros em meio fisiológico sintético acelular. Através do recurso à técnica de ressonância magnética nuclear (RMN) foi identificada uma reacção de desproporcionação do silício, 2Q2- Q0 + Q4, em vidros com elevado teor de óxidos alcalino-terrosos. Esta reacção torna-se mais extensa à medida que a razão MgO/CaO aumenta. O fósforo está presente sob a forma de orto-fosfato. Neste capítulo é apresentado um outro estudo realizado com o objectivo de desenvolver um vidro monofásico com composição e estrutura idênticas às da matriz do vidro bifásico V4. O estudo da matriz do vidro V4 tem interesse na compreensão do processo de cristalização e da viscosidade (determinante do tamanho final das mórulas). O quarto capítulo é dedicado ao comportamento in vitro de vidros e de vidros cerâmicos. Analisou-se a influência da especiação do silício na bioactividade de vidros com a mesma conectividade de rede, tendo-se concluído que a sua presença modifica o comportamento in vitro destes materiais em virtude da diferente solubilidade das espécies Qn. Este comportamento promove os estágios 1-3 do mecanismo proposto por Hench. Embora seja um modificador, concluiu-se que o magnésio tem um papel estrutural diferente do desempenhado pelo cálcio, provocando um aumento da solubilidade dos vidros. O estudo do efeito da topografia e da presença de fases cristalinas no comportamento bioactivo dos vidros cerâmicos encerra esta tese. A existência de flutuações microestruturais e topográficas associada à presença de hidroxiapatite é responsável pela nucleação in vitro de apatite à superfície de vidros cerâmicos.This work extends the subject of the master thesis by the same author, its main aims being the study of the amorphous phase separation process and crystallisation in melted glasses of the SiO2-CaO-P2O5-MgO system. The in vitro behaviour of glasses and glass-ceramics was also evaluated, especially the influence of structure and morphological characteristics. This thesis is mainly composed by sets of scientific articles published in international journals and congresses. The papers are gathered in 3 chapters (Chapters 2, 3 and 4), the first section of each chapter being a introduction. In the first chapter a short historical overview of the development of bioactive glasses and glass-ceramics is presented together with the materials that were used in this investigation. The second chapter is dedicated to the amorphous phase separation and crystallisation in glasses. The two-phase glasses show a dispersed phase that is amorphous silica with a peculiar morphology – morular type formations. The development of these formations depends on melting temperature and time and on the chemical composition of the matrix. It was concluded that a fast coalescence process of a fixed amount of individual droplets was responsible for morulae formation. This fixed amount of droplets depends on melting temperature but is independent of the melting time. The final average morulae dimensions are a function of the matrix viscosity. Additions of TiO2 to an otherwise monophasic glass of the SiO2-3CaO.P2O5-MgO system were able to increase the overall crystallisation rates and to promote amorphous immiscibility in the system. The differential thermal analysis (DTA) technique coupled with theoretical models was used to study the non-isothermal crystallisation kinetics of a phase-separated glass (V4) of nominal composition (wt. %) 30 SiO2, 28.61 CaO, 24.14 P2O5, 17.25 MgO. It was concluded that the kinetics of crystallisation comprises two stages: a nucleation stage at about 730 °C and a particle growth stage for temperatures higher that 800 °C, the activation energy for growth being very high (» 763 kJ/mol) but still lower than the activation energy for viscous flow (» 1000 kJ/mol). It was also concluded that it is necessary to introduce modifications in the kinetic models in order to get a good agreement with the experimental DTA conditions. Chapter three reports structural studies of glasses in order to understand the bioactive properties of these materials. By magic-angle spinning (MAS) nuclear magnetic resonance (NMR) technique a silicon desproportionation reaction, 2Q2 Q0 + Q4, in alkalineearth rich glasses was identified. This reaction is more extensive as the MgO/CaO ratio increases. Besides Q species these glasses are composed by orthophosphate species. This chapter presents further work pursued in order to develop a monophasic glass with the same composition and structure of the matrix in two-phase glass V4. The interest for such material stems from the important role of the V4 glass matrix in the crystallisation behaviour and in the viscosity (which determines the final dimensions of the dispersed phase). The forth chapter presents in vitro studies of glasses and glass-ceramics. The influence of silicon speciation in the bioactivity of glasses with the same network connectivity is analysed and it is concluded that this phenomenon changes the behaviour of glasses due to the different solubilities of Qn species. This behaviour promotes the stages 1-3 of Hench’s mechanism. Although magnesium behaves as a modifier, it plays a structural role different from that played by calcium, the final result being an increase in glass solubility as the MgO/CaO ratio augments. The influence of morphology and nature of crystalline phases on the bioactive behaviour of some glass-ceramics was the last field addressed. It is proposed that microstructural fluctuations together with the presence of crystalline hydroxyapatite are responsible for in vitro apatite nucleation in glass-ceramics

Vidros cerâmicos dos sistema MgO-3CaO.P2O5-SiO2 para implantologia óssea

O presente trabalho teve por finalidade o desenvolvimento de vidros cerâmicos do sistema MgO-3CaO.P2O5-SiO2, para serem aplicados em implantologia óssea. Pretendeu caracterizar-se a estrutura dos materiais e relacioná-la com o desempenho mecânico e comportamento in vitro. [...]Mestrado em Engenharia dos Materiai

Crystallization kinetics of PCL and PCL–glass composites for additive manufacturing

The non-isothermal crystallization kinetics of polycaprolactone (PCL) and PCL–glass composites, used in fused filament fabrication (FFF), was investigated. Films of PCL and PCL reinforced with powders of a bioactive glass, from the CaO.P2O5.MgO.SiO2 system, were prepared by solvent casting process. Crystal structure of the samples was examined by X-ray diffraction (XRD), and thermal properties were assessed by differential scanning calorimetry (DSC), at different cooling rates (5, 10, 15 and 20 C min-1). The DSC curves of non-isothermal crystallization showed a significant dependence of crystallinity (Xc) on the cooling rate. The relevant crystallization kinetic parameters were determined from DSC traces applying a combination of Avrami and Ozawa methods (Mo’s method), Jeziorny method and Friedman method. It was observed that the presence of inorganic particles within the polymeric matrix clearly influenced the composite crystallization. The addition of glass particles allowed a decrease in Xc and accelerated the PCL crystallization rate. The slower cooling rates tested proved to be suitable for the biofabrication of PCL–glass composites by FFF techniques.publishe

Biofabrication of glass scaffolds by 3D printing for tissue engineering

This paper reports a study on the development of bioactive glass powders for biofabrication of scaffolds by an additive manufacturing technique, three-dimensional printing (3DP). Several formulations of the glass were developed from the CaO·P2O5·TiO2 system and prepared on the basis of the results for the commercial powder characterization (average particle size, particle size distribution, microstructural and crystallographic analysis). For printing the glass models in the prototyping machine, a virtual model defined as the “standard model” was produced in commercial powder, and a systematic study of the relevant processing parameters (binder composition, formulation of powder, saturation level in the shell and core, bleed compensation, and printed layer thickness) was carried out in order to determine the most suitable conditions for the fabrication of porous structures for tissue engineering applications. The printed glass models were sintered through specific thermal programs and then characterized in terms of dimensions, structure, morphological features, and mechanical properties. Finally, the sintered models were submitted to mineralization tests in simulated physiological media. In this work, it was demonstrated that it is possible to use a printing machine to manufacture 3DP glassy porous structures with suitable features for tissue engineering applications as temporary scaffolds. The mechanical properties of the produced structures and its mineralization capability in physiological fluids suggest that they have potential to be used in bone tissue regeneration under low load-bearing situations.publishe

Improvement of processability characteristics of porcelain-based formulations toward the utilization of 3D printing technology

A study of the feasibility of porcelain-based formulations for 3D printing was performed. Based on commercial materials characterization, the binder jetting process properties requirements were defined. Porcelain powder-based formulations were prepared and evaluated with different binder solutions. The powder-binder formulations were characterized (e.g., particle size distribution and wettability of powder, viscosity and surface tension of liquid binder) and showed some different and similar characteristics when compared with commercial materials. The addition of solid (sodium alginate, sucrose) and liquid (glycerol, ethanol) additives in the powder-binder composition improved the experimental printed tests. The effect of binder composition and operating process parameters (binder saturation level, bleed compensation, and printed layer thickness) was analyzed and optimized to obtain a printed saucer with different designs. Results revealed some limitations related to the materials and the technology, thus justifying the introduction of technological improvements. This study showed the possibility to process industrial porcelain powders by additive manufacturing, paving the way for a new development challenge in the productive process of ceramic products.publishe

Crystallization kinetics of PCL and PCL–glass composites for additive manufacturing

The non-isothermal crystallization kinetics of polycaprolactone (PCL) and PCL–glass composites, used in fused filament fabrication (FFF), was investigated. Films of PCL and PCL reinforced with powders of a bioactive glass, from the CaO.P2O5.MgO.SiO2 system, were prepared by solvent casting process. Crystal structure of the samples was examined by X-ray diffraction (XRD), and thermal properties were assessed by differential scanning calorimetry (DSC), at different cooling rates (5, 10, 15 and 20 C min-1). The DSC curves of non-isothermal crystallization showed a significant dependence of crystallinity (Xc) on the cooling rate. The relevant crystallization kinetic parameters were determined from DSC traces applying a combination of Avrami and Ozawa methods (Mo’s method), Jeziorny method and Friedman method. It was observed that the presence of inorganic particles within the polymeric matrix clearly influenced the composite crystallization. The addition of glass particles allowed a decrease in Xc and accelerated the PCL crystallization rate. The slower cooling rates tested proved to be suitable for the biofabrication of PCL–glass composites by FFF techniques.publishe

Characterisation of microbial attack on archaeological bone

As part of an EU funded project to investigate the factors influencing bone preservation in the archaeological record, more than 250 bones from 41 archaeological sites in five countries spanning four climatic regions were studied for diagenetic alteration. Sites were selected to cover a range of environmental conditions and archaeological contexts. Microscopic and physical (mercury intrusion porosimetry) analyses of these bones revealed that the majority (68%) had suffered microbial attack. Furthermore, significant differences were found between animal and human bone in both the state of preservation and the type of microbial attack present. These differences in preservation might result from differences in early taphonomy of the bones. © 2003 Elsevier Science Ltd. All rights reserved