Thin films that are functionally gradient improve the mechanical properties of film-substrate layered materials. Mechanical properties of such materials are found by using indentation tests. In this study, finite element models are developed to simulate the indentation test. The models are based on an axisymmetric half space of a specimen subjected to spherical indentation. The film layer through the thickness is modeled to have either homogeneous material properties or nonhomogeneous material properties that vary linearly. Maximum indenter displacement, and maximum normal and shear stresses at the interface are compared between the homogeneous model and the nonhomogeneous model for pragmatic contact length to film thickness ratios of 0.2 to 0.4, and film to substrate moduli ratios of 1 to 200 to 1. Additionally, a coefficient is derived from regression of the stress data produced by these models and compared to that used to define the pressure field in the axisymmetric Hertzian contact model. The results of this study suggest that a displacement boundary condition to an indenter produces the same results as a pressure distribution boundary condition. The critical normal stresses that occur between modeling a film as a nonhomogeneous and as a homogeneous material vary from 19% for a modulus ratio of 2.5:1 to as high as 66% for a modulus ratio of 200:1 indicating that the modeling techniques produced very different maximum normal stresses. The difference in the maximum shear stress between the nonhomogeneous and the homogeneous models varied from 19% for a 2.5:1 modulus ratio to 57% for the 200:1 modulus ratio but reached values as low as 6% for the 50:1 modulus ratio. The maximum contact depth between the nonhomogeneous and the homogeneous models varied from 14% for the 2.5:1 case to as much as 75% in the 200:1 case. The results from the reapplication of the pressure field derived from the regression coefficients and the R2 values from these regression models indicate the correctness of the regression model used as well as its ability to replicate the normal stresses in the contact area and maximum indenter displacements in a FEA model for both the homogeneous and the nonhomogeneous models for modulus ratios ranging from 2.5:1 to 200:1. The agreement between the regression based coefficients and the force based coefficients suggests the validity for the use of the theoretical axisymmetric Hertzian contact model for defining the pressure field in the contact area and displacements for both the homogeneous case and the nonhomogeneous case for the considered film to substrate moduli ratios and contact length to film thickness ratios.

Thermosetting polymers can cure at a gradient of cure temperatures due to a variety of factors, including heat transfer in the thermoset during heating and the exotherm due to the chemical reaction occurring during the cure. A new method for assessing the effect of cure conditions on mechanical behavior of toughened thermosets has been developed. Modeling of the phase separation process of a model thermoset system provided detailed understanding of the mechanism of property variation with cure temperature for this material. Subsequent characterization of gradient temperature cured samples has shown important variations, illustrating not only the importance of cure conditions, but the possibility of producing materials with new and useful properties. A special mold was developed to cure samples in a controlled gradient of temperature. Example systems known to show pronounced variations in microstructure cured in this gradient mold showed large variations of microstructure as a function of position within the sample, corresponding to the cure temperature at that point. A model toughened thermoset system was developed to demonstrate gradients of properties following cure in the gradient temperature mold. Cyanate ester materials were modified with hydroxyl-terminated butadiene-acrylonitrile copolymers as well as low Tg amorphous polyesters. The polyesters showed very desirable properties for a toughener, including relatively good thermo-oxidative stability in comparison with the butadiene-acrylonitrile toughener. However, the variation of properties of the cured materials with temperature was small, and to better understand the property variation possible using a gradient cure temperature technique, the butadiene-acrylonitrile toughened cyanate ester system was chosen for further study. This system showed a significant variation of glass transition temperature of the cyanate-rich phase as a function of cure temperature. Modeling of the phase separation process of this material was varied out employing a modeling procedure developed for epoxy materials. Various characteristics of the system were determined in order to apply the model to the chosen toughened thermoset. These included viscosity, surface, and thermodynamic parameters in addition to a careful characterization of the morphological parameters developed during cure at the chosen temperatures. Results show excellent predictive capability of the model for microstructure. Prediction of phase composition as a function of cure temperature is also possible, again with good agreement with experiment results. Higher cure temperatures result in a non-equilibrium phase composition, depressing the glass transition temperature of the continuous cyanate ester rich phase. This provides a mechanism by which properties of the system change as a function of position within a gradient temperature cured sample. Dynamic mechanical analysis was employed to characterize the relaxation properties of gradient and isothermally cured samples. The Havriliak Negami equation was chosen to describe the relaxation behavior of these samples. Comparison of the fitting of isotherms over the small, experimentally accessible range of frequencies showed that the use of time-temperature superpositioning could more reliably discern relatively small differences. The breadth of the relaxation corresponding to the glass transition of the polycyanurate phase was increased with a gradient cure temperature relative to isothermally cured samples. This increased broadness was expressed in an alternative way through the use of an autocorrelation function, which allows direct comparison of the time-dependent transition from a fully unrelaxed condition to a fully relaxed one. / Ph. D.

Esta pesquisa tem como finalidade desenvolver implantes de alumina com núcleo denso e superfície gradualmente porosa (FGM) recobertos com materiais bioativos - hidroxiapatita e biovidro. Materiais porosos são estudados como solução para a osseointegração, porém apresenta déficit nas suas propriedades mecânicas. Estruturas bifásicas foram desenvolvidas por pesquisadores com o propósito de promover crescimento tecidual, sem afetar significativamente sua propriedade mecânica, no entanto ocorre delaminação. Neste trabalho é proposta uma estrutura em gradiente funcional que visa aprimorar as propriedades mecânicas conjugadas com a sinalização celular e com integração óssea. O tamanho, a morfologia de poros e também a porosidade são parâmetros fundamentais para boa resposta tecidual e integração do implante, pois afetam a viabilidade e a afinidade celular. Para essa finalidade a busca por uma espessura efetiva de porosidade se faz fundamental para alto desempenho do implante. Peças de alumina porosas infiltradas com materiais bioativos foram fabricadas e estudadas in vivo, em tíbias de ratos da raça Wistar durante 14, 18, 21 e 28 dias, para investigar a qualidade do crescimento de tecido ósseo. O estudo com implantes porosos recobertos foi realizado para avaliar e padronizar a superfície porosa do gradiente funcional. Os animais foram analisados com densidade mineral óssea (DMO), as tíbias foram caracterizadas na interface osso-implante e nos poros com histologia, com EDS-line-scan, com radiografias e com ensaios de cisalhamento. Implantes de alumina com 70% de porosidade foram comparados com recobrimento bioativo e sem recobrimento in vivo e ex-vivo. Nos resultados, os implantes recobertos aceleram o processo de osseointegração. Essa característica foi mais evidente no período de 28 dias de implantação com aumento de 24% na tensão de cisalhamento. Após validar uma superfície porosa e osseointegrável para a superfície do gradiente funcional, foram aplicadas técnicas diferenciadas para manufaturar peças com núcleo denso e superfície com gradiente de porosidade. As peças com FGM foram manufaturadas com a utilização de duas técnicas, dipping e co-prensagem e foram analisadas com microscopia eletrônica de varredura. Com o método de manufatura de co-prensagem foram obtidas peças com superfície gradualmente porosa, com transição de densificação contínua, sem delaminação. Os implantes de alumina em gradiente funcional com 70 % de porosidade na superfície mais externa, recobertos por bioativos apresentam potencial para aplicações em implantes ósseos ou dentários. / The present thesis reports on the development of alumina implants with dense core and gradually porous surface (FGM) covered with bioactive materials, hydroxyapatite (HA) and bioactive glass. Porous materials have been studied to provide tissue ingrowth, however they strongly affect the mechanical properties of the implant. Biphasic structures have been developed by some researchers to promote tissue growth without affecting the mechanical properties, although delamination may occur. This study proposes a functional gradient structure to improve both the mechanical properties of the material and cell signaling. The size and morphology of the pores as well as their porosity are key parameters for good tissue response and implant integration, since they affect the viability and cell affinity, and an effective porosity thickness becomes essential for a high performance of the implant. Porous alumina implants coated with bioactive materials were fabricated and studied in vivo in rat tibia for 14, 18, 21, and 28 days to investigate the quality of bone tissue growth. The study of porous coated implants was performed to evaluate and standardize the porous surface of the functional gradient. The animals were examined with bone mineral density (BMD), the tibiae were characterized in the bone-implant interface and the pores were analyzed with histology, EDS line-scan, X-ray and shear tests. Alumina with 70% porosity was compared with and without bioactive coating in vivo and ex vivo. The results showed that the covered implants accelerated the osseointegration process. This characteristic is more evident within 28 days of deployment with a 24% increase in shear stress. After validating a porous and osteointegrated surface for the surface of the functional gradient, several techniques were applied to manufacture parts with dense core and surface with gradient of porosity. The pieces were manufactured with FGM using two techniques, dipping and co-pressing and were analyzed by scanning electron microscopy. The manufacturing method of co-pressing allowed obtaining pieces with gradually porous surface and continuous transition of densification without delamination. On the outermost surface, alumina implants with functional gradient and 70% porosity and coated with bioactive materials presented potential for application to bone or dental implants.

Esta pesquisa tem como finalidade desenvolver implantes de alumina com núcleo denso e superfície gradualmente porosa (FGM) recobertos com materiais bioativos - hidroxiapatita e biovidro. Materiais porosos são estudados como solução para a osseointegração, porém apresenta déficit nas suas propriedades mecânicas. Estruturas bifásicas foram desenvolvidas por pesquisadores com o propósito de promover crescimento tecidual, sem afetar significativamente sua propriedade mecânica, no entanto ocorre delaminação. Neste trabalho é proposta uma estrutura em gradiente funcional que visa aprimorar as propriedades mecânicas conjugadas com a sinalização celular e com integração óssea. O tamanho, a morfologia de poros e também a porosidade são parâmetros fundamentais para boa resposta tecidual e integração do implante, pois afetam a viabilidade e a afinidade celular. Para essa finalidade a busca por uma espessura efetiva de porosidade se faz fundamental para alto desempenho do implante. Peças de alumina porosas infiltradas com materiais bioativos foram fabricadas e estudadas in vivo, em tíbias de ratos da raça Wistar durante 14, 18, 21 e 28 dias, para investigar a qualidade do crescimento de tecido ósseo. O estudo com implantes porosos recobertos foi realizado para avaliar e padronizar a superfície porosa do gradiente funcional. Os animais foram analisados com densidade mineral óssea (DMO), as tíbias foram caracterizadas na interface osso-implante e nos poros com histologia, com EDS-line-scan, com radiografias e com ensaios de cisalhamento. Implantes de alumina com 70% de porosidade foram comparados com recobrimento bioativo e sem recobrimento in vivo e ex-vivo. Nos resultados, os implantes recobertos aceleram o processo de osseointegração. Essa característica foi mais evidente no período de 28 dias de implantação com aumento de 24% na tensão de cisalhamento. Após validar uma superfície porosa e osseointegrável para a superfície do gradiente funcional, foram aplicadas técnicas diferenciadas para manufaturar peças com núcleo denso e superfície com gradiente de porosidade. As peças com FGM foram manufaturadas com a utilização de duas técnicas, dipping e co-prensagem e foram analisadas com microscopia eletrônica de varredura. Com o método de manufatura de co-prensagem foram obtidas peças com superfície gradualmente porosa, com transição de densificação contínua, sem delaminação. Os implantes de alumina em gradiente funcional com 70 % de porosidade na superfície mais externa, recobertos por bioativos apresentam potencial para aplicações em implantes ósseos ou dentários. / The present thesis reports on the development of alumina implants with dense core and gradually porous surface (FGM) covered with bioactive materials, hydroxyapatite (HA) and bioactive glass. Porous materials have been studied to provide tissue ingrowth, however they strongly affect the mechanical properties of the implant. Biphasic structures have been developed by some researchers to promote tissue growth without affecting the mechanical properties, although delamination may occur. This study proposes a functional gradient structure to improve both the mechanical properties of the material and cell signaling. The size and morphology of the pores as well as their porosity are key parameters for good tissue response and implant integration, since they affect the viability and cell affinity, and an effective porosity thickness becomes essential for a high performance of the implant. Porous alumina implants coated with bioactive materials were fabricated and studied in vivo in rat tibia for 14, 18, 21, and 28 days to investigate the quality of bone tissue growth. The study of porous coated implants was performed to evaluate and standardize the porous surface of the functional gradient. The animals were examined with bone mineral density (BMD), the tibiae were characterized in the bone-implant interface and the pores were analyzed with histology, EDS line-scan, X-ray and shear tests. Alumina with 70% porosity was compared with and without bioactive coating in vivo and ex vivo. The results showed that the covered implants accelerated the osseointegration process. This characteristic is more evident within 28 days of deployment with a 24% increase in shear stress. After validating a porous and osteointegrated surface for the surface of the functional gradient, several techniques were applied to manufacture parts with dense core and surface with gradient of porosity. The pieces were manufactured with FGM using two techniques, dipping and co-pressing and were analyzed by scanning electron microscopy. The manufacturing method of co-pressing allowed obtaining pieces with gradually porous surface and continuous transition of densification without delamination. On the outermost surface, alumina implants with functional gradient and 70% porosity and coated with bioactive materials presented potential for application to bone or dental implants.

University of Macau / Faculty of Science and Technology / Department of Civil and Environmental Engineering

Das Potenzial, welches der 3D-Druck im Tissue Engineering für Weichteilgewebe und Knochenersatz hinsichtlich Formgebung und Materialanpassung bietet, wird zunehmend genutzt, weiterentwickelt und ausgebaut. Die Diversität der dabei betrachteten, biologisch aktiven Biomaterialien setzt voraus, dass unterschiedliche Technologien wie Stereolithographie (STL), Fused Deposition Modelling (FDM), Selektives Lasersintern (SLS) in verschiedenen Ausbaustufen zum Einsatz kommen. In medizinischen Anwendungen und besonders im pharmazeutischen Bereich, sind neben den drei räumlichen Dimensionen zusätzlich weitere Dimensionen hinsichtlich der Produkteigenschaften interessant. Einerseits besteht diese Mehrdimensionalität aus strukturellen und geometrischen Gradienten (An, Teoh, Suntornnond & Chua, 2015; Jones et al., 2007; Neri Oxman, Steven Keating & Elizabeth Tsai, 2012). Zusätzlich sind aber auch stoffliche Abstufungen der prozentualen Anteile wichtig. Das betrifft beispielsweise die Einbringung von Wirkstoffen in die generativ aufgebauten Strukturen (Goole & Amighi, 2016; Kalaskar, 2017; Ursan, Chiu & Pierce, 2013). Meist werden dabei scharf abgegrenzte Abstufungen der Materialeigenschaften gezeigt. Dies erfolgt im 3D-Druck beispielsweise durch die Nutzung unterschiedlicher Dosierköpfe in einem Prozess für die jeweiligen Materialien/Materialabstufungen oder durch die getrennte Herstellung der einzelnen Bereiche und anschließendem Fügen der Scaffolds (Diaz-Gomez et al., 2019). Ein allmählich ansteigender/abflachender gradueller Verlauf des zugemischten Anteils (Wirkstoff/Marker) wird bisher nicht beschrieben. Gelingt eine Regelung der Wirkstoffzumischung während des generativen Prozesses, entstehen neue Freiheitsgrade in der Gestaltung der Eigenschaften, wie beispielsweise der pharmazeutischen Wirksamkeit der Produkte. Im biomedizinischen Kontext sind durch Gradientengestaltung innerhalb eines Implantates, unterschiedliche Wirkstoffkonzentrationen oder funktionelle Parameter, wie Festigkeit, Verformbarkeit oder Reaktivität einstellbar. Bei der Zumischung innerhalb des 3D-Drucks ist auch der Einsatz solcher Wirkstoffe denkbar, die bei konventionellen Herstellungsprozessen herausgelöst oder zersetzt würden. Innerhalb der interdisziplinären Zusammenarbeit zwischen der Pharmazeutischen Technologie (Institut für Pharmazie, Medizinische Fakultät) der Universität Leipzig und der Fakultät Maschinenbau und Energietechnik (Maschinenbautechnisches Institut) an der HTWK Leipzig wurde ein miniaturisierter Doppelschneckenextruder (DSE-DK) als Dosierkopf in eine 3D-Druckanlage integriert. Mit der auf dem FDM-Verfahren basierenden Technologie konnte bereits nachgewiesen werden, dass Polymere wie Polycaprolacton (PCL) verarbeitet und mit zugemischten Pulvern homogenisiert werden können (Flath et al., 2016). Es wurden innerhalb eines 3D-Druckprozesses Materialmischungen aus einer aufgeschmolzenen Polymerphase und einem zugeführten Pulver erzeugt. In diesem Aufbau konnten alle Materialien als Pulver zugeführt und dosiert werden. Ziel der hier betrachteten Arbeiten war es, den DSE-DK für pastöse Ausgangsstoffe zur Erweiterung des Einsatzspektrums nutzbar zu machen. Zusätzlich sollten Pasten/Pulver Mischungen und die Möglichkeit der Herstellung von graduellen Zusatzstoffkonzentrationen während der dreidimensionalen Verarbeitung untersucht werden. [... aus der Einleitung]

Functionally graded materials (FGMs) are inhomogeneous materials in which the material properties vary with respect to space. Research has been done by scientific community in developing techniques like photothermal radiometry (PTR) to measure the thermal conductivity and volumetric heat capacity of FGMs. One of the problems involved in the technique is to solve the inverse problem, i.e., estimating the thermal properties after the frequency scan has been obtained. The present work involves finding the unknown thermal conductivity and volumetric heat capacity of the FGMs by using finite volume method. By taking the flux entering the sample as periodic and solving the discretized 1-D thermal wave field equation at a frequency domain, one can obtain the complex temperatures at the surface of the sample for each frequency. These complex temperatures when solved for a range of frequencies gives the phase vs frequency scan which can then be compared to original frequency scan obtained from the PTR experiment by using a residual function. Brute force and gradient descent optimization methods have been implemented to estimate the unknown thermal conductivity and volumetric specific heat of the FGMs through minimization of the residual function. In general, the spatial composition profile of the FGMs can be approximated by using a smooth curve. Three functional forms namely Arctangent curve, Hermite curve, and Bezier curve are used in approximating the thermal conductivity and volumetric heat capacity distributions in the FGMs. The use of Hermite and Bezier curves gives the flexibility to control the slope of the curve i.e. the thermal property distribution along the thickness of the sample. Two-layered samples with constant thermal properties and three layered samples in which one of the layer has varying thermal properties with respect to thickness are considered. The program is written in Fortran and several test runs are performed. Results obtained are close to the original thermal property values with some deviation based on the stopping criteria used in the gradient descent algorithm. Calculating the gradients at each iteration takes considerable amount of time and if these gradient values are already available, the problem can be solved at a faster rate. One of the methods is extending automatic differentiation to complex numbers and calculating the gradient values ahead; this is left for future work.

Au cours des dernières décennies, le développement de nouveaux matériaux a progressé pour les applications liées à la mécanique. De nouvelles générations de composites ont été développées, qui peut offrir des avantages par rapport aux tapis unidirectionnels renforcés de fibres couramment utilisés les matériaux prennent alors le nom de woven fabrics. Le comportement de ce matériau est fortement influencé par la micro-structure du matériau. Dans la thèse, les modèles mécaniques et les schémas numériques capables de modéliser les comportement des tissus et des matériaux de réseau généraux ont été développés. Le modèle prend en compte la micro-structure au moyen d'une technique d'homogénéisation. Les fibres dans le réseau ont été traités comme des micro-poutres, ayant une rigidité à la fois en extension et en flexion, avec différents types de connexions. La procédure développée a été appliquée pour obtenir les modèles mécaniques homogénéisés pour certains types de réseaux de fibres biaxiaux et quadriaxiaux, simulant soit des réseaux de fibres (en ce cas a été supposé parmi les fibres) ou des tissus avec une interaction négligeable entre les faisceaux de fibres et en empêchant tout glissement relatif (dans ce cas, les connexions ont été simulés au moyen de pivots). Différentes géométries ont été analysées, y compris la cas dans lesquels les fibres ne sont pas orthogonales. On obtient généralement un premier milieu à gradient mais, dans certains cas, la procédure d'homogénéisation lui-même indique qu'un continuum d'ordre supérieur est mieux adapté pour représenter la déformation de la micro-structure. Des résultats spéciaux ont été obtenus dans le cas de fibres reliées par pivots. Dans ce cas, un matériau orthotrope à module de cisaillement nul a été obtenu. Un tel matériau a un tenseur constitutif elliptique, il peut donc conduire à des concentrations de contrainte. Cependant, il a été montré que certaines considérations sur le comportement physique de tels réseaux indiqué que les termes d'ordre supérieur inclus dans l'expansion des forces internes et des déformations, de sorte qu'un matériau de gradient de déformation a été obtenu. Les résultats obtenus peuvent être utilisés pour la conception de matériaux spécifiques nécessitant des propriétés. Bien que le modèle de référence soit un matériau de réseau, les résultats obtenus peuvent être appliqué à d'autres types similaires de microstructures, comme des matériaux pantographiques, des micro-dispositifs composé de micro-poutres, etc. Ils étaient limités à la gamme d'élasticité linéaire, qui est petite déformation et comportement élastique linéaire. Ensuite, les simulations numériques ont été axées sur les tests d'extension et les tests de biais. Le obtenu configurations déformées sont conformes aux tests expérimentaux de la littérature, tant pour tissus équilibrés et non équilibrés. De plus, une comparaison entre les premier et deuxième gradients des prédictions numériques ont été effectuées. Il a été observé que les prédictions de deuxième gradient mieux simuler les preuves expérimentales. / In the past decades there has been an impressive progress in the development of new materials for mechanical related applications. New generations of composites have been developed, that can offer advantages over the unidirectional fibre-reinforced mats commonly used then materials take the name of woven fabrics. The behaviour of this material is strongly influenced by the micro-structure of the material. In the thesis mechanical models and a numerical scheme able to model the mechanical behaviour of woven fabrics and general network materials have been developed. The model takes in to account the micro-structure by means of a homogenization technique. The fibres in the network have been treated like microbeams, having both extensional and bending stiffness, with different types of connection, according to the pattern and detail of the network. The developed procedure was applied for obtaining the homogenized mechanical models for some types of biaxial and quadriaxial networks of fibres, simulating either fibre nets (in this case rigid connection were assumed among the fibres) or tissues with negligible interaction between the fibre bundles, and with relative sliding prevented (in this case the connections were simulated by means of pivots). Different geometries were analysed, including the cases in which the fibres are not orthogonal. A first gradient medium is usually obtained but, in some cases, the homogenization procedure itself indicates that a higher order continuum is better fit to represent the deformation of the micro-structure. Special results were obtained for the case of fibres connected by pivots. In this cases an orthotropic material with zero shear modulus was obtained. Such a material has a not elliptic constitutive tensor, thus it can lead to strain concentrations. However, it was shown that some considerations about the physical behaviour of such networks indicated that higher order terms had to be included in the expansion of the internal forces and deformations, so that a strain gradient material was obtained. The results obtained can be used for the design of specific materials requiring ad-hoc properties. Although the reference model is a network material, the results obtained can be applied to other similar kinds of microstructures, like pantographic materials, micro devices composed by microbeams etc. They have been limited at the range of linear elasticity, that is small deformation and linear elastic behaviour. Then, numerical simulations were focused on extension tests and bias tests. The obtained deformed configurations are consistent with the literature experimental tests, both for balanced and unbalanced tissues. Moreover, a comparison between first and second gradient numerical predictions was performed. It was observed that second gradient predictions better simulate the experimental evidences.