Surface modification of zirconia-based bioceramics for orthopedic and dental applications

Debido a sus excelentes propiedades mecánicas y una excelente biocompatibilidad, el uso de las cerámicas de base de circona en aplicaciones dentales y ortopédicas ha crecido rápidamente durante las últimas décadas. Sin embargo, tanto la alúmina como la circona son bioinertes, lo cual dificulta su implantación en contacto directo con el hueso. Además, las infecciones siguen siendo una de las principales causas de fallo de implantes. Para resolver ambos problemas, se requiere un mejor diseño de la superficie: en particular, una topografía adecuada puede promover la osteointegración y limitar la adhesión bacteriana. Por otro lado, la fiabilidad a largo plazo es un asunto crítico para los implantes estructurales, y las cerámicas que contienen circona requieren una atención especial. Como para otras cerámicas, las alteraciones superficiales pueden comprometer sus propiedades mecánicas. Además, la transformación de fase de tetragonal a monoclínica, que les proporciona una tenacidad excepcional, puede ocurrir espontáneamente en presencia de agua, lo cual puede afectar las propiedades del material. La cinética de este fenómeno, conocido como envejecimiento hidrotérmico, es muy sensible a los cambios de procesamiento. Por lo tanto, cualquier modificación de la superficie debe ir acompañada de una evaluación de su impacto en la fiabilidad de los implantes. Basado en estas observaciones, el objetivo de esta tesis fue desarrollar procesos para modificar la superficie de los implantes a base de circona, en particular la topografía, sin comprometer sus propiedades mecánicas y estabilidad hidrotérmica. El esfuerzo de investigación se centró en dos materiales: la circona estabilizada con itria (3Y-TZP), que se utiliza cada vez más para aplicaciones dentales (por ejemplo: coronas, implantes), y la alúmina reforzada con circona (ZTA), que es el estándar actual en ortopedia para la fabricación de componentes cerámicos estructurales. Por lo tanto, este trabajo se puede dividir en dos partes principales. En la primera parte, se llevó a cabo un amplio estudio del ataque de la circona con ácido fluorhídrico (HF). Se demostró que ajustando el tiempo de decapado es posible controlar la rugosidad y la dimensión fractal de la superficie. Además, los resultados indican condiciones adecuadas para incrementar la rugosidad de forma rápida y uniforme, sin comprometer su resistencia mecánica ni tampoco su resistencia al envejecimiento. Basándose en estos hallazgos, se obtuvieron muestras con gradientes de rugosidad mediante inmersión con una velocidad controlada en una solución de ataque. Gracias a este método, que reduce drásticamente los esfuerzos y recursos necesarios para estudiar las interacciones célula-superficie, se realizó un análisis rápido de la influencia de la micro- y nano-topografía inducida por HF en las células madre mesenquimales. Se determinaron correlaciones entre parámetros de rugosidad y morfología celular, destacando la importancia de la optimización de la topografía a múltiples escalas para inducir la respuesta celular deseada. En la segunda parte, una estrategia integrada fue desarrollada para proporcionar propiedades antibacterianas y osteointegrativas a las superficies de ZTA La micro-topografía se controló mediante moldeo por inyección. Mientras tanto, un nuevo procedimiento que implica la disolución selectiva de la circona por HF (ataque selectivo) se utilizó para producir nano-rugosidad y una nanoporosidad superficial interconectada. La utilización potencial de la porosidad para la liberación de antibióticos fue demostrada, y se evidenció que la encapsulación liposomal puede aumentar la cantidad de fármaco cargada. Además, se demostró que el impacto del ataque selectivo sobre las propiedades mecánicas y la estabilidad hidrotermal era limitado. Por lo tanto, la combinación del moldeo por inyección y del ataque selectivo parece prometedora para la fabricación de componentes de ZTA implantables en contacto directo con el huesoDue to their outstanding mechanical properties and excellent biocompatibility, the use of zirconia-based ceramics in dental and orthopedic applications has grown rapidly over the last decades. However, both alumina and zirconia are bioinert, which hampers their implantation in direct contact with bone. Furthermore, infections remain one of the leading causes of implant failure. To address both issues, an improved surface design is required: in particular, an adequate topography can promote osseointegration and limit bacterial adhesion. On the other hand, long-term reliability is a major concern for load-bearing implants, and zirconia-containing ceramics require special attention. As for other ceramics, surface alterations can impair their mechanical properties. Besides, the tetragonal to monoclinic phase transformation, which accounts for their exceptional toughness, can occur spontaneously in the presence of water, potentially deteriorating the material properties. The kinetics of this phenomenon, known as hydrothermal ageing, are highly sensitive to processing changes. Any surface modification of zirconia-containing ceramics should thus be accompanied by a careful assessment of its impact on implant reliability. Based on these observations, the objective of this thesis was to develop processes to modify the surface of zirconia-based implants, in particular the topography, without compromising their mechanical properties and hydrothermal stability. The research effort focused on two materials of particular interest: yttria-stabilized zirconia (3Y-TZP), which is increasingly used for prosthodontic applications (e.g., crowns, implants), and zirconia toughened alumina (ZTA), which is the current gold Standard in orthopedics for the fabrication of load-bearing ceramic components. Accordingly, this work can be divided into two main parts. In the first part, an extensive study of the hydrofluoric acid (HF) etching of zirconia was carried out. It was shown that monitoring etching time allows controlling the roughness and fractal dimension of the surface. Furthermore, the results indicated suitable processing conditions for a fast and uniform roughening of zirconia components, without compromising substantially their strength and ageing resistance. Based on these findings, zirconia samples with roughness gradients were obtained by immersing specimens into an etching solution with a controlled speed. Thanks to this method, which drastically reduces the efforts and resources necessary to study cell-surface interactions, a rapid screening of the influence of HF-induced micro- and nano-topography on mesenchymal stem cell morphology was conducted. Correlations between roughness parameters and cell morphology were evidenced, highlighting the importance of multiscale optimization of topography to induce the desired cell response. In the second part, an integrated strategy was developed to provide both osseointegrative and antibacterial properties to ZTA surfaces. The micro-topography was controlled by injection molding. Meanwhile a novel process involving the selective dissolution of zirconia by HF (selective etching) was used to produce nano-roughness and interconnected Surface nanoporosity. Potential utilization of the porosity for delivery of antibiotic molecules was demonstrated, and it was shown that liposomal encapsulation could improve drug loading. Furthermore, the impact of selective etching on mechanical properties and hydrothermal stability was shown to be limited. The combination of injection molding and selective etching thus appears promising for fabricating a new generation of ZTA components implantable in direct contact with bone

An appreciation of the prescience of Don Gilbert (1930‐2011): master of the theory and experimental unraveling of biochemical and cellular oscillatory dynamics

We review Don Gilbert's pioneering seminal contributions that both detailed the mathematical principles and the experimental demonstration of several of the key dynamic characteristics of life. Long before it became evident to the wider biochemical community, Gilbert proposed that cellular growth and replication necessitate autodynamic occurrence of cycles of oscillations that initiate, coordinate, and terminate the processes of growth, during which all components are duplicated and become spatially re‐organized in the progeny. Initiation and suppression of replication exhibit switch‐like characteristics: i.e., bifurcations in the values of parameters that separate static and autodynamic behavior. His limit cycle solutions present models developed in a series of papers reported between 1974 and 1984, and these showed that most or even all of the major facets of the cell division cycle could be accommodated. That the cell division cycle may be timed by a multiple of shorter period (ultradian) rhythms, gave further credence to the central importance of oscillatory phenomena and homeodynamics as evident on multiple time scales (seconds to hours). Further application of the concepts inherent in limit cycle operation as hypothesized by Gilbert more than 50 years ago are now validated as being applicable to oscillatory transcript, metabolite and enzyme levels, cellular differentiation, senescence, cancerous states, and cell death. Now, we reiterate especially for students and young colleagues, that these early achievements were even more exceptional, as his own lifetime's work on modeling was continued with experimental work in parallel with his predictions of the major current enterprises of biological research

A multiscale model for collagen alignment in wound healing

It is thought that collagen alignment plays a significant part in scar tissue formation during dermal wound healing. We present a multiscale model for collagen deposition and alignment during this process. We consider fibroblasts as discrete units moving within an extracellular matrix of collagen and fibrin modelled as continua. Our model includes flux induced alignment of collagen by fibroblasts, and contact guidance of fibroblasts by collagen fibres. We can use the model to predict the effects of certain manipulations, such as varying fibroblast speed, or placing an aligned piece of tissue in the wound. We also simulate experiments which alter the TGF-β concentrations in a healing dermal wound and use the model to offer an explanation of the observed influence of this growth factor on scarring

Nanoroughness, Surface Chemistry and Drug Delivery Control by Atmospheric Plasma Jet on Implantable Devices

Implantable devices need specific tailored surface morphologies and chemistries to interact with the living systems or to actively induce a biological response also by the release of drugs or proteins. These customised requirements foster technologies that can be implemented in additive manufacturing systems. Here we present a novel approach based on spraying processes that allows to control separately topographic features in the submicron range ( 3d 60 nm - 2 \ub5m), ammine or carboxylic chemistry and fluorophore release even on temperature sensitive biodegradable polymers such as polycaprolactone (PCL). We developed a two-steps process with a first deposition of 220 nm silica and poly(lactic-co-glycolide) (PLGA) fluorescent nanoparticles by aerosol followed by the deposition of a fixing layer by atmospheric pressure plasma jet (APPJ). The nanoparticles can be used to create the nano-roughness and to include active molecule release, while the capping layer ensures stability and the chemical functionalities. The process is enabled by a novel APPJ which allows deposition rates of 10 - 20 nm\ub7s-1 at temperatures lower than 50 \ub0C using argon as process gas. This approach was assessed on titanium alloys for dental implants and on PCL films. The surfaces were characterized by FT-IR, AFM and SEM. Titanium alloys were tested with pre-osteoblasts murine cells line, while PCL film with fibroblasts. Cell behaviour was evaluated by viability and adhesion assays, protein adsorption, cell proliferation, focal adhesion formation and SEM. The release of a fluorophore molecule was assessed in the cell growing media, simulating a drug release. Osteoblast adhesion on the plasma treated materials increased by 20% with respect to commercial titanium alloys implants. Fibroblast adhesion increased by a 100% compared to smooth PCL substrate. The release of the fluorophore by the dissolution of the PLGA nanoparticles was verified and the integrity of the encapsulated drug model confirmed

Histomorphometric, fractal and lacunarity comparative analysis of sheep (Ovis aries), goat (Capra hircus) and roe deer (Capreolus capreolus) compact bone samples

Quantitative and qualitative studies dealing with histomorphometry of the bonetissue play a new role in modern legal medicine/forensic medicine and archaeozoologynowadays. This study deals with the differences found in case of humerusand metapodial bones of recent sheep (Ovis aries), goat (Capra hircus) and roedeer (Capreolus capreolus) specimens, both from a qualitative point of view, butmainly from a quantitative perspective. A novel perspective given by the fractalanalysis performed on the digital histological images is approached. This studyshows that the qualitative assessment may not be a reliable one due to theclose resemblance of the structures. From the quantitative perspective (severalmeasurements performed on osteonal units and statistical processing of data),some of the elements measured show significant differences among 3 species(the primary osteonal diameter, etc.). The fractal analysis and the lacunarity ofthe images show a great deal of potential, proving that this type of analysis canbe of great help in the separation of the material from this perspective