Desarrollo de titanio con porosidad gradiente radial y longitudinal para aplicaciones biomédicas

La diferencia de rigidez entre el tejido óseo y los implantes de Titanio (Ti) implica un apantallamiento de tensiones que promueve la reabsorción ósea y un incremento de la probabilidad de fractura. Los objetivos principales de este trabajo son el diseño, la fabricación y la caracterización de sólidos cilíndricos de Ti comercialmente puro, Ti cp, con porosidad gradiente (longitudinal y radial); su propósito es “replicar” la estructura altamente jerarquizada del tejido óseo y mejorar la biofuncionalidad del futuro implante. Se implementan dos rutas de procesamiento: pulvimetalúrgia (PM) convencional y la técnica de espaciadores (cloruro de sodio, NaCl). Las muestras obtenidas con porosidad gradiente longitudinal son: i) 6 capas, 0-90 MPa, y ii) 30/40/50 %vol. de NaCl y 800 MPa. Las muestras obtenidas con porosidad gradiente radial (núcleo, centro y exterior) son: i) 500, 250 y 125 MPa, y (ii) 20/40/60 %vol. de NaCl y 800 MPa. Las temperaturas de sinterización fueron de 1100ºC (PM) y de 1250ºC (espaciadores), ambas durante 2 h y en alto vacío. La caracterización incluye: la densidad, el tamaño y distribución de la porosidad gradiente, el Módulo de Young (convencional y dinámico) y el límite de fluencia. Los diseños de porosidad gradiente evaluados permiten alcanzar un equilibrio biomecánico (resistencia y rigidez acorde a los requisitos de reemplazo del hueso cortical) y biofuncional (facilitar el crecimiento del tejido óseo hacia el interior del implante). A pesar que la técnica de espaciadores es más costosa, esta permite una mayor variación y control de la porosidad. El novedoso dispositivo de compactación secuencial diseñado, optimizado e implementado en esta tesis, resulta adecuado para fabricar cilindros con porosidad gradiente radial y una excelente integridad estructural en verde. El mismo permite controlar la porosidad radial, variando la presión de compactación y/o el contenido de espaciador, así como combinar polvos de familia de materiales distintos. El desarrollo tiene una aplicación potencial elevada, destacando los implantes óseos, las piezas auto lubricadas, los disipadores de calor de alta eficiencia y la simulación de combustibles nuclear gastado

Influence of the Compaction Pressure and Sintering Temperature on the Mechanical Properties of Porous Titanium for Biomedical Applications

In the present work, the use of porous titanium is proposed as a solution to the difference in stiffness between the implant and bone tissue, avoiding the bone resorption. Conventional powder metallurgical technique is an industrially established route for fabrication of this type of material. The results are discussed in terms of the influence of compaction pressure and sintering temperature on the porosity (volumetric fraction, size, and morphology) and the quality of the sintering necks. A very good agreement between the predicted values obtained using a simple 2D finite element model, the experimental uniaxial compression behavior, and the analytical model proposed by Nielsen, has been found for both the Young’s modulus and the yield strength. The porous samples obtained by the loose sintering technique and using temperatures between 1000 °C −1100 °C (about 40% of total porosity) are recommended for achieving a suitable biomechanical behavior for cortical bone partial replacement.Ministry of Economy and Competitiveness of the State General Administration of Spain grant MAT2015-71284-

Advanced titanium scaffolds obtained by directional freeze-drying: on the influence of processing conditions

Ministry of Science and Innovation of Spain under Grant No. MAT2010-20855Junta de Andalucía (Spain) / FEDER (EU), through the project Ref. P12-TEP-140

Porous Titanium for Biomedical Applications: Evaluation of the Conventional Powder Metallurgy Frontier and Space-Holder Technique

Titanium and its alloys are reference materials in biomedical applications because of their desirable properties. However, one of the most important concerns in long-term prostheses is bone resorption as a result of the stress-shielding phenomena. Development of porous titanium for implants with a low Young’s modulus has accomplished increasing scientific and technological attention. The aim of this study is to evaluate the viability, industrial implementation and potential technology transfer of different powder-metallurgy techniques to obtain porous titanium with stiffness values similar to that exhibited by cortical bone. Porous samples of commercial pure titanium grade-4 were obtained by following both conventional powder metallurgy (PM) and space-holder technique. The conventional PM frontier (Loose-Sintering) was evaluated. Additionally, the technical feasibility of two different space holders (NH4HCO3 and NaCl) was investigated. The microstructural and mechanical properties were assessed. Furthermore, the mechanical properties of titanium porous structures with porosities of 40% were studied by Finite Element Method (FEM) and compared with the experimental results. Some important findings are: (i) the optimal parameters for processing routes used to obtain low Young’s modulus values, retaining suitable mechanical strength; (ii) better mechanical response was obtained by using NH4HCO3 as space holder; and (iii) Ti matrix hardening when the interconnected porosity was 36–45% of total porosity. Finally, the advantages and limitations of the PM techniques employed, towards an industrial implementation, were discussed.Ministry of Economy and Competitiveness of Spain Grant MAT2015-71284-PJunta de Andalucía Grant P12-TEP-1401Comisión Nacional de Investigación, Científica y Tecnológica (CONICYT) of the Chilean government project FONDECYT 1116086

Surface Modification of Porous Titanium Discs Using Femtosecond Laser Structuring

[Abstract] The failure of titanium implants is associated with two main problems that include the bone resorption and fracture of the surrounding bone tissue (stiffness incompatibility) and implant loosening (poor osseointegration). The development of porous titanium implants with low Young modulus solve the stress shielding phenomenon, while the modification of the implant surface must be implemented to promote a fast bond between the implant and bone. In this work, femtosecond laser micromachining was applied to modify the topography of the surface of Ti porous samples obtained by a space-holder technique to obtain hierarchical structures (micro and nano roughness patterns) to enhance osseointegration. Scanning electron microscopy, confocal laser microscopy, and image analysis were used for characterization of the surface morphology, roughness, and porosity before and after performing the laser treatment. Based on these results, the effect of the treatment on the mechanical behavior of the samples was estimated. In addition, a preliminary in-vitro test was performed to verify the adhesion of osteoblasts (filopodia presence) on modified titanium surface. Results revealed that laser texturing generated clusters of micro-holes and micro-columns both on the flat surface of the samples and inside the macro-pores, and periodic nanometric structures across the entire surface. The porous substrate offers suitable biomechanics (stiffness and yield strength) and bio-functional behavior (bone ingrowth and osseointegration), which improves the clinic success of titanium implantsJunta de Andalucía; US-1259771The regional government from Andalusia through FEDER-Junta de Andalucía Research Project US-1259771, (Modeling and implementation of the freeze casting technique: gradients of porosity with a tribomechanical equilibrium and electro-stimulated cellular behavior) funded this research

Designing, processing and characterisation of titanium cylinders with graded porosity: An alternative to stress-shielding solutions

Bone resorption events and consequent failure of titanium implants are frequently related to stress-shielding problems, due to stiffness mismatch with respect to bone. This is a mechanical incompatibility problem, which is difficult to resolve because of the challenge of replacing highly anisotropic biomechanical systems, as is the case of dental implants. This work describes the designing, processing and characterisation of cylindrical titanium samples with a longitudinally graded porosity obtained by conventional powder-metallurgy techniques. The design concept used was biomimetic, based on the stiffness properties of the tissues to be in contact with titanium dental implants. Processing conditions were optimised in terms of different parameters: structural integrity, porosity and mechanical properties. The influence of sintering temperature was evaluated in search of optimum results under the above criteria. The behaviour of longitudinal porosity and Young’s modulus were consistent with the preliminary design concept from the original biomechanical system. Mechanical strength results were reasonably suitable for dental applications and they were favourably sensitive to increasing sintering temperature, due to a stronger adhesion between initial green layers of cylindrical samples. Results showed that it is possible to obtain a desired longitudinal gradient in Young’s modulus, as well as suitable yield strength values. The optimised processing described suggests that it is a plausible candidate for manufacturing dental implants with a good balance between reduced stress shielding and suitable mechanical strength, which encourages us to undertake further work along the same lines.Ministerio de Economía y Competitividadt MAT2010- 2085

Fabrication and characterization of superficially modified porous dental implants

Stress-shielding and loosening compromise the success of dental implants under real-life service conditions. This work evaluates the mechanical behavior of superficially modified porous titanium dental implants fabricated by two different routes: conventional powder metallurgy and space-holder techniques. A novel, feasible and repetitive protocol of micro-milling of the implant thread (before sintering), as well as surface modification treatments (after sintering) are also implemented. The discussion is conducted in terms of the influence of porosity and surface roughness on the stiffness and yield strength of implants. The macro-pores concentrate stress locally, and, at the same time, they could act as a barrier to the propagation of micro-cracks. Higher rugosity was observed for virgin implants obtained with spacer particles. Concerning superficially modified implants, while bioglass 1393 was the most effective coating due to its greater infiltration and adhesion capacity, chemical etching could improve osteoblast adhesion because modifies the roughness of the implant surface.Ministry of Science and Innovation of Spain PID2019-109371GB-I00Junta de Andalucía–FEDER (Spain) US-125977

Porous Titanium Cylinders Obtained by the Freeze-Casting Technique: Influence of Process Parameters on Porosity and Mechanical Behavior

 The discrepancy between the stiffness of commercially pure titanium and cortical bone  tissue compromises its success as a biomaterial. The use of porous titanium has been widely studied,  however, it is still challenging to obtain materials able to replicate the porous structure of the bones  (content, size, morphology and distribution). In this work, the freeze‐casting technique is used to  manufacture cylinders with elongated porosity, using a home‐made and economical device. The  relationship between the processing parameters (diameter and material of the mold, temperature  gradient), microstructural features and mechanical properties is established and discussed, in terms  of ensuring biomechanical and biofunctional balance. The cylinders have a gradient porosity  suitable for use in dentistry, presenting higher Young’s modulus at the bottom, near the cold spot  and, therefore better mechanical resistance (it would be in contact with a prosthetic crown), while  the opposite side, the hot spot, has bigger, elongated pores and walls.  Ministry of Economy and Competitiveness of Spain  grant  MAT2015‐71284‐P  FEDER‐Junta de Andalucía Research  Project (Modeling and implementation of the freeze casting technique: gradients of porosity with a tribomechanical equilibrium and electro‐stimulated cellular behavior).

Bioactive Bilayer Glass Coating on Porous Titanium Substrates with Enhanced Biofunctional and Tribomechanical Behavior

The use of porous titanium samples fabricated by space-holder powder metallurgy with bioactive coatings has already been reported to prevent resorption of the bone surrounding the implant and improve osseointegration, respectively. However, the presence of pores as well as the poor adherence and the brittle behavior inherent to glassy coatings affect the service behavior of implants fabricated from these samples. Therefore, they need to be optimized. In this work, 50 vol.% of porosity titanium substrates were manufactured with different pore range size (100–200 and 355–500 µm) spacer particles and coated with a bilayer of bioactive glasses (45S5/1393). The effect of the pores on the tribomechanical properties and infiltration of the bioactive glass 1393 along with the bioactivity of the bioactive glass 45S5 were evaluated by instrumented micro-indentation and scratch tests and the formation of hydroxyapatite in simulated body fluid. The results obtained were very promising as potential implants for the replacement of small tumors in cortical bone tissues, mainly due to the smaller pores that present an improved biomechanical and biofunctional balance.Ministry of Science and Innovation of Spain grant PID2019-109371GB-I00Junta de Andalucía-FEDER (Spain) Project US-1259771Junta de Andalucía-FEDER (Spain) Project US-1380878Junta de Andalucía (Spain) Project PAIDI 2020 P20_0067

Surface Modification of Porous Titanium Discs Using Femtosecond Laser Structuring

The failure of titanium implants is associated with two main problems that include the bone resorption and fracture of the surrounding bone tissue (stiffness incompatibility) and implant loosening (poor osseointegration). The development of porous titanium implants with low Young modulus solve the stress shielding phenomenon, while the modification of the implant surface must be implemented to promote a fast bond between the implant and bone. In this work, femtosecond laser micromachining was applied to modify the topography of the surface of Ti porous samples obtained by a space-holder technique to obtain hierarchical structures (micro and nano roughness patterns) to enhance osseointegration. Scanning electron microscopy, confocal laser microscopy, andimageanalysiswereusedforcharacterizationofthesurfacemorphology,roughness,andporosity before and after performing the laser treatment. Based on these results, the effect of the treatment on the mechanical behavior of the samples was estimated. In addition, a preliminary in-vitro test was performed to verify the adhesion of osteoblasts (filopodia presence) on modified titanium surface. Resultsrevealedthatlasertexturinggeneratedclustersofmicro-holesandmicro-columnsbothonthe flat surface of the samples and inside the macro-pores, and periodic nanometric structures across the entire surface. The porous substrate offers suitable biomechanics (stiffness and yield strength) and bio-functional behavior (bone ingrowth and osseointegration), which improves the clinic success of titanium implants.FEDER-Junta de Andalucía Research Project US-125977