J.A. Rodríguez-Ortiz

Electrophoretic Deposition of PEEK/45S5 Bioactive Glass Coating on Porous Titanium Substrate: Influence of Processing Conditions and Porosity Parameters

Commercially pure titanium (cp Ti) is typically accepted as one of the best in vitro and in vivo bone replacement biomaterial, due to its excellent balance between biomechanical and biofunctional properties. In that context, the aim of this work is to prove the hypothesis of a simultaneous solution to certain specific limitations of cpTi, which can often compromise the reliability of implants: (i) stress-shielding phenomenon, and (ii) a deficient biointerface with bone, which reduces the osseointegration. Porous samples of cp Ti, grade IV, were obtained by space-holder technique (50 vol.% NH4HCO3, 800 MPa, at 1250 oC during 2h, under high vacuum), to produce a good balance between Young ́s Modulus and yield strength. Different types of porous samples were manufactured by considering different size particles ranges of NH4HCO3: 100-200μm, 250-355μm and 355-500μm. Afterwards, they were coated with a PEEK/45S5 bioactive glass composite by electrophoretic deposition, to be finally sintered at 350oC for 1h. The coatings homogeneity, infiltration efficiency, adhesion and cracking, were studied in order to establish correlations with processing conditions (time of deposition, applied voltage, composition, concentration and stability of the colloidal suspension). Detailed structural characterization of the coatings was performed (SEM and XRD), besides the contact angle and contact profilometry testing. Additional mechanical and chemical insights were achieved by evaluating both the tribo-mechanical (instrumented microindentation and micro-scratch testing) and electrochemical behaviors (potentiodynamic polarization and in vitro corrosion tests in SBF). All these results allowed us to determine the optimal balance of properties for a porous substrate (space holder of 250-355μm) with a coating obtained for 65 V, 2 min, 6 mm (distance between electrodes), 10 g/L bioactive glass and 20 g/l PEEK. The high adhesion estimated between the bioactive/biopolymer coatings and the porous titanium substrates (excellent infiltration) suggest that this new biocomposite is a good candidate for load-bearing applications.

Electrophoretic Deposition and Characterization of Chitosan/45S5 Bioactive Glass Composite Coatings on Porous Titanium for Biomedical Applications

Porous titanium samples of cp Ti grade IV were obtained by space-holder technique (50%vol of NH4HCO3, 800 MPa, 1250 oC during 2h in high vacuum), producing a good balance between stiffness and mechanical strength. The samples were coated with chitosan/45S5 bioactive glass composite by electrophoretic deposition. Homogeneity, infiltration efficiency, and coatings integrity (cracking and adhesion) were evaluated in order to establish correlations with processing parameters. SEM, FTIR, and contact profilometry were performed for detailed characterization of the coatings; and micro-mechanical properties (P-h curves and scratch testing) were set-up as well. Optimum EPD parameters were 25V, 7 min and suspension containing 0.5 g/L chitosan and 1.5 g/L BG a titanium structure with pore sizes greater than 200 μm are required.

Design, Processing and Characterization of Materials with Controlled Radial Porosity for Biomedical and Nuclear Applications

The manufacture of graded materials has gained an enormous interest during the last decade due to the diversity of industrial and biological materials systems that require or are actually designed to implement that criterion; those natural or artificial materials offer multiple possibilities of applications. In this work, a novel uniaxial and sequential compaction device has been successfully designed and fabricated, in order to obtain samples with three different layers; this new device is suitable for both conventional and non-conventional powder metallurgy (PM) techniques. In addition, this device allowed us to use different combinations of powders and space-holder particles, irrespective of their nature, sizes, morphologies and proportions. It has no restriction about applying different compaction pressures for every layer, which may result in increasing or decreasing porosity. This compaction device is especially powerful if the aim is obtaining samples with radial graded porosity for biomedical applications (replacement of cortical bone involved in different joints and dental restorations) and nuclear applications (mimicking burnt used nuclear fuel). Specifically in this work, different samples with radial graded porosity were fabricated and then microstructurally and mechanically characterized: i) Commercially pure titanium (CP Ti) samples, starting from blends CP Ti with 20 vol.%, 40 vol.% and 60 vol.% of Sodium Chloride (NaCl) as space holder, which were placed in core, intermediate and external layers, respectively; processing conditions were 800 MPa of compaction pressure and 1250 °C for 2h in high vacuum of sintering; and ii) CeO2 samples, starting from blends CeO2 with 0.5 vol.%, 3.0 vol.% and 7.5 vol.% of Ethylene Bis Stearamide (EBS) as space holder, which were placed in core, intermediate and external layers, respectively; processing conditions were 460 MPa in external layer and 700 MPa in core and intermediate layers of compaction pressure, and 1700 °C during 4h in static air of sintering. This new device has proved to have unique advantages for solving problems of structural integrity in conventional PM manufacturing in a simple, economic and reproducible way.

A Simple and Economical Device to Process Ti Cylinders with Elongated Porosity by Freeze-Casting Techniques: Design and Manufacturing

Nowadays, the development of materials with gradient porosity is an important aim for many applications, especially in the field of bone tissue replacement. This research work shows the design and manufacture of a simple and economical device to process Ti cylinders with elongated and high interconnected porosity by freeze-casting techniques. The influence of the vessel material on the internal lamellar structure of the porous Ti samples is also studied. The device has been validated with: (i) a thermal gradient from-10 °C at the cold surface to 20 °C at the hot surface; and (ii) a cylindrical vessel with 12 mm diameter of alumina or Teflon. These working conditions have allowed maintaining the freezing conditions during the full process (alumina vessel: 30 minutes; Teflon vessel: 3 hours). After the solidification of the Ti aqueous suspension, the ice is sublimated (24 hours at-50 °C and 0.070 mbar). Then, the resulting powder is sintered (1150 °C for 5 h at high vacuum ~ 10-5 mbar), obtaining the porous Ti sample with the final structural strength. Finally, a detailed study of the most relevant porosity features is performed: porosity ratio and interconnectivity degree by Archimedes’ method, and size and shape of the pores by image analysis at three different zones along the longitudinal cross section. The results indicate the viability of the device to enhance the directional freezing and thus, the elongated porosity. Teflon vessel present the best results, with an average porosity ratio and porosity size of 39.5 % and 123.3 μm, respectively. This suggests an optimal biomechanical and bifunctional balance of this porous material.

Preparation and characterization of titanium—segmented polyurethane composites for bone tissue engineering

Segmented polyurethanes were prepared with polycaprolactone diol as soft segment and 4,4-methylene-bis cyclohexyl diisocyanate and l-glutamine as the rigid segment. These polyurethanes were filled with 1 wt.% to 5 wt.% titanium particles (Ti), physicochemically characterized and their biocompatibility assessed using human dental pulp stem cells and mice osteoblasts. Physicochemical characterization showed that composites retained the properties of the semicrystalline polyurethane as they exhibited a glass transition temperature (Tg) between −35°C and −45°C, melting temperature (Tm) at 52°C and crystallinity close to 40% as determined by differential scanning calorimetry. In agreement with this, X-ray diffraction showed reflections at 21.3° and 23.6° for polycaprolactone diol and reflections at 35.1°, 38.4°, and 40.2° for Ti particles suggesting that these particles are not acting as nucleating sites. The addition of up to 5 wt.% of Ti reduced both, tensile strength and maximum strain from 1.9 MPa to 1.2 MPa, and from 670% to 172% for pristine and filled polyurethane, respectively. Although there were differences between composites at low strain rates, no significant differences in mechanical behavior were observed at higher strain rate where a tensile stress of 8.5 MPa and strain of 223% were observed for 5 wt.% composites. The addition to titanium particles had a beneficial effect on both human dental pulp stem cells and osteoblasts viability, as it increased with the amount of titanium in composites up to 10 days of incubation.

Processing and characterization of surrogate nuclear materials with controlled radial porosity

ABSTRACT Irradiated fuel pellets present radial gradient porosity. CeO2 has been proven as a surrogate material to understand irradiated mixed oxide (MOX) due to its similar structural and mechanical properties. A novel compaction device was developed to produce CeO2 cylindrical pellets with controlled radial porosity. Three blends of CeO2 with different binder contents (0.5, 3 and 7.5 vol.% of ethylene-bis-stearamide, EBS) were prepared and used to obtain three different porosities for the core, intermediate and outer rings of pellets, respectively. Different compaction pressures were employed in each region to get the intended porosities. The whole pellet was subjected to a heating rate up to 500 °C to remove the EBS binder. Finally, a pressureless sintering step was performed at 1700 °C for 4 h. A microstructural characterization was performed in the three areas, including grain size and porosity. Mechanical properties like hardness, fracture toughness and tribo-mechanical response, as scratch resistance, were also determined. Pellets fabricated from this device have shown microstructural and mechanical properties with a good correlation to those of irradiated nuclear fuel.