Flexural strength of 3Y-TZP bioceramics obtained by direct write assembly as function of residual connected-porosity.

OBJECTIVES: The present work reports the effect of the extrusion nozzles' size and consequent residual porosity on the flexural strength of 3Y-TZP bioceramics fabricated by direct write assembly technology. METHODS: A printable ink containing a volume fraction of 45% of 3Y-TZP (ZrO2 stabilized with 3 mol% Y2O3) submicron powder, carboxymethyl cellulose and polyethyleneimine as additives was fine-tuned by rheological measurements. Different nozzle diameters (0.41 mm, 0.33 mm, and 0.25 mm) were used to print 3D specimens with proper dimensions for structural and mechanical characterization after sintering, namely relative density, linear shrinkage, and three-point flexural strength. Bulk surface sample and exposed fractured surfaces after flexural strength tests were analyzed by X-ray diffraction, Rietveld refinement and scanning electronic microscopy. Strength reliability and failure probability of the three sample groups were analyzed by Weibull statistics. RESULTS: The sintered samples exhibited relative densities in the range of 78% (nozzle Ø 0.41 = mm) and 82% (nozzle Ø 0.25 = mm), i.e., a slight increase in the residual interfilamentous porosity is observed, as the extrusion tip diameter increases, while linear shrinkage is statistically similar (≈25%). Likewise, a progressive reduction of flexural strength and Weibull modulus as nozzle diameter increases was noticeable, being respectively σf = 337,5 ± 49 MPa and m = 6.6 for the smallest nozzle diameter (Ø = 0.25 mm) and σf = 261.4 ± 79 MPa and m = 3.2 for the biggest one (Ø = 0.41 mm). Unlike nozzle diameter, the material is constituted by 79-81 wt% tetragonal t-ZrO2 and 19-21 wt% cubic c-ZrO2 with equiaxed grain sizes between 0.3 and 0.6 µm. CONCLUSION: X-ray diffraction analyses on the fracture surface of flexural test samples suggests that the toughening mechanism by tetragonal→ monoclinic phase transformation is the main responsible for the mechanical strength of this structural ceramic. Additionally, the reduction of flexural strength for samples printed with extrusion nozzle of 0.41 mm could be explained by the surface roughness of the bending surfaces, as well as the lower effective resistance to crack-propagation arising from the higher size of residual pores on the fracture surface.

Novel sintering-free scaffolds obtained by additive manufacturing for concurrent bone regeneration and drug delivery: Proof of concept.

Advances on the fabrication of sintering-free biphasic calcium phosphate (BCP)/natural polymer composite scaffolds using robocasting as additive manufacturing technique are presented in the present work. Inks with high amounts of BCP powders (45 vol%) containing different HA/ß-TCP ratios, in presence of crosslinked polymer, were successfully fine-tuned for extrusion by robocasting. The non-existence of sintering step opened the possibility to obtain drug loaded scaffolds by adding levofloxacin to the extrudable inks. The drug presence induced slightly changes on the rheological behaviour of the inks, more emphasized for the BCP compositions with higher amounts of ß-TCP, and consequently, on the microstructure and on the mechanical properties of the final scaffolds. The strong interaction of ß-TCP with chitosan difficult the preparation of suitable rheological inks for printing. Drug delivery studies revealed a fast release of levofloxacin with a high burst of drug within the first 30 min. Levofloxacin loaded samples also presented bacteria growth inhibition ability, proving that antibiotic was not degraded during the fabrication process and its bactericidal efficacy was preserved. From the results obtained, the composite scaffolds containing higher amounts of HA (around 80% HA/20% ß-TCP) constitute a promising bi-functional synthetic bone substitute for simultaneous local bone regeneration and infection treatments.

Surface passivation of MgAl2O4 spinel powder by chemisorbing H3PO4 for easy aqueous processing.

A stoichiometric MgAl 2O 4 spinel (MAS) powder was synthesized by heat treating at 1400 degrees C for 2 h a compacted mixture of alpha-Al 2O 3 and calcined caustic MgO, followed by crushing and milling. The surface of this powder was then passivated against hydrolysis with H 3PO 4 and Al(H 2PO 4) 3 in an ethanol solution. The as-passivated powder could then be dispersed in water using tetramethylammonium hydroxide (TMAH) and an ammonium salt of poly(acrylic acid) (Duramax D-3005) as dispersing agents and gelcast to form green consolidates with relatively high strength (>15 MPa). The good dispersing behavior of the passivated powder in water was confirmed by the low viscosity of its suspension containing 41-45 vol % solids, demonstrating the viability of replacing organic solvents by water in colloidal processing of MAS-based ceramics. The Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and energy dispersive X-ray (EDAX) studies revealed that only negligible amounts of phosphate ions at the surface are required to effectively protect the powder from reacting with water.