Accelerated hardening of nanotextured 3D-plotted self-setting calcium phosphate inks

Direct ink writing (DIW) techniques open up new possibilities for the fabrication of patient-specific bone grafts. Self-setting calcium phosphate inks, which harden at low temperature, allow obtaining nanostructured scaffolds with biomimetic properties and enhanced bioactivity. However, the slow hardening kinetics hampers the translation to the clinics. Different hydrothermal treatments for the consolidation of DIW scaffolds fabricated with an a-tricalcium phosphate /pluronic F127 ink were explored, comparing them with a biomimetic treatment. Three different scaffold architectures were analysed. The hardening process, associated to the conversion of a-tricalcium phosphate to hydroxyapatite was drastically accelerated by the hydrothermal treatments, reducing the time for complete reaction from 7¿days to 30 minutes, while preserving the scaffold architectural integrity and retaining the nanostructured features. ß-tricalcium phosphate was formed as a secondary phase, and a change of morphology from plate-like to needle-like crystals in the hydroxyapatite phase was observed. The binder was largely released during the treatment. The hydrothermal treatment resulted in a 30% reduction of the compressive strength, associated to the residual presence of ß-tricalcium phosphate. Biomimetic and hydrothermally treated scaffolds supported the adhesion and proliferation of rat mesenchymal stem cells, indicating a good suitability for bone tissue engineering applications. Statement of Significance 3D plotting has opened up new perspectives in the bone regeneration field allowing the customisation of synthetic bone grafts able to fit patient-specific bone defects. Moreover, this technique allows the control of the scaffolds’ architecture and porosity. The present work introduces a new method to harden biomimetic hydroxyapatite 3D-plotted scaffolds which avoids high-temperature sintering. It has two main advantages: i) it is fast and simple, reducing the whole fabrication process from the several days required for the biomimetic processing to a few hours; and ii) it retains the nanostructured character of biomimetic hydroxyapatite and allows controlling the porosity from the nano- to the macroscale. Moreover, the good in vitro cytocompatibility results support its suitability for cell-based bone regeneration therapiesPeer ReviewedPostprint (author's final draft

In vivo performance of novel soybean/gelatin-based bioactive and injectable hydroxyapatite foams

Major limitations of calcium phosphate cements (CPCs) are their relatively slow degradation rate and the lack of macropores allowing the ingrowth of bone tissue. The development of self-setting cement foams has been proposed as a suitable strategy to overcome these limitations. In previous work we developed a gelatine-based hydroxyapatite foam (G-foam), which exhibited good injectability and cohesion, interconnected porosity and good biocompatibility in vitro. In the present study we evaluated the in vivo performance of the G-foam. Furthermore, we investigated whether enrichment of the foam with soybean extract (SG-foam) increased its bioactivity. G-foam, SG-foam and non-foamed CPC were implanted in a critical-size bone defect in the distal femoral condyle of New Zealand white rabbits. Bone formation and degradation of the materials were investigated after 4, 12 and 20 weeks using histological and biomechanical methods. The foams maintained their macroporosity after injection and setting in vivo. Compared to non-foamed CPC, cellular degradation of the foams was considerably increased and accompanied by new bone formation. The additional functionalization with soybean extract in the SG-foam slightly reduced the degradation rate and positively influenced bone formation in the defect. Furthermore, both foams exhibited excellent biocompatibility, implying that these novel materials may be promising for clinical application in non-loaded bone defects. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd.Peer ReviewedPostprint (published version

Interpenetrated Magnesium–Tricalcium Phosphate Composite: Manufacture, Characterization and In Vitro Degradation Test

Magnesium and calcium phosphates composites are promising biomaterials to create biodegradable load-bearing implants for bone regeneration. The present investigation is focused on the design of an interpenetrated magnesium–tricalcium phosphate (Mg–TCP) composite and its evaluation under immersion test. In the study, TCP porous preforms were fabricated by robocasting to have a prefect control of porosity and pore size and later infiltrated with pure commercial Mg through current-assisted metal infiltration (CAMI) technique. The microstructure, composition, distribution of phases and degradation of the composite under physiological simulated conditions were analysed by scanning electron microscopy, elemental chemical analysis and X-ray diffraction. The results revealed that robocast TCP preforms were full infiltrated by magnesium through CAMI, even small pores below 2 lm have been filled with Mg, giving to the composite a good interpenetration. The degradation rate of the Mg–TCP composite displays lower value compared to the one of pure Mg during the first 24 h of immersion test.Magnesium and calcium phosphates composites are promising biomaterials to create biodegradable load-bearing implants for bone regeneration. The present investigation is focused on the design of an interpenetrated magnesium–tricalcium phosphate (Mg–TCP) composite and its evaluation under immersion test. In the study, TCP porous preforms were fabricated by robocasting to have a prefect control of porosity and pore size and later infiltrated with pure commercial Mg through current-assisted metal infiltration (CAMI) technique. The microstructure, composition, distribution of phases and degradation of the composite under physiological simulated conditions were analysed by scanning electron microscopy, elemental chemical analysis and X-ray diffraction. The results revealed that robocast TCP preforms were full infiltrated by magnesium through CAMI, even small pores below 2 lm have been filled with Mg, giving to the composite a good interpenetration. The degradation rate of the Mg–TCP composite displays lower value compared to the one of pure Mg during the first 24 h of immersion test

Brushite foamsthe effect of Tween (R) 80 and Pluronic (R) F-127 on foam porosity and mechanical properties

Resorbable calcium phosphate based bone void fillers should work as temporary templates for new bone formation. The incorporation of macropores with sizes of 100 -300 mu m has been shown to increase the resorption rate of the implant and speed up bone ingrowth. In this work, macroporous brushite cements were fabricated through foaming of the cement paste, using two different synthetic surfactants, Tween (R) 80 and Pluronic (R) F-127. The macropores formed in the Pluronic samples were both smaller and less homogeneously distributed compared with the pores formed in the Tween samples. The porosity and compressive strength (CS) were comparable to previously developed hydroxyapatite foams. The cement foam containing Tween, 0.5M citric acid in the liquid, 1 mass% of disodium dihydrogen pyrophosphate mixed in the powder and a liquid to powder ratio of 0.43 mL/g, showed the highest porosity values (76% total and 56% macroporosity), while the CS was >1 MPa, that is, the hardened cement could be handled without rupture of the foamed structure. The investigated brushite foams show potential for future clinical use, both as bone void fillers and as scaffolds for in vitro bone regeneration

Magnesium Strengthening in 3D Printed TCP Scaffold Composites

This study reports the production of a Mg/15%β-tricalcium phosphate Ca3(PO4)2 composite by combining direct ink writing for the β-TCP preform and liquid infiltration technique to obtain a continuous metal matrix composite. The influence of the volume fraction of β-TCP and the in situ reaction between ceramic and metal on the microstructure and mechanical properties were investigated in detail. The β-TCP preform was uniformly distributed in the matrix, forming a continuous three-dimensional (3D) network. The obtained composite was characterized by means of relative density (He pycnometry), X-ray diffractometry (XRD), scanning electron microscopy (SEM), and electron spectroscopy (EDX). The results suggested that a highly densified composite was processed. Three phases were identified as products generated by an exothermic reaction (Mg2Ca, CaO, and MgO); based on this, the chemical reaction mechanism for MgO formation was proposed. The compression and hardness tests showed that the Mg/15%β-tricalcium phosphate Ca3 (PO4)2 composite significantly improved its mechanical properties, i.e., 27% and 15% higher than pure Mg in compressive strength and yield strength, respectively. This behavior was attributed to the high densification of the resulting composite, strong chemical interfacial bonding, phase dispersion hardening (in situ phase formation), and the geometry and continuity of the reinforcement. These provided good load transfer from the Mg matrix to the reinforcement and contributed as strengthening mechanisms. The results reported in this investigation can help to design Mg/calcium phosphate continuous composites for biomedical applications

Effect of powder milling on sintering behavior and monotonic and cyclic mechanical properties of Mo and Mo–Si lattices produced by direct ink writing

Molybdenum is a refractory metal regarded as a promising basis for producing high-temperature components. However, the potential of manufacturing molybdenum-based structures by direct ink writing (DIW) has not been explored. In this study, three-dimensional porous molybdenum (Mo) and molybdenum-silicon (Mo–Si) composite lattices were fabricated using DIW with non-milled and milled powders. The effects of Mo powder morphology (resulting from milling) and chemical composition (alloying Mo with 3 and 10 wt% of Si) on the microstructure, phase composition, and static and cyclic compression properties at room temperature were investigated. Lattices fabricated from commercial spherical Mo powder exhibited the highest intra-filament porosity. Conversely, lattices fabricated from milled Mo powder were denser and had higher compressive strength, offset stress, and quasi-elastic gradient. Alloying Mo with Si during sintering resulted in composite lattices with Mo + Mo3Si microstructure. A low content of Mo3Si slightly decreased monotonic compression properties but did not affect the cyclic compression response compared to Mo lattices made from milled powder. In contrast, a high content of Mo3Si produced quasi-brittle lattices with reduced compressive strength and increased damage accumulation during cyclic loading. The cyclic behavior of all lattices was characterized by a ratcheting-dominated stress-strain response. Lattices fabricated from milled Mo and milled Mo-3 wt.%Si powders demonstrated superior performance compared to those fabricated from commercial spherical Mo and milled Mo-10 wt%Si powders. The results suggest that using milled powders can enhance the mechanical reliability and promote the use of DIW as preferred additive manufacturing technology for the fabrication of Mo–Si composite lattices

Pressure-less spark plasma sintering of 3D-plotted titanium porous structures

Additive manufacturing of titanium porous structures by direct ink writing involves the removal of the binder needed for powder extrusion and subsequent sintering to consolidate the 3D-plotted body. In this work, pressure-less spark plasma sintering (PL-SPS) was systematically studied for fast consolidation of titanium porous structures. Furthermore, poloxamer 407 was used as the binder and the lowest temperature possible was identified for its thermal elimination. The results show for the first time that PL-SPS generated sintering conditions similar to those generated by conventional pressure-less sintering, producing hierarchical titanium porous structures with equivalent densification, shrinkage, and surface roughness, but with minimal grain growth. The thermal responses of the die and material showed efficient radiation heat transfer, allowing fast heating (100 °C/min) of one sample per run, promoting the formation of sintering necks and powder densification in 10 min, which is much faster than conventional sintering that requires at least 2 h of dwell time. However, the process operates at a sintering temperature 200–300 °C above the conventional sintering temperature, and at the expense of high consumption of electrical energy to achieve such a high heating rate. The mechanical strength of the resulting titanium structures increases with increasing strand densification at nearly constant strand separation, resulting in strong and plastic porous structures