Effect of Cold-Sintering Parameters on Structure, Density, and Topology of Fe-Cu Nanocomposites.

The design of advanced nanostructured materials with predetermined physical properties requires knowledge of the relationship between these properties and the internal structure of the material at the nanoscale, as well as the dependence of the internal structure on the production (synthesis) parameters. This work is the first report of computer-aided analysis of high pressure consolidation (cold sintering) of bimetallic nanoparticles of two immiscible (Fe and Cu) metals using the embedded atom method (EAM). A detailed study of the effect of cold sintering parameters on the internal structure and properties of bulk Fe-Cu nanocomposites was conducted within the limitations of the numerical model. The variation of estimated density and bulk porosity as a function of Fe-to-Cu ratio and consolidation pressure was found in good agreement with the experimental data. For the first time, topological analysis using Minkowski functionals was applied to characterize the internal structure of a bimetallic nanocomposite. The dependence of topological invariants on input processing parameters was described for various components and structural phases. The model presented allows formalizing the relationship between the internal structure and properties of the studied nanocomposites. Based on the obtained topological invariants and Hadwiger's theorem we propose a new tool for computer-aided design of bimetallic Fe-Cu nanocomposites.

Microstructure, mechanical characteristics and cell compatibility of ß-tricalcium phosphate reinforced with biodegradable Fe-Mg metal phase.

The use of beta-tricalcium phosphate (ß-TCP) ceramic as a bioresorbable bone substitute is limited to non-load-bearing sites by the material׳s brittleness and low bending strength. In the present work, new biocompatible ß-TCP-based composites with improved mechanical properties were developed via reinforcing the ceramic matrix with 30 vol% of a biodegradable iron-magnesium metallic phase. ß-TCP-15Fe15Mg and ß-TCP-24Fe6Mg (vol%) composites were fabricated using a combination of high energy attrition milling, cold sintering/high pressure consolidation of powders at room temperature and annealing at 400 °C. The materials synthesized had a hierarchical nanocomposite structure with a nanocrystalline ß-TCP matrix toughened by a finely dispersed nanoscale metallic phase (largely Mg) alongside micron-scale metallic reinforcements (largely Fe). Both compositions exhibited high strength characteristics; in bending, they were about 3-fold stronger than ß-TCP reinforced with 30 vol% PLA polymer. Immersion in Ringer׳s solution for 4 weeks resulted in formation of corrosion products on the specimens׳ surface, a few percent weight loss and about 50% decrease in bending strength. In vitro studies of ß-TCP-15Fe15Mg composite with human osteoblast monocultures and human osteoblast-endothelial cell co-cultures indicated that the composition was biocompatible for the growth and survival of both cell types and cells exhibited tissue-specific markers for bone formation and angiogenesis, respectively.

ß-TCP-polylactide composite scaffolds with high strength and enhanced permeability prepared by a modified salt leaching method.

A modified particulate leaching method for fabrication of strong calcium phosphate-polymer composite scaffolds with improved pore interconnectivity is reported. The scaffolds were produced by mixing precompacted composite granules (ß-TCP with 40vol% PLA) of different size and density with salt particles followed by high pressure consolidation (at room temperature or 120°C) and porogen dissolution. The scaffolds' compressive strength and Darcy's permeability were found to be inversely related and to be strongly dependent on the processing parameters. The use of precompacted granules instead of the loose ß-TCP-PLA powder allowed us to increase permeability by three orders of magnitude while maintaining load bearing characteristics. Scaffolds with 50% porosity prepared from large (300-420µm) composite granules of ß-TCP-40vol% PLA and salt porogen particles of comparable size exhibited the best combination of compressive strength (4-6MPa) and permeability (1.3-1.6×10(-10)m(2)) falling within the range of trabecular bone.

Mesenchymal stem cell proliferation and differentiation on load-bearing trabecular Nitinol scaffolds.

Bone tissue regeneration in load-bearing regions of the body requires high-strength porous scaffolds capable of supporting angiogenesis and osteogenesis. 70% porous Nitinol (NiTi) scaffolds with a regular 3-D architecture resembling trabecular bone were produced from Ni foams using an original reactive vapor infiltration technique. The "trabecular Nitinol" scaffolds possessed a high compressive strength of 79 MPa and high permeability of 6.9×10(-6) cm2. The scaffolds were further modified to produce a near Ni-free surface layer and evaluated in terms of Ni ion release and human mesenchymal stem cell (hMSC) proliferation (AlamarBlue), differentiation (alkaline phosphatase activity, ALP) and mineralization (Alizarin Red S staining). Scanning electron microscopy was employed to qualitatively corroborate the results. hMSCs were able to adhere and proliferate on both as-produced and surface-modified trabecular NiTi scaffolds, to acquire an osteoblastic phenotype and produce a mineralized extracellular matrix. Both ALP activity and mineralization were increased on porous scaffolds compared to control polystyrene plates. Experiments in a model coculture system of microvascular endothelial cells and hMSCs demonstrated the formation of prevascular structures in trabecular NiTi scaffolds. These data suggest that load-bearing trabecular Nitinol scaffolds could be effective in regenerating damaged or lost bone tissue.

Strong bioresorbable Ca phosphate-PLA nanocomposites with uniform phase distribution by attrition milling and high pressure consolidation.

Highly dense bioresorbable Ca-deficient HA-PLA (CDHA-PLA) and ß-TCP-PLA nanocomposite materials with high (up to 80 vol%) contents of the calcium phosphate (CaP) phase and homogeneous phase distribution were prepared via attrition milling followed by high pressure consolidation at ambient temperature. The microstructure and mechanical properties of the materials obtained were studied as a function of milling time and PLA amount. Attrition milling resulted in disintegration of ß-TCP powder agglomerates down to 50-150 nm, disintegration of CDHA agglomerates and refinement of 15 × 150 nm(2) CDHA nanoparticles to a size of 8 × 20 nm(2), and in a uniform distribution of the polymer component. Very high compressive strengths up to 400 MPa and high bending strengths up to 70 MPa were obtained. For both ß-TCP and CDHA-based nanocomposites, the strength characteristics increased with milling time and decreased with increasing PLA content. For CDHA-based nanocomposites, attrition milling resulted in decrease of ductility while for ß-TCP-40 vol% PLA the ductility increased. The observed behavior may be a result of formation of homogeneous, relatively thick (tens of nanometers), ductile PLA layers in ß-TCP-PLA nanocomposites, but very thin (several nanometers) PLA layers in attrition milled CDHA-PLA nanocomposites. Degradation of compressive and bending strength in aqueous solutions was observed for all the studied CaP-PLA nanocomposites.

Phosphonate-anchored monolayers for antibody binding to magnetic nanoparticles.

Targeted delivery of magnetic iron oxide nanoparticles (IONPs) to a specific tissue can be achieved by conjugation with particular biological ligands on an appropriately functionalized IONP surface. To take best advantage of the unique magnetic properties of IONPs and to maximize their blood half-life, thin, strongly bonded, functionalized coatings are required. The work reported herein demonstrates the successful application of phosphonate-anchored self-assembled monolayers (SAMs) as ultrathin coatings for such particles. It also describes a new chemical approach to the anchoring of antibodies on the surface of SAM-coated IONPs (using nucleophilic aromatic substitution). This anchoring strategy results in stable, nonhydrolyzable, covalent attachment and allows the reactivity of the particles toward antibody binding to be activated in situ, such that prior to the activation the modified surface is stable for long-term storage. While the SAMs do not have the well-packed crystallinity of other such monolayers, their structure was studied using smooth model substrates based on an iron oxide layer on a double-side polished silicon wafer. In this way, atomic force microscopy, ellipsometry, and contact angle goniometry (tools that could not be applied to the nanoparticles' surfaces) could contribute to the determination of their monomolecular thickness and uniformity. Finally, the successful conjugation of IgG antibodies to the SAM-coated IONPs such that the antibodies retain their biological activity is verified by their complexation to a secondary fluorescent antibody.