MECHANICAL PROPERTIES, AND BIOACTIVITY IN BIPHASIC AND PREFERENTIALLY ORIENTED CALCIUM PHOSPHATE BIOCERAMICS

Calcium phosphate bioceramics are recognized as one of the most biocompatible materials and are widely used in systems for bone repair and regeneration. In this work, the pulsed laser deposition technique was chosen to produce calcium phosphate materials with different crystallographic texture and chemical composition for investigation of their structural, mechanical, as well as biological properties. Various calcium phosphate coatings were obtained on Ti6Al4V substrates using a KrF excimer laser (248 nm) with an energy density of 4−8 J/cm 2 and substrate temperature of 625−730 °C. A method was developed that enables the deposition of highly textured hydroxyapatite by controlling the laser fluence and angle of incidence of the plume. Nanoindentation results showed enhancement of hardness and Young’s modulus in c-axis-oriented hydroxyapatite coatings, compared to randomly oriented coatings. Human mesenchymal stem cells attached in greater numbers to the hydroxyapatite with c-axis texture, as compared with randomly-oriented coatings. These results indicate that calcium phosphates with surfaces exhibiting controlled crystallographic texture achieved improved mechanical propertie

Synthesis and control of microstructure, mechanical properties, and bioactivity in biphasic and preferentially oriented calcium phosphate bioceramics

Calcium phosphate bioceramics are recognized as one of the most biocompatible materials and are widely used in systems for bone repair and regeneration. In this work, the pulsed laser deposition technique was chosen to produce calcium phosphate materials with different crystallographic texture and chemical composition for investigation of their structural, mechanical, as well as biological properties. Various calcium phosphate coatings were obtained on Ti6Al4V substrates using a KrF excimer laser (248 nm) with an energy density of 4–8 J/cm2 and substrate temperature of 625–730°C. A method was developed that enables the deposition of highly textured hydroxyapatite by controlling the laser fluence and angle of incidence of the plume. Nanoindentation results showed enhancement of hardness and Young’s modulus in c-axis-oriented hydroxyapatite coatings, compared to randomly oriented coatings. Human mesenchymal stem cells attached in greater numbers to the hydroxyapatite with c-axis texture, as compared with randomly-oriented coatings. These results indicate that calcium phosphates with surfaces exhibiting controlled crystallographic texture achieved improved mechanical properties and bioactivity and may be a promising system for clinical applications requiring superior biological and biomechanical performance. In addition, highly crystalline, biphasic hydroxyapatite/tetracalcium phosphate coatings were produced by laser ablation of targets of pure crystalline hydroxyapatite. The fraction of tetracalcium phosphate phase in the coatings was controlled either by varying substrate temperature or partial pressure of water vapor in the chamber during deposition. Systematic studies of phase composition in the hydroxyapatite/tetracalcium phosphate biphasic coatings were performed using X-ray diffraction. Tetracalcium phosphate in the coatings seems to be formed not by decomposition of hydroxyapatite but by local nucleation during deposition. In-vitro dissolution studies revealed that the hydroxyapatite phase in the coatings remain essentially unaltered for periods of up to one week after immersion in a physiological solution while all the tetracalcium phosphate phase in the coating dissolves within 12 h. In-vivo studies showed a higher percentage of mineralized bone contiguous to the bone-coating interface for the biphasic coatings, suggesting enhanced osteoconduction in comparison to pure hydroxyapatite. Results suggest that biphasic coatings with well-controlled tetracalcium phosphate content may represent a route for triggering optimized biological response shortly after implant insertion, followed by a period of osteointegration on a still mechanically robust nonresorbable coating

Single Inductor Multiple Output Auto-Buck-Boost DC–DC Converter with Error-Driven Randomized Control

We propose a single inductor multiple output (SIMO) auto-buck-boost DC–DC converter with error-driven randomized control (EDRC). The conventional controls in a SIMO DC–DC converter supply power to outputs that have been selected in a sequential order. Furthermore, they control the inductor current levels at either edge of a switching period in a steady state to be at the same level to alleviate cross-regulation. However, this limits the flexibility of the converter to respond to changes in load requirements. A sequential selection of light loads results in these loads being selected more often than a load demand, degrading the efficiency for light loads. In addition, limited flexibility leads to delayed responses. This paper introduces an auto-buck-boost topology that selects outputs based on output errors, and instantaneously adjusts the inductor current level. Moreover, we propose a technique for allowing any output to avoid selection when all outputs are fully supplied. The proposed EDRC scheme achieves improvements in efficiency in regards to light loads, cross-regulation, and output driving capability

Effect of post weld heat treatment on weldability of high entropy alloy welds

The effect of post weld heat treatment (PWHT) temperature on laser beam welds in high-entropy alloys (HEAs) using a cold-rolled cantor system (CoCrFeMnNi) was investigated. Laser welding of low heat input was applied to reduce thermal distortion. The cold-rolled HEA welds indicated larger grain size and inferior tensile/hardness properties as compared to the base metal (BM). By applying PWHT, the welds showed superior hardness to the BM with no variation in the face-centred cubic phase and a decrease in the size and fraction of CrMn oxide inclusions. As the PWHT temperature increased (800-1000 degrees C), the variation in the grain size decreased between the weld metal and heat-affected zone, thus resulting in approximately the same tensile strength and elongation of the transverse welds as compared to the BM.11Nsciescopu

Development of a genome-targeting mutator for the adaptive evolution of microbial cells

Methods that can randomly introduce mutations in the microbial genome have been used for classical genetic screening and, more recently, the evolutionary engineering of microbial cells. However, most methods rely on either cell-damaging agents or disruptive mutations of genes that are involved in accurate DNA replication, of which the latter requires prior knowledge of gene functions, and thus, is not easily transferable to other species. In this study, we developed a new mutator for in vivo mutagenesis that can directly modify the genomic DNA. Mutator protein, MutaEco, in which a DNA-modifying enzyme is fused to the alpha-subunit of Escherichia coli RNA polymerase, increases the mutation rate without compromising the cell viability and accelerates the adaptive evolution of E. coli for stress tolerance and utilization of unconventional carbon sources. This fusion strategy is expected to accommodate diverse DNA-modifying enzymes and may be easily adapted to various bacterial species.N