anlong Hu

Structure and properties of nano-hydroxypatite scaffolds for bone tissue engineering with a selective laser sintering system

In this study, nano-hydroxypatite (n-HAP) bone scaffolds are prepared by a homemade selective laser sintering (SLS) system based on rapid prototyping (RP) technology. The SLS system consists of a precise three-axis motion platform and a laser with its optical focusing device. The implementation of arbitrary complex movements based on the non-uniform rational B-Spline (NURBS) theory is realized in this system. The effects of the sintering processing parameters on the microstructure of n-HAP are tested with x-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The particles of n-HAP grow gradually and tend to become spherical-like from the initial needle-like shape, but still maintain a nanoscale structure at scanning speeds between 200 and 300 mm min(-1) when the laser power is 50 W, the light spot diameter 4 mm, and the layer thickness 0.3 mm. In addition, these changes do not result in decomposition of the n-HAP during the sintering process. The results suggest that the newly developed n-HAP scaffolds have the potential to serve as an excellent substrate in bone tissue engineering.

In vitro bioactivity and degradability of β-tricalcium phosphate porous scaffold fabricated via selective laser sintering

Porous scaffolds consisting of β‐tricalcium phosphate (β‐TCP) were successfully fabricated via selective laser sintering. The scaffolds had a controlled microstructure and totally interconnected porous structure. The microstructure and mechanical properties were studied. The bioactivity and degradability of scaffolds were evaluated through the simulated body fluid (SBF) cultivation experiment. The formation of a biologically active carbonate apatite layer on the surface after immersion in SBF was demonstrated using scanning electron microscope, energy dispersive X‐ray, and Fourier transform infrared spectroscopy. Fast nucleation and growth of the carbonate apatite crystals were observed to occur all through the specimen surfaces. The phenomenon was explained in terms of the distribution and dispersion of inorganic phases in the scaffolds and the ionic activity products of the apatite in the SBF. The calculation results of weight loss and Ca/P molar ratio also suggest the good bioactivity and degradability of the scaffolds. These indicate that the β‐TCP porous ceramic scaffold is a potential candidate scaffold for bone tissue engineering.

Correlation between properties and microstructure of laser sintered porous β-tricalcium phosphate bone scaffolds

Abstract A porous β-tricalcium phosphate (β-TCP) bioceramic scaffold was successfully prepared with our homemade selective laser sintering system. Microstructure observation by a scanning electron microscope showed that the grains grew from 0.21 to 1.32 μm with the decrease of laser scanning speed from 250 to 50 mm min−1. The mechanical properties increased mainly due to the improved apparent density when the laser scanning speed decreased to 150 mm min−1. When the scanning speed was further decreased, the grain size became larger and the mechanical properties severely decreased. The highest Vickers hardness and fracture toughness of the scaffold were 3.59 GPa and 1.16 MPa m1/2, respectively, when laser power was 11 W, spot size was 1 mm in diameter, layer thickness was 0.1–0.2 mm and laser scanning speed was 150 mm min−1. The biocompatibility of these scaffolds was assessed in vitro with MG63 osteoblast-like cells and human bone marrow mesenchymal stem cells. The results showed that all the prepared scaffolds are suitable for cell attachment and differentiation. Moreover, the smaller the grain size, the better the cell biocompatibility. The porous scaffold with a grain size of 0.71 μm was immersed in a simulated body fluid for different days to assess the bioactivity. The surface of the scaffold was covered by a bone-like apatite layer, which indicated that the β-TCP scaffold possesses good bioactivity. These discoveries demonstrated the evolution rule between grain microstructure and the properties that give a useful reference for the fabrication of β-TCP bone scaffolds.

Fabrication and Characterization of Porous 45S5 Glass Scaffolds via Direct Selective Laser Sintering

A porous 45S5 bioactive glass-ceramic scaffold has been designed and fabricated via direct selective laser sintering (SLS). The scaffolds sintered over a range of laser powers from 6.0 to 30.0 W were characterized using scanning electron microscope (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). Their mechanical properties were tested with Vickers hardness tester. The testing results showed that the 45S5 glass in an initial amorphous state was transformed into the favorable crystallization phase Na2Ca2Si3O9 by inhibiting other phase transformation due to the rapid heating and cooling of laser. The 45S5 particles began to soften and fuse together with the increased laser power during the sintering process. At a proper range of laser power (around 15.0 W), the sample became denser and had higher degree of crystallinity with superior fracture toughness. Further increasing the laser power, the 45S5 glass powders were melted, and there are the undesired holes or even sinking emerged on the surface of the sample. So, the scaffold with the superior fracture toughness and degree of crystallinity has been obtained under the laser power of 15.0 W.

Enhanced sintering ability of biphasic calcium phosphate by polymers used for bone scaffold fabrication.

Biphasic calcium phosphate (BCP), which is composed of hydroxyapatite [HAP, Ca10(PO4)6(OH)2] and β-tricalcium phosphate [β-TCP, β-Ca3(PO4)2], is usually difficult to densify into a solid state with selective laser sintering (SLS) due to the short sintering time. In this study, the sintering ability of BCP ceramics was significantly improved by adding a small amount of polymers, by which a liquid phase was introduced during the sintering process. The effects of the polymer content, laser power and HAP/β-TCP ratios on the microstructure, chemical composition and mechanical properties of the BCP scaffolds were investigated. The results showed that the BCP scaffolds became increasingly more compact with the increase of the poly(l-lactic acid) (PLLA) content (0-1 wt.%) and laser power (6-10 W). The fracture toughness and micro-hardness of the sintered scaffolds were also improved. Moreover, PLLA could be gradually decomposed in the late sintering stages and eliminated from the final BCP scaffolds if the PLLA content was below a certain value (approximately 1 wt.% in this case). The added PLLA could not be completely eliminated when its content was further increased to 1.5 wt.% or higher because an unexpected carbon phase was detected in the sintered scaffolds. Furthermore, many pores were observed due to the removal of PLLA. Micro-cracks and micro-pores occurred when the laser power was too high (12 W). These defects resulted in a deterioration of the mechanical properties. The hardness and fracture toughness reached maximum values of 490.3±10 HV and 1.72±0.10 MPa m(1/2), respectively, with a PLLA content of approximately 1 wt.% and laser power of approximately 10 W. Poly(l-lactic-co-glycolic acid) (PLGA) showed similar effects on the sintering process of BCP ceramics. Rectangular, porous BCP scaffolds were fabricated based on the optimum values of the polymer content and laser power. This work may provide an experimental basis for improving the mechanical properties of BCP bone scaffolds fabricated with SLS.

Optimize Structure Dimension of Spacer-Free Nozzle of Aluminum Roll Casting Using Orthogonal Experiment Design

Optimization of nozzle was crucial process to improve quality of production of roll casting, and nozzle without spacer should be pre-designed before placing spacer in it. Using orthogonal experiment design, three main structure dimensions and the speed of roll casting sheet were chosen as 4 factors, to make orthogonal array of 3 levels for simulation of nozzle. It was concluded that an optimal combination of factors and levels could be provided by orthogonal experiment, which might achieve optimum of good distributing of velocity and temperature of flow fluid in nozzle. The optimized result would be used as the original condition of integrated design of nozzle and spacers.

FABRICATION OPTIMIZATION OF NANOHYDROXYAPATITE ARTIFICIAL BONE SCAFFOLDS

Serious microcracks often occur on the surface of nanohydroxyapatite (n-HAP) artificial bone scaffolds prepared by selective laser sintering (SLS) technology. In this study, we found that appropriate preheating before sintering can reduce and attenuate the cracks. The microstructure and morphology of sintered n-HAP were tested at different preheating temperature and laser sintering speed with scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). The experiments showed that the cracks gradually reduced and then disappeared when the preheating temperature increased from 0°C to 600°C while other parameters remain unchanged. The n-HAP particles gradually fused and grew up, while the grain size of sintered n-HAP will be attenuated with the increase of preheating temperature. As the thermal conductivity of n-HAP increases with increased preheating temperature, the temperature drops quickly, inhibiting greatly the grain growth of n-HAP. We obtained a group of optimum parameters when the sintered n-HAP still maintains nanostructure and possesses the optimal comprehensive performances, that is, laser power is 26 W, spot diameter is 4 mm, sintering speed is 200 mm/min, layer thickness is 0.4 mm, layer density is 852 kg/m3, and optimized preheating temperature is 600°C. These data illustrated that the cracks of sintered n-HAP can be eliminated at appropriate preheating temperature and sintering speed. This provided experimental optimal condition for the preparation of artificial bone scaffolds with nanohydroxyapatite ceramics.

Pre-design and Analysis of Flow Field of Spacer-Free Nozzle of Aluminum Roll-Casting

To optimize the design of flow field of nozzle in aluminum roll-casting, a coupled fluid-thermal finite element analysis using ANSYS software, was performed to explore the distributing of velocity and temperature of melt aluminum in spacer-free nozzle by MATLAB. Curve of velocity was gently sloping but curve of temperature was steep at outlet of spacer-free nozzle. It was explored that geometrical sizes of nozzle should be pre-designed to get better curves of velocity and temperature before placing spacers into nozzle.

An Optimization Scheme of Single-Spacer Nozzle of Aluminum Roll Casting Using Coupled Fluid-Thermal Finite Element Analysis

To optimize shape of single-spacer nozzle, width of single spacer was chosen to be the only optimized parameter to discover its influence on distribution of velocity and temperature of nozzle’s flow fluid. According to results of coupled fluid-thermal finite element analysis with four different widths of single spacer in nozzle, it was discovered that the greater width was, the more uneven of distribution of velocity and temperature of flow fluid at outlet of nozzle would be.

Quantitative Analysis of Orthogonal Experiment on Simulation of Fluid Field in Spacer-Free Nozzle in Aluminum Roll-Casting

Conventional analysis of orthogonal experimental results was applied to optimize structure of spacer-free nozzle in aluminum roll-casting. Weighting factors of velocity and temperature were determined in analysis with extreme deviation. It was concluded that good distributing of flow fluid’s velocity and temperature, would come with bigger width of outlet, modest thickness of entrance, and smaller thickness of outlet. The main effects in order of priority were investigated and advices on improving performance were given for industrial experiments.