Poly (l-lactide acid) improves complete nano-hydroxyapatite bone scaffolds through the microstructure rearrangement

Abstract Cracks often occur when nano-hydroxyapatite bone scaffolds are fabricated with selective laser sintering, which affect the performance of scaffolds. In this study, a small amount of poly (l-lactide acid) (PLLA) was added into nano-hydroxyapatite (nano-HAP) powder by mechanical blending in order to improve the sintering properties. The nano-HAP powder combined with 1wt % PLLA was sintered under different laser power (5W, 7.5W, 10W, 12.5W, 15W and 20W). The fabricated scaffolds were characterized using Scanning Electron Microscope (SEM), X-ray Diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and Micro Hardness Tester. The results showed that nano-HAP particles grew up quickly with the laser power increasing, and there were many strip-like cracks on the surface of sintering zone. The cracks gradually reduced until disappeared when the laser power increased to 15W, together with a great improvement of density. Large pores were observed on the specimen when the laser power further increases, accompanied with the decomposition of HAP into \u3b2-tricalcium phosphate (\u3b2-TCP) and tetracalcium phosphate (TTCP). And the optimum parameters were eventually obtained with laser power of 15W, scanning speed of 1000 mm/min, powder bed temperature of 150\ubaC, laser spot diameter of 2 mm and layer thickness of 0.2 mm. We summarized that the molten PLLA enhanced the particle rearrangement of nano-HAP by capillary force and may absorb thermal stress in laser sintering process, while PLLA would be oxidized gradually until completely excluded from the sintered nano-HAP scaffolds, which was confirmed by FTIR analysis. This study provides a novel method to improve the sintering properties of nano-HAP with no adverse effects which would be used in the application of bone tissue engineering potentially

Formation and characteristic corrosion behavior of alternately lamellar arranged α and β in as-cast AZ91 Mg alloy

Formation and characteristic corrosion resistance of alternately lamellar arranged α and β in as-cast AZ91 Mg alloy were investigated as an independent micro-constituent identity. As-cast AZ91 presented three microstructural entities, i.e. α-Mg grain, (α+β) lamellae and coarse β particle, and each had its own Al content and microstructural morphology. The lamellae occurrence was due to the precipitation of β particle from the divorced eutectic Al-rich-α phase during solidification, because the Al composition can not exceed its maximum solubility. The evidences that were obtained from electrochemical tests, micro-corrosion morphology and hydrogen evolution rate certified that the (α+β) lamellae was beneficial to corrosion resistance, which was different from the reported deleterious influence for its original eutectic Al-rich-α phase. This different corrosion behavior was explained to be ascribed to the changes in fine structure and local composition that resulted in combined electrochemical effects of the changes in α and β phases on the corrosion

Nano SiO2 and MgO Improve the Properties of Porous β-TCP Scaffolds via Advanced Manufacturing Technology

Nano SiO2 and MgO particles were incorporated into β-tricalcium phosphate (β-TCP) scaffolds to improve the mechanical and biological properties. The porous cylindrical β-TCP scaffolds doped with 0.5 wt % SiO2, 1.0 wt % MgO, 0.5 wt % SiO2 + 1.0 wt % MgO were fabricated via selective laser sintering respectively and undoped β-TCP scaffold was also prepared as control. The phase composition and mechanical strength of the scaffolds were evaluated. X-ray diffraction analysis indicated that the phase transformation from β-TCP to α-TCP was inhibited after the addition of MgO. The compressive strength of scaffold was improved from 3.12 ± 0.36 MPa (β-TCP) to 5.74 ± 0.62 MPa (β-TCP/SiO2), 9.02 ± 0.55 MPa (β-TCP/MgO) and 10.43 ± 0.28 MPa (β-TCP/SiO2/MgO), respectively. The weight loss and apatite-forming ability of the scaffolds were evaluated by soaking them in simulated body fluid. The results demonstrated that both SiO2 and MgO dopings slowed down the degradation rate and improved the bioactivity of β-TCP scaffolds. In vitro cell culture studies indicated that SiO2 and MgO dopings facilitated cell attachment and proliferation. Combined addition of SiO2 and MgO were found optimal in enhancing both the mechanical and biological properties of β-TCP scaffold

Magnetostrictive bulk Fe-Ga alloys prepared by selective laser melting for biodegradable implant applications

Magnetostrictive Fe-Ga alloys have received extensive attention for their good mechanical properties and low saturation magnetization. However, the magnetostriction of bulk polycrystalline Fe-Ga alloys always suffers a major setback due to random grain orientation. In this study, selective laser melting (SLM) was used for the first time to manufacture bulk polycrystalline Fe81Ga19 alloys, and different scanning paths were determined for revealing the influences on grain orientation and magnetostrictive properties. The results showed that elongated columnar grains formed in the building direction due to the epitaxial growth during SLM. For zigzag scanning, grain orientation was parallel to the building direction while tilted by ∼17° for unidirectional scanning. And annular scanning resulted in mixed grain orientations in the alloys. Moreover, Fe81Ga19 alloy with zigzag scanning exhibited 〈100〉 preferred grain orientation. Consequently, the saturated magnetostriction of bulk Fe81Ga19 alloy with zigzag scanning reached ∼77.2 ppm, representing 23.9% and 25.1% increments compared with unidirectional and annular scanning, respectively. The Fe81Ga19 alloy with zigzag scanning also presented an ultimate compressive strength of 448.6 MPa and a degradation rate of 0.09 mm/y, as well as favorable biocompatibility. These results indicated that SLM might be a potential method for manufacturing magnetostrictive bulk Fe-Ga alloys for implant applications

Heterogeneous grain structure in biodegradable Zn prepared via mechanical alloying and laser powder bed fusion for strength-plasticity synergy

ABSTRACTThis study presented a unique process of mechanical alloying (MA) and laser powder bed fusion (LPBF) to prepare heterogeneous grain structure (HGS) Zn with strength-plasticity synergy. The results showed that the MA-treatment significantly refined the grain sizes of Zn to nano-scale and contributed to the formation of HGS powders. Moreover, the ultra-fast heating and cooling of LPBF process inhibited the grain growth and resulted in a specific core/shell HGS, wherein the cores (micron-grains) were surrounded by interconnected shells (nano-grains). Notably, the prepared HGS Zn demonstrated synergistically improved compressive yield strength (2.1 times) and plasticity (58%) compared with the homogeneous coarse-grained counterparts. This impressive strength-plasticity synergy was primarily attributed to high back stress and dislocation pile-ups generated by the deformation incompatibility between the nano-grained shells and micron-grained cores. These findings open up new fields for MA and LPBF in the preparation of HGS metals and their applications in solving strength-plasticity tradeoff

Functionalization of Calcium Sulfate/Bioglass Scaffolds with Zinc Oxide Whisker

There are urgent demands for satisfactory antibacterial activity and mechanical properties of bone scaffolds. In this study, zinc oxide whisker (ZnOw) was introduced into calcium sulfate/bioglass scaffolds. Antimicrobial behavior was analyzed using Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). The results showed that the scaffolds presented a strong antibacterial activity after introducing ZnOw, due to the antibacterial factors released from the degradation of ZnO. Moreover, ZnOw was also found to have a distinct reinforcing effect on mechanical properties. This was ascribed to whisker pull-out, crack bridging, crack deflection, crack branching and other toughening mechanisms. In addition, the cell culture experiments showed that the scaffolds with ZnOw had a good biocompatibility

Mechanical Reinforcement of Diopside Bone Scaffolds with Carbon Nanotubes

Carbon nanotubes are ideal candidates for the mechanical reinforcement of ceramic due to their excellent mechanical properties, high aspect ratio and nanometer scale diameter. In this study, the effects of multi-walled carbon nanotubes (MWCNTs) on the mechanical properties of diopside (Di) scaffolds fabricated by selective laser sintering were investigated. Results showed that compressive strength and fracture toughness improved significantly with increasing MWCNTs from 0.5 to 2 wt %, and then declined with increasing MWCNTs to 5 wt %. Compressive strength and fracture toughness were enhanced by 106% and 21%, respectively. The reinforcing mechanisms were identified as crack deflection, MWCNTs crack bridging and pull-out. Further, the scaffolds exhibited good apatite-formation ability and supported adhesion and proliferation of cells in vitro

Ag-Introduced Antibacterial Ability and Corrosion Resistance for Bio-Mg Alloys

Bone implants are expected to possess antibacterial ability and favorable biodegradability. Ag possesses broad-spectrum antibacterial effects through destroying the respiration and substance transport of bacteria. In this study, Ag was introduced into Mg-3Zn-0.5Zr (ZK30) via selective laser melting technology. Results showed that ZK30-Ag exhibited a strong and stable antibacterial activity against the bacterium Escherichia coli. Moreover, the degradation resistance was enhanced due to the comprehensive effect of positive shifted corrosion potential (from -1.64 to -1.53 V) and grains refinement. The positive shifted corrosion potential reduced the severe galvanic corrosion by lowering the corrosion potential difference between the matrix and the second phase. Meanwhile, the introduction of Ag caused the grain refinement strengthening and precipitated-phase strengthening, resulting in improved compressive yield strength and hardness. Furthermore, ZK30-0.5Ag exhibited good biocompatibility. It was suggested that Ag-modified ZK30 was potential candidate for bone implants

A Novel Collaborative Optimization Framework for Web Video Event Mining Based on the Combination of Inaccurate Visual Similarity Detection Information and Sparse Textual Information

The high speed and low latency of 5G mobile network have accelerated the speed and amount of information transmission. Web video is likely to become the main mode of news production and dissemination in the future for its richer information and more convenient dissemination, which will subvert the traditional mode of event mining. Therefore, event mining based on web videos has become a new research hotspot. However, web videos are vulnerable to video editing, lighting, shooting perspective and shooting angle, and other factors, resulting in the inaccurate visual similarity detection problem. Generally speaking, effectively integrating humungous volumes of cross-model information would give a great help. However, web videos are described with few terms, and thus sparse text information becomes a challenge for cross-model information combination. To address this issue, this paper proposes a new collaborative optimization framework with the combination of inaccurate visual similarity detection information and sparse textual information. This framework is composed of three steps. After obtaining the statistics of the distribution characteristics of each word in all Near-Duplicate Keyframes (NDKs), the high-level semantic cross-correlation between NDKs is first mined with the help of textual features, forming a new set of semantic relevant NDKs with different visual expressions. Next, textual distribution features are enriched through finding more semantically related words by the new NDK set with various forms of visual expressions, solving the sparse distribution problem for each word in all NDKs. Finally, Multiple Correspondence Analysis (MCA) is used to mine the events. Experimental results with a large number of real data demonstrate that the proposed model outperforms the existing methods for web video event mining