Jiaxing Wang

Refining of coarse lamellar microstructure of TiAl alloys by rapid heat treatment

Abstract It is now well known that the room temperature tensile yield strength (σy), fracture strength (σf), and ductility (δ) of TiAl alloys increase with decreasing the colony size (d) of the fully lamellar (FL) structure. Therefore searching a cost-effective approach to refine the coarse FL structure becomes critical to the engineering application of this material. In this study, we report our recent observation that, merely by rapid heat treatment, a FL structure with d=500 μm in a TiAl alloy of Ti–48Al–2Cr (at%) could be refined to a FL structure with d being as fine as 10 μm. No thermomechanical processing such as canned forging and hot extrusion, as done in previous studies, was required. Mechanical testing in tension shows that with the refinement from 500 to 10 μm, the room temperature yield strength increased from 330 to 610 MPa, fracture strength from 415 to 825 MPa and ductility from 0.7 to 3.3%, although the fracture toughness decreased only slightly from 24 to 19 MPa m1/2.

A Systematic Review and Meta-Analysis of Antibiotic-Impregnated Bone Cement Use in Primary Total Hip or Knee Arthroplasty

Background Antibiotic-impregnated bone cement (AIBC) has been widely used for the treatment of infected revision arthroplasty, but its routine use in primary total joint arthroplasty (TJA) remains considerably controversial. With this meta-analysis of published randomized controlled trials, we intended to assess the antimicrobial efficacy and safety of AIBC for its prophylactic use in primary TJA. Methods A literature search was performed in MEDLINE, Embase, CBMdisc and the Cochrane Library until June, 2013. The studies were divided into two sub-groups according to the type of the control group. Outcomes of interest included postoperative infection rates, radiographic outcomes and clinical joint score. Study quality was evaluated using the Jadad scale (five points). Results In total, eight studies were included, with a sample size of 6,381 arthroplasties. The overall pooled data demonstrated that, compared with the control (plain cement or systemic antibiotic), AIBC did not reveal an advantage in decreasing the rate of superficial infection (relative risk [RR] = 1.47; 95% CI, 1.13–1.91; P=0.004), while there were significant differences in deep infection rate between the AIBC and control group (RR = 0.41; 95% CI, 0.17–0.97; P=0.04). For the analysis of gentamicin and cefuroxime subgroups, the gentamicin was superior to the cefuroxime in reducing deep infection rate (P=0.0005 versus P= 0.10). However, no significant differences were found in their radiographic outcomes and clinical joint score. Conclusion This meta-analysis had proven that the prophylactic use of AIBC could lower the deep infection rate in primary TJA, while AIBC did not show an improvement in reducing the superficial infection rate compared with the control. More sufficiently powered studies would be required to further evaluate the efficacy and safety of AIBC for primary TJA.

Antibacterial Surface Design of Titanium-Based Biomaterials for Enhanced Bacteria-Killing and Cell-Assisting Functions Against Periprosthetic Joint Infection.

Periprosthetic joint infection (PJI) is one of the formidable and recalcitrant complications after orthopedic surgery, and inhibiting biofilm formation on the implant surface is considered crucial to prophylaxis of PJI. However, it has recently been demonstrated that free-floating biofilm-like aggregates in the local body fluid and bacterial colonization on the implant and peri-implant tissues can coexist and are involved in the pathogenesis of PJI. An effective surface with both contact-killing and release-killing antimicrobial capabilities can potentially abate these concerns and minimize PJI caused by adherent/planktonic bacteria. Herein, Ag nanoparticles (NPs) are embedded in titania (TiO2) nanotubes by anodic oxidation and plasma immersion ion implantation (PIII) to form a contact-killing surface. Vancomycin is then incorporated into the nanotubes by vacuum extraction and lyophilization to produce the release-killing effect. A novel clinical PJI model system involving both in vitro and in vivo use of methicillin-resistant Staphylococcus aureus (MRSA) ST239 is established to systematically evaluate the antibacterial properties of the hybrid surface against planktonic and sessile bacteria. The vancomycin-loaded and Ag-implanted TiO2 nanotubular surface exhibits excellent antimicrobial and antibiofilm effects against planktonic/adherent bacteria without appreciable silver ion release. The fibroblasts/bacteria cocultures reveal that the surface can help fibroblasts to combat bacteria. We first utilize the nanoarchitecture of implant surface as a bridge between the inorganic bactericide (Ag NPs) and organic antibacterial agent (vancomycin) to achieve total victory in the battle of PJI. The combination of contact-killing and release-killing together with cell-assisting function also provides a novel and effective strategy to mitigate bacterial infection and biofilm formation on biomaterials and has large potential in orthopedic applications.

Silver nanoparticles promote osteogenic differentiation of human urine-derived stem cells at noncytotoxic concentrations

In tissue engineering, urine-derived stem cells are ideal seed cells and silver nanoparticles (AgNPs) are perfect antimicrobial agents. Due to a distinct lack of information on the effects of AgNPs on urine-derived stem cells, a study was conducted to evaluate the effects of silver ions and AgNPs upon the cytotoxicity and osteogenic differentiation of urine-derived stem cells. Initially, AgNPs or AgNO3 were exposed to urine-derived stem cells for 24 hours. Cytotoxicity was measured using the Cell Counting kit-8 (CCK-8) test. The effects of AgNPs or AgNO3 at the maximum safety concentration determined by the CCK-8 test on osteogenic differentiation of urine-derived stem cells were assessed by alkaline phosphatase activity, Alizarin Red S staining, and the quantitative reverse transcription polymerase chain reaction. Lastly, the effects of AgNPs or AgNO3 on “urine-derived stem cell actin cytoskeleton organization” and RhoA activity were assessed by rhodamine-phalloidin staining and Western blotting. Concentration-dependent toxicity was observed starting at an AgNO3 concentration of 2 μg/mL and at an AgNP concentration of 4 μg/mL. At these concentrations, AgNPs were observed to promote osteogenic differentiation of urine-derived stem cells, induce actin polymerization and increase cytoskeletal tension, and activate RhoA; AgNO3 had no such effects. In conclusion, AgNPs can promote osteogenic differentiation of urine-derived stem cells at a suitable concentration, independently of silver ions, and are suitable for incorporation into tissue-engineered scaffolds that utilize urine-derived stem cells as seed cells.

Effects of alloying elements on creep of TiAl alloys with a fine lamellar structure

Abstract Creep experiments were conducted on five powder-metallurgy TiAl alloys with fine grains (65–80 μm), fine lamellar spacings (0.1–0.16 μm), and different compositions [Ti–47Al (+Cr, Nb, Ta, W, Si)] at temperatures of 760°C and 815°C and stresses from 35 to 723 MPa. Results show that at a given lamellar spacing, replacing 1% Nb (atomic percent) with 1% Ta and replacing 0.2% Ta with 0.2% W induced little effect, but addition of 0.3% Si decreased the creep resistance by a factor of 3–4 under otherwise identical conditions. Field emission TEM was used to characterize the changes of microstructure and alloy element distribution before and after creep. It was found that thinning and dissolution of α 2 lamellae and continuous coarsening of γ lamellae were the main creep processes and the microalloying elements tended to segregate at lamellar interfaces, especially at ledges during creep. The effects of different alloying elements are interpreted in terms of the interaction of alloy segregants with misfit and/or misorientation dislocations at the lamellar interface. That is, the interaction retards the climb of interfacial dislocations and thus the creep process in the case of large segregants (Nb, Ta, W), but facilitates the climb and creep in the case of small segregants (Si).

On the grain size refinement of TiAl alloys by cyclic heat treatment

Abstract In this study, we report a new approach, alternative to the previous thermomechanical processing, for achieving the refinement of TiAl alloys from a coarse fully lamellar structure of 1–3 mm to a fine one of 10–20 μm merely by heat treatment. This approach involves the refining pathway via massive transformation, or feathery γ transformation, or discontinuous coarsening. Although these different routes led to similar results, they included different treating conditions such as annealing time and cooling rate and thus may be used to ingots or components of different sizes.

Preparation and mechanical properties of a new Zr–Al–Ti–Cu–Ni–Be bulk metallic glass

Abstract In this paper, a new Zr 37 Al 10 Ti 12.5 Cu 11.25 Ni 9 Be 20.25 bulk metallic glass is reported. The present alloy was prepared by water quenching in a silica tube of φ10×85 mm. The amorphicity of the quenched bulk samples was examined using X-ray diffraction analysis and optical microscopy. The thermal stability was evaluated by differential scanning calorimetry (DSC) at a heating rate of 10 K/min. The characteristic data of the bulk metallic glass are presented, including glass transition temperature ( T g ) and crystallization temperature ( T x ). Results show that the present alloy exhibits large glass forming ability. For comparison with the well-known Zr–Ti–Cu–Ni–Be metallic glass, it was found that aluminum has a little effect on the vitrification of the present alloy but influences physical properties. Specifically, Al enhances the Youngs modulus by 21.4% and Vickers hardness by 20% and reduces density by 7.2%.

On the massive transformation in TiAl-based alloys

Abstract As one of the important phase transformation modes in TiAl-based alloys, massive transformation (MT) is of great importance for microstructure control. It has been successfully ultilized for refining a coarse lamellar structure with a colony size ∼1000 μm to a fine one of ∼10 μm without any thermomechanical process. In this paper, various controlling factors and the underlying mechanisms of MT in Ti–46Al–2Cr–2Nb and Ti–48Al–2Cr alloys are studied. It is found that high cooling rate is needed for implementation of this transformation, but further increasing cooling rate may induce the competing transformation of α → α 2 and result in a decrease in the volume fraction of the MT product ( γ m ). Solution treating temperature and time before quenching should be high and long enough for the formation of γ m , but too long time holding at high temperature would result in fewer γ m . The MT takes place more easily in those alloys with a fine initial microstructure than in those with a coarse one. The γ m can be observed in the present alloy with only 46Al but alloyed with Nb and Cr even after oil quenching. It is emphasized that the controlling factors including cooling rate, solution treating temperature and time, initial microstructure, and chemical composition are not independent from each other, but mutually related.

Microstructure refinement of a TiAl alloy by beat treatment

Abstract In order to refine the as-cast microstructure of a TiAl based alloy with a grain size of ∼1000 μm, a multiple-stage heat treatment was carried out and a fine fully lamellar microstructure (FFL) with a grain size of 10–20 μm was obtained. This treatment route includes pretreatment in the single α field, cyclical treatment and aging in the α+γ field and solution treatment in the α field again. The final refining result appears to be influenced by the aging length.

Silver-nanoparticles-modified biomaterial surface resistant to staphylococcus: new insight into the antimicrobial action of silver.

Titanium implants are widely used clinically, but postoperative implant infection remains a potential severe complication. The purpose of this study was to investigate the antibacterial activity of nano-silver(Ag)-functionalized Ti surfaces against epidemic Staphylococcus from the perspective of the regulation of biofilm-related genes and based on a bacteria-cell co-culture study. To achieve this goal, two representative epidemic Staphylococcus strains, Staphylococcus epidermidis (S. epidermidis, RP62A) and Staphylococcus aureus (S. aureus, USA 300), were used, and it was found that an Ag-nanoparticle-modified Ti surface could regulate the expression levels of biofilm-related genes (icaA and icaR for S. epidermidis; fnbA and fnbB for S. aureus) to inhibit bacterial adhesion and biofilm formation. Moreover, a novel bacteria-fibroblast co-culture study revealed that the incorporation of Ag nanoparticles on such a surface can help mammalian cells to survive, adhere and spread more successfully than Staphylococcus. Therefore, the modified surface was demonstrated to possess a good anti-infective capability against both sessile bacteria and planktonic bacteria through synergy between the effects of Ag nanoparticles and ion release. This work provides new insight into the antimicrobial action and mechanism of Ag-nanoparticle-functionalized Ti surfaces with bacteria-killing and cell-assisting capabilities and paves the way towards better satisfying the clinical needs.