Abrasion-corrosion of cast CoCrMo in simulated hip joint environments

Metal-on-metal (MoM) hip joint replacements have been increasingly used for younger and more active patients in recent years due to their improved wear performance compared to conventional metal-on-polymer bearings. MoM bearings operate at body temperature within a corrosive joint environment and therefore are inevitably being subjected to wear and corrosion as well as the combined action of tribo-corrosion. Issues such as metal sensitivity/metallosis associated with high levels of metal ion release triggered by the wear and corrosion products remain critical concerns. During the past few decades, significant research has been conducted into understanding the wear/lubrication mechanisms within the MoM hip joints in order to improve their performance and thereby prolonging their life. However, not much attention has been given to the combined effect of wear and corrosion of such devices in the hip joint environment, in addition, the role of third body particles and the effects of proteins have not been well understood.In this work, a systemic approach is presented for the first time for the mapping of abrasion and tribo-corrosion performance of a cast CoCrMo (F75) in simulated hip joint environments. The effects of third body particles have been studied in the MoM context using 4 ?m SiC, 1 ?m and 300 nm Al2O3, as well as sub-micron BaSO4. Modified tribo-testers (micro-abrasion, nanoindenter/scratching) incorporating a novel electrochemical cell have been used to monitor the abrasion-corrosion behaviour of the alloy in situ. The effects of solution chemistry, abrasives size / concentration and presence of proteins on the wear / corrosion level, wear-corrosion mechanisms, and the depassivation/repassivation kinetics of the CoCrMo have been explored. A variety of surface and sub-surface characterization techniques have been employed to identify the microstructual wear mechanism interactions. Results show that the change of protein concentration (0, 25% and 50% bovine serum) and pH (pH 7.4 and pH 4.0) of the test solutions can significantly influence the protein adsorption behaviour, which subsequently influence the wear rates (synergy), wear mechanisms as well as the wear-induced corrosion currents of the CoCrMo. For abrasion-corrosion tests, reducing abrasive size from 4 ?m to 300 nm and/or abrasive volume concentration from 0.238 vol% to 0.006 vol% results in different abrasion-corrosion wear mechanisms (rolling or grooving abrasion) and the average wear-induced corrosion currents show a linear correlation with wear rates for 4 ?m and 1 ?m abrasives. For low volume concentration (< 0.03 vol%) slurries containing bovine serum, organo-metallic conglomerates have been found within the wear scars. These conglomerates help separate the surfaces, impose less damage to the surface passive film and polish the wear scars through a chemical mechanical polishing mechanism. In addition, tribo-corrosion tests at micro-/nano- scales reveal the effects of single abrasive particle on the surface/sub-surface microstructual change. This investigation has revealed the nanoscale wear mechanisms that generate nanoscale wear debris, the mechanical mixing of the surface nanostructure with adsorbed denatured protein and also the slip/dislocation systems that are present near and on abraded surfaces that are likely to disrupt the surface passive films. The findings give a better understanding of the evolution of the sub-surface nanocrystalline structures and tribo-layers formation seen for the retrieved implants. This near surface nanostructure layer and phase transformation might offer better wear resistance through these inherent self-protecting mechanisms (i.e. increased hardness); conversely, it may become the precursors to debris ejection and enhanced ion-release into the CoCrMo joints.This work established an experimental technique that gives greater understanding of the tribocorrosion behaviour of cast CoCrMo in simulated hip joint environments. In particular, the roles of third body abrasive particles and proteins have been addressed, which are relevant to clinical applications. The material multi-scale wear mechanisms as well as the evolution of the surface / subsurface microstructures and tribo-layers have been elucidated, which provide new insights into the in vivo wear mechanisms of CoCrMo. The findings of this study may provide some important indications for improved MoM joint materials, design, manufacture and evaluation

A Role for Protein Phosphatase 2A in the Proliferation-Quiescence Decision

Metazoans precisely control the number of cell divisions during organ or tissue development throughout their lifetime. In adult metazoans, most differentiated cells no longer proliferate and lie in a quiescent state, also termed cell cycle exit. The decision to proliferate or to lie in quiescence is essential for development and its dysregulation may lead to defects in organogenesis, wound healing and regeneration as well as tumor formation. However, at what stage of the cell cycle the proliferation-quiescence decision occurs and what molecular mechanisms control this decision remain controversial. Here my thesis work revealed a novel role for PP2A in promoting the transition to quiescence upon terminal differentiation during tissue development. Using Drosophila eyes and wings as a model, I found that compromising PP2A activity during the final cell cycle prior to a developmentally controlled cell cycle exit leads to extra cell divisions and delayed normal exit. By systematically testing the regulatory subunits of Drosophila PP2A, I discovered that the B56 family member widerborst (wdb) is required for the role of PP2A in promoting the transition to quiescence. In particular, the PP2A/B56 complex targets cyclin-dependent kinase 2 several hours after mitosis to promote entry into quiescence, indicating when the decision occurs and how PP2A impacts the decision. I also investigated the dynamic features of the proliferation-quiescence transition using time-lapse, live imaging in mammalian cell culture. By monitoring the proliferation-quiescence transition without cell synchronization, I discovered that the quiescent state is heterogeneous. Mammalian cells can enter into either a transient or a prolonged quiescent state after mitosis, prior to the next round of cell cycle even under conditions of abundant nutrients. Notably, I showed that two sister cells born of the same mitosis can make different cell cycle decisions, with one cell entering long-term quiescence while the other re-entering the cell cycle. Consistent with my work in the Drosophila model, PP2A in mammals also plays a conserved role in promoting the entry into quiescence. The novel role of PP2A in modulation of the proliferation-quiescence decision may contribute to its tumor suppressor role and impact the emerging problem of tumor dormancy.PhDMolecular, Cellular and Developmental BiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133389/1/sunda_1.pd

Landau-Zener-Stuckelberg interference in a multi-anticrossing system

We propose a universal analytical method to study the dynamics of a multi-anticrossing system subject to driving by one single large-amplitude triangle pulse, within its time scales smaller than the dephasing time. Our approach can explain the main features of the Landau-Zener-Stuckelberg interference patterns recently observed in a tripartite system [Nature Communications 1:51 (2010)]. In particular, we focus on the effects of the size of anticrossings on interference and compare the calculated interference patterns with numerical simulations. In addition, Fourier transform of the patterns can extract information on the energy level spectrum.Comment: 6 pages, 5 figure

Examining Cultural and Technological Change: A Study of Cultural Affordances on WeChat

This study presents qualitative research of the interaction among WeChat, users, and Chinese culture by applying a three-dimensional notion of cultural affordances. The data is collected by conducting 10 semi-structured in-depth interviews, follow-up discussions, and observations, and is analyzed via the grounded theory approach. The study provides preliminary findings that demonstrate the roles of WeChat, users, and Chinese culture in terms of cultural and technological changes. The study also discusses the interactive patterns among three dimensions of cultural affordances and offers possible direction for further study