Effects of select fluids on the friction of metal-on-polyethylene joint replacement surfaces

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005.Includes bibliographical references (leaves 57-61).Lubricants are important factors in the tribology of total joint arthroplasty (TJA) surfaces, which are primarily comprised of a polished metallic or ceramic component articulating on an Ultra-High Molecular Weight Polyethylene (PE) surface. Wear particles released from the PE surface are the primary cause of TJA failure. The human body responds to the foreign, micro- scale particles by activating a cascade of cytokine responses that ultimately leads to osteolysis and aseptic loosening. Although research in the materials selection and design of TJA components is continually advancing, one of the major intrinsic components that affect the tribological response in joints is overlooked. In particular, the properties and composition of joint fluid directly affect the fluid film and boundary lubrication of artificial prostheses. Since the characteristics of joint fluids are likely to differ from patient to patient as a result of varying disease indications, age, health, gender, and activity level, tribological behavior is also likely to vary significantly. The primary objective of this thesis is to examine the effects of variation in joint fluid composition on tribology. Due to the relative high stresses applied to the knee, the tribological effects related specifically to total knee arthroplasty (TKA) are investigated in detail. Before any joint fluid samples are examined, however, an assay capable of determining appropriate tribological properties is adapted. A unidirectional pin-on-disk (POD) tribometer is therefore selected to measure friction between PE and cobalt-chromium-molybdenum alloy (Co-Cr). Its sufficient precision, short testing time-frame and low cost enables rapid evaluations.(cont.) Preliminary friction data collected on fluids such as distilled water and bovine serum are used as standards and controls against lubricants in subsequent tests. From this data, the contributions to friction of boundary and fluid-film lubrication in PE on Co-Cr POD systems are discussed. Analysis of these friction properties in conjunction with previously published differences in wear between water and bovine serum leads to a rejection of a hypothesis directly correlating friction and wear. However, since ultimately wear is the important factor in the failure mechanism of TJA, an indirect relationship between friction and wear is investigated and proposed. Friction is then recorded using joint fluids as the lubricant and compared to the standards. Analysis of the joint fluid data demonstrates significance in frictional behavior, indicating that compositional properties affect friction. Moreover, examination of the data reveals large variation in joint fluids. Comparisons of the data to standard lubricants exhibit the potential for large variations in wear among joint fluids.by Timothy C. Chang.S.M

Maelstrom Represses Canonical RNA Polymerase II Transcription in Drosophila Dual-Strand piRNA Clusters

Transposons constitute much of the animal genome. While many transposons are ancient and inactivated, numerous others are intact and must be actively repressed. Uncontrolled transposons can cause genomic instability through DNA damage or mutations and must be carefully silenced in the germline or risk sterility or mutations that are passed on to offspring. In Drosophila melanogaster, 23–30 nt long piRNAs direct transposon silencing by serving as guides for Aubergine, Argonaute3, and Piwi, the three fly PIWI proteins. piRNAs derive from piRNA clusters—large heterochromatic DNA loci comprising transposons and transposon fragments. piRNAs are loaded into PIWI proteins via the ping-pong cycle which serves to amplify guide piRNAs. Loaded Piwi then enters the nucleus to transcriptionally repress transposons by establishing heterochromatin. Therefore, to silence transposons, transposon sequences must also be expressed. To bypass this paradox, the HP1 homolog Rhino (Rhi) allows non-canonical, promoter-independent, transcription of transposons embedded in heterochromatin. Transposon RNAs produced in this manner are “incoherent” and have little risk of being translated into transposon-encoded proteins required for transposition. This thesis focuses on understanding how piRNA clusters permit non-canonical transcription yet restrict canonical transcription. We found that although Rhi promotes non-canonical transcription in piRNA clusters, it also creates a transcriptionally permissive environment that is amenable to canonical transcription. In addition, we discovered that the conserved protein, Maelstrom, is required to repress promoter-driven transcription of individual, potentially active, transposons within piRNA clusters and allows Rhi to transcribe such transposon sequences into incoherent piRNA precursors

MARINE TOURISM RESOURCE DEVELOPMENT IN KOREA

Resource /Energy Economics and Policy,

Ediacaran Macro Body Fossils

This paper, Ediacaran Macro Body Fossils, reports a new discovery of well preserved three dimensional macro body fossils of the Ediacaran Period in central YunNan province in the People's Republic of China. These body fossils will enable more detailed and in-depth exploration of the evolution of multi-cellular macro organisms on this planet, whereas in the past, researches could only rely on cast or imprint fossils