Human T cell leukemia virus type-I (HTLV-I) is the etiologic agent of adult T cell leukemia and has also been suggested to be involved in other diseases such as chronic arthritis or myelopathy. To elucidate pathological roles of the virus in disease, transgenic mice were produced that carry the HTLV-I genome. At 2 to 3 months of age, many of the mice developed chronic arthritis resembling rheumatoid arthritis. Synovial and periarticular inflammation with articular erosion caused by invasion of granulation tissues were marked. These observations suggest a possibility that HTLV-I is one of the etiologic agents of chronic arthritis in humans.

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Commercially pure titanium powder is subjected to mechanical milling (MM)-a severe plastic deformation process-for various periods of time. The MM powder has two different kinds of microstructure, which can be controlled by the MM conditions. They include ultra fine and coarse grain structures known as ''shell'' and ''core'', respectively. Subsequently, these MM powder is sintered using a hot roll sintering (HRS) process. The HRS materials with the shell and the core have a network structure of continuously connected shells, which is known as a harmonic structure. The HRS materials with the harmonic structure simultaneously demonstrate both high strength and elongation. These outstanding mechanical properties are influenced by the harmonic structure characteristics such as shell and core grain sizes, and shell fraction and shell network size. Thus, the harmonic structure can be considered as a remarkable design for improving the mechanical properties of commercially pure titanium as well as other metallic materials.

Pure titanium and Ti-6Al-4V alloy powders are treated by a mechanical milling process, which is one of the severe plastic deformation processes. The mechanical milling enables to produce a nano-grain microstructure very easily and has been applied to many powder materials. The bimodal microstructure in those mechanically milled powders is composed of a nano-grain structure with a grain size of about 50 nm in the surface-near region and of a work-hardened microstructure in the core region of the powder. We applied the hot roll sintering process to pure titanium and to Ti-6Al-4V mechanically milled powders. These compacts have a hybrid microstructure that consists of nano-grain structure with grain sizes of 200 to 500 nm and a micron size meso-grain structure. The hybrid microstructure materials demonstrate enhanced mechanical properties compared to conventional materials.