1. Silk Materials Research Unit; National Institute of Agrobiological Sciences (NIAS); 1-2 Owashi Tsukuba Ibaraki 305-8634 Japan
2. Faculty of Textile Science and Technology; Shinshu University; 3-15-1 Tokida Ueda Nagano 386-8567 Japan
3. Department of Mechanical Engineering and Science, Graduate School of Engineering; Kyoto University; Kyoto-Daigaku-Katsura Nishikyo-Ku, Kyoto 615-8540 Japan