1. a Silk Materials Research Unit, Genetically Modified Organism Research Center, National Institute of Agrobiological Sciences , 1-2 Owashi, Tsukuba , Ibaraki , 305-8634 , Japan
2. b Department of Mechanical Engineering , Graduate School of Engineering, Kyoto University , Yoshida-Honmachi, Sakyo-ku , Kyoto , 606-8501 , Japan