1. Division of Environmental Engineering Science, Gunma University, Kiryu, Japan;
2. Graduate School of Engineering, Gunma University, Kiryu, Japan;
3. Takasaki Advanced Radiation Research Institute, National Institutes for Quantum and Radiological Science and Technology, Takasaki, Gunma, Japan;
4. Department of Macromolecular Science and Engineering, Kyoto Institute of Technology, Kyoto, Japan;
5. Silk Materials Research Unit, National Agriculture and Food Research Organization (NARO), Tsukuba, Ibaraki, Japan;
6. Department of Applied Biology, Shinshu University, Ueda, Japan