1. Department of Bioengineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
2. The Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
3. Superabsorbents Research Department, Nippon Shokubai Co., Ltd., 992-1 Aza Nishioki Okihama, Aboshi-ku, Himeji, Hyogo 671-1292, Japan
4. Neutron Science and Technology Center, Comprehensive Research Organization for Science and Society, 162-1 Tokai, Ibaraki 319-1106, Japan