1. a Department of Geriatric Dentistry, Peking University School and Hospital of Stomatology, Beijing 100081, P. R. China
2. b Department of Geriatric Dentistry, Peking University School and Hospital of Stomatology, Beijing 100081, P. R. China
3. c The Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, P. R. China
4. d The Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, P. R. China
5. e The Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, P. R. China
6. f Department of Geriatric Dentistry, Peking University School and Hospital of Stomatology, Beijing 100081, P. R. China
7. g Department of Geriatric Dentistry, Peking University School and Hospital of Stomatology, Beijing 100081, P. R. China. kqdengxuliang@bjmu.edu.cn