Liu, D
Huang, Y
Bu, D
Liu, A D
Holmberg, L
Jia, Y
Tang, C
Du, J
Jin, H
Article History
Received: 29 November 2013
Revised: 18 April 2014
Accepted: 22 April 2014
First Online: 22 May 2014
Competing interests
: The authors declare no conflict of interest.