First Online: 3 August 2018
Compliance with ethical guidelines
: L. Kong, X.-F. Shang, W.-Z. Zhang, L.-Q. Duan, Y. Yu, W.-J. Ni and Y. Huang declare that they have no competing interests.
: All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the Helsinki Declaration of 1975 (in its current revised form). Informed consent was obtained from all individual participants included in the study.