Declaration of interest
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the article.
Funding
This study was financially supported by the Science and Technology support project of Sichuan Province (grant no: 2014SZ0012).
Supplemental data
Supplemental data for this article is available at http://dx.doi.org/10.3109/13816810.2015.1101776.