According to distribution over time (Fig. 1), a significant percentage of papers belongs to the years 2014 and 2015 with 16 and 11 papers respectively. Specifically, there are 13 papers written from 2000 to 2005, 29 papers from 2006 to 2010, 40 papers from 2011 to 2015. The trend of papers on this topic is therefore one of growth in recent years.