The thematic evolution of these keywords is analyzed starting from the "milestone year" shown in Fig. 10, namely 2013, and including smaller time-steps until present time, since it can be considered a recent topic in fast evolution. The time slices analyzed are eventually four:
- up to 2013
- 2013-2016
- 2016-2018
- 2018-present time
The evolution of keywords is well represented in figure 14 and we can observe how after 2013 the topic was more fragmentated than the previous years and how the field expanded up to present day.