## 2 Materials and Methods
### Science Mapping Tools  
This section deals with the first step of the work sequence. Some tools to carry out a scientific mapping on any area of knowledge were studied. Currently there are online bibliographic databases where thousands of scientific papers are stored and can be retrieved. Some important bibliometric sources are Google Scholar(tm), Web of Science(tm) and Scopus(tm). Google Scholar(tm) is a free tool open to the general public. The Google Scholar(tm) database is part of the Google search engine and provides a large amount of information, but in a poorly ordered and updated way [25]. In addition, downloading large data sets from Google Scholar(tm) is a difficult process and does not allow to download the entire set [15]. Web of Science(tm) and Scopus(tm) are databases of commercial suppliers that require a fee for access. Web of Science(tm) offers publications with records indexed and archived since 1900, Scopus(tm) includes articles from 1966. [PERSON] et al. [26] compared the data collected from Web of Science(tm) and Scopus(tm) and found a high correlation between the articles and the citations. Hence, they demonstrated that there were no major differences in the results of a scientific mapping carried out with the two databases. To avoid problems of duplication of publications between the different databases, only one of them was used. After comparing the three proposed databases, Scopus(tm) was finally used due to its wide range of coverage of scientific publications [27] and its faster indexing process which makes it possible to compile the latest literature [28]. The Scopus(tm) database was developed by Elsevier [29]. A comprehensive collection of scientific mapping software is available for bibliometric analysis, which allows for the mapping and visualization of a given set of large-scale academic data in a field of knowledge. [PERSON] et al. [15] and [PERSON] et al. [30] provided a generic list of software tools used by the scientific community to perform scientometric analysis. In order to select the tool needed for this study, the characteristics and limitations of various scientometric software were analysed: CiteSpace(tm), CitNetExplorer(tm) and VOSviewer(tm). CiteSpace(tm) is a software tool used to identify, examine and display patterns and trends in the scientific literature. It allows dynamic visualizations of the evolution of bibliometric networks over time. Its graphical interface offers many options available to the user which is quite confusing for new users. The graphic results are not as easy to interpret as those offered by other tools [31]. CitNetExplorerTM interactively visualises maps of the relationship between documents. This tool allows to explore a network of publication citations interactively: it is possible to examine a network that includes thousands of publications until it is gradually reduced to a small sub-network of less than one hundred documents [31]. VOSviewerTM is used to create and visualize bibliometric network maps with options similar to CiteSpaceTM and CitNetExplorerTM. This tool focuses the graphic representation of network maps to help in the interpretation of the extracted data. Against to the other tools, VOSviewerTM constructs the bibliographic networks in distance-based visualisations which makes it the most suitable method for visualizing large networks (e.g., at least 100 items). It is appropriate to represent big maps since zoom functionality, special labelling algorithms, and density metaphors are used. In addition, VOSviewerTM allows to view any two-dimensional map based on distance, regardless of the mapping technique used to construct the map. VOSviewerTM can display maps constructed using any suitable mapping technique. This software integrates the VOS cartography which enables it to be used to build and visualize the maps without the use of any other tool [32]. In addition, the scientific community has validated the VOSviewerTM software as a useful tool for performing scientometric analysis in different fields of science [33; 34; 35].