## 2 Materials and Methods
### Literature Review  
The authors performed a systematic literature review using several databases such as the Web of Science, Google Scholar, and Scopus to identify existing Industry 4.0 KT initiatives and platforms.  
The foundation of _Plattform Industrie 4.0_ by several German associations with more than 6000 member companies in April 2013 was the first attempt to create a platform supporting industry KT and the further development of the recent trend in manufacturing technologies, termed as Industry 4.0. The platform, officially announced and kicked off at the Hannover Fair in 2013, seeks to promote the development of Industry 4.0 in Germany and thus strengthen the competitiveness of Germany as a production location [23; 24]. In this context, the excellent Industry 4.0 map is worth highlighting due to its transformation of knowledge related to successful and sustained Industry 4.0 test cases and support of other SMEs accessing Industry 4.0 technologies by providing already proven solutions and saving the initial trial and error phase. In addition, the SME Transfer Network submodule organizes meetings and workshops to promote KT within SMEs.  
Using the benefits of the digital transformation, the _Digital Innovation Hub_ (DIH) from the European Commission is another excellent initiative to further promote Industry 4.0 KT within SMEs. It intends to build a European network of DIHs that are one-stop shops to help companies improve their processes, products, and services by using digital technologies. DIHs provide access to technical expertise and experimentation by acting as knowledge transfer platforms [25; 26; 27]. To support this initiative, the European Commission has proposed the creation of the first-ever Digital Europe Programme, which will invest EUR 9.2 billion to align the next long-term EU budget (2021-2027) with increasing digital challenges [27; 28; 29]. The current numbers show the astonishing progress of the DIHs, as shown in Figure 2. By May 2021, there were 350 fully operational DIHs in Europe. Countries like Spain and Italy already have more than 50 DIHs fully operational, while other countries like Germany and France have more than 20 DIHs in preparation that will open their doors in the following months [30]. The Digital Europe Programme will foster the creation of new DIHs even further.  
During recent years, a new trend has been observed: some DIHs are getting connected and creating a cluster to expand their area of influence to a national level and to lead the development not only of a region, but also of a nation or even a pan-European area [31]. One example of this development is the cluster de:hub, which comprises twelve DIHs creating one digital ecosystem [29]. Furthermore, a relevant part of the DIHs is the project DIHNET.EU, which aims to create a sustainable pan-European network of networks to interconnect the regional DIHs and promote networking and collaboration [32]. This project might help to create a great online community to foster interaction among hubs, information exchange, and peer learning.  
To conclude, and to highlight the relevance of the abovementioned platforms, the interest and research in recent years on the DIH and _Plattform Industrie 4.0_ is growing, as shown in Figure 3. Moreover, the number of searches for DIH in Google in the last four years shows popularity peaks during the last two years, which emphasizes the need for Industry 4.0 knowledge transfer platforms, as shown in Figure 4.  
_Appl. Sci._**2021**, _11_, 5548  
Figure 3: Development during the last four years of the number of results in Google Scholar related to DIH and _Plattform Industrie 4.0_.  
Figure 2: Top 7 EU countries by number of fully operational DIHs.