## 4 Discussion  
A bibliometric analysis is a profitable opportunity for highlighting potential gaps in the literature. Consequently, it can offer valuable suggestions to direct the research [12]. To our knowledge, this is the first bibliometric analysis conducted on the applications of PENS therapy for pain management. Therefore, results are potentially useful for planning further research in the field. The results of the bibliometric investigations indicated an upward annual trend in the number of scientific output and the publication of articles in top-ranked journals. On the other hand, partnerships among researchers should be implemented. The annual trend of scientific production on the subject is progressively growing. This finding suggests an interest in the technique and its applications. However, the comparative analysis conducted by time intervals (trend analysis) highlighted a significant increase in  
Figure 10: Co-authorship analysis for countries. An amount of 431 elements (countries) met the threshold (minimum number of documents per country was five). The software indicated 161 links and a total link strength of 540. Partnership involved mainly the United States (497 documents with 26 links). Figure 9: Co-authorship analysis for authors (n = 5140). The software found 5 clusters of scientific collaboration, 107 links, and a total link strength of 169. Researchers with the same color in the view belong to the same clustered cooperation network. The curved line indicates the connection strength (i.e., relationships between authors). The shorter the space between the two nodes, the larger the extent of the scientific links between authors. articles only up to the year 2020. On the contrary, from 1 January 2021 to April 2022, the increase remains, but it was not significant. These data need a necessary update to verify whether they express a trend reversal or are only a transitory phenomenon. For example, in the years of the pandemic, other topics may have ceased to be preferred by researchers. Concerning the quality of the journals, two-thirds of the articles were published in top-ranked journals (Q1 plus Q2, 75%). Many articles were published in reference journals in the field of pain medicine with a high impact factor. For example, the journal _\"Pain\"_ published 52 papers. The analysis of sources is helpful for authors to search the literature and choose a journal for the submission of their articles. Overall, most of the papers (98%) reached less than 100 citations, although we calculated a high mean of citations per article (n = 42.4) and approximately one-half of the documents received an important number of citations for over 20 years. Although the citation trend should indicate the visibility of an article rather than its overall goodness [29], taken together, these findings are suggestive of the high quality of the research. Nevertheless, only a quarter of the articles concern clinical research and 44% of the articles are meeting abstracts, book chapters, corrections, and other non-research or -review articles. These data represent an element of weakness, as clinical studies are essential to obtaining the scientific evidence useful for recommendations and guidelines. Approximately 10% of the articles concerned preclinical research and, in same the percentage, the topic \"mechanism of action\" was extracted. This demonstrates that there is interest in explaining the mechanisms underlying the analgesic effect of the technique. The precise processes of PENS-induced analgesia are not yet recognized [30]. Therefore, a better understanding of PENS's actions will certainly stimulate translational research. For example, telecommunication engineering focused on the study communication between neurons and transmitting signals through nanomachines can offer important data and an appropriate research methodology [31; 32]. The most relevant topic was \"efficacy\" (13.6%). Data on the actual efficacy of the treatment are of fundamental importance in order to plan further research. Recently, [PERSON] et al.