A scientometric review of the growing trends in transcranial alternating current stimulation (tACS)

1 Introduction

Transcranial alternating current stimulation (tACS), as a non-invasive neuromodulation technique, has attracted much attention in neuroscience and clinical medicine in recent years. The technique provides new possibilities for the study and treatment of neurological disorders by applying a weak alternating current to the scalp, which acts on the cerebral cortex at a specific frequency and intensity, thereby modulating neuronal excitability and network connectivity (Chai et al., 2018). tACS acts directly on the brain, avoiding the metabolic process and possible side effects of drugs in the body, as well as reducing the risk and complexity of surgical treatment (Alexander et al., 2019). Therefore, transcranial alternating current electrical stimulation is considered as a neuromodulation technique with promising applications.

Transcranial electrical stimulation (tES) encompasses a range of techniques, including transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS), transcranial pulsed current stimulation (tPCS), and transcranial random noise stimulation (tRNS) (Dissanayaka et al., 2017). While all these methods are generally considered safe, noninvasive, and painless, tACS particularly stands out due to its ability to entrain specific neural oscillations, making it a preferred choice for many researchers exploring a broad range of applications. tACS is able to control the brain’s transitions between different activity states and has emerged as a key neuromodulator for revealing the link between behavior and brain oscillations (He et al., 2023). Studies have shown that tACS is capable of inducing patterns of neural oscillations that match the frequency of the stimulus, which in turn improves or restores impaired cognitive functioning and even has an effect on somatic states, among others (Antal and Herrmann, 2016; Cruciani et al., 2024).

At this stage, tACS is recognized as a potential therapeutic option for the treatment of a wide range of neurological and psychiatric disorders. For example, in the treatment of depression, tACS has been used to modulate the activity of the prefrontal cortex to alleviate depressive symptoms and improve patients’ mood states (Haller et al., 2020). In Alzheimer’s disease research, tACS was found to enhance memory function and cognitive performance in patients (Nissim et al., 2023). At the same time, tACS has shown significant potential for use in motor rehabilitation. tACS has been used to promote the recovery and reconstruction of damaged nerves to improve motor function in patients with stroke, gait disorders, and others (Koganemaru et al., 2020). These research results are not only encouraging, but also provide new options for the treatment of various psychiatric disorders. To this end, we present in Table 1 a detailed description of tACS applications in different fields and their unique features, in order to gain insight into the rich diversity and great potential of this technology.

Table 1. Overview of tACS application areas.

Notably, tES is not an isolated treatment and can be combined with existing treatments to form an integrated treatment strategy. For example, research by Tatti et al. (2022) has shown that tES can enhance the effects of psychotherapy by modulating the excitability of neural networks in the brain, facilitating the production and consolidation of psychological change (Tatti et al., 2022). This combination offers new hope for patients who do not respond well to traditional psychotherapy. It has also been shown that the combination of medication, psychotherapy and non-invasive stimulation can also reduce certain prevalence rates (Allida et al., 2020). In addition, in the face of the growing problem of cognitive decline in an aging society, tES can maintain or enhance cognitive functioning in older adults, which opens up new possibilities for developing personalized rehabilitation programs for older adults (Tatti et al., 2016). By stimulating specific brain regions, tES can improve learners’ higher cognitive functions such as attention, which is important for improving the quality of education and promoting learning progress (Boetzel and Herrmann, 2021).

Bibliometric analysis is a quantitative analytic method that focuses on the quantitative analysis of literature indicators and can assess researches’ prospective impacts on a field based on bibliometric indicators (Ahmad et al., 2019). Identification of scientific activity at the author, country, institution, journal, reference, and keyword levels is used for comparison analysis within the same category to help find research hotspots in a certain field as well as prospective future trends.

By conducting bibliometric analysis, an overall evaluation of existing literature can be achieved, and indicators such as research hotspots, major research directions, and influence within the tACS field can be determined. Thus, conducting bibliometric analysis on literature in the tACS field not only can reveal the development trajectory and provide direction and reference for future research and researchers but also promote the development and application of tACS technology.

2 Material and methods 2.1 Data acquisition and search tactics

The entire retrieval process takes place in Web of Science (WOS). The search strategy included the following: topic= “transcranial alternating current stimulation” or “tACS” not “TACs”. During the search, it was found that many other factors with the same acronym were also included in the search results, so “TACs” were excluded from the search to ensure accurate results. Database selected=WOS Core Collection, time span=2013–2022, index=Science Citation Index Expanded (SCI-EXPENDED) and Social Sciences Citation Index (SSCI). The current inquiry was only interested in articles and review articles. Only English was permitted as a language. Finally, 602 articles met all of the criteria, and database searches and downloads were conducted uniformly on 25 April 2023. A detailed flowchart of each analysis is reported in Figure 1.

Figure 1. Flow chart of the scientometric analysis.

2.2 Analysis tool

CiteSpace is a Java-based application that combines scientific knowledge mapping and bibliometric analysis to identify advances and research Frontiers in a field and predict trends (Cheng et al., 2022). VOSviewer, which uses citation relationships, co-citations, co-authorship, and other relevant factors to build a visual network diagram, provides a comprehensive understanding of the structure and trends of scientific research (van Eck and Waltman, 2010). The bibliometrix R package is a flexible free open-source tool programmed in R for bibliometric analysis, enabling comprehensive scientific mapping analysis (Sabe et al., 2023). As a code-free tool, Scimago Graphica is suitable not only for visual analysis of communication data, but also for exploratory data analysis (Hassan-Montero et al., 2022).

3 Results 3.1 Publication outputs

Using the search method and selection procedure in Figure 1, a total of 602 publications met the search requirements. Figure 2 depicted the annual distribution, citation counts, and publication trends of tACS research publications from 2013 to 2022. By the date of search, the cumulative number of tACS-related publications from 2013 to 2022 was 602. The publications of papers on tACS researches have generally shown a fluctuating upward trend during the last decade. In addition, the continuous development and optimization of technology; the increasing research investment from governments, research institutions and enterprises; the cross-fertilization of multiple disciplines such as neuroscience and biomedical engineering, and other factors have combined to provide a strong impetus for the widespread application and in-depth exploration of tACS in the field of neuroscience and clinical research. When fitting the number of publications per year, there was a strong association between year and number of publications (R2 = 0.98).

Figure 2. Annual publications, citation counts, and the fitting equation for annual publications in tACS research from 2013 to 2022.

3.2 Journals distribution

Table 2 listed the top 20 journals publishing tACS-related publications. The open-source journal Brain Stimulation has published the largest number of articles on tACS, with 55 articles, accounting for about 9.14% of the total number of publications. At the same time, this journal has an impact factor of 9.184, ranking among the top 20 journals. The next highest ranked journal was the open-source journal Frontiers in Human Neuroscience with 47 articles (IF = 3.473), accounting for approximately (7.81%) of the total number of articles published. The journal with the highest impact factor is the non-open access journal Current Biology (IF = 10.900), but it is only ranked 15th in terms of number of publications, which shows the difficulty of publishing articles about tACS in journals with high research quality.

Table 2. Top 20 journals that published tACS researches.

Bradford’s law, known as one of the three laws of bibliometrics, reflects the unbalanced distribution of specialized papers within the same disciplinary field across different journals. It proposes an empirical law describing the structure of literature: journals are categorized into core journals, related journals, and marginal journals, according to the number of research papers on related topics or disciplines in descending order, with a ratio of 1:n:n2. This law has not only had an important impact on the theoretical study of intelligence but has also been widely used in practical applications such as determining core journals and core readers (Venable et al., 2016). Figure 3 demonstrated the distribution of the main 6 journals in which the literature was published. In the current study, there are 159 journals, with about 201 publications in each region. 6 of the 159 journals were classified as the core journal area of the field, accounting for about 3.77%. The six core journals can be classified into the field of neuroscience, specifically involving research in neuromodulation, neuroimaging, and neurological diseases. This shows from the side that tACS is a hot topic of more attention in the field of neuroscience.

Figure 3. Bradford’s law of borderline personality disorder (BPD). The horizontal coordinate is the logarithm of the serial number (level) of the rank-ordered journal, and the vertical coordinate is the corresponding cumulative number of papers.

3.3 Authors distribution

Figure 4 showed the collaborative relationships among authors. Nodes of different colors in the figure indicate different academic clusters, while nodes of the same color represent authors with similar research directions. The size of each node was proportional to its number of posted publications, which means that the larger the number of posts, the larger the node. The linkage between nodes indicated the collaboration between authors (Qiu et al., 2022). At least eight clusters are formed in the figure. The purple cluster is represented by Christoph S. Herrmann, whose research findings are more prominent in the mechanism of action of tACS and its modulation of cognitive processes (Herrmann et al., 2013). Flavio Frohlich is the main representative of the red cluster, whose research focuses on how to modulate and intervene in cortical oscillations through non- invasive brain stimulation techniques to modulate and intervene in cortical oscillations and the effects of these oscillations on cognitive processes (Fröhlich, 2015).

Figure 4. Network map of authors engaged in BPD researches (threshold = 5). The thickness of the connecting lines between the nodes indicates the strength of their association; the size usually indicates their importance or centrality, such as the number of citations, frequency of occurrence, etc.; the distance between the nodes can reflect the similarity or relevance between them; the color can indicate the different research fields or the categories to which they belong.

The top 20 authors in tACS in terms of number of publications are shown in Table 3. Christoph S. Herrmann published 51 related articles, accounting for 8.47% of the total number of papers, with the highest number of related articles published. This was followed by Flavio Frohlich, who published 28 related articles, accounting for 4.65% of the total number of articles. Andrea Antal followed with 21 papers, accounting for 3.49% of the total. Among the top 20 authors, Alvaro Pascual-leone had the highest H-index (10 publications; H-index = 148). Alvaro Pascual-leone does not have much advantage in terms of the number of publications, but his related researches have a high quality and academic influence.

Table 3. Top 20 authors that published tACS researches.

3.4 Country and institution distribution

As shown in Figure 5, a total of 51 countries have published publications on tACS researches. It is clear from the figure that the publishing countries are mainly concentrated in the European and North American regions. The vast majority of the links are issued from the European region, indicating that the European region maintains close cooperation with other countries, and the scope of cooperation shows a globalization trend. The North American region is particularly prominent with the node in the United States, which has a rich research record in tACS. The United States has been actively cooperating with other countries in this field, as shown by the multiple lines of connection from the United States to other countries. As seen by the node sizes, the results of China, Canada and Australia in tACS are also of great interest.

Figure 5. Map of the distribution of countries/regions engaged in tACS researches. Different colors of nodes represent different research subjects; the size of nodes is related to the number of articles published; the connecting lines between nodes indicate the existence of cooperation between different subjects; the thickness of the connecting lines reflects the strength of inter-subject association.

The top 20 countries ranked by number of publications are shown in Table 4. Germany tops the list with 171 publications, accounting for about 28.41% of the total number of publications. The United States followed with 152 publications, accounting for about 25.25% of the total. The vast majority of the top 20 countries in terms of number of publications are from Europe, implying that Europe has a strong research strength and advantage in this field. The largest centrality in the table is the United States (0.62), which indicates that the research results of the United States in the field of tACS are very valuable and have an important position in this field.

Table 4. Top 20 countries that published tACS researches.

The top 20 institutions in the field of tACS in terms of number of publications are listed in Table 5. In the current study, Harvard University is the institution with the highest number of publications with a total of 68 papers, which is about 11.30% of the total number of papers. It was followed by Carl von Ossietzky Universitat Oldenburg and City University of New York Cuny System with 53 (8.80%) and 52 (8.64%) publications, respectively. Of the top 20 institutions in terms of number of publications, five are from the United States and seven are from Germany. To a certain extent, this reflects that research institutions in the United States and Germany are in the leading position in the field of tACS. Similarly, it can be found that most of the top 20 institutions are from universities in various countries, which usually have high-level talents, diversified research resources and abundant opportunities for collaboration, providing the necessary support and conditions for research in this field and promoting the improvement of academic standards.

Table 5. Top 20 institutions that published tACS researches.

3.5 Analysis of references

CiteSpace clustering function was used to cluster and analyze the co-cited references, and a total of 11 common themes of tACS-related references were identified. As shown in Figure 6, a co-citation network of references was generated for the period 2013–2022. The modularity value (Q value) of 0.6567 implies a significant cluster structure; the mean profile value (S value) of 0.8686 indicates that the clustering is convincing. Among them, #0 “alpha oscillations” was the largest cluster. Huang et al. (2021) found through experimental studies that alternating current stimulation enhances the frequency and intensity of alpha waves in the cerebral cortex and proposed that this effect is achieved by stimulating specific phases of fast-spiking cortical neurons (Huang et al., 2021). This study has important implications for understanding the mechanisms of oscillatory regulation in the brain and for developing novel neuromodulation methods. This is followed by #1 “entrainment” and #2 “consolidation.”

Figure 6. Reference co-citation network with cluster visualization in tACS. A connecting line between nodes indicates that two documents are co-cited by the same document or documents; each cluster of a different color represents a relatively independent research topic or subfield.

The top 20 highly cited references for research on tACS are shown in Table 6. Woods et al. (2016) research was the most cited article on both WOS and Google Scholar, with 661 and 1,154 citations, respectively. They provide a technical guide for tACS, transcranial magnetic stimulation (TMS), tDCS, and other related non-invasive brain stimulation tools, describing the basic principles, application methods, and safety issues (Woods et al., 2016). The next study is that of Helfrich et al. (2014), who found through experimental studies that tACS can induce frequency-specific oscillations in the cerebral cortex and that this effect is reproducible and persistent. The effects of different frequencies of tACS on cognitive task performance are also discussed, and it is proposed that tACS may enhance information processing in neural networks by modulating brain oscillations (Helfrich et al., 2014).

Table 6. Top 10 cited references that published BPD researches.

3.6 Keywords analysis

A word cloud map of the most frequently used keywords in tACS publications was generated using Bibliometrix, as shown in Figure 7. The position and font size of the keywords reflect the frequency of their use in the study. In the current study, “oscillations” was the most frequently used word, with 146 uses. Many researches have shown that tACS can regulate the oscillatory rhythms generated in the brain. Specifically, tACS can increase or decrease the synchronization of specific oscillatory frequencies in the brain, thereby affecting the transmission of information between brain regions and brain function (Guerra et al., 2023). This is followed by “alternating current stimulation” and “electrical stimulation,” which are used 128 and 102 times, respectively.

Figure 7. Word cloud map on tACS researches. The size of a word usually indicates its frequency or importance in the textual data; each word in a word cloud map represents a keyword or theme in the study; and the color usually does not directly indicate a specific data dimension.

Figure 8 showed the top 17 most bursting keywords from 2013 to 2022. The nature of the keywords over time reflected the shift in research trends in the field. Notably, the strongest outburst in the past decade was “mechanisms” with an outburst value of 4.41. tACS, a non-invasive neuromodulation technique that affects neuronal activity in the brain by delivering alternating currents on the scalp surface, has attracted many researchers to explore the mechanism behind the problem (Huang et al., 2021). Similarly, the keywords “gamma,” “transcranial direct current stimulation” and “Alzheimer’s disease” citation outbursts have continued through 2022 and the outburst continues.

Figure 8. Keywords with the strongest occurrence burst on tACS researches. Keywords may represent new research directions, technological breakthroughs, or important discoveries; an indicator of burst intensity quantifies the increase in the frequency of that keyword within a given time period relative to other time periods; and time intervals indicate when a research hotspot begins and ends.

4 Discussion

The unique feature of tACS technology is its ability to precisely modulate electrical currents at specific frequencies, allowing for the fine tuning of brain activity. It is this characteristic that has led to a wider and deeper interest in tACS than in other forms of electrical stimulation (Kim et al., 2021). Since its inception, tACS has grown in popularity in the medical business due to its advantages of safety, non-invasiveness, low cost, and ease of use. Looking ahead, with the continuous progress of science and technology and researchers’ in-depth understanding of the tACS mechanism, tACS is expected to become an important tool to promote the improvement of human cognitive and behavioral abilities. Of course, all this requires researchers to continue to relentlessly explore its potential and translate it into innovative results in practical applications.

However, while tACS has shown encouraging promise in a number of areas, controversy remains as to whether it is capable of producing long-lasting neuroplastic after-effects. tACS, as a non-invasive neuromodulation tool by applying alternating current electrical stimulation of a specific frequency to the scalp, has been suggested to have the ability to modulate neural oscillatory patterns in the cerebral cortex, and potentially to further induce long-term synaptic plasticity changes that thereby profoundly affecting brain function and cognitive performance (Antal and Paulus, 2013). Several studies have shown that tACS can indeed produce long-lasting neuroplasticity after-effects. For example, in a motor training task, individuals stimulated with tACS showed significant performance gains that were maintained for some time after the end of the stimulation (Sale and Kuzovina, 2022). These results seem to indicate that tACS does have the ability to trigger lasting changes in the brain that promote learning and skill retention. However, it has also been suggested that the effects of tACS may be limited and that combining it with other stimuli can extend the duration (Guerra et al., 2018). In summary, although there have been a number of studies that provide support for the ability of tACS to produce long-lasting neuroplasticity after-effects, there are still many unanswered questions and controversies in this area. Future studies need to delve more deeply into the mechanisms of action of tACS, particularly its potential to trigger long-term synaptic plasticity changes, and how to most effectively combine tACS with other interventions to achieve more lasting and significant gains in brain function.

From the results of the analysis of the data obtained, the four Frontiers of tACS can be predicted as follows:

(1) Neural network stimulation. tACS can modulate neural networks and thus influence brain activity through a pre-determined external frequency. Neural oscillations in the θ range (4–6 Hz) have been shown to affect associative memory functions in hippocampal-cortical networks (Meng et al., 2021). tACS has an even more important impact in terms of sensorimotor enhancements. It has been shown that γ tACS promotes movement and contributes to effective changes in ongoing exercise programs, whereas β tACS acts as an anti-motor agent (Santarnecchi et al., 2017). Current evidence suggests that tES holds great potential for application in exercise rehabilitation and therapy (Veldema and Gharabaghi, 2022). In recent years, neural network simulations have made significant progress in neuroscience research with the popularization of high-density, multi-electrode tACS protocols and the development of precise mathematical models of current flow. These innovations not only provide higher spatial resolution and more precise stimulus control, but also enable precise focusing of stimulation on specific brain regions, which is expected to significantly improve the effectiveness of tACS in neuromodulation and rehabilitation therapy. Thus, future studies could continue to examine the effects of different frequencies of tACS on other regions of the brain network, exploring how more specific frequency bands affect various cognitive and emotional functions of the brain.

(2) Multi-people stimulation. In recent years, there has been a growing interest in applying tES techniques to modulate brain activity. While traditional methods of transcranial alternating current stimulation have focused on stimulating individual brain regions of an individual, recent advances have explored the use of multi-electrode monoliths for more precise modulation of neural networks (Zhou et al., 2021). In their study, Chen et al. (2022) used the dual-tACS technique to investigate potential brain-to-brain electroencephalogram (EEG) coupling and possible enhancement effects during conceptual alignment. It was shown that neural activity between different regions of the brain is synchronized when people engage in conceptual alignment, and the degree of this synchronization varies with the frequency of dual tACS (Chen et al., 2022). Importantly, the use of tES (such as dual tACS) has the potential to enhance cognitive processes related to social interaction and joint action. In this situation, the research of Tatti et al. (2022) becomes particularly important. Their research emphasizes the potential benefits of tES in promoting psychotherapy courses and its impact on improving social outcomes (Tatti et al., 2022). Future research could consider applying it to a wider range of multiplayer interaction scenarios, such as teamwork, classroom teaching, and family interactions, in order to explore how multi-people stimulation affect team performance, learning outcomes, and interpersonal relationships.

(3) Feedback stimulation. The use of tACS combined with feedback stimulation is a method employed to investigate changes in brain activity. Lustenberger et al. (2016) have revealed the functional role of sleep spindle waves in motor memory consolidation through the use of feedback-controlled tACS. Special EEG feedback mechanisms are employed to monitor sleep spindle wave activity and identify appropriate timing for brain stimulation (Lustenberger et al., 2016). As technological advances continue to improve the precision and effectiveness of neuromodulation techniques, there is a growing need to understand how tACS interacts with feedback stimuli and how to optimize this interaction to modulate brain activity and cognitive function. In this context, recent work by Soleimani et al. (2023) on closing the loop between the brain and electrical stimulation provides valuable insights into the potential of closed-loop tACS in precise neuromodulation therapy. Their study emphasizes the importance of considering individual differences and real-time brain states when designing effective neuromodulation interventions. The ability of closed-loop tES-fMRI systems to continuously monitor brain activity and adjust stimulation parameters in real time offers unprecedented opportunities for personalized neuromodulation therapy (Soleimani et al., 2023). Future studies should aim to deeply explore how closed-loop tACS interacts with feedback stimuli and how this interaction affects brain activity and cognitive function.

(4) Combination with other techniques. In addition to exploring the use of tACS techniques alone, researchers are gradually trying to combine tACS with other techniques to achieve better results. Functional magnetic resonance imaging and tACS have been used in the same study to investigate the effects of bilateral prefrontal cortex stimulation with 40 Hz gamma waves on brain activity (Mencarelli et al., 2022). Other researchers have used EEG and tACS together to investigate the effects of left hemisphere homophasic anterior parietal and parieto-occipital theta wave tACS on large-scale functional network connectivity in patients with schizophrenia (Yeh et al., 2023). tACS used in combination with other techniques can provide a better understanding of the effects of tACS on individuals and expand the field of application. However, in the future, simultaneous application of multiple techniques, such as the simultaneous use of fMRI, EEG, and tACS, can be considered to probe the function and structure of the brain more comprehensively in a multimodal manner, and to further explore the exact mechanism of action and therapeutic effects of tACS in combination with other techniques.

As far as we know, this is the first study to conduct a bibliometric analysis of tACS-related publications. The current study yielded some useful results through bibliometric analysis, but there were inevitably some limitations. First, we used WOS exclusively to obtain literature from the SCI-EXPENDED and SSCI databases, which may have prevented us from identifying certain significant studies from other databases. Second, because only articles and review articles were chosen, rather than all tACS-related publications, the results may be incomplete. Finally, by restricting the language to English, important literature in other languages may have been overlooked.

We recognize that only 602 tACS articles were included in this study for bibliometric analysis, which appears limited compared to established disciplines and affects the comprehensiveness and depth of the study. This may be due to the fact that tACS is an emerging field with insufficient accumulation of literature and fragmented research that lacks a unified consensus and direction. These factors result in an analysis that may not be able to fully cover the details of the field, and the conclusions may lack in generalizability and representativeness. To compensate for the limitations, it is recommended to continuously update the literature database, combine other research methods, and strengthen interdisciplinary cooperation and communication to jointly promote the development of tACS research.

5 Conclusion

The current study used a bibliometric approach to visualize and evaluate tACS-related publications from 2013 to 2022. In the previous ten years, 602 researches were published. tACS researches were evolving, and publications were increasing in general. Brain Stimulation ranked the first in number of publications in tACS. Christoph S. Herrmann has contributed the most publications. Germany is the world leader in the field of tACS, and the United States has a significant global presence. Harvard University has published the most tACS-related publications.

Author contributions

YLi: Data curation, Formal Analysis, Investigation, Methodology, Project administration, Resources, Software, Visualization, Writing – original draft, Writing – review & editing. YLu: Data curation, Formal Analysis, Investigation, Methodology, Project administration, Resources, Writing – original draft, Writing – review & editing. NZ: Conceptualization, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing. XZ: Conceptualization, Data curation, Formal Analysis, Funding acquisition, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing. SL: Conceptualization, Data curation, Formal Analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing.

Funding

The authors declare that financial support was received for the research, authorship, and/or publication of this article. SL was supported by the Outstanding Youth Program of Philosophy and Social Sciences in Anhui Province (2022AH030089) and the Starting Fund for Scientific Research of High-Level Talents at Anhui Agricultural University (rc432206). XZ was supported by the National Natural Science Foundation of China (71942003, 32171080, 32100886, 32161143022, and 32200914), the Chinese National Programs for Brain Science and Brain-like Intelligence Technology (2021ZD0202101), and the Global Select Project (DJK-LX-2022008) of the Institute of Health and Medicine, Hefei Comprehensive National Science Center. Shanghai Key Laboratory of Brain-Machine Intelligence for Information Behavior.

Acknowledgments

We thank the Bioinformatics Center of the University of Science and Technology of China, School of Life Science, for providing supercomputing resources for this project.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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