A Scientometric Study on Graphene and Related Graphene-Based Materials in Medicine A Scientometric Study on Graphene and Related Graphene-Based Materials in Medicine

Here we carried out a scientometric analysis of scientific literature published referred to the use of graphene and graphene-based materials. We found that in the last 15 years, more than 1200 issues have been produced, with an H -index of 67 cited 2647 times. The countries that have a larger production, in terms of number of issues published, are China, the United States, South Korea, India, and Iran, and the most relevant subject categories in which they are indexed are materials science, chemistry, science and tech- nology, physics, and engineering, while the biological and medical specialties seem to be actually not deeply involved.


Introduction
Graphene consists of a single layer of carbon atoms packed into a honeycomb lattice. Its particular atomic organization of the carbon atoms affords graphene a set of very unique characteristics that justify the attention researcher of all fields have given it. The more standing out properties are a high mechanical strength, thermal and electrical conductibility, high surfaceto-mas ratio, and relative transparency [1]. Many studies use graphene oxide (GO) or reduced graphene oxide (rGO) instead of pristine graphene, because the oxidized forms are easier to process and can be dispersed in water while at the same time maintaining most of graphene's properties.
Graphene oxide has shown great potential enhancing differentiation and proliferation of human stem cells in vitro, which tend to adhere to graphene plates. In particular, it favors differentiation of human neuronal stem cells (hNSC) toward neurons rather than glia cells [2]. Combined with its inherent flexibility and strength, the possibility of creating a 3D structure that mimics the original organ, graphene appears to be a great scaffold for stem cell-based therapy [3].
Furthermore, a lot of research has come forward regarding the use of graphene in biosensors. Compared to previously used materials, graphene shows increased resistance and sensitivity. Also, being biocompatible it can be worn, allowing for the possibility of a permanently used sensor. Additionally, graphene can be bound to a wide range of molecules and proteins that allow for better selectivity [4].
Another field to which graphene's ability to be bound to specific molecules has been applied is drug carrying and delivery. In particular, it has been successfully used for specific anticancer drug delivery [5]. It presents novel perspective in combining site detection and drug delivery. Peptides bound to the GO plates allow for detection by specific cell types, minimizing uptake by other healthy cells [6].
Graphene's use in the medical field raises a lot of questions regarding its safety and toxicity. In this regard, there are many conflicting studies and opinions. It appears that the matter of toxicity varies greatly depending on the physicochemical characteristics of the administrated graphene, also on the form of administration, and the model, varying between different species and cell types. The characteristics of graphene like concentration, dimensions (lateral and number of layers), surface structure functional groups, and protein corona influence its toxicity in biological systems. Despite its relevance to the effect, some toxicological studies do not give a proper characterization of the form of graphene used. Though most agree on the interaction of graphene with the cellular membrane, the question of its uptake is more controversial [7]. For example, the studies of Yue et al. on the viability of six different cell lines when treated with GO of varying dimensions show that only two phagocytic cell lines were able to internalize both nano-and micro-sized GO sheets. Furthermore, there was no difference in the viability of any of the six cell line studies when the concentration was lower than 20 μg/mL. On the other hand, inhalation of GO particles may lead to an accumulation in the pulmonary surfactant and initiate an inflammatory process [8].
Interestingly, although GO does not show to be absorbed through the gastrointestinal tract, a low dose of GO can cause more damage to the gastrointestinal surface being drank as a suspension than a high dose of GO [9]. Most toxic effects seem to surge from the use of high doses of GO and the sequential aggregation and formation of conglomerates than can block small blood vessels and result in dyspnea [10]. However, recent publications detect no pathological effects in mice exposed to low dosages of GO and functionalized graphene when administrated by intravenous injection [11].
While studying toxicity it is very important to analyze the effect on the reproductive system and development because this can lead to more lasting effects. Graphene plaques seem unable to penetrate the blood-testis barrier in mice, and therefore sperm function and male reproductive activity show no alteration even for high doses of graphene [12]. In the female, there are no alterations if GO is administered before mating or during early gestation, and the female can give birth to healthy litters. However, if administered during late gestation, it leads to abortion and even death of the pregnant mice for high dose [13]. Injection of chicken eggs leads to reduced vascularization of the heart [14]. Despite showing no obvious malformation or mortality in zebrafish embryo, GO aggregates were retained in many organelles leading to hypoxia and ROS generation in these areas [15].
Even though graphene toxicity has drawn a lot of attention from scientists, there is a remarkable lack of understanding of the mechanisms underlying this effect. The use of different models and forms of graphene seem to lead to very dissimilar conclusions. There is a clear need for more systematic and in-depth studies, before graphene can be brought to its full potential use [7].
In this context, it is evident that, in one hand, graphene and graphene-related materials are even more used in medicine and bioengineering; on the other one, the information about their safety, their toxicity, and about the way of their possible interaction with living being (and human body and fluids) are still incomplete.
For this reason, here, we carried out a scientometric study on this very interesting topic, with the aim to study the scientific literature and to identify the most relevant topic and the countries that are more involved in this research activity.

Data collection and dataset
We accessed the data from Web of Science repository (https://apps.webofknowledge.com/) in December 2017-January 2018. The data have been filtered using the Advanced Search tool with the following syntax: where TS is the topic; AND is the Boolean operator.
In our queries, we used as topic 1 "graphene" or "graphene oxide" or "graphene-related material," combined with the following keywords as topic "medicine," "biomaterials," "scaffold," "regenerative medicine," or "bioengineering." Then all the data sets obtained were merged with the "Combine Sets" tool. As a result, we obtained a dataset in .txt format containing a list of 1208 articles with their attributes. All the following analyses have been carried out on this data set: • Number of citable issues: are considered exclusively articles, reviews, and conference papers.
• Number of cites per documents: it is the number of citation of documents published in specific years.
• H index: a topic/journal/author has index h if h of its N p papers has at least h citations each, and the other (N p − h) papers have no more than h citations each.

Temporal and geospatial analysis
The data were processed for temporal and geospatial analysis by Sci 2 Tool (Sci 2 Team). We generated temporal visualization of burst detection analysis of ISI keywords used in the papers. The geolocation of author collaboration was realized using Citespace (http://cluster. cis.drexel.edu/~cchen/citespace/) and Google Earth (https://www.google.it/intl/it/earth/).

Results and discussion
Overall, we found 1248 issues characterized by the bibliometric parameters shown in Table 1.
The number of issues published per year is described in The time trend of citations (sum of cited per year) has a different pattern, described by more than linear pattern, as reported in Figure 2.
Interestingly, the distribution of cites per year, as shown in Figure 3, in keeping with the Bedford's, follows a power law, with a negative exponent.
In addition it has been possible to compute the main parameters of cites/year distribution (see Table 2).
To explore the temporal pattern of the most important themes studied, we analyzed the burst in citations referred to specific keywords (see Table 3 for the list of citation bursts identified).

H-index 67
Average citation per item 17.65

Sum of time cited 2647
Citing articles 14,055 Table 1. Bibliometric parameters referred to the studied dataset.

Scientometrics
Interestingly, we investigated the number of issues published by each country, thus estimating the contribution of different countries in research on graphene application in medicine ( Table 4). These data demonstrate that graphene and graphene-based material are used in a wide variety of application in biomedicine such as cell and stem cell culture, translational medicine, bioengineering, toxicology, and development, thus confirming that these materials are becoming to represent a reality in life sciences.  As it is evident, the most of issues have been published in China, with a total number of issues that accounts for about a third of worldwide production, followed by the United States and South Korea, India, and Iran. This datum is very interesting, because it demonstrates that Asiatic countries are the most important contributor, at least quantitative point of view, to this such important field of research.
To better explore the context to which the research is referred, we assessed the subject categories, as reported in Table 5.
As it is evident from the analysis of Table 5, the most of issues are indexed in nonbiological fields (materials science, chemistry, science and technology, physics, and engineering) rather than in biological fields. This seems to indicate that, to date, the research is led and defined by hard science scientist and, possibly, the contribution of researched belonging to biological and medical areas could be markedly increased in next years.  The same trend could be identified looking on the WC, i.e., the classification system adopted by Web of Science (see Figures 4 and 5).
From these data, we could infer that we are seeing a first phase of the use of graphene and graphene-based materials, in which the studies on basic issues (synthesis, chemical characterization, description of chemical and physical properties) rather than the application in biology  and medicine are predominating. Likely, it is possible to hypothesize that in next years, the contribution of life scientists and researchers and clinicians involved in medical field could acquire higher importance.

Conclusion
The use of graphene and graphene-based materials in biomedicine and bioengineering is an emergent technology that promises a wide variety of application in human health, diagnostics, and therapeutics. Here, for the first time, we carried out a scientometric analysis on this topic, finding as a result that the number of published issues and of their citations is quickly and markedly increasing, as proof of the intense activity in this field. The countries that display a more active production (in quantitative term) are from Asia (China, South Korea, India, and Iran) and from North America (the USA). The issues published are mainly referred to hard sciences (materials science, chemistry, science and technology, physics, and engineering) rather than biology or medicine. Despite that these materials are used in a wide variety of biomedical and bioengineering applications (from cell culture to stem cell differentiation, from the realization of scaffolds to toxicological studies), the research activity on these issues seems still in an early stage, characterized by the physical and chemical characterization of materials, rather than the massive application in biomedicine and bioengineering.