Analysis of cognitive framework and biomedical translation of tissue engineering in otolaryngology

Tissue engineering is a relatively recent research area aimed at developing artificial tissues that can restore, maintain, or even improve the anatomical and/or functional integrity of injured tissues. Otolaryngology, as a leading surgical specialty in head and neck surgery, is a candidate for the use of these advanced therapies and medicinal products developed. Nevertheless, a knowledge-based analysis of both areas together is still needed. The dataset was retrieved from the Web of Science database from 1900 to 2020. SciMAT software was used to perform the science mapping analysis and the data for the biomedical translation identification was obtained from the iCite platform. Regarding the analysis of the cognitive structure, we find consolidated research lines, such as the generation of cartilage for use as a graft in reconstructive surgery, reconstruction of microtia, or the closure of perforations of the tympanic membrane. This last research area occupies the most relevant clinical translation with the rest of the areas presenting a lower translational level. In conclusion, Tissue engineering is still in an early translational stage in otolaryngology, otology being the field where most advances have been achieved. Therefore, although otolaryngologists should play an active role in translational research in tissue engineering, greater multidisciplinary efforts are required to promote and encourage the translation of potential clinical applications of tissue engineering for routine clinical use.

Science mapping analysis.Scientific mapping allows us to visually assess the relationships established between documents thanks to the spatial representation established between the different nodes 10 .SMA is considered an important area in bibliometrics 11 .To perform this type of analysis, we used SciMAT (Science Mapping Analysis Tool), a freely available bibliometric software that allows the construction of scientific maps based on the co-occurrence of keywords 19 .
Keywords from documents were loaded to SciMAT, creating a single time period (1994-2020) and submitted to three different stages, as previously described 2 .Briefly, a normalization of authors and words by singulars, plurals and Levenshtein distances was applied.Keywords are clustered in conceptual nodes, named themes.Then, networks based on the co-occurrence of the unit of analysis (keywords clustered on themes) were constructed.Once networks were developed, each of the obtained themes was plotted on a low dimensional space layout based on Callon's centrality and density 12,13 .These indicators allow to classify each theme into four quadrants: 1. Motor themes present a strong density and high centrality, and show the most intensely developed topics in the research area of interest (upper right quadrant).2. Basic and transversal themes represent themes shared by several disciplines, so their fundamentals are well defined (lower right quadrant).3. Emerging or declining topics they have a weak density and low centrality, thus signifying marginal areas of knowledge (lower left quadrant).The differentiation between emerging and declining themes requires a prospective assessment of their centrality.4. Highly developed or isolated themes show a high density showing significant internal development.However, they are less connected with other themes in the research field because of their low centrality values (upper left quadrant) Finally, the number of documents and citations received were used as bibliometric indicators to analyze the thematic areas.

Biomedical translation analysis.
In order to get the information about the biomedical translation, iCite (https:// icite.od.nih.gov) translation module was used on the set of documents previously identified.This module assigns each document a score in three different categories (Human, Animal and Molecular/Cellular Biology) based on the number of Medical Subject Heading (MeSH) terms.These scores represent the focus of the article content on these dimensions.The papers are represented in the "translational space" of the Triangle of Biomedicine 15 .Articles with high scores in "Human" will appear closer to the Human vertex, while those with high scores in "Animal" and "Molecular/Cellular Biology" will appear closer to those respective vertices.
This module also uses this information to track and predict citations for clinical articles by calculating the Approximate Potential to Translate (APT).This is an estimate obtained by a machine-learning algorithm, with a value of 0, 25, 50, 75, 95 or 100%, which shows the probability that a document will be cited by a clinical article 20 .

Science mapping analysis.
The analysis performed by SciMAT shows the themes of interest in TE in the field of otolaryngology research (Fig. 1).First, those themes considered basic or transversal were described, followed by the motor themes.Third, highly developed and isolated themes were analyzed, and finally, those that were emerging or declining.In each of these four cases, the concepts that make up each cluster were presented.www.nature.com/scientificreports/Within the basic or transversal themes, we found those themes that brought together the largest number of documents and citations received, with the term scaffold occupying first place, followed by chondrocytes and stem-cells.Furthermore, three themes were identified as motor themes, corresponding to tympanic-membraneperforation, microtia, and flap.Figure 2 shows the thematic networks of the basic and transversal and motor themes.
Both highly developed or isolated themes and emerging or declining themes are characterized by low centrality (below 0.5).The themes distraction-osteogenesis, type-I-collagen-oligomers and laryngotracheal-reconstruction were identified as the main highly developed or isolated themes, while tracheal-stenosis was the only theme considered emerging or declining.Figure 3 shows the thematic networks of the highly developed or isolated themes and emerging or declining themes.
Biomedical translation.iCite tool uses information obtained from PubMed, so the PMIDs of 331 documents were obtained (there is a loss of 12 records, which represents 3.49% of the total).This application categorizes only nine documents as clinical articles (2.72%) corresponding to clinical trials or clinical guidelines.
The different scores generated in the iCite translation module were obtained from the total number of documents and clinical articles (Table 1).From the total number of documents, it can be observed that the average "Human" score is 0.41, "Animal" score is 0.35 and "Molecular/cellular" score is 0.23.The distribution of the articles according to these scores is shown in Fig. 4. The average APT of this set of analyses is 25.4%.Only 79 papers (23.86%) have been cited, at the time, by clinical articles.The two papers that received the most citations from clinical articles were a non-clinical article, with 13 citations 21 , and one clinical article that received a total of 12 citations 22 , both aimed to tympanic perforation closure.Table 2 shows the five most cited by clinical articles, if it is a clinical article or not, its main theme, and the number of citations it has received.
When performing the analysis of the translation according to the different themes identified by SciMAT and distributing them into the Triangle of Biomedicine, it is possible to observe those that have a position closer to the human and those topics that are still in an intermediate phase or closer to the animal vertex.As presented in Fig. 5, three groups of themes can be identified according to their location within the triangle.In the first place, those themes located at a central position are located intermediately between the three vertices.This would be the case of the basic or transversal themes chondrocytes and scaffold, together with the motor theme microtia and the highly developed theme laryngotracheal-reconstruction.The set formed by the themes tracheal-stenosis, type-I-collagen-oligomers and distraction-osteogenesis, the latter two highly developed and isolated themes, are in a more displaced position to the animal vertex.These two groups do not present Human as the nearest vertex, thus they were considered as basic research (orange).Finally, the themes flap, stem-cells and the main motor theme, tympanic-membrane-perforation, are found in the upper segment of the triangle, presenting the human vertex as the nearest vertex and being considered as clinical research (blue).

Discussion
TE is an area of research aimed at the development of artificial tissues and organs that can restore, maintain or even improve the anatomical and/or functional integrity of injured tissues 1 .Although it is a relatively young discipline, recent studies have shown a trend towards its consolidation through the bibliometric analysis of its scientific production [2][3][4] .Therefore, the time is ripe to evaluate whether the advances achieved in this area of research are applied in clinical practice, which is known as biomedical translation.This work tries to quantitatively assess the cognitive structure of TE in otolaryngology, as well as estimate its degree of translation for the first time in the biomedical literature.
Different knowledge-based methods to perform this evaluation have been described in literature 14 .Using SMA allows the characterization of the cognitive structure of TE in otolaryngology.The main tool used in this work was the analysis of science maps based on the co-occurrence of keywords, which allowed us to discover a series of complex relationships between the different concepts (themes) in the area of study.The application of software such as SciMAT makes a series of relevant themes emerge and classify them on the basis of their centrality and density, which are bibliometrics indicators measuring the external (inter-node) and internal (intra-node) coherence of the different themes (nodes) constituting the thematic network that lead to a more logical and plausible interpretation of results.Particularly noteworthy is the presence of three transversal themes in this area, such as scaffold, chondrocytes and stem-cells.
The search for a scaffold with adequate biomechanical characteristics is an area of interest in different fields, for example, in the reconstruction of the larynx, trachea or for the repair of cranial bone defects 26,27 .All these concepts are linked to the central term scaffold, together constituting the scaffold theme.In the case of cranial bone defect repair, there is a significant link between this theme, the use of tricalcium beta-phosphate and the application of in situ TE.This connection derives from the application of matrices composed of tricalcium betaphosphate for the induction of in situ formation of new bone for the closure and repair of cranioencephalic bone defects 28,29 .This theme occupies a central position in the basic research area of the Triangle of Biomedicine, which shows its profile as a basic and transversal theme shared by several disciplines.
The second of the themes highlighted in the cognitive structure of the analyzed corpus was chondrocytes.The procurement of chondrocytes and the subsequent generation of cartilage tissue through TE is a basic research topic, being the only theme found in the area of basic cellular/molecular research in the Triangle of Biomedicine.TE of cartilage is a fundamental aspect in the development of grafts for the reconstruction of structures made up of a cartilaginous skeleton such as the trachea or nasal septum, both terms present in the thematic network of chondrocytes 30,31 .Likewise, this network includes some of the most important points from the perspective of TE in the generation of cartilage tissue.Adipose tissue stem cells also appear in this network since they are the most common cells used to differentiate into chondrocytes 32,33 .We also find the two main biomaterials in the www.nature.com/scientificreports/development of artificial cartilage, which are collagen and, above all, alginate 34,35 as its properties closely resemble those of the native cartilage matrix 36,37 .Finally, the set defining the stem-cell thematic network also proved to be a cross-cutting theme in this study.A link was observed with the themes rhinology and vocal fold injury, given the potential use of stem cells in the regeneration of nasal cavity mucosa 38,39 or altered vocal folds 40 .There is also a relationship with the salivary glands and xerostomia, and a strong link between these two subjects, since one of the main applications being investigated is their use for xerostomia, especially in patients who have received radiotherapy in the head and neck area 41,42 .Interestingly, its position within the human area in the triangle of medicine suggests that possible clinical applications of stem cell use have been tested.

TYMPANIC-MEMBRANE-PERFORATION
The theme related to tympanic-membrane-perforation emerged as the main motor theme.Tympanic perforation is a prevalent pathology, mainly caused by trauma or repeated otitis media 43 , which is associated with constant hearing loss or otorrhea 44,45 .This comorbidity, although mild, significantly affects the quality of life of patients.Surgical closure of these by tympanoplasty surgery has a variable and inconsistent success rate 46 .In the network of this theme are elements and scaffolds, such as fibrin glue or gelatin sponge, and fibroblast-derived growth factors obtained.Using these compounds obtained by TE techniques is postulated as alternatives to conventional surgery for minimally invasive closure of tympanic perforations 47,48 .Another finding that confirms the important role of tympanic membrane perforation research in TE in the field of otolaryngology is the fact that, of the nine papers that were identified as clinical papers according to iCite, five dealt with this clinical problem 22,24,[47][48][49] .This is another theme that falls within the human area in the Triangle of Biomedicine.As we have seen, it is one of the main topics of TE research within otolaryngology, constituting the research objective  of the main clinical articles.This not only reflects the great interest in this field of study, but also the driving force to promote clinical translation.One of the best established themes was microtia as it showed high Callon's density.Microtia theme lies on the borderline between highly developed and motor themes, with a high number of nodes in its thematic network.This is because the creation of artificially created pinnae for the reconstruction of microtia or anotia is one of the most popular and well-known research topics in otolaryngology 50,51 , with the milestone of the regeneration of a tissue-engineered human pinna in the back of a rat in 1997 52 .This theme can be associated with chondrocytes,  www.nature.com/scientificreports/but in this case it is specifically investigated with auricular chondrocytes for the generation of cartilage similar to that present in an auricle 53,54 .It can also be extracted from interpreting the thematic network that computer design and 3D printing of these TE products play an important role in this research topic 55 .However, despite all these advances, this is a subject that occupies a central position in the triangle of biomedicine, which shows that its translation to humans has not yet been applied.Other clinical themes like tracheal-stenosis or laryngotrachealreconstruction are in a more displaced position to the animal vertex, showing that they also are still far from a human application phase.Despite this, their character as emerging and highly developed themes, respectively, may be an indication that laryngotracheal reconstruction techniques for pathology such as tracheal stenosis, using TE techniques, may in the future occupy a place in the area of human clinical research with possible clinical applications.Despite the results presented in this study could be of interest to the research community in TE and otorhinolaryngology, some limitations need to be addressed.First, we have used the citation count as a proxy of scientific impact.The fact that one paper cites another is an indication that the cited paper has had some influence, or impact, on the paper citing it.However, the authors can cite a paper because of different reasons (to give credit to their mentors, to reinforce an idea or to discuss their findings with the previous literature).As stated by Garfield and Belter 56,57 , which is true is that highly cited papers are highly useful to authors for writing other papers, but what those papers are useful for, however, is not clear.Citation count, as all the bibliometric indicators (i.e.journal impact factor, H-index, etc.) need to be used responsibly, and following the international recommendations 58,59 .Last, documents were obtained from the Core Collection database of WoS, which could www.nature.com/scientificreports/include certain limitations, such as language bias or limited inclusion of grey literature (thesis dissertations, patents or conference abstracts), which can be important sources of information.However, WoS includes over 21,000 peer-reviewed, high-quality scholarly journals published worldwide (including Open Access journals) in over 250 disciplines, and is considered a reference by the scientific community as one of the most complete and reliable databases of scientific information 60,61 .

Mol/Cell
In conclusion, TE is still in an early translational stage in otolaryngology.Regarding the analysis of the cognitive structure, we find consolidated research lines, such as the generation of cartilage for use as a graft in reconstructive surgery, reconstruction of microtia or the closure of perforations of the tympanic membrane.This last research area occupies the most relevant clinical translation with the rest of the areas presenting a lower translational level, placing otology as the field with the most advances in TE in otorhinolaryngology.Therefore, although otolaryngologists and head and neck surgeons should play an active role in translational research in TE, greater multidisciplinary efforts are required to promote and encourage the translation of potential clinical applications of TE for routine clinical use.

Figure 2 .
Figure 2. Thematic networks of the basic or transversal themes (a-c): (a) Scaffold thematic network; (b) Chondrocytes thematic network; (c) Stem-cells thematic network.Thematic networks of the motor themes (df): (d) Thematic network of tympanic-membrane-perforation; (e) Thematic network of microtia; (f) Thematic network of flap.

Figure 3 .
Figure 3. Thematic network of highly developed and isolated themes (a-c): (a) Thematic network of distraction-osteogenesis; (b) Thematic network of type-I-collagen-oligomers; (c) Thematic network of laryngotracheal-reconstruction.(d) Thematic network of the emerging or declining theme tracheal-stenosis.

Figure 4 .
Figure 4. Bubble plot showing the distribution of the set of documents in the Triangle of Biomedicine.

Figure 5 .
Figure 5. Triangle of Biomedicine with the distribution of the different themes identified in the SMA.This triangle is segmented into two areas according to the nearest vertex.The basic research area (orange) is the area whose nearest vertices are Mol/Cell or Animal.The clinical research area (blue) is the area of the triangle whose nearest vertex is Human.

Table 1 .
Average scores and APT obtained from the translational module for the total publications and clinical articles. Avg.

Table 2 .
Top-cited documents referred to tissue engineering in otolaryngology by citation count.