To further shed light on the status of development in biomedical applications of 3D printing, we have also analyzed the prevalence and connections between different concepts. The results are shown in Figure , where the size of a node reflects the number of times the corresponding concept occurred in the literature, lines between every two nodes denote co-occurrences in the same publication, and distances between nodes indicate the frequencies at which the concepts co-occurred. Concepts with most frequent co-occurrence are grouped into the same color: green nodes depict concepts related to 3D printing technologies and materials, blue and yellow nodes represent biomedical-related concepts, whereas red nodes correspond to characterizations and properties of 3D printed biomaterials. Several observations can be made from the diagram: (1) the prevalence of hydrogel materials in bioprinting technology is illustrated by the short distance between the concepts "hydrogels" and "bioprinting"; (2) concepts related to 3D printing techniques (except "three-dimensional printing" and "additive manufacturing") are small and have few connections (co-occurrences) with other concepts, suggesting that 3D printing methods studies have been much isolated from research involving applications or properties of printed materials;