Hydrogels for Bioprinting: A Systematic Review of Hydrogels Synthesis, Bioprinting Parameters, and Bioprinted Structures Behavior

Abstract

Nowadays, bioprinting is rapidly evolving and hydrogels are a key component for its success. In this sense, synthesis of hydrogels, as well as bioprinting process, and cross-linking of bioinks represent different challenges for the scientific community. A set of unified criteria and a common framework are missing, so multidisciplinary research teams might not efficiently share the advances and limitations of bioprinting. Although multiple combinations of materials and proportions have been used for several applications, it is still unclear the relationship between good printability of hydrogels and better medical/clinical behavior of bioprinted structures. For this reason, a PRISMA methodology was conducted in this review. Thus, 1,774 papers were retrieved from PUBMED, WOS, and SCOPUS databases. After selection, 118 papers were analyzed to extract information about materials, hydrogel synthesis, bioprinting process, and tests performed on bioprinted structures. The aim of this systematic review is to analyze materials used and their influence on the bioprinting parameters that ultimately generate tridimensional structures. Furthermore, a comparison of mechanical and cellular behavior of those bioprinted structures is presented. Finally, some conclusions and recommendations are exposed to improve reproducibility and facilitate a fair comparison of results.

Keywords: bioink; biomaterial; bioprinting; hydrogel; systematic review.

Figure 1

Schematic representation of three bioprinting stages: pre-printing (material selection, hydrogel synthesis, and bioink generation), extrusion-based bioprinting (parameters and cross-linking methods), and post-printing analysis (cellular and mechanical tests).

Figure 2

PRISMA flow diagram depicting literature search, exclusion process, eligibility criteria, and final included papers. One hundred and eighteen papers were included without publication date restriction (search performed on April 15th, 2019).

Figure 3

Research trend in hydrogels for bioprinting using a bar chart (papers per year) combined with a stacked area chart (total uses of a material per year). The three most used materials (alginate, gelatin, and GelMA) are shown individually while the rest of materials are grouped in the category “other.” It is important to note that some materials are used in more than one paper, so some stacked areas overtake its corresponding bar chart.

Figure 4

Venn diagram of journals' categories selected in this review according to JCR/SJR categories. Diagram information is organized as follows: Topic of the journal (number of papers, number of journals) and some of the most represented journals are listed. We noted that intersection areas are exclusive, and sizes are not proportional.

Figure 5

Stacked columns graph that shows evolution per year of natural (white) and synthetic (blue) materials on the percentage of total.

Figure 6

Combination of the 10 most used materials in hydrogels among them. The materials are shown in the external ring (total of papers and name). The middle ring segments represent one-material hydrogels and hydrogels, marked with (*), that are a combination of this material with non-selected materials (one-material papers mixed with non-selected materials papers). Inner lines represent hydrogels with two of the selected materials, but in some case other non-selected materials can be included (number of papers next to each inner line).

Figure 7

Cross-linking used for the three most common materials and its combinations. Blue, yellow, and red circles represent chemical, physical and thermal cross-linking, respectively. Superposed circles indicate that two cross-linking classifications have been combined. Each circle contains a classification codification (upper code) and the number of papers for the corresponding code combination (lower number). Chemical cross-linking has been chosen as primary classification, therefore papers not using it have been codified under C7 for graphical representation purposes and no papers are allocated to this code alone, e.g., alginate C2/13 – P4/1 (blue and yellow circles superposed) indicates that 13 papers used C2 ([100–500] mM Ca²⁺ solution) to cross-link alginate and another paper used C2 and P4 (UV light—no data of wavelength) to perform the cross-linking.

Figure 8

Cell viability for the most important materials: (A) alginate, gelatin and GelMA in the five selected periods, (B) material combinations for selected periods.

Figure 9

Pie chart with the most representative cell lines used for bioprinting. Angle represents number of papers that use each cell line (#) and radius indicates the mean cell viability of all related papers (%). All circle sections are proportionally scaled with viability in order to facilitate comparison.