Collagen Bioinks for Bioprinting: A Systematic Review of Hydrogel Properties, Bioprinting Parameters, Protocols, and Bioprinted Structure Characteristics

Abstract

Bioprinting is a modern tool suitable for creating cell scaffolds and tissue or organ carriers from polymers that mimic tissue properties and create a natural environment for cell development. A wide range of polymers, both natural and synthetic, are used, including extracellular matrix and collagen-based polymers. Bioprinting technologies, based on syringe deposition or laser technologies, are optimal tools for creating precise constructs precisely from the combination of collagen hydrogel and cells. This review describes the different stages of bioprinting, from the extraction of collagen hydrogels and bioink preparation, over the parameters of the printing itself, to the final testing of the constructs. This study mainly focuses on the use of physically crosslinked high-concentrated collagen hydrogels, which represents the optimal way to create a biocompatible 3D construct with sufficient stiffness. The cell viability in these gels is mainly influenced by the composition of the bioink and the parameters of the bioprinting process itself (temperature, pressure, cell density, etc.). In addition, a detailed table is included that lists the bioprinting parameters and composition of custom bioinks from current studies focusing on printing collagen gels without the addition of other polymers. Last but not least, our work also tries to refute the often-mentioned fact that highly concentrated collagen hydrogel is not suitable for 3D bioprinting and cell growth and development.

pH Modification of High-Concentrated Collagen Bioinks as a Factor Affecting Cell Viability, Mechanical Properties, and Printability

“…The printability of a hydrogel depends on several factors related to both the printing parameters and the properties of the bioink itself. Bioink viscosity is a crucial parameter for bioprinting, but is influenced by other factors, in particular collagen concentration, cell concentration, temperature and acidity [ 23 ]. However, it is not possible to determine the appropriate viscosity for optimal printing; instead, the printing parameters must be adjusted to the viscosity of the hydrogel.…”

“…The effect of heat transfer on the print was crucial, and in general, the setup where the syringe and nozzle were cooled with water at about 4 °C resulted in a 10 °C temperature of the extruder head while the platform was circulated at about 40 °C; therefore, 37 °C proved to be the best temperature for the samples. This is supported by the fact that gel crosslinking occurs as the temperature increases and the pH changes (increases to alkaline values) and is fastest around 37 °C [ 23 ]. On the contrary, not maintaining a low gel temperature in the syringe leads to solidification before printing.…”

“…However, in the case of gel crosslinking under changes in temperature and pH, the use of high concentrations of collagen (at least 5 mg/mL) is rare. According to a systematic review published earlier in our article [ 23 ], only the study of Ren [ 24 ] (10 mg/mL) used concentrations higher than 5 mg/mL. Other studies used the addition of additives for collagen crosslinking, e.g., Diamantidese’s study [ 25 ] used concentrations of 4, 8 and 12 mg/mL with riboflavin as the crosslinking agent, Moncal [ 26 ] used 3 and 6 mg/mL, respectively, of Pluronic F127, and Rhee el al.…”

“…Three types of bioprinting are distinguished in terms of their bioprinting method: microextrusion printing, inkjet and laser-assisted bioprinting (LaBP) [ 31 ]. A detailed description of the methods, including the effect of the parameters on printing, is given in our recent systematic review [ 23 ]. Due to the relative ease of implementation, collagen nozzle printing is most commonly used and is present in both the extrusion and inkjet methods [ 23 ].…”

“…A detailed description of the methods, including the effect of the parameters on printing, is given in our recent systematic review [ 23 ]. Due to the relative ease of implementation, collagen nozzle printing is most commonly used and is present in both the extrusion and inkjet methods [ 23 ]. These methods are also characterized as relatively suitable for printing hydrogels with incorporated cells, as the cells do not come into contact with cytotoxic materials [ 32 ].…”