Hydrogels as Bioink for 3D Printing

A robust and valid alternative to other commercial  systems for the fabrication of 3D hydrogels to be used as an advanced platform in tissue engineering.

An innovative and fully synthetic PolyIsoCyanide (PIC) thermoresponsive material, capable of mimicking characteristics of the natural ExtraCellular Matrix (ECM). This material exhibits a reverse thermoresponsive behavior due to the hydrophobic interactions of the oligoglycol substituent present along its backbone, with a steep increase of the storage modulus (G0) above 18 ºC. Moreover, they demonstrated that the mechanical properties and network characteristics of the resulting hydrogel can be precisely controlled in a rather broad range. Furthermore, this new class of material exhibits cytocompatibility and unique reverse thermoresponsive properties, representing an ideal candidate for bioink formulation to be used for the assembly of 3D matrices via printing and bioprinting technologies.
In this paper, they first demonstrate that PIC solutions can be deposited in 3D with good accuracy, thanks to their rheological temperature-dependent properties. They performed a thorough optimization of the 3D printing process, showing the possibility of fabricating 3D objects with a good shape fidelity.

The changes in the viscosity of the PIC solution within a range of temperatures (12–15ºC) and the relative pressure needed to extrude hydrogel struts were studied. The obtained results are shown in Figure 3. As it can be noticed, the viscosity of the PIC solution at 12ºC was ~0.39 Pas, and 0.1 bar was sufficient to attain regular struts (Figure 3b). However, when the temperature was increased to 15 ºC, we measured a 10% increase in viscosity, and the required pressure for strut deposition increased to 0.4 bar. The printing pressure is a crucial parameter during 3D bioprinting experiments, as this may negatively affect the cell viability. The pressure values used in this study were significantly lower when compared to previous studies using different reverse thermoresponsive hydrogels.







Figure 3. Viscosity and 3D printing parameters of the PIC hydrogels for a range of temperatures. In (a) the effect of temperature over the viscosity of a 5 mg/mL PIC solution is shown while in (b) the printability of PIC solution and the dependence of printing pressure on cartridge temperature are reported.

The printed scaffolds showed structural stability and endurance both during and after printing, when they were kept at room temperature or at 37 ºC without any additional crosslinker and any further process.The simplicity of 3D printing of PIC hydrogels, their structural stability at 37 °C, the high water content of the hydrogel network (99.5% of the overall weight), their ECM-like characteristics, and their cytocompatibility demonstrate the suitability of PIC hydrogels for 3D bioprinting.

More information: Nehar Celikkin et al. Polymers 2018, 10, 555