Bioprinting created revolutionary perspectives not only limited to fabrication of structures geometrically similar to the native tissues, but also in providing biochemically similar complex microenvironment for cells to induce and guide functionalities in tissue regeneration. In this respect, hydrogels showed great potential in providing a favorable microenvironment mimicking the natural three-dimensional (3D) extracellular matrix. The challenges arise with bioprinting process for the hydrogel, which needs to have a particular sol-gel transition state without clogging the extrusion nozzle and to keep its shape fidelity after deposition. In addition, it is difficult to keep the structure of such a low mechanical strength of printed biomaterials in overhanging structures.
3D bioprinting in a sacrificial support-bath, is an emerging solution for fabrication of complex hydrogel-based overhanging structures. The support-bath material must possess a rigid matrix that yields by a passing nozzle and rapidly recovers itself after nozzle motion. Meanwhile it needs to hold bioprinted structure by providing necessary gelation to the extruded hydrogel before spreading into support-bath and to allow the integration of the subsequent layers without clogging the nozzle. A number of materials have been implemented as a sacrificial support-bath in direct free form writing of hydrogels. However, their applications are limited for several reasons including ionic sensitivity, working temperature range and possible reactions with the extruded hydrogel. Pluronic (PF) is one of the biocompatible materials which has been employed as fugitive bioink and support bath due to its thermoreversible sol-gel phase transition property. However, due to its mechanical weakness and tendency of quick dissolving in physiological conditions, it might not be able to provide enough support required for long-lasting printing processes. Moreover, pure PF as a sacrificial support-bath, did not show promising properties regarding harvesting of the printed structure.
Laponite, a synthetic biocompatible nanoclay mostly known as a rheology-modifier with enhanced thixotropic behavior, has been utilized as a support-bath material. However, instability in the rheological properties does not allow printing of low-viscosity hydrogels in Laponite support bath, which is unfavorable for the encapsulated cells and hence low efficiency. Despite their unique properties, PF and Laponite RDS have various drawbacks when they are employed as a support bath individually. In the work we described in the recent Scientific Reports publication, a blend of PF and nanoclay (Laponite RDS) in the presence of calcium chloride (CaCl2) was utilized with the aim of benefit the distinct characteristics of each, namely the thermoresponsive gelation of PF and the thixotropic behavior of Laponite. To have optimum rheological properties for printing in a long time printing and recovery process of the structure, various concentrations of PF-Laponite RDS and CaCl2 were evaluated and, an optimum rheological properties were achieved for support-bath. Optimized printing parameters allowed printing of computer aided design (CAD) hollow and overhanging structures from low viscous sodium alginate solution in a long-time process. The homogeneous distribution of cells with high viability revealed a cell friendly bioprinting process and to be further used for various tissue engineering applications. We believe that PF-Laponite RDS composite support-bath will be further modified for other types of hydrogels by providing appropriate crosslinking mechanism.
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Members of 3D Bioprinting Lab in Sabanci University (Ferdows Afghah, Dr. Mine Altunbek and Caner Dikyol) have contributed in this research under supervision of Prof. Bahattin Koc.