Gelatin vs GelMA in alginate-based bioinks as a platform for versatile 3D bioprintable in vitro systems

3D in vitro model systems, such as hydrogels, have garnered popularity due to their ability to more accurately recapitulate in vivo environments compared to 2D cell culture systems. However, methods which involve casting hydrogels by hand may be time consuming, have poor reproducibility, and reduced capacity to generate complex structures. Hence, 3D bioprinting has emerged as a useful tool for the high throughput production of in vitro tissue models such as hydrogels and complex constructs. Here, we demonstrate the mechanical properties, printability, and ability to support single cells and spheroids in culture for two highly characterised composite bioinks: Alginate/Gelatin (AlgGel), which is ionically crosslinked, and Alginate/Gelatin Methacrylate (GelMA) (AlgGelMA), whereby the GelMA is crosslinked by illumination with UV light. In this study, we engineered gels that exhibit a wide range of stiffnesses, which vary due to the concentration of crosslinking polymer present. AlgGel hydrogels were softer (1.5-4.5 kPa), and stiffness decreased with time in culture, however, AlgGelMA hydrogels were stiffer (6-40 kPa), and the stiffness increased with time. Microarchitectural studies using Scanning Electron Microscopy and Microcomputed Tomography (μCT) revealed that hydrogels produced using both bioinks bore a highly porous structure, further simulating in vivo conditions. To assess the ability of both bioink families to support cell culture, the Acute Myeloid Leukaemia cell line THP-1 and human Mesenchymal Stem Cells (hMSCs) as single cells and spheroids were bioprinted in each bioink. Interestingly, THP-1 cells formed larger clusters when cultured within AlgGel bioinks compared to AlgGelMA. Additionally, hMSCs appeared to be unable to migrate through the AlgGel matrix, as single hMSCs displayed rounded morphologies and hMSC spheroid shape was not disrupted after seven days. Contrastingly, hMSCs and spheroids cultured within AlgGelMA hydrogels were able to invade the gel matrix and migrate. Together, these data demonstrate that both AlgGel and AlgGelMA bioinks show promise for use as the basis of 3D bioprinted in vitro tissue models.