RESOURCES

Abstract
Gelatin Methacryloyl (GelMA) is one of the most used biomaterials for a wide range of applications, such as drug delivery, disease modeling and tissue regeneration. GelMA is obtained from gelatin, which can be derived from different sources (e.g., bovine skin, and porcine skin), through substitution of reactive amine and hydroxyl groups with methacrylic anhydride (MAA). The degree of functionalization (DoF) can be tuned by varying the MAA amount used; thus, different protocols, with different reaction efficiency, have been developed, using various alkaline buffers (e.g., phosphate-buffered saline, DPBS, or carbonate-bicarbonate solution). Obviously, DoF modulation has an impact on the final GelMA properties, so a deep investigation on the features of the obtained hydrogel must be carried on. The purpose of this study is to investigate how different gelatin sources and synthesis methods affect GelMA properties, as literature lacks direct and systematic comparisons between these parameters, especially between synthesis methods. The final aim is to facilitate the choice of the source or synthesis method according to the needs of the desired application. Hence, chemical and physical properties of GelMA formulations were assessed, determining the DoFs, mechanical and viscoelastic properties by rheological analysis, water absorption by swelling capacity and enzymatic degradation rates. Biological tests with lung adenocarcinoma cells (A549) were performed. Moreover, since 3D bioprinting is a rapidly evolving technology thanks to the possibility of precise deposition of cell-laden biomaterials (bioinks) to mimic the 3D structures of several tissues, the potential of different GelMA formulations as bioinks have been tested with a multi-material approach, revealing its printability and versatility in various applications.

Abstract
Abstract Gelatin-methacryloyl (GelMA) hydrogel has gained huge success in the last decades thanks to its versatilities in many applications. Notably, one of them is 3D bioprinting, as GelMA physical-mechanical properties and biocompatibility of uncured formulation perfectly suit the requirements of a bioink. Nevertheless, before the photopolymerization, the hydrogel shows weak mechanical properties and long recovery time after stress application, which results in the inability to obtain complex and self-standing forms due to structure collapse. In this work, Carbopol ETD 2020 NF, dissolved in cell culture medium, was used as supporting bath to optimize GelMA bioprinting and overcome its stability limitations. The achieved results demonstrated the possibility of printing shapes containing hollows with lumens or non-planar surfaces, also by using nozzles with larger inner diameter, which reduced cell death during printing process, but were usually avoid because of low resolution. Moreover, constructs' extraction was easier when Carbopol solution was prepared in culture medium rather than in water, reducing sample handling. In conclusion, thanks to this supporting bath, it was possible to print cellularized scaffold, with channels that were then seeded, obtaining inner structure. Further, this Carbopol formulation could be considered an eligible candidate as a supporting bath to obtain GelMA 3D self-standing-shaped and vascularized scaffold.

Abstract
The study proposes a platform for the formation and culture of non-small cell lung cancer (NSCLC) spheroids, to obtain an in vitro model suitable for drug and therapy testing. To achieve that, traditional cell culture is compared to methacrylated gelatin (GelMA) 3D bioprinting, in order to explore not only the potential of the matrix itself, but also the impact of different architectures on spheroid formation. Starting from a systematic analysis, where GelMA concentration, methacrylation degree and cell seeding concentration is set; three different architectures (round, ring and grid) are analyzed in terms of spheroid formation and growth, using 3D bioprinting. The study reveals that Very High GelMA 7.5% w/v formulation, with single cells dispersed in, is the best bioink to obtain NSCLC spheroids. Moreover, grid architecture performs in the best way, because of the highest volume-surface area ratio. The designed GelMA platform can be used as a powerful in vitro tool for drug testing and therapy screening, that can be designed playing with four different parameters: cell concentration, GelMA methacrylation degree, GelMA concentration and geometry.

Abstract
(1) Background: Synovial tissue plays a fundamental role in inflammatory processes. Therefore, understanding the mechanisms regulating healthy and diseased synovium functions, as in rheumatic diseases, is crucial to discovering more effective therapies to minimize or prevent pathological progress. The present study aimed at developing a bioartificial synovial tissue as an in vitro model for drug screening or personalized medicine applications using 3D bioprinting technology. (2) Methods: The volumetric extrusion technique has been used to fabricate cell-laden scaffolds. Gelatin Methacryloyl (GelMA), widely applied in regenerative medicine and tissue engineering, was selected as a bioink and combined with an immortalized cell line of fibroblast-like synoviocytes (K4IM). (3) Results: Three different GelMA formulations, 7.5–10–12.5% w/v, were tested for the fabrication of the scaffold with the desired morphology and internal architecture. GelMA 10% w/v was chosen and combined with K4IM cells to fabricate scaffolds that showed high cell viability and negligible cytotoxicity for up to 14 days tested by Live & Dead and lactate dehydrogenase assays. (4) Conclusions: We successfully 3D bioprinted synoviocytes-laden scaffolds as a proof-of-concept (PoC) towards the fabrication of a 3D synovial membrane model suitable for in vitro studies. However, further research is needed to reproduce the complexity of the synovial microenvironment to better mimic the physiological condition.