Alginate-based tissue-specific bioinks for multi-material 3D-bioprinting of pancreatic islets and blood vessels: A step towards vascularized pancreas grafts

Although allogeneic islet transplantation has been proposed as a therapy for type 1 diabetes, its success rate remains low. Disruption of both extracellular matrix (ECM) and dense vascular network during islets isolation are referred to as some of the main causes of their poor engraftment. Therefore, the recapitulation of the native pancreatic microenvironment and its prompt revascularization should be beneficial for long-term islet survival. In this study, we developed novel bioinks suitable for the microfluidic-assisted multi-material biofabrication of 3D porous pancreatic and vascular structures. The tissue-specific bioactivity was introduced by blending alginate either with pancreatic decellularized extracellular matrix powder (A_ECM) or fibrinogen (A_FBR). The formulated bioinks, although they exhibited different rheological properties, were 3D-printed with high shape fidelity using a multichannel microfluidic platform coupled with a co-axial needle system. The A_FBR scaffolds were subjected to secondary crosslinking using thrombin, which significantly decreased their swelling and increased mechanical properties. To investigate the bioactivity, the A_ECM and A_FBR bioinks were laden with, respectively, porcine pancreatic islets and a mixture of vessel-forming cells (HUVEC and HMSC) and 3D-bioprinted with high viability. Insulin secretion upon glucose stimulation and developed vessel-like structures composed of CD31-positive cells validated the biomimetic potential of the two formulated bioinks. Moreover, we demonstrated the feasibility of the secondary crosslinking to modulate cell behaviour and angiogenic potential both in vitro and in vivo using a chick chorioallantoic membrane model. Finally, the successful 3D-printing of heterogeneous 3D scaffolds with three different configurations demonstrated that our approach could be considered as a potential step towards the biofabrication of a vascularized pancreas construct.