Inadequate vascularization leading to insufficient oxygen and nutrient supply in deeper layers of bioartificial tissues remains a limitation in current tissue engineering approaches to which pre-vascularization offers a promising solution. Hypoxia triggering pre-vascularization by enhanced vascular endothelial growth factor (VEGF) expression can be induced chemically by dimethyloxalylglycine (DMOG). Nanoporous silica nanoparticles (NPSNPs, or mesoporous silica nanoparticles, MSNs) enable sustained delivery of molecules and potentially release DMOG allowing a durable capillarization of a construct. Here we evaluated the effects of soluble DMOG and DMOG-loaded NPSNPs on VEGF secretion of adipose tissue-derived stem cells (ASC) and on tube formation by human umbilical vein endothelial cells (HUVEC)-ASC co-cultures. Repeated doses of 100 µM and 500 µM soluble DMOG on ASC resulted in 3- to 7-fold increased VEGF levels on day 9 (P < 0.0001). Same doses of DMOG-NPSNPs enhanced VEGF secretion 7.7-fold (P < 0.0001) which could be maintained until day 12 with 500 µM DMOG-NPSNPs. In fibrin-based tube formation assays, 100 µM DMOG-NPSNPs had inhibitory effects whereas 50 µM significantly increased tube length, area and number of junctions transiently for 4 days. Thus, DMOG-NPSNPs supported endothelial tube formation by upregulated VEGF secretion from ASC and thus display a promising tool for pre-vascularization of tissue-engineered constructs. Further studies will evaluate their effect in hydrogels under perfusion.

Vascularization of tissue engineered implants is crucial for their survival and integration in the recipient’s body. Pre-vascularized, fibrin-based implants offer a solution since low concentration fibrin hydrogels (1 mg/mL) have been shown to promote tube formation of endothelial cells in co-culture with adipogenic stem cells. However, higher fibrinogen concentrations (> 20 mg/mL) enabling the fabrication of stable implants are necessary. We here characterized fibrin gels of 1–30 mg/mL for their rheological properties and whether they support tube formation of endothelial cell–adipogenic stem cell co-cultures for up to 7 days. Moreover, 20 mg/mL gels containing preformed channels and endothelial cell–adipogenic stem cell co-culture were perfused continuously in a customized flow chamber with 3.9 dyn/cm2 for 12 days and analyzed for capillary formation. Rheology of fibrin gels showed increasing stability proportional to fibrinogen concentration with 20 mg/mL gels having a storage module of 465 Pa. Complex tube networks stable for 7 days were observed at 1–5 mg/mL gels whereas higher concentrations showed initial sprouting only. However, perfusion of 20 mg/mL fibrin gels resulted in endothelialized pore formation in several layers of the gel with endothelial cell–adipogenic stem cell co-culture. Thus, perfusion supports the formation of capillary-like structures in fibrin gels that are too dense for spontaneous tube formation under static conditions. Future studies are necessary to further increase pore density and to investigate proper nutrition of tissue-specific target cells in the scaffold.

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Facilitating sufficient nutrient and oxygen supply in large‐scale bioartificial constructs is a critical step in organ bioengineering. Immediate perfusion not only depends on a dense capillary network, but also requires integrated large‐diameter vessels that allow vascular anastomoses during implantation. These requirements set high demands for matrix generation as well as for in vitro cultivation techniques and remain mostly unsolved challenges up until today. Additionally, bioartificial constructs must have sufficient biomechanical stability to withstand mechanical stresses during and after implantation. We developed a bioartificial tissue construct with a fibrin matrix containing human umbilical vein endothelial cells and adipose tissue‐derived stem cells facilitating capillary‐like network formation. This core matrix was surrounded by a dense acellular fibrin capsule providing biomechanical stability. Two fibrin‐based macrovessels were integrated on each side of the construct and interconnected via four 1.2 mm thick microchannels penetrating the cellularized core matrix. After 4 days of perfusion in a custom‐built bioreactor, homogeneous capillary‐like network formation throughout the core matrix was observed. The fibrin capsule stabilized the core matrix and facilitated the generation of a self‐supporting construct. Thus, the encapsulated fibrin tissue construct could provide a universal prevascularized matrix for seeding with different cell types in various tissue engineering approaches.

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