In vivo, tissues are drained of excess fluid and macromolecules by the lymphatic vascular system. How to engineer artificial lymphatics that can provide equivalent drainage in biomaterials remains an open question. This study elucidates design principles for engineered lymphatics, by comparing the rates of removal of fluid and solute through type I collagen gels that contain lymphatic vessels or unseeded channels, or through gels without channels. Surprisingly, no difference was found between the fluid drainage rates for gels that contained vessels or bare channels. Moreover, solute drainage rates were greater in collagen gels that contained lymphatic vessels than in those that had bare channels. The enhancement of solute drainage by lymphatic endothelium was more pronounced in longer scaffolds and with smaller solutes. Whole-scaffold imaging revealed that endothelialization aided in solute drainage by impeding solute reflux into the gel without hindering solute entry into the vessel lumen. These results were reproduced by computational models of drainage with a flow-dependent endothelial hydraulic conductivity. This study shows that endothelialization of bare channels does not impede the drainage of fluid from collagen gels and can increase the drainage of macromolecules by preventing solute transport back into the scaffold.

Current methods to treat large soft-tissue defects mainly rely on autologous transfer of adipocutaneous flaps, a method that is often limited by donor site availability. Engineered vascularized adipose tissues can potentially be a viable and readily accessible substitute to autologous flaps. In this study, we engineered a small-scale adipose tissue with pre-patterned vasculature that enables immediate perfusion. Vessels formed after one day of perfusion and displayed barrier function after three days of perfusion. Under constant perfusion, adipose tissues remained viable and responded to lipoactive hormones insulin and epinephrine with lipid accumulation and loss, respectively. Adipocyte growth correlated inversely with distance away from the feeding vessel, as predicted by a Krogh-type model.

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