In 2018, Nichols’ team published a paper in Science Translational Medicine revealing their success with a bioengineered lung that was transplanted in a clinically relevant porcine model. And while their model functioned to produce systemic vascular circulation, it was not suitable for development of functional pulmonary circulation with bioengineered pulmonary artery and vein. Nichols attributed the lack of vascular tissue development to limited dispersal of the growth factor needed for vascular tissue development — vascular endothelial growth factor (VEGF) — and other additives in the whole organ scaffold, combined with the inability of the system to retain the growth factor delivery suspension within the scaffold framework. VEGF is critical in vascular development and has been shown to support development of vascular tissue in bioengineering, but it needs to attach to the scaffold and remain in place to do the job. Generating vascular tissue with microvasculature networks capable of perfusion at physiological pressures without leaking is perhaps the most common challenge in organ and tissue bioengineering. The biomimetic vascular models available have been unable to model vascular cell responses and lacked the appropriate vascular culture platforms focused on three-dimensional whole vessel or whole lung vascular bioengineering. While microfluidic-supported platforms are the standard for investigating endothelial cell culture methods and vascular tissue responses, they are unable to accommodate evaluation of whole vessel or whole organ culture. So, Nichols’ group did what they do best and developed what they needed to advance their work: novel in vitro biomimetic vascular models with appropriate culture- or microfluidic-supported vascular platforms to assess vascular cell responses to growth factors and factor delivery vehicles. These platforms include flattened pieces of vascular scaffold with standard plate culture; microfluidic system-supported whole-vessel scaffolds and whole acellular lung scaffolds. The whole vessel and whole lung platforms are microfluidic-supported culture chambers that mimic native tissue function to allow medium exchange and provide nutrient and oxygen circulation and waste removal from the culture micro-environment.

In the study published last summer in Nanomedicine: Nanotechnology, Biology and Medicine, Nichols’ team used these platforms to investigate delivery of VEGF via microparticle (MP) or hydrogel vehicles to the scaffolds and the effects of VEGF on vascular cell attachment and subsequent tissue formation in in vitro bioreactor culture. Their results are exciting for several reasons. In vivo imaging of their system shows that VEGF-MP size and shape allowed retention in the mesh-like collagen/elastin network comprising the acellular whole-lung scaffold but did not impede MP dispersal through its vascular tree. Importantly, they detected no evidence of VEGF-MPs or degraded VEGF-MPs outside of the vascular scaffold, and particles were distributed throughout whole left lung scaffolds including in the small capillary regions. Ten days post culture, VEGF could be detected in degrading VEGF-MPs. The effects of VEGF on the scaffolds: endothelial cell attachment and microvascular and vascular tissue formation, were found only in the MP-targeted vascular regions. In addition, they found that a 50/50 mixture of 30 and 60 nm pore-sized silicon wafer MPs was superior to the hydrogel for delivering VEGF. While hydrogels are thermosensitive and cannot provide slow, controlled release of growth factors, MPs with the appropriate pore size provide temporospatial release of factors to facilitate both protein loading and load release. Furthermore, they demonstrated that MPs could fit into even the smallest vascular regions and in whole organ scaffolds. These results show that MPs are more effective than hydrogel at delivering bioactive factors in acellular whole lung scaffolds to facilitate vascular tissue bioengineering. The success of silicon MPs for factor delivery is especially exciting for the future of vascular and organ bioengineering because standardization of pharma-grade MPs can be achieved by leveraging well-characterized large-scale manufacturing techniques borrowed from the semiconductor industry. These techniques have the scalability to support research and development studies, clinical trials and, ultimately, clinical commercial use. Their results also demonstrate that the newly developed platforms provide the modeling capabilities researchers in this field need to keep advancing their work. These platforms not only signal important advancement for whole organ and vascular bioengineering; they can also be adapted for use in other studies. Additionally, they allow assessment of growth factors (alone or in combination) and delivery systems that can be easily modified to evaluate therapeutics for clinical vascular graft production, vascular regeneration, vascular health and more. Currently, Nichols and her team are working on using these platforms to model select agent and SARS-CoV-2 infections and test candidate countermeasures.