From arteries to capillaries: approaches to engineering human vasculature

Abstract

From micro-scaled capillaries to millimeter-sized arteries and veins, human vasculature spans multiple scales and cell types. The convergence of bioengineering, materials science, and stem cell biology has enabled tissue engineers to recreate the structure and function of different hierarchical levels of the vascular tree. Engineering large-scale vessels has been pursued over the past thirty years to replace or bypass damaged arteries, arterioles, and venules, and their routine application in the clinic may become a reality in the near future. Strategies to engineer meso- and microvasculature have been extensively explored to generate models to study vascular biology, drug transport, and disease progression, as well as for vascularizing engineered tissues for regenerative medicine. However, bioengineering of large-scale tissues and whole organs for transplantation, have failed to result in clinical translation due to the lack of proper integrated vasculature for effective oxygen and nutrient delivery. The development of strategies to generate multi-scale vascular networks and their direct anastomosis to host vasculature would greatly benefit this formidable goal. In this review, we discuss design considerations and technologies for engineering millimeter-, meso-, and micro-scale vessels. We further provide examples of recent state-of-the-art strategies to engineer multi-scale vasculature. Finally, we identify key challenges limiting the translation of vascularized tissues and offer our perspective on future directions for exploration.

Keywords: Tissue engineering; engineered microvasculature; tissue engineered blood vessels; tissue vascularization.

Figure 1.. Organization of the vascular tree.

The vascular tree is organized into a hierarchical network of arteries, arterioles (blue), capillary beds, veins and venules (red) that span several orders of magnitude in diameter. All vessels are characterized by an inner layer of endothelium and an outer layer of basement membrane. Arterioles and venules are further bound by a second layer of SMCs, as well as elastin and collagen fibers. In arteries and veins this layer is thicker. Capillaries have a varying extent of basement membrane and pericyte coverage and can be continuous, fenestrated, or discontinuous.

Figure 2.. Overview of fundamental and more recent developments in vascular bioengineering;

sub-fields are broken down into engineered large vessel grafts, anastomosis, in vitro models of vasculature and whole organ vascularization. (A) top: engineered blood vessel post-implantation ^[63]; bottom: schematic for translation of engineered vessels ^[13b]. (B) top: human CD31+ staining in vitro and once implanted in vivo ^[139d]; bottom: stromal support of endothelial cells in fibrin gels ^[98]. (C) top: perfusable mesovasculature in collagen gels ^[72]; bottom: microfluidic platform to study endothelial cell interactions with supportive populations during angiogenesis ^[94]. (D) top: macroscopic view of whole heart decellularization and re-cellularization with ECs and cardiomyocytes ^[145]; bottom: bioprinting of soft proteins like collagen in various organizations ^[3a]. (E) top: acellular vessel grafts from bioreactor-based tissue culture from Humacyte, Inc. ^[1e]; bottom: incorporation of iPSC-derived SMCs strengthens engineered vessel organization once implanted ^[13e] (F) top: macroscopic view of “Angiochip” scaffold for improved anastomosis ^[84]; bottom: patterned endothelial cells within hepatocyte aggregates prior to implantation ^[155]. (G) top: perfused endothelial cells in microvasculature-on-a-chip ^[2a]; bottom: iPSC-derived blood vessel organoids show cell cross-talk and overview of organoid ^[6a]. (H) top: bioprinted vascular structures emulate alveolar capillary structure ^[152]; bottom: live/dead staining in high density organoids within vascularized tissues ^[151]. Images reproduced with permission.

Figure 3.. Fabrication approaches for small-caliber vessels.

Tissue engineered small-caliber vessels are fabricated by electrospinning, molding, decellularization, cellular sheets or 3D printing to generate either cellular or acellular grafts. Acellular grafts are either implanted immediately post fabrication or seeded with an autologous cell source. Cellularized grafts could be further stimulated in bioreactors to mature vessels prior to implantation.

Figure 4.. Engineered meso- and microvasculature.

Approaches to engineering meso-and microvasculature can be divided into top-down and bottom-up approaches. In top-down the vasculature is pre-designed, while bottom-up relies on cellular and extracellular stimuli to promote vessel formation. The vasculature can be further matured or stimulated to induce sprouting to then generate a stable and functional vascular network. Both approaches are utilized to generate in vitro vasculature models for basic research and drug screening applications and/or to vascularize tissues for regenerative medicine applications.

Figure 5.. Overview of approaches for engineering meso- and microscale vasculature.

(A) Uniaxial mesochannel fabricated by needle molding ^[72]. (B) Patterned mesochannels in PDMS ^[77c]. (C) Mesochannel casting within collagen hydrogels ^[80]. (D) Laser patterning microvascular networks within PEG hydrogels ^[3d]. (E) Layer-by-layer assembly of a branched 3D mesochannel network ^[84]. (F) Mesovasculature 3D printing using sacrificial bioinks ^[91c]. (G) Vasculogenesis-based vascular self-assembly ^[100]. Images reproduced with permission.

Figure 6.. Overview of approaches for engineering complex and multi-scale vasculature.

(A) Decellularized (left) and reendothelialized (right) whole heart ^[145]. (B) Hierarchical vasculature generated by programmable photodegradation ^[83]. (C) Projection based 3D printing of multi-scale channels ^[147]. (D) 3D printed hierarchically branched tubular networks ^[150]. (E) 3D printed perfusable model of the heart ^[3b]. (F) Transport between 3D intertwining microchannels generated by laser degradation ^[6d]. Images reproduced with permission.