Digital Light Processing 3D Bioprinting of Gelatin-Norbornene Hydrogel for Enhanced Vascularization

Digital light processing (DLP) bioprinting can be used to fabricate volumetric scaffolds with intricate internal structures, such as perfusable vascular channels. The successful implementation of DLP bioprinting in tissue fabrication requires using suitable photo-reactive bioinks. Norbornene-based bioinks have emerged as an attractive alternative to (meth)acrylated macromers in 3D bioprinting owing to their mild and rapid reaction kinetics, high cytocompatibility for in situ cell encapsulation, and adaptability for post-printing modification or conjugation of bioactive motifs. In this contribution, the development of gelatin-norbornene (GelNB) is reported as a photo-cross-linkable bioink for DLP 3D bioprinting. Low concentrations of GelNB (2-5 wt.%) and poly(ethylene glycol)-tetra-thiol (PEG4SH) are DLP-printed with a wide range of stiffness (G' ≈120 to 4000 Pa) and with perfusable channels. DLP-printed GelNB hydrogels are highly cytocompatible, as demonstrated by the high viability of the encapsulated human umbilical vein endothelial cells (HUVECs). The encapsulated HUVECs formed an interconnected microvascular network with lumen structures. Notably, the GelNB bioink permitted both in situ tethering and secondary conjugation of QK peptide, a vascular endothelial growth factor (VEGF)-mimetic peptide. Incorporation of QK peptide significantly improved endothelialization and vasculogenesis of the DLP-printed GelNB hydrogels, reinforcing the applicability of this bioink system in diverse biofabrication applications.

Ultrasound standing wave spatial patterning of human umbilical vein endothelial cells for 3D micro-vascular networks formation

Generating functional and perfusable micro-vascular networks is an important goal for the fabrication of large and three-dimensional tissues. Up to now, the fabrication of micro-vascular networks is a complicated multitask involving several different factors such as time consuming, cells survival, micro-diameter vasculature and strict alignment. Here, we propose a technique combining multi-material extrusion and ultrasound standing wave forces to create a network structure of human umbilical vein endothelial cells within a mixture of calcium alginate and decellularized extracellular matrix. The functionality of the matured microvasculature networks was demonstrated through the enhancement of cell-cell adhesion, angiogenesis process, and perfusion tests with microparticles, FITC-dextran, and whole mouse blood. Moreover, animal experiments exhibited the implantability including that the pre-existing blood vessels of the host sprout towards the preformed vessels of the scaffold over time and the microvessels inside the implanted scaffold matured from empty tubular structures to functional blood-carrying microvessels in two weeks.

Double-layered blood vessels over 3 mm in diameter extruded by the inverse-gravity technique

One of the most promising techniques for treating severe peripheral artery disease is the use of cellular tissue-engineered vascular grafts (TEVGs). This study proposes an inverse-gravity (IG) extrusion technique for creating long double-layered cellular TEVGs with diameters over 3 mm. A three-layered coaxial laminar hydrogel flow in an 8 mm-diameter pipe was realised simply by changing the extrusion direction of the hydrogel from being aligned with the direction of gravity to against it. This technique produced an extruded mixture of human aortic smooth muscle cells (HASMCs) and type-I collagen as a tubular structure with an inner diameter of 3.5 mm. After a 21 day maturation period, the maximal burst pressure, longitudinal breaking force, and circumferential breaking force of the HASMC TEVG were 416 mmHg, 0.69 N, and 0.89 N, respectively. The HASMC TEVG was endothelialised with human umbilical vein endothelial cells to form a tunica intima that simulated human vessels. Besides subcutaneous implantability on mice, the double-layered blood vessels showed a considerably lower adherence of platelets and red blood cells once exposed to heparinised mouse blood and were considered nonhaemolytic. The proposed IG extrusion technique can be applied in various fields requiring multilayered materials with large diameters.

Angiogenesis in Free-Standing Two-Vasculature-Embedded Scaffold Extruded by Two-Core Laminar Flow Device

Rapid construction of pre-vascular structure is highly desired for engineered thick tissue. However, angiogenesis in free-standing scaffold has been rarely reported because of limitation in growth factor (GF) supply into the scaffold. This study, for the 1st time, investigated angiogenic sprouting in free-standing two-vasculature-embedded scaffold with three different culture conditions and additional GFs. A two-core laminar flow device continuously extruded one vascular channel with human umbilical vein endothelial cells (HUVECs) and a 3 mg/ml type-1 collagen, one hollow channel, and a shell layer with 2% w/v gelatin-alginate (70:30) composite. Under the GF flowing condition, angiogenic sprouting from the HUVEC vessel had started since day 1 and gradually grew toward the hollow channel on day 10. Due to the medium flowing, the HUVECs showed elongated spindle-like morphology homogeneously. Their viability has been over 80% up to day 10. This approach could apply to vascular investigation, and drug discovery further, not only to the engineered thick tissue.

Cell Attachment on Inside-Outside Surface and Cell Encapsulation in Wall of Microscopic Tubular Scaffolds for Vascular Tissue-Like Formation

By using the microfluidic spinning technology we generated tiny hydrogel tubular scaffolds. Fibroblast (NIH/3T3) cell cultures were performed for seventeen days to demonstrate the potential of cell attachment on surfaces and encapsulation in the wall of he microscopic scaffolds for blood vessel-like structure forming. Over theculture period, the NIH/3T3 confluence reached around 80\%, and 100\% on the inside and outside scaffolds' surface respectively while cells proliferated and coalesced in cell group in the hydrogel wall. These results could further be applied to endothelial co-culturing for forming engineered blood vessel.