Thermo-sensitive Sacrificial Microsphere-based Bioink for Centimeter-scale Tissue with Angiogenesis.

Centimeter-scale tissue with angiogenesis has become more and more significant in organ regeneration and drug screening. However, traditional bioink has obvious limitations such as balance of nutrient supporting, printability, and vascularization. Here, with "secondary bioprinting" of printed microspheres, an innovative bioink system was proposed, in which the thermo-crosslinked sacrificial gelatin microspheres encapsulating human umbilical vein endothelial cells (HUVECs) printed by electrospraying serve as auxiliary component while gelatin methacryloyl precursor solution mixed with subject cells serve as subject component. Benefiting from the reversible thermo-crosslinking feature, gelatin microspheres would experience solid-liquid conversion during 37°C culturing and form controllable porous nutrient network for promoting the nutrient/oxygen delivery in large-scale tissue and accelerate the functionalization of the encapsulated cells. Meanwhile, the encapsulated HUVECs would be released and attach to the pore boundary, which would further form three-dimensional vessel network inside the tissue with suitable inducing conditions. As an example, vascularized breast tumor tissue over 1 cm was successfully built and the HUVECs showed obvious sprout inside, which indicate the great potential of this bioink system in various biomedical applications.

Electro-Assisted Bioprinting of Low-Concentration GelMA Microdroplets.

Low-concentration gelatin methacryloyl (GelMA) has excellent biocompatibility to cell-laden structures. However, it is still a big challenge to stably fabricate organoids (even microdroplets) using this material due to its extremely low viscosity. Here, a promising electro-assisted bioprinting method is developed, which can print low-concentration pure GelMA microdroplets with low cost, low cell damage, and high efficiency. With the help of electrostatic attraction, uniform GelMA microdroplets measuring about 100 µm are rapidly printed. Due to the application of lower external forces to separate the droplets, cell damage during printing is negligible, which often happens in piezoelectric or thermal inkjet bioprinting. Different printing states and effects of printing parameters (voltages, gas pressure, nozzle size, etc.) on microdroplet diameter are also investigated. The fundamental properties of low-concentration GelMA microspheres are subsequently studied. The results show that the printed microspheres with 5% w/v GelMA can provide a suitable microenvironment for laden bone marrow stem cells. Finally, it is demonstrated that the printed microdroplets can be used in building microspheroidal organoids, in drug controlled release, and in 3D bioprinting as biobricks. This method shows great potential use in cell therapy, drug delivery, and organoid building.

Vessel-on-a-chip with Hydrogel-based Microfluidics.

Hydrogel structures equipped with internal microchannels offer more in vivo-relevant models for construction of tissues and organs in vitro. However, currently used microfabrication methods of constructing microfluidic devices are not suitable for the handling of hydrogel. This study presents a novel method of fabricating hydrogel-based microfluidic chips by combining the casting and bonding processes. A twice cross-linking strategy is designed to obtain a bonding interface that has the same strength with the hydrogel bulk, which can be applied to arbitrary combinations of hydrogels. It is convenient to achieve the construction of hydrogel structures with channels in branched, spiral, serpentine, and multilayer forms. The experimental results show that the combination of gelatin and gelatin methacrylate (GelMA) owns the best biocompatibility and can promote cell functionalization. Based on these, a vessel-on-a-chip system with vascular function in both physiological and pathological situations is established, providing a promising model for further investigations such as vascularization, vascular inflammation, tissue engineering, and drug development. Taken together, a facile and cytocompatible approach is developed for engineering a user-defined hydrogel-based chip that can be potentially useful in developing vascularized tissue or organ models.