Researchers from Vienna University of Technology (TU Wien) and Keio University have found a way to create artificial blood vessels in miniature organ models in a quick and reproducible manner. The method utilizes ultrashort laser pulses in the femtosecond range to write highly 3D structures into a hydrogel. In biomedical research, organs-on-a-chip are becoming increasingly important: By cultivating tissue structures in precisely controlled microfluidic chips, it is possible to conduct research much more accurately than in experiments involving living humans or animals. However, there has been a major obstacle: such mini-organs are incomplete without blood vessels. To facilitate systematic studies and ensure meaningful comparisons with living organisms, a network of perfusable blood vessels and capillaries must be created — in a way that is precisely controllable and reproducible. Schematic representation of a hepatic lobule (left) and 3D view of the vascularized hepatic lobule on-chip after nine days of culture (right). Courtesy of TU Wien. “If you want to study how certain drugs are transported, metabolized and absorbed in different human tissues, you need the finest vascular networks,” said TU Wien researcher Alice Salvadori. Until now, the shape and size of these microvascular networks have been difficult to control. In self-organization-based approaches, vessel geometry varies significantly from one sample to another. This makes it impossible to run the reproducible, precisely controlled experiments needed for reliable biomedical research. “We can create channels spaced only a hundred micrometers apart. That’s essential when you would like to replicate the natural density of blood vessels in specific organs,” said Aleksandr Ovsianikov. It’s not just about precision: The artificial blood vessels have to be formed quickly and remain structurally stable once they are populated with living cells. “We know that cells actively remodel their environment. That can lead to deformations or even to the collapse of vessels,” Salvadori said. “That’s why we also improved the material preparation process.” Instead of using the standard single-step gelation method, the team used a two-step thermal curing process, in which the hydrogel is warmed in two phases, using different temperatures, rather than just one. This alters its network structure, producing a more stable material. The vessels formed within such material remain open and maintain their shape over time. “We have not only shown that we can produce artificial blood vessels that can actually be perfused. The even more important thing is: We have developed a scalable technology that can be used on an industrial scale,” said Aleksanr Ovsianikov. “It takes only 10 minutes to pattern 30 channels, which is at least 60 times faster than other techniques.” If biological processes are to be realistically modeled on a chip, the artificial tissues must behave like their natural counterparts. “We showed that these artificial blood vessels are colonized by endothelial cells that respond just like real ones in the body,” Salvadori said. “For example, they react to inflammation in the same way – becoming more permeable, just like real blood vessels.” According to the researchers, this marks an important step toward establishing lab-on-a-chip technology as an industrial standard in many fields of medical research. “Replicating the liver’s dense and intricate microvasculature has long been a challenge in organ-on-chip research. By building multiple layers of microvessels spanning the entire tissue volume, we were able to ensure adequate nutrient and oxygen supply — which, in turn, led to improved metabolic activity in the liver model. We believe that these advancements bring us a step closer to integrating organ-on-a-chip technology into preclinical drug discovery,” said Masafumi Watanabe of Keio University. The research was published in Biofabrication (www.doi.org/10.1088/1758-5090/add37e).