A chip with natural blood vessels

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. This is exactly what has now been achieved at TU Wien: the team established a method using ultrashort laser pulses to create tiny blood vessels in a rapid and reproducible manner. Experiments show that these vessels behave just like those in living tissue. Liver lobules have been created on a chip with great success.

Real Cells in Artificial Microchannels

"If you want to study how certain drugs are transported, metabolized and absorbed in different human tissues, you need the finest vascular networks," says Alice Salvadori, a member of the Research Group 3D Printing and Biofabrication established by Prof. Aleksandr Ovsianikov at TU Wien.

Ideally such blood vessels have to be created directly within special materials called hydrogels. Hydrogels provide structural support for living cells, while being permeable similarly to natural tissues. By creating tiny channels within these hydrogels, it becomes possible to guide the formation of blood vessel-like structures: endothelial cells -- the cells that line the inside of real blood vessels in the human body -- can settle inside these channel networks. This creates a model that closely mimics the structure and function of natural blood vessels.

The major challenge so far has been geometry: 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 reproducible, precisely controlled experiments -- yet that is exactly what is needed for reliable biomedical research.

Improved Hydrogel and Laser Precision

The team at TU Wien therefore relied on advanced laser technology: with the help of ultrashort laser pulses in the femtosecond range, highly precise3D structures can be written directly into the hydrogel -- quickly and efficiently.

"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," says Aleksandr Ovsianikov.

But it's not just about precision: the artificial blood vessels have to be formed quickly and also 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," explains Alice Salvadori. "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: the hydrogel is warmed in two phases, using different temperature, 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," says Aleksanr Ovsianikov. "It takes only 10 minutes to pattern 30 channels, which is at least 60 times faster than other techniques."

Simulating Inflammation: Natural Reactions on a Chip

If biological processes are to be realistically modeled on a chip, the artificial tissues must behave like their natural counterparts. And this, too, has now been demonstrated:

"We showed that these artificial blood vessels are colonized by endothelial cells that respond just like real ones in the body," says Alice Salvadori. "For example, they react to inflammation in the same way -- becoming more permeable, just like real blood vessels."

This marks an important step toward establishing lab-on-a-chip technology as an industrial standard in many fields of medical research.

Big Success with Liver Tissue

"Using this approach, we were able to vascularize a liver model. In collaboration with Keio University (Japan), we developed a liver lobule-on-chip that incorporates a controlled 3D vascular network, closely mimicking the in vivo arrangement of the central vein and sinusoids," says Aleksandr Ovsianikov.

"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," says Masafumi Watanabe (Keio University).

"OoC technology and advanced laser technology work well together to create more reliable models of blood vessels and liver tissues. One important breakthrough is the ability to build tiny tissues on a chip that allow liquid to flow through them, similar to how blood flows in the body. This helps researchers better understand how blood flow affects cells. OoC technology also makes it possible to closely observe how cells react under a microscope. These models will help scientists study how the body works and may lead to better treatments and healthcare in the future," says Prof. Ryo Sudo at Keio University.