To a scientist, trial and error is common in laboratories to refine new tests and methods, and look for potential problems on the small scale. But what happens when these techniques are meant for human tissues, such as for surgeries, medical tests, and treatments, where multiple different trials are not possible?
For Yun Wu, who just finished her PhD in Professor Shirley Tang’s lab, 3-Dimensional (3D) printing may hold the answer. In her latest research, Wu has been developing inks compatible with cells, known as bio-inks, to 3D print the structural unit of livers, called liver lobules.
Liver lobules are microscopic-scale hexagon shaped structures that are the structural unit of the liver and contain blood vessels and bile ducts for fluid transfer in and out of the organ. These hexagonal lobules fit together to create a honeycomb like pattern, separated by connective tissues. Each lobule surrounds a main artery and is filled with cells that contribute to and support liver functions. Since the structure is 3D, it makes it difficult to replicate the conditions of these cells in traditional lab methods like petri dishes, which are primarily 2-dimensional.
“By developing a method for bio-compatible 3D printing, you could imagine a future where doctors can 3D print liver tissues with their patient’s own cells,” says Wu. “This way, they could test the effectiveness of different treatments, such as chemotherapy drugs, in order to create treatments that were personalized for each patient.”
Unfortunately, there are a limited number of bio-inks available to researchers at the moment. Wu is working to change that. The development of bio-inks is an important step in the process of achieving 3D bio-printing, because the inks need to balance being able to flow through the printer nozzle, while also being stiff enough to hold the shape printed. While this challenge has been solved for plastic 3D printing, biologically compatible inks also have the added challenge of needing to keep cells alive and provide the right conditions for cell growth after printing occurs.
In a recent paper titled 3D bioprinting of bicellular liver lobule‑mimetic structures via microextrusion of cellulose nanocrystal‑incorporated shear‑thinning bioink, published in Nature’s Scientific Reports, Wu, Tang and their co-authors have found a way of achieving this fine balance. They report the development of a bio-ink by combining nanocrystals of cellulose, a compound that gives structure to plant cells, with two gel-like substances – gelatin methacryloyl and alginate, and have used this ink to print 3D structures mimicking liver lobules.
The cellulose nanocrystals induce a property called “shear thinning”, which allows the ink to flow through the printer nozzle more like a liquid, as it experiences the “shearing forces” of being forced through the needle. Then, once through the nozzle, it thickens up again to hold its shape. The gelatin methacroyloyl is added to support cell adhesion, spreading, and proliferation, and also mimic the stiffness of real liver lobule tissues.
After developing this bio-ink, and ensuring that it was able to print 3D structures, Wu added two different types of cells to the ink, in order to print the liver lobule structures. Multi-cellular bio-printing that mimicked the structure of a liver lobule was achieved by precisely printing the hexagonal framework with bio-ink containing one cell type and then depositing a second type of cell into the middle of each hexagon. Improved liver cell functions were observed in this multi-cellular system. These results showed that complex structures with multiple types of cells and encapsulating bio-inks can be 3D printed, which could have future applications in tissue engineering and biomedical research.
“We have only begun to see the impacts of 3D bio-printing on various research fields and industries,” says Tang. “With further breakthroughs in material and instrumentation, 3D bio-printing holds the promise to manufacture tissues and organs with ever increasing precision, higher complexity, and larger scale, bringing innovations to medical, space, pharmaceutical, beauty, and food industries.”