Engineers have demonstrated that strands of cow cartilage can substitute for ink in a 3-D bioprinting process, which opens a door to creating cartilage patches for worn-out joints.
Cartilage is a good tissue to target for such bioprinting because it has no blood vessels within the tissue and is made up of only one cell type -- but it cannot repair itself after being damaged. Previous attempts at growing cartilage began with cells embedded in a hydrogel -- a substance composed of polymer chains and about 90 percent water -- that is used as a scaffold to grow the tissue.
"Hydrogels don't allow cells to grow as normal," said Ibrahim T. Ozbolat, associate professor of engineering science and mechanics at Penn State. "The hydrogel confines the cells and doesn't allow them to communicate as they do in native tissues."
That leads to tissues that do not have sufficient mechanical integrity, and degradation of the hydrogel also can produce toxic compounds detrimental to cell growth.
Ozbolat and his research team developed a method to produce larger scale tissues without using a scaffold. They create a tiny (3- to 5/100 of an inch in diameter) tube made of alginate, an algae extract. They inject cartilage cells into the tube and allow them to grow for about a week and adhere to each other. Because cells do not stick to alginate, they can remove the tube and are left with a strand of cartilage.
The cartilage strand substitutes for ink in the 3-D printing process. Using a specially designed prototype nozzle that can hold and feed the cartilage strand, the 3-D printer lays down rows of cartilage strands in any pattern the researchers choose. After about half an hour, the cartilage patch self-adheres enough to move to a petri dish. The researchers put the patch in nutrient media to allow it to further integrate into a single piece of tissue. Eventually the strands fully attach and fuse together.
The artificial cartilage produced by the team is very similar to native cow cartilage. However, the mechanical properties are inferior to those of natural cartilage, but better than the cartilage that is made using hydrogel scaffolding. Natural cartilage forms with pressure from the joints, and Ozbolat thinks that mechanical pressure on the artificial cartilage will improve the mechanical properties.
If this process is eventually applied to human cartilage, each individual treated would probably have to supply their own source material to avoid tissue rejection. The source could be existing cartilage or stem cells differentiated into cartilage cells.
Citation: Yin Yu, Kazim K. Moncal, Jianqiang Li, Weijie Peng, Iris Rivero, James A. Martin & Ibrahim T. Ozbolat, 'Three-dimensional bioprinting using self-assembling scalable scaffold-free “tissue strands” as a new bioink', Scientific Reports 6, Article number: 28714 June 27, 2016 doi:10.1038/srep28714 (open access)