Strands of cow cartilage stand in for ink in a 3D bio-printing technique that may some day create cartilage patches for worn-out joints. Cartilage is a good tissue to target for scale-up bio-printing because it is made up of only one cell type and has no blood vessels within the tissue.
It is also a tissue that cannot repair itself. Once cartilage is damaged, it remains damaged. Ibrahim T. Ozbolat, associate professor of engineering science and mechanics at Penn State, said:
“Our goal is to create tissue that can be used to replace large amounts of worn out tissue or design patches. Those who have osteoarthritis in their joints suffer a lot. We need a new alternative treatment for this.”
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,” says Ozbolat. “The hydrogel confines the cells and doesn’t allow them to communicate as they do in native tissues.”
This leads to tissues that do not have sufficient mechanical integrity. Degradation of the hydrogel also can produce toxic compounds that are detrimental to cell growth.
Ozbolat and his research team developed a method to produce larger scale tissues without using a scaffold.
The researchers create a tiny (from 3 to 5 one hundredths 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 3D printing process. Using a specially designed prototype nozzle that can hold and feed the cartilage strand, the 3D 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.
“We can manufacture the strands in any length we want,” says Ozbolat. “Because there is no scaffolding, the process of printing the cartilage is scalable, so the patches can be made bigger as well. We can mimic real articular cartilage by printing strands vertically and then horizontally to mimic the natural architecture.”
Bring Your Own Cartilage?
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.
The work was funded by the National Science Foundation, Grow Iowa Value Funds, and the China Scholarship Fund.
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 (2016) doi:10.1038/srep28714
Image: Ozbolat Lab / Penn State. A piece of printed cartilage on a plug of bone in a petri dish of nutrient media.