Engineering news
Strands of cow cartilage were used as a substitute for ink in a 3D bio-printing process that may one day create cartilage patches for worn out joints, according to a team of engineers at Pennsylvania State University in the US.
"Our goal is to create tissue that can be used to replace large amounts of worn out tissue or design patches," said Ibrahim T. Ozbolat, associate professor of engineering science and mechanics. "Those who have osteoarthritis in their joints suffer a lot. We need a new alternative treatment for this."
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.
Previous attempts at growing cartilage began with cells embedded in a hydrogel – a substance composed of polymer chains and about 90% water – that is used as a scaffold to grow the tissue.
"Hydrogels don't allow cells to grow as normal," said 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 may also 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. They create a tiny tube – from 3 to 5 one hundredths of an inch in diameter –made of alginate, an algae extract. Cartilage cells are injected into the tube and allowed 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, which is used as a substitute 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.
Ozbolat said: "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."
The artificial cartilage produced by the team is very similar to native cow cartilage. 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, either from existing cartilage or stem cells differentiated into cartilage cells.