Technique guides the growth of grafts to mimic structure and strength of natural cartilage
Articular cartilage is a substance that few people are aware of until they begin to experience pain, often debilitating, in their knees, shoulders and other joints. Then it is simple to understand that articular cartilage is the smooth surface that allows bones to interact efficiently at the joints during movement—performing flawlessly until nicked and scraped from injury or years of wear and tear.
Using tissue-engineering techniques to repair this unique tissue is the focus of the five-year $6.7 million grant from the National Institute of Biomedical Imaging and Bioengineering (NIBIB) to establish the Center for Multimodal Evaluation of Engineered Cartilage at Case Western University.
In a paper published in the journal Acta Biomaterialia,1 bioengineers at the center describe a new “roll-up” technique based on a novel cylindrical scaffolding design seeded with collagen-secreting cells. The new technique yields grafts for articular cartilage repair with superior mechanical strength and durability.
“What the group at Case Western has discovered is the importance of scaffold design and how it influences growth of the graft at the level of individual collagen fibers,” said Seila Selimovic, Ph.D., director of the NIBIB program in Tissue Engineering. “Engineered grafts that match the strength and durability of natural cartilage moves us closer to the goal of a tissue-engineering solution to the pervasive public health problem of joint pain and dysfunction.”
Scaffold form drives function
The recipe for growing cartilage in the laboratory has been known for some time, and basically consists of culturing scaffolds seeded with mesenchymal stem cells (MSCs) and growth factors. When exposed to growth factors the MSCs secrete an extracellular matrix (ECM), which works as a lubricant and shock absorber for the bones that meet and move against each other at the joints. The ECM is rich in collagen and glycosaminoglycans, two of the primary components of the ECM of natural articular cartilage. However, this recipe creates cartilage that is similar, but significantly less sturdy than the real thing.
The new scaffold was designed to create long continuous collagen fibers—the structure seen in natural articular cartilage that appears to be key to its strength and durability. To grow constructs containing long, organized collagen fibers the researchers made scaffolds with parallel rows of channels. The MSCs were seeded in the channels, sort of like crops in a field. As hoped, the channels of MSCs secreted collagen in long continuous fibers that aligned with the channels.
Tests demonstrated that cartilage grown on scaffolds with the “guide channels” was significantly stronger than cartilage grown on scaffolds that are basically flat sheets with MSCs spread evenly over the surface without any guiding channels.
Moving from 2-D to 3-D scaffolds to cover more territory
For future practical use it was necessary to move from the 2-D structure of the scaffold sheet with guide channels, to a scaffold design that would create larger pieces of tissue-engineered cartilage. This was achieved using the roll-up method. The thin 2-D layer construct with guide channels was seeded with MSCs and growth factors and then rolled to create a 3-D cylindrical structure.
The cylinders were rolled into five or six layers and placed in culture media to allow further growth of the cartilage throughout the cylinder.
The result was a sturdy piece of cartilage about an inch long and as thick as a pencil. The tissue-engineered cartilage displayed all the desired characteristics. Unlike previous tissue-engineered cartilage, the roll up displayed mechanical strength similar to natural cartilage. In addition, the matrix contained the mix of collagen and glycosaminoglycans of natural cartilage.
“The current roll-up method would allow us to grow pieces of cartilage large enough to repair a typical crack or hole in the articular cartilage that occurs from acute sports or accident injuries,” explained Harihara Baskaran, Ph.D., Professor of Chemical Engineering at the Case Western School of Engineering and senior author of the study.
Baskaran explained that the idea is to take slices of the cartilage roll and use the button shaped disks to graft over pits and cracks in acutely damaged cartilage. “To repair larger areas of damage from chronic wear and tear, we are now experimenting with much thicker rolls of 30 or 40 layers, which would allow us to create a larger slice capable of covering and repairing defects of a square inch or so,” said Baskaran.
The group now plans to test their cartilage grafts in animal studies. Although growing cartilage grafts requires painstaking attention to detail and constant testing and refinement to improve the strength and durability of the grafts, Baskaran and his colleagues are committed to the work.
“Articular cartilage is a unique, and frankly, amazing tissue that is built to perform the difficult function of allowing our skeletal system to function smoothly,” said Baskaran. “Unfortunately, it does not regenerate, and the current procedures to attempt to patch injuries are not very successful and usually short-lived. The concept of making lasting repairs with our grafts is one we think can make a real difference in the lives of the many people who want to remain active but are hampered by joint damage and often debilitating pain.”
The collaboration included researchers from the Departments of Chemical Engineering, Biomedical Engineering, Mechanical and Aerospace Engineering Biology, Skeletal Research Center, and the Center for Multimodal Evaluation of Engineered Cartilage, Case Western Reserve University, Cleveland, OH; The Lerner Research Institute, Cleveland Clinic; and the Department of Molecular and Clinical Pharmacology, Institute of Translational Medicine, University of Liverpool, Liverpool, UK.
The work was supported by grant # EB021911 from the National Institute of Biomedical Imaging and Bioengineering to establish the Center for Multimodal Evaluation of Engineered Cartilage, grant # AR053622 from the National Institute of Arthritis and Musculoskeletal and Skin Diseases, and the National Science Foundation.
1. Micrometer scale guidance of mesenchymal stem cells to form structurally oriented large-scale tissue engineered cartilage. Chou CL, Rivera AL, Williams V, Welter JF, Mansour JM, Drazba JA, Sakai T, Baskaran H. Acta Biomater. 2017 Sep