Various synthetic materials including metals, ceramics and ultrahigh molecular weight polyethylene (UHMWPE) have found some success when used as replacement materials for human cartilage. These materials often lack the complete strength, stiffness or water content that is present in natural human cartilage, resulting in a need to develop more suitable materials for these purposes.
To address this dilemma, a recent joint effort between researchers at the University of Michigan and Jiangnan University of China has led to the development of “Kevlartilage,” a nanofibrous hydrogel.
The Complexity of Human Cartilage
The primary function of articular cartilage is to provide a smooth and lubricated surface between bones to allow for efficient articulation and ability to withstand large loads. Typically measuring in a thickness of 2-4 millimeters (mm), articular cartilage is composed of a dense extracellular matrix (ECM) and sparsely distributed chondrocytes, or highly specialized cartilage cells.
The ECM of cartilage is made up of water, collagen, proteoglycans and small amounts of other proteins that allow the ECM to retain water and maintain its unique functional properties1. Articular cartilage retains water during resting periods, and when joints are put under stress, water is released from the network to ensure its flexibility and strength during strenuous activities.
Various synthetic materials have been used for joint replacement procedures, however these materials are often incapable of maintaining each property of high stiffness, toughness and water content that is required to adequately replace human cartilage. One of the most promising options for cartilage replacement procedures is the hydrogel, as its specific design incorporates a long network of flexible molecules with a high water content required to support the growth of chondrocytes.
Unfortunately, traditional hydrogels are unable to exhibit the tensile strength that is required for most cartilage functions, as these materials are more susceptible to tears and strains.
The Kevlar-based hydrogel developed by the Michigan and Jianguan researchers combined a network of nano-aramid fibers, the material that is typically used for bulletproof vests and other types of body armor, with polyvinyl alcohol (PVA), a material that has already been used for hydrogel cartilage replacements.
The PVA within the aramid nanofiber network traps water in the same mechanism found in natural cartilage, and under periods of stretching or compression, the Kevlartilage releases the water to comply with the activity.
The team of researchers evaluated the mechanical properties of the synthetic cartilage, in which they determined that the tensile moduli of the cartilage measured to approximately 9.1 MPA, an ultimate tensile strain of approximately 325%, as well as a compressive strength of 26 MPa2. These tensile properties were found to be 92% comparable to that of natural cartilage. The Kevlartilage material also exhibited an inability to cause harm to adjacent cells, therefore allotting this synthetic material as an ideal replacement option for biological purposes.
Further Biological Applications
The primary application of the Kevlartilage was originally for cartilage replacements required for knee replacement procedures, however primary investigator Nicholas Kotov of the University of Michigan believes that other areas of the body could benefit from this material. Hydrogels exhibit a unique biocompatibility and ease of synthesis that allows this material to be used for various regenerative medical applications. Such applications of hydrogels are often found in biological scaffolds that provide structural integrity to the system, such as tissue barriers and bioadhesive materials.
Hydrogels can also be used as encapsulating materials to deliver drugs and other bioactive agents. Therefore, Kotov believes that the unique properties of the Kevlartilage could prove beneficial in improving current hydrogels that are used in medicine.
- “The Basic Science of Articular Cartilage” A. Fox, A. Bedi, et al. Sports Health. (2009). DOI: 10.1177/1941738109350438.
- “Water-Rich Biomimetic Composites with Abiotic Self-Organizing Nanofiber Netowrk” L. Xu, X. Zhao, et al. Advanced Materials. (2017). DOI: 10.1002/adma.201703343.