The Michigan Engineer News Center

Kevlar-based artificial cartilage mimics the magic of the real thing

In spite of being 80 percent water, cartilage is tough stuff. Now, a synthetic material can pack even more H2O without compromising on strength.| Medium Read
EnlargeResearchers demonstrate the flexibility of the substance
IMAGE:  The artificial cartilage is very flexible yet resistant to tearing. Photo: Joseph Xu, Michigan Engineering Communications & Marketing

The unparalleled liquid strength of cartilage, which is about 80 percent water, withstands some of the toughest forces on our bodies. Synthetic materials couldn’t match it – until the “Kevlartilage” developed by researchers at the University of Michigan and Jiangnan University.

“We know that we consist mostly of water – all life does – and yet our bodies have a lot of structural stability,” said Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering at U-M, who led the study. “Understanding cartilage is understanding how lifeforms can combine properties that are sometimes unthinkable together.”

Many people with joint injuries would benefit from a good replacement for cartilage, such as the 850,000 patients in the US who undergo surgeries removing or replacing cartilage in the knee.

While other varieties of synthetic cartilage are already undergoing clinical trials, these materials fall into two camps that choose between cartilage attributes, unable to achieve that unlikely combination of strength and water content.

Understanding cartilage is understanding how lifeforms can combine properties that are sometimes unthinkable together.Nick Kotov

The other synthetic materials that mimic the physical properties of cartilage don’t contain enough water to transport the nutrients that cells need to thrive, explained Kotov. Meanwhile, hydrogels – which incorporate water into a network of long, flexible molecules – can be designed with enough water support the growth of chondrocytes, cells that build up natural cartilage. Yet those hydrogels aren’t especially strong, tearing under strains a fraction of what cartilage can handle.

The new Kevlar-based hydrogel recreates the magic of cartilage by combining a network of tough nanofibers from Kevlar – the “aramid” fibers best known for making bulletproof vests – with a material commonly used in hydrogel cartilage replacements, called polyvinyl alcohol (PVA).

In natural cartilage, the network of proteins and other biomolecules gets its strength by resisting the flow of water among its chambers. The pressure from the water reconfigures the network, enabling it to deform without breaking. Water is released in the process, and the network recovers by absorbing water later.

This mechanism enables high impact joints, such as knees, stand up to punishing forces. Running repeatedly pounds the cartilage between the bones, forcing water out and making the cartilage more pliable as a result. Then, when the runner rests, the cartilage absorbs water so that it provides strong resistance to compression again.

EnlargeResearcher in the lab
IMAGE:  Lizhi Xu, a research fellow in chemical engineering, demonstrates the strength of the artificial cartilage under stretching by attaching a weight to one end of a sample. Photo: Joseph Xu, Michigan Engineering Communications & Marketing

The synthetic cartilage boasts the same mechanism, releasing water under stress and later recovering by absorbing water like a sponge. The aramid nanofibers build the framework of the material, while the PVA traps water inside the network when the material is exposed to stretching or compression. Even versions of the material that were 92-percent water were comparable in strength to cartilage, with the 70-percent version achieving the resilience of rubber.

As the aramid nanofibers and PVA don’t harm adjacent cells, Kotov anticipates that this synthetic cartilage may be a suitable implant for some situations, such as the deeper parts of the knee. He also wonders whether chondrocytes might be able to take up residence inside the synthetic network to produce a hybrid cartilage. But his potential applications are not limited to cartilage: he suspects that similar networks, with different proportions of aramid nanofibers, PVA and water, may be able to stand in for other soft tissues.

“We have a lot of membranes in the body that require the same properties. I would like to evaluate the space,” said Kotov. “I will talk to doctors about where the acute need is and where this intersection of the properties will allow us to make best headway and biggest impact.”

Kotov is a member of the Biointerfaces Institute, which provides shared space for researchers from U-M’s top-ranked engineering and medical schools.

The paper, recently published in Advanced Materials, is titled “Water-rich biomimetic composites with abiotic self-organizing nanofiber network.”

The study was supported by the National Science Foundation (grant numbers ECS-0601345, CBET 0933384, CBET 0932823 and CBET 1036672), with additional funding from the Department of Defense (MURI W911NF-12-1-0407).

The University is seeking patent protection and partners to bring the technology to market.

Kotov is also a professor of chemical engineering, materials science and engineering, and macromolecular science and engineering.

  • Jars containing the substance
    Like natural cartilage, the artificial cartilage withstands stresses by releasing water and can later recover by absorbing water. Photo: Joseph Xu, Michigan Engineering Communications & Marketing
  • Cells survive well in the synthetic cartilage matrix: green cells are alive, and red cells are dead. Image: Xueli Zhao, Kotov Lab, University of Michigan.
  • An electron microscope image of the synthetic cartilage matrix. Image: Lizhi Xu, Kotov Lab, University of Michigan.
Advanced Ideas, Breakthrough Care

Michigan Engineers solve complex healthcare challenges by working together. We work across disciplines, span various industries and bridge generations to develop technologies to treat the future. Join us.

Researchers demonstrate the flexibility of the substance
Researcher in the lab
Portrait of Kate McAlpine

Contact

Kate McAlpine
Senior Writer & Assistant News Editor

Michigan Engineering
Communications & Marketing

(734) 763-4386

3214 SI-North

Man pours liquid concrete into a small container

Roads and bridges would last longer, save money with new concrete formula

U-M researchers to offer non-proprietary concrete formula free of charge. | Medium Read