Cartilage-mimicking material inspired by nature

UK engineers have developed a material that mimics human cartilage — the body’s shock-absorbing and lubrication system — and it could herald the development of a new generation of lightweight bearings.

Cartilage is a soft fibrous tissue found around joints which provides protection from the compressive loading generated by walking, running or lifting. It is a biphasic porous material, meaning it exists in solid and fluid phases, and switches to its fluid phase by absorbing a viscous substance produced in the joints called synovial fluid. This fluid not only lubricates the joints but, when held in the porous matrix of the cartilage, it provides a hydroelastic cushion against compressive forces.

Because cartilage is porous, the synovial fluid eventually drains away; as it does, it helps dissipate the energy forces travelling through the body, protecting joints from wear and tear and impact injuries. At this point the cartilage returns to its solid phase, ready for the cycle to be repeated.

For years scientists have been trying to create a synthetic material with the properties of cartilage, with mixed results. Now researchers at the University of Leeds and Imperial College London have announced the creation of a material that truly functions like cartilage, describing their work in the journal Applied Polymer Materials.

“We have now developed a material for engineering applications that mimics some of the most important properties found in cartilage, and it has only been possible because we have found a way to mimic the way nature does it,” said Dr Siavash Soltanahmadi from the University of Leeds, who led the new research.

Earlier attempts at developing a synthetic cartilage system have focused on the use of hydrogels — materials that absorb water. Hydrogels are good at reducing friction but perform poorly when under compressive force, taking time to return to their normal shape after being compressed.

The researchers have overcome this problem by creating a synthetic porous material made of a hydrogel held in a matrix of polydimethylsiloxane (PDMS), a silicone-based polymer. The matrix keeps the shape of the hydrogel and the hydrogel also provides a lubricating layer.

The scientists report that the load-bearing behaviour of the hydrogel held in the PDMS matrix was 14 to 19 times greater than the hydrogel on its own. The equilibrium elastic modulus of the composite was 452 kPa at a strain range of 10–30%, close to the values reported for the modulus of cartilage tested.

The researchers believe a cartilage-like material would have a wide range of uses across engineering, with Dr Soltanahmadi saying, “There are many applications in engineering for a synthetic material that is soft but can withstand heavy loading with minimum wear and tear, such as in bearings.”

The scientists further believe that future applications of a new material based on the function of cartilage could challenge traditional oil-lubricated engineering systems. As noted by research supervisor Dr Michael Bryant, from the University of Leeds, “The ability to use water as an effective lubricant has many applications, from energy generation to medical devices. However, this often requires a different approach when compared to traditional engineering systems, which often use oil-based lubricants and hard-surface coatings.

“This project has helped us to better understand these requirements and develop new tools to address this need.”

Image caption: The interface between the hydrogel (left-hand side) and the PDMS (on the right-hand side). The image was taken at 100,000 times magnification. Image credit: University of Leeds.

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