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 that provides protection from the compressive loading generated by walking, running, or lifting. It also provides a protective, lubricating layer, allowing bones to pass over one another in a frictionless way. Cartilage is a bi-phasic porous material, meaning it exists in solid and fluid phases. It 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 also when held in the porous matrix of the cartilage, it provides a hydroelastic cushion against compressive forces.
Because the cartilage is porous, the synovial fluid eventually drains away and as it does, it helps dissipate the energy forces traveling through the body, protecting joints from wear and tear and impact injuries. At this point, the cartilage returns to its sold phase, ready for the cycle to be repeated.
Earlier attempts at developing a synthetic cartilage system focused on the use of hydrogels — materials that absorb water. Hydrogels are good at reducing friction but perform poorly when under compressive force. One of the problems is that it takes time for the hydrogel to return to its normal shape after it has been compressed.
The researchers overcame this problem by creating a synthetic porous material made of a hydrogel held in a matrix of polydimethylsiloxane — a silicone-based polymer. The matrix keeps the shape of the hydrogel. The hydrogel also provides a lubricating layer. The load-bearing behavior of the hydrogel held in the 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 percent to 30 percent, close to the values reported for the modulus of cartilage tested.
The researchers believe future applications of a new material based on the function of cartilage could challenge many traditional oil-lubricated engineering systems. 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 that often use oil-based lubricants and hard-surface coatings.
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