Nature has created probably one of the most powerful optimization algorithms of the Universe – evolution by natural selection. Engineers like to borrow Nature’s ideas and there is even a special name for this activity – biomimicry. One of the most popular examples of biomimicry is probably the exercise of kids mimicking birds using self-made wings (although rarely successful). Jokes aside, there are many successful examples where the borrowed ideas worked very well: velcro, sharks skin type of coatings on boats, whales fin shape on wind turbine blades, etc.
Tribology is not an exception and Nature has helped in developing slippery surfaces mimicking Salvinia molesta surface, snake skin, or gripping systems mimicking gecko feet. Another inspiration example comes from the operation of a human cartilage in knees. These load bearing machines designed so well and operate for so many years that the best artificial replacements are not even kind of close to the natural one. The long life of a natural knee is believed to be related to a very special structure of the human cartilage that allows for optimum lubrication. Recently, researchers from UK have proposed a new material that mimics human cartilage that could be used in the applications where lubrication is important.
Cartilage is a soft fibrous tissue found around human joints and it ensures smooth operation under compressive loading generated during walking, running or lifting. It also provides supply of sufficient amount of lubricant into the contact to minimize the friction forces. Of course, researchers have been trying to replicate such behavior for many years which was not really successful up to date. However, the UK team published an article where they have announced development of a material that functions like cartilage.
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 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.
Dr. Siavash Soltanahmadi, Research Fellow in the School of Mechanical Engineering at Leeds, who led the research, said: “Scientists and engineers have been trying for years to develop a material that has the amazing properties of cartilage. 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.
“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. There is potential across engineering for a material that behaves like cartilage.”
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.
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 have overcome this problem by creating a synthetic porous material made of a hydrogel held in a matrix of polydimethylsiloxane or PDMS—a silicone-based polymer. The matrix keeps the shape of the hydrogel.
In the paper, the scientists report that the load-bearing behavior 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 hydrogel also provided a lubricating layer.
The scientists believe future applications of a new material based on the function of cartilage would challenge many traditional oil-lubricated engineering systems.
Dr. Michael Bryant, Associate Professor in the School of Mechanical Engineering, who supervised the research, said; “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.”
The paper, Fabrication of Cartilage-Inspired Hydrogel/Entangled Polymer–Elastomer Structures Possessing Poro-Elastic Properties, is published in the journal Applied Polymer Materials.
Further information: Siavash Soltanahmadi et al, Fabrication of Cartilage-Inspired Hydrogel/Entangled Polymer–Elastomer Structures Possessing Poro-Elastic Properties, ACS Applied Polymer Materials (2021). DOI: 10.1021/acsapm.1c00256
Material adapted from: Inspired by nature, the research to develop a new load-bearing material