Tribology for Biomedical Applications

Tribological properties associated with wear, friction and lubrication are important to the implementation of many biomedical applications. Medicine now allows for the replacement of biological tissue with artificial devices.

Over time, the implants can fail due to friction and wear as seen commonly in artificial joints but also because of the friction and corrosion produced from simple contact with body fluids such as blood. Tribology is an important aspect in the design of many biomedical devices, including artificial joints, implants and cardiovascular valves. Moreover, the ability to measure properties such as friction, levels of tension and liquid cohesion have enabled the comfortable wearability of the common contact lens.

Tribology and The Design of Artificial Joints and Implants

The most prominent biomedical application of tribology is for the design of artificial joints and implants. Metallic artificial implants are usually fabricated from surgical stainless steel, cobalt alloys or titanium alloys and the materials have different tribological properties that can be used to predict the service period of the implanted device.  Artificial knee and hip replacements must currently be replaced every 10-15 years because friction and wear cause implant failure through the aseptic loosening of the bond between device and bone as well as the formation of osteolysis.  

Cobalt alloys provide the material with the highest resistance to wear but are vulnerable to tension. Titanium alloys are preferable for biomedical devices because of their high level of biocompatibility but provide a limited service period because of their poor friction and wear properties. Techniques are currently being developed to improve the tribological properties of metallic devices to increase the time between artificial joint replacements. This includes reducing friction and the risk of damage by the modification of the outer surface of the metal alloy with ions and the production of surface coatings that can harden the material.

Tribology in Cardiovascular Devices

Friction and wear can lead to the failure of cardiovascular devices such as artificial heart valves and synthetic vascular grafts to repair weak blood vessels. Pathological conditions have also been proven to occur due to cardiovascular device wear including the formation of blood clots and the rupture of red blood cells. Friction can occur between moving components of the medical device or from blood cells. Mechanical wear through friction has been reported in the hinge regions and pivots of mechanical heart valves.  

The friction force that occurs on a blood vessel wall cell is calculated as shear stress and is proportional to blood flow viscosity. The shear stress from blood flow has been identified in numerous cardiovascular devices. The tribological properties of medical implants used to treat heart conditions are being improved through the choice of material pairs that reduce friction. Pyrolitic carbon shows particular promise as a material for mechanical heart valve fabrication because of its good wear performance and biocompatibility.

Tribology and Contact Lenses

Contact lenses are medical devices that correct vision without the reduced peripheral vision and problems with condensation that spectacle wearing causes. The increased understanding of various tribological properties has allowed for the development of new contact lenses that provide increased comfort for the wearer.

One property that has to be balanced when designing contact lenses is the modulus, which is a measure of how a material strains and deforms under tension. If the modulus of the lens is too low it will increase the difficulty of handling and reduce movement on blinking, if the modulus is too high it may cause pathological lesions of the cornea.

The coefficient of friction measures the lubricity of the lens and designs with low coefficients of friction have been shown to provide more comfort for the wearer. A further tribological property of contact lenses is wettability or the degree to which a liquid maintains contact with a solid surface. By improving the wettability property of a contact lens, a thick coverage of the tear film can be developed, allowing for good visual acuity after eye closure.

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Sources:

1. American Society of Mechanical Engineers: modern applications of tribology. https://www.asme.org/engineering-topics/articles/tribology/modern-application-of-tribology
2. Zhou, Z.R & Jin, Z.M. 2015. Biotribology: Recent progresses and future perspectives, Biosurface and Biotribology, 1, pp. 3-24. https://doi.org/10.1016/j.bsbt.2015.03.001
3. Matthew, M.T. et al. 2009. Significance of Tribocorrosion in Biomedical Applications: Overview and Current Status, Advances in Tribology, 1, e250986. http://dx.doi.org/10.1155/2009/250986
4. Xie, D. et al. 2015. A brief review of bio-tribology in cardiovascular devices, Biosurface and Biotribology, 1, pp. 249-262. https://doi.org/10.1016/j.bsbt.2015.11.002
5. Johnson & Johnson: contact lens properties. https://jnjvisionpro.co.uk/