Carbon Fiber Prosthetic

The integration of carbon fiber reinforced polymer (CFRP) composites into the structure of prosthetics has changed the way that amputees are able to participate in sports.Figure 1shows the first below-knee prosthesis made purely from CFRP, the Flex Foot, which was  developed by Phillips in 1985. The mechanical properties of the composite including its lightweight, flexibility and high strength are what make it a good material to use for this application. Those properties allow for energy to be stored when the compressive load of a person’s weight is put on the prosthetic. Lifting body weight off as in a step results in decompression and the material returns to its original shape releasing the stored energy. So the carbon fiber composite stores elastic energy while in the stance phase and releases it during the step which aids in the swinging of the leg. This energy store and release system is what enhances a prosthetic’s performance.  When describing  how the amputee/prosthetic system behaves one could assume that it behaves like a perfect spring meaning Hooke’s law would apply and there’d be a linear relationship between stress and strain and no energy would be lost. However, a more realistic model would account for sources of energy loss such as friction, heat and sound. 

Figure 1. Schematic representation of the Flex Foot design (Scholz M.S. et al. Composites Science and Technology. 2011)

In terms of the make up of the material, the carbon fibers imbue strength and the polymer contributes its flexibility.  The concept of property averaging explains that the properties of a composite represent some average of the constituent properties. Because fibers respond in a different mechanical manner  when load is applied parallel  to their orientation vs transversely, the average material property is sensitive to geometry. Therefore, not only is the design of the prosthetic important for its function, but in order to tune performance for individuals the materials composition can be changed. Adjustments that can be made for possible customizations include differing laminate thickness and or fiber orientation.

In conclusion, the mechanical properties of CFRP make it a good material to design prosthetics with that can closely match human limb performance. An  example of what this means in terms of sports and competition is that in 2012 Summer Olympics Oscar Pistorius, shown in figure 2, became the first amputee runner to compete at an Olympic game.

Figure 2. Oscar Pistorius in the Men’s 4×400-Meter Relay, Track and Field finals at the 2012 Summer Olympics in London (Chang W. Lee/The New York Times)

Source: M.S. Scholz et al., The use of composite materials in modern orthopedic medicine and prosthetic devices: A review, Composites Science and Technology 2011

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