Materials used in making Prosthetic limbs

A variety of metals are used for prosthetics limbs; Aluminum, Titanium, Magnesium, Copper, Steel, and many more. They are each used in a varied amount and for various applications, either pure or alloyed. Copper, iron, aluminum and nickel have all been used for the load bearing structure in the past, but are currently used  primarily as alloys or for plating.
This paper will focus on analyzing Titanium and the primary load bearing structure and current favorite in the biomedical field. Titanium was discovered in the late 18th century. It is a common metal used for medical and engineering applications because of its many favorable properties. It has good strength to weight ratio, goo strength to density ratio, excellent corrosion resistance, low density and it is lightweight. It is commonly alloyed with other metals to improve certain
properties, most commonly aluminum and vanadium. In its unalloyed condition,  titanium is as strong as some steels, but less dense. Being lightweight, strong,
resistant to corrosion and bio compatibility are its most desirable properties for the application of prosthetics. Its low modulus of elasticity makes it similar to that of
bone. This means that the skeletal load of its user will be distributed relatively  evenly between the bone and the implant making for a more natural gait. When its
characteristics are well understood and designed properly, this can be a very economical option for the lifetime of the product.

Polymers are not often used for as the main load bearing structure for limbs. They are more common with phalanges, joints, and other smaller body parts. When it comes to limb prostheses, polymers are more common for the smaller components
or specialized features.  Common polymers used are polyoxymethylene (POM), which is a hard polymer,
pliable polyurethane (PU), which is much softer, and poly vinyl chloride (PVC),  which is used as a coating.
Polyethylene is a more flexible form of plastic and it ideally used in larger quantities
when the prosthetic needs to be waterproof. The design, fit and material are all highly specialized because it need to be waterproof,  capable of performing swim motions, and comfortable while doing so. Everyday prosthetics are not intended to be used in such an environment nor in such a motion. PVC first developed in the early part of the 20th century and by 50s it was one of the
most important plastics PVC is very durable but has limited color range. Silicone resists stains but is less durable. PVC is unstable when exposed to heat and light so it requires the addition of stabilizers.
Ionometic polymer metal composites (IPMC) are attractive types of electro-active polymer actuation materials because of their characteristics of large electrically induced bending, mechanical flexibility, low excitation voltage, low density, and ease
of fabrication.

The use of carbon fibers came about in the 20th century when medics and engineers were in search of a lighter load bearing material. The properties of carbon fibers,
such as high stiffness, high tensile strength, low weight, high chemical resistance, high temperature tolerance and low thermal expansion, high specific strength and
specific modulus. It was determined that it could be strong enough for even a heavy weight amputee. Materials with high elastic modulus are usually not very ductile: the specific modulus of wood is comparable to that of steel, magnesium, titanium, or aluminum, whereas that of carbon fiber reinforced composites is about three times as high. Carbon fiber reinforced composites also have very high specific tensile and
compressive strengths, as well as high responsive elastic deformation.
The Northrup Aircraft Corporation was doing research after being contracted by the Veteran’s Administration. It was determined that the material was brittle and
susceptible to impact damage that was great cause for concern. Carbon fiber can also be costly compared to other material with similar properties.

Bio compatibility refers to materials that are not harmful to living tissue. This is most often considered when making surgical tools or other objects that interact with the body internally. Another aspect of bio compatibility is how a material interacts with the surface of the skin or the external body. When prosthetics are  attached to the exterior of the limb, and constant movement is occurring, the skin can be subject to a variety of painful and uncomfortable side effects. The distribution of mechanical stress at body support interfaces can influence the risk of tissue breakdown. Excessive pressure and shear stress can lead to skin blisters,  cysts, or ulceration. Interface materials influence the pressure and shear distribution on skin and underlying tissues principally via their elastic property and  their frictional characteristics with skin. Supporting materials used in prosthetics
are Spenco, Poron, Nylon-reinforced silicone, Nickelplast, to name a few. These are  all commonly used and have been carefully tested and selected based on their performance during compression testing. They have all been evaluated based on
their coefficients of friction with some exceptions. Nylon-reinforced silicone was not tested because it tended to crack during shear loading and Spenco was not tested because it became extremely thin after short term loading.

The goal as a doctor is to improve the quality of life of patients. The goal as an engineer is to enhance a system and improve the quality of life for the world. In creating a prosthetic, those two goals come together and make a huge difference in the life of the user. Prosthetics are not only becoming more functional and comfortable but also lifelike and aesthetically pleasing. The more life like a piece is, the less social stigma or pity a user will receive. Between the improvements in design, fit, and appearance an amputee can walk with a more normal and confident gait. This new technology and better material selection, an amputee can gain its
mobility, freedom, and life back.
The next step in this research is to find a suitable material to take to third world
countries. A metal alloy that is strong, lightweight, and affordable for amputees of developing nations should be next in this line of research. 

Producing the material locally has many socioeconomic advantages. It will create commerce for the community and present a more affordable prosthetic for the user.