Dot point 3.2.4
Identify the properties of silicone that make it suitable for use in bionics
Silicon is an element that can be found in trace proportions in the human body. Silicones are
a bit like hydrocarbons except the carbon chain is replaced by silicon. Silicon forms a highly
versatile rubber-like substance called silicone.
Today silicone is used for many applications including oven door seals, heart valves, and in
medicines and bionics.
Silicone’s useful properties include:
o Resistance to acids, ozone and UV light
o Long life expectancy of about 500 years
o Totally inert, meaning it will not react with human body chemistry and so can be used
in implants, medicines and food processing
o Silicon is easy to shape
o It is permeable to oxygen which means it will allow oxygen to reach all parts of the
body even when used in implants
o Able to withstand radiation used to sterilise implants and other medical devices
o At high or low temperatures it is able to maintain its flexibility and elasticity
o Silicon does not dissolve or react with water and is water resistant
Dot point 3.2.5
Explain why silicone joints would be suitable substitutes for small joints in the fingers and
toes that bear little force
Artificial joints
Artificial joints are used in the body when natural joints are inflamed due to chronic
rheumatoid arthritis.
o These joints have swollen synovial membranes filled with synovial fluid – the
swelling and subsequent breakdown of cartilage of the joint make it less flexible,
painful and even deformed.
o Silicone can be used to coat the end of small bones in a joint where the cartilage has
been destroyed.
o This provides a smooth, frictionless surface that will not be broken down by disease.
o Although silicone can withstand the low forces in small joints, it is not able to
withstand the much larger forces found in larger joints such as the hip.
o They are biocompatible, as they allow the flow of oxygen and do not react with living
tissue. They would last a long time
Explain why silicone is a suitable material for replacing small joints in fingers and toes.
(3M)
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Dot point 3.2.6
Describe the properties that make ultrahigh molecular weight polyethylene (UHMWPE) a
suitable alternative to cartilage surrounding a ball and socket joint in terms of its
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biocompatability with surrounding tissue
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low friction
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durability
For artificial joints that are required in areas that bear a great deal of force, such as the hip,
then other substances are used – Ultra high molecular weight polyethylene (UHMWPE).
UHWMPE is an extremely long chain of CH2 units which form the raw material for a range
of items including plastic containers, bottles, medical tubes and specific medical devices such
as hip replacements.
Advantages of UHMWPE include:
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Resistant to abrasion
Strong
Doesn’t deform easily
Has good strength against fatigue
Has lower density than high density polyethylene
Forms a suitable alternative to cartilage because it is compatible with surrounding
tissue, durable and forms low friction coating in place of cartilage
UHMWPE and hip joints/replacements
o UHMWPE is used in hip replacements to coat the metallic parts forming the
articulating (moving) ends of a joint to give it a smooth surface.
o Coating the ball and socket joint with UHMWPE eliminated the metal debris that
would have accumulated if the two metal parts in contact rubbed against each other –
this would have led to infection and loosening of the joint.
1. What is meant by the term “biocompatability”?
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2. Describe three properties that make ultrahigh molecular weight polyethylene
(UHMWPE) a suitable alternative to cartilage in a ball and socket joint. (3M)
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3. Describe the frictional characteristics of ultrahigh molecular weight polyethylene
(UHMWPE) that make it suitable for use in ball and socket joints.
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4. Outline why durability would be an important property of ultrahigh molecular
weight polyethylene (UHMWPE) in relation to its use in an artificial hip joint.
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Ultra-high molecular weight polyethylene (UHMWPE) is biocompatible with
surrounding tissue. It possesses a similar density to living tissue, and therefore tends
not to cause problems in the body.
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UHMWPE has a low friction coefficient and this, along with other exhibited
characteristics, such as high hardness, high tensile strength, high elasticity, add to its
suitability of it for use in joints as artificial cartilage.
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UHMWPE is durable: it has no known effective solvent at mild temperatures. High
temperatures and pressures must be used to manipulate the material and gain the
desired product. UHMWPE also exhibits a very high creep resistance. Creep
resistance is the tendency for polymers to deform when under constant stress.