Case Studies of Common Biomaterials Flashcards
What is UHMWPE made from?
Repeating units of Ethene or ethylene
Where is UHMWPE used in medical applications?
Joint bearing surfaces in hip, shoulder and knee implants
Is UHMWPE degradable? and why?
no, due to the incredible strong c-c chain backbone it doesn’t degrade
It is also incredibly tightly packed which makes it hard for enzymes and catalysts to access and begin hydrolysis
Where is ethylene naturally produced?
It is naturally produced as a hormone in plants that triggers ripening
-It can be sourced from sugarcane, corn and wheat
Explain the reactions that convert glucose to ethene
1) Glucose is fermented with yeast to produce 2ethanol and 2CO2
2) The ethanol is heated with a catalyst to produce ethylene and water
How is ethylene converted into a a repeating polyethylene and then polymerised
- The c=c double bond is broken open
- Using a Ziegler catalyst and special polymerisation techniques the monomers are linked together to form extremely long chains
How does the Ziegler-Natta catalyst that helps to form UHMWPE operate?
1) An activator chemical (Methyl Aluminoxane) MOA, activates the Ziegler-Natta catalyst by taking away a chloride ion from the catalysts transition metal centre -> Making it highly active and capable of stating polymerisation
2) The active metal centre in the catalyst begins coordinating ethylene monomers together, into long chains
3) Control over the reaction conditions ensures the healthy formation of UHMWPE
What happens if the reaction conditions are changed when trying to form UHMWPE?
Different forms of PEs will begin to form such as:
HDPE and Linear Low density PE
How long is the chain length of UHMWPE?
Over 36000 repeating units normally, with minimal branching
How does manufacturability change with chain length?
As chain length increases, so does the polymer molecular weight. This makes it more viscous at molten temperatures and therefore harder to process and form
What happens to UHMWPE when it is melted and reformed?
The chains are so long that when melted and reformed the crystallinity most likely decreases. Ordering long chains into a tightly packed crystalline structure is incredibly hard and amorphous regions will form
What are some of the desirable properties of UHMWPE?
- High wear resistance
- Biocompatible
- Long lasting and resists biodegradation
- Similar mechanical properties to hard cartilage
- High impact strength
What is a potential issue of UHMWPE?
Delamination
How does delamination of UHMWPE occur>
During moulding and extrusion under high pressures and temps, some polymer chains can diffuse, which means they delaminate from the surface layer
How can we prevent the likelihood of delamination of UHMWPE?
Crosslink the polymer chains using gamma radiation
1) Irradiate to add crosslinks
2) Thermally treat it t solidify the links
3) Sterilise it
Explain Ram Extrusion
1) A hopper feeds powder/pellets into a heated column
2) This melts the UHMWPE, the molten polymer is then extruded through a die by a hydraulic ram into a rod formation
3) These rods are then turned and milled into the desired shape
How does gel spinning work?
1) The polymer is dissolved into a medium to create a gel
2) This gel is extruded through a narrow die and spun into fibres
What is the issue with processability and stress strength of UHMWPE fibres?
The more structured and less entangled the polymer strains the greater the strength profile
- However to achieve more uniform and structured polymer chains, they need to be processed at significantly lower temperatures which is not industrially viable
What does caprolactone look like and how is it converted into its repeating unit?
cyclic ring hydrocarbon
- This process is called ring opening polymerisation
- It uses a stannous octoate catalyst
