Week 10 - Biopolymers Flashcards
How are polymers made?
By linking together small molecules using covalent bonding along backbone
What affects the physical properties of polymers?
Their chemical composition, particularly molecular weight, functional groups and side chains
What effect does the length of the polymer chains have?
On the mechanical properties, due to the entanglements between chains and their mobility
What forms can polymers be fabricated into?
- Solid polymer parts
- Fibres for composites
What are the three main types of polyethylene?
- Low density (soft)
- High density (stronger)
- UHMWPE which is the toughest most wear resistant polymer known to mankind
What are the advantages of UHMWPE?
- High stability
- Low toxicity
- Stiff
- Strong
- Easily produced
- No additives or leachants
- Low friction coefficient
- AMAZING wear resistance
What are the disadvantages of UHMWPE?
Can fail by fatigue and creep, inflexible processing (must be compression moulded and machined), can absorb fluid and is hard to injection mould
What are methods for enhancing polyethylene wear resistance?
Carbon fibre, cross-linking, vitamin E
What are polyethylene fibres?
- Fabric woven from solvent spun UHMWPE
- Can be hotpressed with a HPPE or LDPE matrix to make bullet proof panels
What is the strength of polyethylene fibres?
120GPa, but modulus can be engineered directly to match bone
What is polyether ether ketone?
- Expensive
- High melting point
- Very bioinert
- Similar properties to PTE
- Used almost exclusively for spinal fusion cages
What is PMMA?
- Thermoset which comes as two parts that can be combine via curing
- Often used for dental materials and bone cement
What are the advantages of PMMA?
- Outstanding bond strength
- Many different mixing methods are used to prevent porosity and minimise shrinkage
What are the disadvantages of PMMA?
- Deterioration over time
- Potential for leachants and wear particles
- Sets rapidly, heating to 70 degrees, causing potential necrosis
- Precoatings required for bonding to metal
What elastomers have been used for implants?
- Silicon used when large recoverable deformations at low stress are required e.g. wrist and finger joint, breast implants
- However absorbs blood lipids and causes swelling means no longer applicable for ball in cage valves
What natural polymers have been used for implants?
- Cellulose for dialysis membranes
- Collagen for dialysis membranes
What have synthetic polymers been used for?
- Dacron for composites and on surface of blood interfacing implants
- Hydron for breast implants, soft contact lenses and wound coverings
What are hydrogels used for and why?
- Opthalmic devices
- Spinal disc nucleus implants
- Very soft but difficult to sterilise
What are biodegradable biopolymers used for and why?
- Mainly used in sutures and TE scaffolds
- Degradation products are acidic monomers which cause inflammatory response
e. g. PGLA
What are the factors affecting the lifespan of biopolymers?
- Can deteriorate due to chemical, thermal and physical factors
- In vivo environment may cause polymers to degrade after implantation due to ionic attack, dissolved oxygen
- Mechanical effects can also cause polymer degradation
What sort of chemical affects impact polymers and how?
- Depolymerisation, chain scission and loss of cross linking
- Can affect mechanical properties
What are the implications of sterilisation for biopolymers?
- Anything that can kill bacteria can damage polymers
- Causes changes to polymer’s structure, inducing early deterioration
What does heat sterilisation involve?
- High temperatures and pressures
- Can take polymers above their melting or softening points
What does chemical sterilisation involve?
Can cause chemical reactions in polymer and induce degradation
What does radiation sterilisation involve?
Can break polymer chains and therefore change mechanical properties
