Biomaterials for sutures

Question 1

What are the uses of biodegradable polymers?

Answer 1

Degradable polymers have a number of uses – drug delivery, sutures and scaffolds.
– The material chosen depends on when it degrades; it needs to have hold tissue long enough for it to heal before it degrades.

Question 2

Describe the features of a polymer.

Answer 2

– A polymer is a material that consists of many units, referred to as monomers.
– Polymers have one main quality –> they have high tensile strength, e.g. a plastic bag. They also have toughness – resistance to cracking.
– The number of monomers (i.e. length of the polymer chain) can affect the polymer’s properties. Generally, a longer chain will make a stronger polymer.
- -Polymers can be natural (e.g. collagen which has strong triple helix structure, but it’s difficult to manipulate or synthesise) or synthetic polymers which are mostly thermoplastic or thermosetting.
o Thermoplastic are most common polymers – tough, long thin molecules which can become entangled ‘string like’ molecules. The tangles can hold polymer together and give it toughness (resistant to cracks). Make up plastic bags - polyethylene.
o Thermosetting - e.g. bone cement, brittle and hard to break as they criss-cross. q

Question 3

Explain the difference between solubility and degradation?

Answer 3

1. Solubility of polymer – when a polymer dissolves, its chains are separated and dispersed but not degraded.
   - - PVA is soluble (used in glue).
   - - The molecular weight determines size of chains in solubility – should be small enough not to clog up kidneys.
2. Degradation of polymers – involves the chains being ‘cut up’. Occurs by two mechanisms
   - - Biodegradation – digestion by enzymes.
   - - Commercial – by hydrolysis with water.

Question 4

Describe features of biodegradable polymers and their autocatalysis phenomenon.

Answer 4

They are designed to be degraded after they have completed their functions.

• • The metabolites produced from degradation must be safe and non-toxic to body.
• • They are broken down by natural enzymes in body or by hydrolysis of their (ester) bonds.
• • Polylactide and polyglycolide are polymers (polyesters) which are bioresorbable – they are degraded by hydrolysis of their ester bonds which creates naturally occurring glycolic/lactic acid.
• • Problems can arise if the polymer is degraded too quickly, e.g. build-up of acid in body which is why the polylactide/glycolide are degraded over long period of time.
• • The rate of degradation depends largely on water uptake –> more hydrophobic molecules have less water absorption.
• • Simple changes – e.g. replacing H by CH3 (in polylactide) can significantly increase time for degradation since it’s now more hydrophobic. However it doesn’t affect rate. (?)
• • Polylactide degradation is very fast –> exponential decrease in chain length due to water chopping the chains up. Eventually forms lactic acid. Also during degradation there will be –COOH groups present, which will lower pH – a lower pH will speed up the degradation process. Buildup of lactic acid can be dangerous.
• • Autocatalysis –>
• — Random chain excision which halves the Molecular weight.
• — The rate of cutting will increase as reaction goes on as molecules formed (carboxylic acids) will catalyse their own incision.
• — This will cause sudden degradation, referred to as the ‘whoosh’ effect and this can cause loss of mechanical properties very early on which is an issue.
• — The sudden degradation can be altered to when it happens, but it will always occur.

Question 5

What are the external factors affecting degradation rate?

Answer 5

1. Sterilisation –
   - - Methods include gamma radiation, ethylene oxide and plasma etching or electron beam irradiation.
   - - Radiation can result in damage in polymer chain.
   - - Ethylene oxide gas is mainly used, which needed de-gassing of fibres after for two weeks to prevent any residual gas and harm to the patient.
   - - Temperature needs to be kept low to prevent dimension change in polymer during sterilisation.
2. Shape
   - - A screw will have ‘hollowing-out’ problems due to the whoosh effect, the water from blood will enter screw and cause some degradation forming –COOH groups, lowering pH in the screw.
   - - Since there was no fluid transport, it started degrading from inside out due to the rapid release of lactic acid and polyglycolide oligomers. This is overcome by using a basic filler which will neutralise acidic carboxyl groups formed by hydrolysis.
3. Packaging - most polymers are packaged in some way
   - - Moisture will degrade them in storage so polymers are usually stored in vacuum in a freezer to minimise the amount of moisture. Once opened, they are handled as little as possible in room temp to minimise the condensation.
   - - The suture is wrapped around dried paper holder to act as dessicant.
   - - Final packaging consists of placing the suture/device in an airtight moisture proof container.