Biodegradable materials

Question 1

*Describe applications of biodegradable materials.

Answer 1

• Sutures (stitches), made from PGA (polyglycolic acid), PGA/PLA, PDO (polydioxanone).
• Drug delivery devices
• Orthopaedic fixation devices
• Adhesion prevention
• Temporary vascular grafts and stents

Question 2

Discuss the four main types of degradable implants.

Answer 2

• Temporary scaffold: this includes sutures, bone fixation devices, vascular grafts
• Temporary barrier: such as adhesion prevention caused by blood clotting, inflammation and fibrosis.
• Drug delivery device: PLA and PGA are the most widely used materials for such applications.
• Multifunctional implant: e.g. a biodegradable bone nail that also stimulates repair (BMP (bone morphogenic protein) plus TGF-beta (transforming growth factor)).

Question 3

*Define biodegradation and bioerosion.

Answer 3

Biodegradation:
A degradation process wherein a biological agent is the dominant component.
Bioerosion: Water insoluble polymer than is converted under physiological conditions into water soluble material without regard to the specific mechanism involved in the erosion process.

Question 4

*State the changes observed due to bioerosion, and differentiate between surface and bulk bioerosion.

Answer 4

Macroscopic changes observed due to bioerosion include swelling, deformation, structural disintegration, and weight loss. These all contribute to loss of function.

Bulk erosion: Water penetration rate exceeds rate of transformation to water soluble product (hydrophilic)
Surface erosion: Water penetration rate is less than rate of transformation to water soluble product (hydrophobic)

Question 5

*List currently available degradable materials and describe applications and properties.
1. Polylactic and polyglycolic acid

Answer 5

Polylactic and polyglycolic acid are both synthetic polymers.

• PGA: the simplest linear aliphatic polyester.
• PGA/PLA: PLA is hydrophobic, therefore limits water uptake.
• PLA: lactic acid is a chiral molecule, there exists D-PLA and L-PLA, with L-PLA most commonly used.
  
  • D,L-PLA amorphous used for drug delivery
  • L-PLA semi-crystalline used for bone pins

Properties include: biocompatible, biodegrade to normal body constituents (lactic acid and glycolic acid), minimal inflammatory response, and degradation controlled by variation of molecular weight, lactide/glycolide ratio, and polymer crystallinity.

Used in: sutures, adsorbable meshes, vascular grafts, artificial nerve grafts, bone repair.

Question 6

*Describe factors which influence the degradation rates of common polymeric biomaterials.

Answer 6

For polylactic polymers:
- Molecular weight
- Crystallinity: crystalline PLA degrades more slowly than amorphous poly (DL-lactide). Fluid adsorption retarded in crystalline phase.
- Copolymerisation: Copolymers tend to be amorphous, homopolymers are crystalline.
- Molar ratio of copolymers: e.g. with PLA/PGA copolymers, methyl groups of repeating lactic acid units protect carbonyl groups of glycolic acid units from tissue fluid, limiting auto catalysis of degradation. Copolymers rich in lactic acid generally degrade more slowly than copolymers rich in glycolic acid units.
- Chain sequence of PLA and PGA: LLLLL-GGGGG-LLLLL block copolymer
LLGLGLGGLLGLGLG random copolymer