Polymer Composites 5

Question 1

Identifying a need

Answer 1

In the 1970s, metallic implants were recognized as not fully adequate for bone replacement due to high modulus (stress shielding)
- synthetic composite system may better match the mechanical properties of bone as bone is a composite material through the association of HA with collagen

Question 2

Stress shielding

Answer 2

the reduction in local stresses in an adjacent bone due to a load bearing implant taking a portion of the tensile load or bending moment
- leads to an adaptive remodeling process of bone that is akin to osteoporosis

Question 3

Device Design: bioactive composites as bone analogue devices

Answer 3

HA particulate reinforced HDPE (high density polyethylene)

• HA is chemically similar to inorganic component of bone, bioactive, stiff, strong and brittle
• HDPE is a non degradable thermoplastic, ductile, tough, bioinert
• path to clinic simplified as both materials are already used clinically

Question 4

material synthesis and testing

Answer 4

range of composites of HA and HDPE with HA volume fractions of 0.1-0.6 for a modulus of 1-8GPA and failure strain of >100-3%

Question 5

Fabrication - flow diagram of fabrication HA/HDPE composites

Answer 5

raw materials (HA and HDPE) –> mechanical mixing at HA Vf of 0.1-0.6 –> compounding (Betol BTS40L, co-rotating twin screw extruder, 210-250C) –> HA/HDPE composite wire formation, cooled in distilled water, room temp –> powdering, centrifugal mill, liquid nitrogen temp –> compression moulding, 205C, step wise increase of pressure –> composite plates

Question 6

Action of compounding twin screw extruders

Answer 6

• blend components are def into the feed throat
• motion of screw against barrel wall heats polymer wile conveying it to higher shear section of screw
• mixing occurs within intermeshing screw elements
• vents downstream to allow moisture, air and degradation products to escape

Question 7

co-rotating twin screw extruders

Answer 7

• barrel with a figure 8 profile encasing 2 screws that rotate in same direction
• mixing arises from high shear introduced by movement of the screw again barrel and by screws relative to one another

Question 8

Sterilization

Answer 8

radiation, which is an air and surface disinfectant that kills bacteria (some viruses and fungi) and leaves no residue.

Question 9

Device testing

Answer 9

• upper limit of incorporating HA was 0.4 Vf
• fracture toughness comparable to that of bone
  HA particle size of 4um was optimum for mechanical fixation of filler and matrix
• 0 toxicity in vitro
• less than 0.2 Vf were inert in vivo –> fibrous encapsulation was formed
• 0.4HA Vf was confirmed as bioactive
  –> designated as HAPEX TM.

Question 10

Regulatory

Answer 10

• HAPEX considered for non-load bearing applications
• 18 patients received implants with good outcome
• licensed to commercial manufacturer as a bone subtitute wih primary application in ear nose and throat surgery

Question 11

Clinical Use

Answer 11

• hearing is transferred from the eardrum to the inner ear by three bones; the hammer, anvil and stirrup
• the middle ear devices can replace all three bones
• composite offers better advantage than bioactive glass as it can be shaped in the operating theater

Question 12

explant analysis

Answer 12

implant extrusion (failure) was 5.3%
- hearing success rate was 56.8% in 233 patients