Polymer Composites 5 Flashcards

1
Q

Identifying a need

A

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

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2
Q

Stress shielding

A

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

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3
Q

Device Design: bioactive composites as bone analogue devices

A

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
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4
Q

material synthesis and testing

A

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%

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5
Q

Fabrication - flow diagram of fabrication HA/HDPE composites

A

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

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6
Q

Action of compounding twin screw extruders

A
  • 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
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7
Q

co-rotating twin screw extruders

A
  • 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
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8
Q

Sterilization

A

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

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9
Q

Device testing

A
  • 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.
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10
Q

Regulatory

A
  • 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
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11
Q

Clinical Use

A
  • 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
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12
Q

explant analysis

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