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Metallic Biomaterials. Metals Are Crystalline. Body-Centered Cubic.

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metals are crystalline
Metals Are Crystalline

Body-Centered Cubic

The body-centered cubic (bcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells. Source: W. G. Moffatt, et al., The Structure and Properties of Materials, Vol. 1, John Wiley & Sons, 1976.

CHEE 340

metal bonding
Metal Bonding
  • The electrons in metals are mobile and surround a core of cations. This gives rise to their high electrical conductivity.

CHEE 340

crystals and grain formation

Crystals

Grain Boundary

Grain or Crystalline Structure

Crystals and Grain Formation

CHEE 340

product manufacture
Product Manufacture
  • There are 4 main methods of metal product manufacture:
  • machining
  • melt casting
  • forging
  • hot isostatic pressing

CHEE 340

influence of manufacturing process
Influence of Manufacturing Process

Casting Defect

Polished-etched view of a cast ASTM F75 femoral hip stem. Note dendrites and large grains

In vivo fracture initiated from an inclusion formed during the casting process

From : H. Alexander et al., Chapter 2, Biomaterials Science, BD Ratner et al., Academic Press, 1996.

CHEE 340

metallic biomaterials7
Metallic Biomaterials
  • There are 3 main groups of metals used as biomaterials:
    • stainless steels
    • Co-based alloys
    • titanium-based alloys

CHEE 340

design considerations
Design Considerations
  • typically want to match mechanical properties of tissue with mechanical properties of metal
  • have to consider how the metal may fail in vivo
    • corrosion
    • wear
    • fatigue
  • need to consider cost

CHEE 340

corrosion
Corrosion
  • The extra-cellular environment is a chemically aggressive space. Metallic biomaterials are good conductors in an electrolyte solution, leading to galvanic corrosion.

CHEE 340

corrosion11
Corrosion

CHEE 340

mechanisms of corrosion
Mechanisms of Corrosion
  • Crevice Corrosion
  • Pitting Corrosion
  • Intergranular Corrosion

CHEE 340

mechanisms of corrosion13
Mechanisms of Corrosion
  • Fretting Corrosion Stress Corrosion Cracking

CHEE 340

mechanisms of corrosion14
Mechanisms of Corrosion

Contribution of biological environment

From : S.H. Teoh, International Journal of Fatigue 22 (2000) 825–837

CHEE 340

slide15
Wear
  • The effects of wear are most predominant in joint prostheses.
  • There are two types of wear :
  • Interfacial Wear
  • Fatigue Wear

CHEE 340

fatigue
Fatigue
  • Recall that fatigue is progressive failure of a material due to the application of cyclical stresses below the ultimate stress of the material causing crack propagation.
  • Crack usually starts at a stress concentrator or stress riser.
  • Methods for reducing fatigue failure :

CHEE 340

fatigue17
Fatigue

CHEE 340