Materials Selection and Design

1
Materials Selection and Design Ex
-Bioapplications Hip Replacement
Recall that for selection one must establish a
link between materials and function, with shape
and process playing also a possibly important
role.
DESIGN CONCERNS Function - support a load
without non-failure. Objectives - make cheaply,
light weight, bio-compatible, reduce friction
and wear. Constraints - shape, match modulus of
elasticity of bone. What is non-negotiable? What
is negotiable?
2
Bio-Materials Artificial Hip Replacement
Anatomical overview of a hip joint
Joint is susceptible to fracture. Hip can get
osteoarthritis, where small lumps of bone for on
the rubbing surfaces causing pain.

• Bone tissue consists of soft and strong
  protein collagen and brittle apatite, with
  density of 1.6 to 1.7 g/cm3.
• Mechanical properties of bone are highly
  anisotropic (axial and radial).

3
Artificial Hip Replacement

• Mechanical properties of bone are highly
  anisotropic (axial and radial).

• modulus of elasticity of bone 2-30 GPa,
  depending on porosity.
• Match bone, or else bone will die around implant
  due to lack of use.

4
Artificial Hip Replacement

• There are four components to the artificial hip
  femoral stem, ball, acetabular cup, and possible
  fixation agent (e.g., epoxy, but highly damaging
  - more and more not used).

• Property constraints on materials are stringent
  because of chemical (corrosive, biocompatibility)
  and mechanical (compatibility) needs.

5
Artificial Hip Replacement

• Biocompatibility produce minimum degree of
  rejection.
• Products resulting from reactions with body
  fluids must be tolerated in surrounding tissue
  (toxicity).

• Body fluid is warm 1 wt NaCl plus other salts
  and organic compounds.
• Corrosion in metals this leads both to uniform
  corrosion and to crevice attack and pitting, AND,
  to stress-corrosion cracking and fatigue.

Rivet joint in aqueous solution
Pitting in 304 SS plate by acid-chloride solution.
Stress-corrosion cracking in brass
(intergranular).
Electrochemical corrosion - crevice corrosion.
6
Artificial Hip Replacement

• Corrosion effects
• Mechanical behavior plus reduce wear and
  friction.

• Max. tolerable corrosion rate for implant metal
  alloys is about 2.5 x 10-4 mm/yr, or 0.01
  mils/yr.
• (See chapt. on corrosion.)
• Stem
• minimum YS (500 MPa) and UTS (650 MPa).
• min. ductility 8EL.
• min. fatigue 400 MPa at 107 cycles.
• elasticity 2-30 GPa, depends on porosity.

• Very hard material will reduce wear.
• Frictional forces should be considered to reduce
  wear.
• Three other major factors density (light
  weight), property reproducibility
  (manufacturing), and cost (reasonable).
• Last a long time! (No additional operations to
  replace again.

7
Artificial Hip Replacement

• Corrosion and Mechanical properties.

• Metal Ti-6Al-4V is most biocompatible, but
  stiffer so carriers more load.
• 316 SS and Ti alloys not magnetic - important
  for MRI use, e.g.
• Ceramics, like Alumina, too brittle but are wear
  resistant and lower friction stress --- thus use
  as coating (hydroxyapatite) as the porous ceramic
  scaffolding allows bone to grow into its pores
  (integrated systems).
• CUP polyethylene, with low friction and good
  wear-resistance.

8
Always Consider the BIG PICTURE

• Engineering design and application requires
  multifaceted approach.
• What you do as an engineering impacts humanity!
• help communities in need,
• reducing labor needs,
• increasing productivity,
• extend exploration,
• enhance safety,
• improve the human condition

• Popular Internet Photo.
• Reality is composite picture from 4 photos made
  by underwater photographer R.A. Clevenger (1999).
• Two halves of the iceberg are from one taken in
  Alaska and one in Antarctica (neither is
  underwater).
• Underwater part is the background taken off the
  coast of California.
• The sky is the last component.
• GREAT PICTURE, anyway!

9
SUMMARY

• Materials for biological implants selected based
  on
• - biocompatibility, mechanical match,
  corrosion resistance, friction and wear
  resistance, toxicity of local reaction chemistry,
  longevity, ease of installation, etc.
• Materials for other biological applications
  must be similarly considered
• - designer proteins for drug-delivery
• toxicity, site-specific delivery,
  containment of pharmaceutical,
• mechanical properties (rheology).
• BIG PICTURE considerations Teams
• Engineered materials for biological applications
  require materials, engineering, biology,
  chemistry, surface science, reaction chemistry,
  toxicology, etc.