Biocompatibility

1
Biocompatibility

• Filip Ilievski
• DMSE, MIT
• 5.22J Production/Engineering

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Overview

• Biomaterials and rules of selection
• Basic materials science
• Surface properties influencing host response
• Modes of failure of biomaterials
• Look at stainless steel and cobalt chromium
  alloys
• Conclusions
• Questions

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What are Biomaterials?

• A nonviable material used in a medical device,
  intended to interact with biological systems.

Williams, 1987
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What governs materials choice?

• Historically
• Bulk properties matched to those of natural
  organs
• Mechanical (ex. modulus)
• Chemical (ex. degradation)
• Optical (ex. whiteness)
• Ability to process
• Federal Regulations

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What governs materials choice? (cont.)

• Today
• Rational design of biomaterials based on better
  understanding of natural materials and their
  material/biological organism interface
• Adoption of the Materials Engineering Paradigm

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Materials Engineering Paradigm
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What is Structure?

• Primary Chemical Structure
• Primary Bonds
• Secondary Bonds
• Higher Order Structure (1-100nm)
• 3D arrays of atoms or molecules
• Microstructure (1um)
• Crystal Grains

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Higher Order Structure
BCC
FCC
webelements.com
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Microstructure
Lehigh U Institute for Metal Forming
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Surface Properties

• Wettability
• Roughness
• Electrical Charge
• Crystallinity
• Composition
• Mobility

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Surface Energy

• Surface tension, g, is the work required to
  create unit surface area at constant T, P and
  composition.
• Minimize Gibbs free energy
• g(metals) gt g (oxides) gt g (water) gt
  g(organics)

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Surface Phenomena

• Adsorption of a species from environment
• Surface segregation of a species from bulk
• Surface reconstruction
• Surface reaction

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Oxidation of Metals
xM ½y O2 ? MxOy
Metal Oxide
O2
O2
O2
Metal alloy
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Acid/Base Reactions on Oxides
M2 acts as an electron pair acceptor for oxygen
lone pairs
H2O cleavage with H transfer to surface basic
O-2 site OH- coordination with M2
O2- M2 O2- M2
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Aqueous Corrosion of Metals

• Why does it happen?
• Mz diffusion will ALWAYS occur
• Oxide may dissolve
• What is it?
• Corrosion is the destructive result of chemical
  reaction between a metal or metal alloy and its
  environment, and involves an electronic charge
  transfer (i.e. and electrochemical reaction)

Ratner et. al.
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Corrosion reactions

• Typically, metal surface acts both as anode and
  cathode in different regions.
• Anodic reaction
• M ? Mz ze-
• Cathodic reactions
• O2(dissolved) 4H 4e- ? 2H2O
• 2H 2e- ?H2?
• O2(dissolved) 4H2O 4e- ? 4OH-

acidic

basic
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In vivo environment

• pH 7.4
• T 37oC
• Anions Cl-, PO3-, HCO3-
• Cations Na, K, Ca2, Mg2

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Pitting corrosion of SS
Fe2 ? Fe3 e- O2 4H2O 4e- ? 4OH-
Fe ? Fe22e- Fe22Cl-2H2O ? Fe(OH)2 2HCl
Fe3 OH- ? Fe(OH)3
(Fe, Cr)2O3
Fe-9Cr
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Other types of corrosion

• Crevice corrosion
• Fretting corrosion (mechanical damage)
• Intergranular corrosion (2 phases)
• Microbiological corrosion
• Stress corrosion cracking

Ratner, et. al.
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Stainless steel - composition

• 316L (bar, wire, sheet, strip..)
• Fe 60-65
• Cr 17-19
• Ni 12-14
• 2-3 of
• Mn max 2
• Cu max 0.5
• C max 0.03
• N max 0.1
• P max 0.025
• Si max 0.75
• S max 0.01

Ratner, et. al.
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SS microstructure

• Cr is added to permit development of passivating
  strongly adherent Cr2O3
• Cr stabilizes the BCC, weaker than FCC
• Ni added for austenitic phase stabilization
• Low carbon because Cr23C6 can precipitate at
  grain boundaries, depleting Cr from neighbouring
  grains.

Ratner, et. al.
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Microstructure affects corrosion
Lehigh U Institute for Metal Forming
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SS mechanical properties

• Annealed
• Youngs modulus 190 GPa
• Yield strength 331 MPa
• Tensile strength 586 MPa
• Fatigue 241-276 x 107 cycles
• 30 Cold worked
• Youngs modulus 190 GPa
• Yield strength 792 MPa
• Tensile strength 930 MPa
• Fatigue 310-448 x 107 cycles

Ratner, et. al.
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Co-Cr alloys and composition

• F75 (Co-Cr-Mo), F799 (Forged Co-Cr-Mo)
• Co 58.9-69.5
• Cr 27-30
• Mo 5-7
• lt1 Mn, Si, Ni, Fe, C
• F90 (Co-Cr-W-Ni, Wrought Co-Cr)
• Co 45-56
• Cr 19-21
• W 14-16
• Mo 9-11
• Fe max 3
• lt1 of Mn, C, P, Si, S

Ratner, et. al.
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Co-Cr alloys continued

• F562 (Co-Ni-Cr-Mo-Ti, Biophase)
• Co 29-38.8
• Ni 33-37
• Cr 19-21
• Mo 9-10.5
• lt1 Ti, Si, S, Fe, Mn

Ratner, et. al.
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Co-Cr microstructure

• Strongly influenced by processing
• Investment casting
• Powder isotactic pressing
• Cold and hot forging
• Annealing

Ratner, et. al.
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Co-Cr mechanical properties

• F75 as cast or annealed/F90 annealed
• Youngs modulus 210 GPa
• Yield strength 445-517 MPa
• Tensile strength 655-889 MPa/ 951-1220MPa
• Fatigue 207-310 x 107 cycles /NA
• F562 cold forged
• Youngs modulus 232 GPa
• Yield strength 1500 MPa
• Tensile strength 1795 MPa
• Fatigue 689-793 x 107 cycles

Ratner, et. al.
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Conclusions

• Processing ? Structure ? Properties ? Application
• Effects of structure and surfaces on properties
  of materials
• A wide selection of materials available
• Future work biocompatibility of each of these
  alloys

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References

• B. Ratner,A. Hoffman, F. Schoen and J. Lemons
  An Introduction to Materials in Medicine,
  Academic Press 1996
• C. Clerc, M. Jedwab, D. Mayer, P. Thompson, J.
  Stinson, Assessment of Wrought ASTM F1058 Cobalt
  Alloy Properties for Permanent Surgical Implants,
  Wiley Interscience 1997
• I. De Scheerder, J. Sohier, E. Verbeken,L. Froyen
  and J. Van Humbeeck, Mat.-wiss. u. Werkstofftech.
  32, 142-148 (2001)
• David R. Haynes, Tania N. Crotti, Michael R.
  Haywood, CCC 0021-9304/00/020167-09 Wiley
  Interscience (2000)
• Hallie E. Placko, Stanley A. Brown, Joe H. Payer,
  CCC 0021-9304/98/020292-08 Wiley Interscience
  (1998)

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Questions