bio metals
TRANSCRIPT
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ME4253Biomaterials Engineering
Biometals
Thian Eng San, Assistant Professor
Department of Mechanical Engineering9 Engineering Drive 1Singapore 117 576
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Outline
Reading List
Introduction
Fabrication
Bioinert Metals
Bioactive Metals
Mechanical Properties
Stress Shielding
Corrosion
Applications
Summary
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Reading List
Ratner BD, Hoffman AS, Schoen FJ, Lemons JE.Biomaterials Science: An Introduction to Materials inMedicine. Academic Press, 2nd Edition, c2004. Chapter
2, Section 2.9 (CL RBR: R857Mat.Bi2004)
Teoh SH. Engineering Materials for Biomedical
Applications. World Scientific, c2004. Chapter 2 (CL RBR:
R856Teo2004)
Callister WD, Rethwisch DG. Materials Science andEngineering. World Scientific, 8th Edition, c2004.Chapter 11, Sections 11.4, 11.5, 11.6 (CL RBR:
TA403Cal2011)
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Introduction
Inorganic materials
Metallic bonding
Polycrystalline
Bioinert
Usually used in a form of alloy
Achieve required properties
Stainless steel
Cobalt alloy Titanium alloy
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Factors influencing materials properties
Microstructure (Grain size)
Porosity
Grain size distribution
Applications of biometals
Orthopaedic implants and fixation
Orthodontic implants
Internal electrical devices
Introduction
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Material undergoes an elastic
deformation (blue) initially until it
reaches a point - yield stress, wherebysecond stage of deformation called
plastic deformation (red) dominates.
Deformation will proceed till the metal
fractures. Before fracture, the materialwill reach its maximum (ultimate)
tensile stress
Plastic DeformationNon-reversible
Fracture
TensileStrength
YieldStress
Recoverable
Elastic Strain
x
Stress Strain Graph
Introduction
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Fabrication
Forming
Forging
Extrusion
Casting
Sand
Die
Others
Powder Metallurgy
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Fabrication
A force is applied to both
top and bottom die halves
Hot metal is deformed
in the cavity
Forging
Image extracted from doitpoms.ac.uk
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Fabrication
A force is applied to a ram
Metal is forced through a
die orifice
Reduction in x-sectionalarea
Extrusion
Image extracted from Materials Science and Engineering
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Fabrication
Sand is used as the mold material
A two-piece mold is formed by packing sandaround a pattern
A gating system is used to allow the flow ofmolten metal into the cavity
Sand Casting
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Fabrication
A two-piece permanent mold is clamped together
to form the desired shape
Molten metal is forced into the mold cavity under
pressure and at high speed
The molds are opened and the cast piece isejected once solidification is completed
Die Casting
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Fabrication
Compaction of powdered metal
Heat treatment of green part to produce a moredense piece
Powder Metallurgy
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Bioinert Metals
316L Stainless Steel
Composition
Fe (bal.)
Cr (16-18 %)
Ni (10-14 %)
Mo (2-3 %)
Mn (2 %)
C (0.03 %)
Excellent corrosion resistant
Abundantly available
Photo extracted from
emedicine.medscape.com
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316L Stainless Steel
Relatively cheap
Ease of manufacturing
Mainly used in orthopaedic fixation
Bioinert Metals
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Cobalt Chromium Alloy
Composition
Co (bal.)
Cr (26 %)
Mo (5 %)
Very excellent corrosion resistant
Better wear resistance than 316L stainlesssteel and titanium-based alloy
Difficult to machine due to its relatively low ductility
More expensive than 316L stainless steel
Mainly used in orthopaedic implants
Photo extracted fromemedicine.medscape.com
Bioinert Metals
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Titanium 6-4 Alloy
Composition
Ti (bal.)
Al (6 %)
V (4 %)
Very excellent corrosion resistant
Excellent strength to weight ratio
Less stiffer than 316L stainless steel and cobalt-chromium alloy means stress shielding is minimised
Poorer wear resistance than 316L stainless steel andcobalt-chromium alloy
Bioactive Metals
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Titanium 6-4 Alloy
Significantly more expensive than 316Lstainless steel
Mainly used in orthopaedic implantsand fixation
Photo extracted fromemedicine.medscape.com
Bioactive Metals
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Mechanical Properties
Mechanical Properties of 316L SS, Co-Cr and Ti6Al4V
E (GPa) Y (MPa) UTS (MPa) % Elongation
316L SS 200 210-1200 200-1200 ~10
Co-Cr 230 430-1000 430-1000 ~10
Ti6Al4V 110 780-1100 800-1100 ~10
Cortical Bone 7-30 n/a 50-150 1-3
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Stress Shielding
A reduction in bone density due to the removal ofnormal stresses from the host bone by animplanted prosthesis
Governed by Wolffs Law
In healthy human, bone will remodel itself in responseto the applied load
If loading on the bone increases, it will become
stronger over time due to the continued stimulus thatis required to maintain bone mass
Stress Shielding
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Corrosion
Galvanic Corrosion
Occurs when two dissimilar metals/alloys areelectrically connected in the presence of an electrolyte
Metal/alloy having the more negative potential
becomes the anode and corrodes preferentially
Metal/alloy having the less negative potential becomes
the cathode
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Corrosion
Galvanic Corrosion
Rate of Corrosion
Potential Difference
The higher the potentialdifference, the higher rate
of corrosion
PlatinumGoldGraphiteTitaniumSilverCopperTinLead316 Stainless SteelIron/Steel
Aluminum AlloysCadmiumZincMagnesiummo
reanodic
(active)
morecat
hodic
(inert)
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Applications
Total Hip Replacements
Femoral stem (Ti6Al4V)
Pelvis
Femur
Femoral head (Co-Cr)
Acetabular cup liner
(UHMWPE)
Bone cement (PMMA)
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Summary
Describe the class of biometal materials, itsadvantages and disadvantages when used asimplant materials
Describe the engineering design principles foreffective functioning of implants, in terms ofmaterial properties
Describe the processing routes for manufacturingof biometals
Describe the phenomenon of stress shieldingeffect and galvanic corrosion