Bio Metals

7/22/2019 Bio Metals

1/23

ME4253Biomaterials Engineering

Biometals

Thian Eng San, Assistant Professor

Department of Mechanical Engineering9 Engineering Drive 1Singapore 117 576


2/23

Outline

Reading List

Introduction

Fabrication

Bioinert Metals

Bioactive Metals

Mechanical Properties

Stress Shielding

Corrosion

Applications

Summary


3/23

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)


4/23

Introduction

Inorganic materials

Metallic bonding

Polycrystalline

Bioinert

Usually used in a form of alloy

Achieve required properties

Stainless steel

Cobalt alloy Titanium alloy


5/23

Factors influencing materials properties

Microstructure (Grain size)

Porosity

Grain size distribution

Applications of biometals

Orthopaedic implants and fixation

Orthodontic implants

Internal electrical devices

Introduction


6/23

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


7/23

Fabrication

Forming

Forging

Extrusion

Casting

Sand

Die

Others

Powder Metallurgy


8/23

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


9/23

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


10/23

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


11/23

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


12/23

Fabrication

Compaction of powdered metal

Heat treatment of green part to produce a moredense piece

Powder Metallurgy


13/23

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


14/23

316L Stainless Steel

Relatively cheap

Ease of manufacturing

Mainly used in orthopaedic fixation

Bioinert Metals


15/23

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


16/23

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


17/23

Titanium 6-4 Alloy

Significantly more expensive than 316Lstainless steel

Mainly used in orthopaedic implantsand fixation

Photo extracted fromemedicine.medscape.com

Bioactive Metals


18/23

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


19/23

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


20/23

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


21/23

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)


22/23

Applications

Total Hip Replacements

Femoral stem (Ti6Al4V)

Pelvis

Femur

Femoral head (Co-Cr)

Acetabular cup liner

(UHMWPE)

Bone cement (PMMA)


23/23

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

Documents

Bio Metals