Metals as biomaterials - Aalto · PDF fileMetals 1/ 55 Emilia Peltola, DSc Properties and challenges ... Can not be hardened via heat treatment and do not weld to a high standard.

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Metals 1/ 55

Emilia Peltola, DSc

Properties andchallenges

Commonly used metalsStainless steel

Titanium

Cobalt alloys

Smart materialsShape memory metals

Magnetostrictive materials

Metals as biomaterials

Emilia Peltola, DSc

21.9.2015

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Titanium

Cobalt alloys



Contents

Properties and challenges

Commonly used metalsStainless steelTitaniumCobalt alloys

Smart materialsShape memory metalsMagnetostrictive materials

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Titanium

Cobalt alloys



Brainstorming I

What kind of properties do metals have?

To what kind of implants are metals best suitable for?

http://presemo.aalto.fi/biomaterials/

http://presemo.aalto.fi/biomaterials/

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Tensile properties of metals

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Cobalt alloys



Applications

Biomet Zimmer

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Brainstorming II

I Why is biological environment particularly hostilefor metals?

I What is the consequence of the hostileenvironment?

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Cobalt alloys



Ion concentrations in body

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Cobalt alloys



Corrosion

This etched metallograp-hic micrograph demon-strates the pitting corro-sion of stainless steel.

Extensive corrosion onthe titanium stem of amodular hip prosthesis.

[Ratner et al., 2013]

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Cobalt alloys



Failure of orthopaedic implants


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Requirements for metallic implantmaterials

I Must be corrosion resistantI Mechanical properties must be appropriate for

desired applicationI Areas subjected to cyclic loading must have good

fatigue properties

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Cobalt alloys



Commonly used metals

Stainless steel Titanium CoCr alloys

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Stainless steel

Stainless steels were the first metals to be used inorthopaedics in 1926.

Steel used in the hip implants until the end of 1970s

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Stainless steel

Stainless steel

2015

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Stainless steelStainless steel

Austenitic (FCC)Can be formed and welded with successful results.

Martensitic (FCT)Can be tempered and hardened and are thus highly useful insituations where the strength of the steel is more important than itsresistance to corrosion.

Ferritic (BCC)Can not be hardened via heat treatment and do not weld to a highstandard.

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Stainless steel

α ferrite; σ martensite; γ austenite

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Austenitic steel

Specially AISI 316L is used in implants (316 =Mo-containing ja L = low carbon)

I Ni stabilises the austenitic microstructure of steel(note: allergic reactions)

I Cr-containing steel produces a thin and relativelydurable passivating oxide layer

I Mo has a strong positive effect on pitting andcrevice corrosion resistance in chloride-containingsolutions

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Chromium oxide passivating film

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Grain boundaries

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Role of Mo

[Hospadaruk and Petrocelli, 1966]

Role of Mo

[Hospadaruk and Petrocelli, 1966]

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Stainless steelRole of Mo

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Steel: pros & cons

+ Good corrosion and fatigueresistance in short-termapplications

+ Low cost+ Easy to be machined- Tend to be corroded in

long-term applications- High modulus (stress

shielding effect)- Ni and Cr allergy

Typical applications: Temporary implants such asfixation screws and plates

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Cobalt alloys



Titanium

I More flexible than SS and closer to the stiffness ofbone

I Good behaviour under fretting corrosion andfatigue although not specially good with respect towear

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Ti O phase diagram

What can you tell about the microstructure of titanium?

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Cobalt alloys



Ti

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Ti alloying

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Mechanical properties

Alloy Micro-structure

E (GPa) YS (MPa) UTS(MPa)

cpTi α 105 692 785Ti-6Al-4V α/β 110 850–900 960–970AISI 316L – 205–210 170–750 465–950Co–Cr–Mo – 220–230 275–1585 600–1785Bone – 10–40 90–140


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Ti crystal structure

α

+ corrosion resistance, biocompatibility+ weldability- poor forgeability, low strength

α− β+ can be strengthened by heat treatment

β

+ high hardenability+ good ductility and toughness- high density- low creep strength- low tensile ductility in the aged state- low wear resistance

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Ti-6Al-4V

I Al is added to the alloy as α-phase stabilizer andhardener due its solid solution strengtheningeffect

I V stabilizes ductile β-phase, providing hotworkability of the alloy

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Titanium

I cpTi has the best corrosion durabilityI Ti6Al4V has improved strength

Ti and its alloys tolerate pit and crevice corrosion butare sensitive for fretting corrosion.

I e.g. under the heads of screwsI this is the most significant limitation for titanium as

a biomaterial

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Cobalt alloys



Ti applications: joints

I since the 1970s

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Cobalt alloys



Ti applications: dental

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Osseointegration

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Osseointegration

Per-Ingvar Branemark in 1952

Osseointegration

Per-Ingvar Branemark in 19522015

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TitaniumOsseointegration

• First Step: formation of a carbonated hydroxyapatite onsurface via ion exchange

• Second Step: collagen fibers of host bone insert intothe carbonated apatite layer

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Ti applications: vascular implants

I Heart valve (Starr-Edwards1961)

I Packaging of pacemakerI Artificial heartsI Stents (nitinol)

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Cobalt alloys



Ti applications: others

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Ti: pros & cons

+ Light+ Greatest corrosion resistance+ Excellent biocompatibility+ Relatively low Young’s modulus- Lower shear strength- Low wear resistance- Expensive- High modulus (stress shielding effect)

Typical applications: stem of hip prostheses, dentalscrews (permanent implant)

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Cobalt alloys



Cobalt alloys

Alloys contain 19-21%chromium, 4.5-7% molybdenumand carbon (carbides: MoC, WC)

Mechanical properties aredependent on processing

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Cast and wrought cobalt-based alloys

Alloys E(GPa)

UTS (MPa) 0.2% σy(MPa)

F75/Cast, ann. 210 650–890 450–520F75/P/M HIP 250 1280 840F799/forged 210 1400–1590 900–1030F90/ann. 210 950–1220 450–650F90/44% cold w. 210 1900 1610F562/forged 230 1210 960–1000F562/cold w., aged 230 1800 1500F563/ann. 230 600 280F563/cold w. 230 1000–1310 830–1170F563/cold w., aged 230 1590 1310F1058 wire 230 1860–2280 1240–1450

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Cobalt alloying

I Cr increases corrosion resistanseI Mo is added to produce finer grains which results

in higher strength after casting or forging

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Cobalt alloys



CoCr: pros & cons

+ Long-term corrosion resistance+ Super fatigue and wear resistance+ Biocompatibility- Difficult to machine and thus expensive to process- High modulus (stress shielding effect)- Ni and Cr allergy

Typical applications: Permanent implants

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Cobalt alloys



Brainstorming III

I Alloys are increasingly used versus pure metalsfor biomedical applications. What two majoradvantages do they impart?

I Bone compatibility of Ti and CoCr alloys isimproved by making porous implants. Why arethere no porous stainless steel implants?

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Cobalt alloys



Strength of pure metals versus alloys

A pure metal

lateralforce→

Dislocation occurs especially at grain boundaries

An alloy

Packing of two different sizes of metal elementsprevents physical dislocation of the lattice structure

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Smart materials

I Shape Memory (Nitinol)I Magnetostriction (Terfenol-D)I Piezoelectric

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Cobalt alloys



Shape memory metals


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Shape memory metals

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Cobalt alloys



Nitinol stent

[Chen and Thouas, 2015]

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Magnetostriction video

https://www.youtube.com/watch?v=MmopHPp4dQA

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Bone lengthening now...

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Bone lengthening in the future

*Synoste is a start-up from Aalto

http://synoste.com/

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Future metallic biomaterials

Biodegradable metalsI Mg, MgAl, FeMn

Self-cleaning surfacesI Ag, ZnO, CuO, TiO2

NanoparticlesI drug delivery

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Cobalt alloys



Muddy cards

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Next time: Ceramics

Properties of ceramics

I Inert ceramicsI Nearly inert, porous ceramicsI Bioactive ceramicsI Absorbable/degradable ceramics

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Recommended watching

Charnley’s hip replacement technique turns 50

Smart materials (3 of 5): shape shifting material, drugdelivering nano particles

Osseointegration

https://www.youtube.com/watch?v=UDHswbA5aRk

https://www.youtube.com/watch?v=i6n8cpLKzHE

https://www.youtube.com/watch?v=i6n8cpLKzHE

https://www.youtube.com/watch?v=4YM7TkAM7Fo

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References

Chen, Q. and Thouas, G. A. (2015).Metallic implant biomaterials.Mater Sci Eng R, 87:1–57.

Hospadaruk, V. and Petrocelli, J. V. (1966).The pitting potential of stainless steels in chloride media.J. Electrochem. Soc., 113:878–883.

Ratner, B. D., Hoffman, A. S., Schoen, F. J., and Lemons, J. E., editors (2013).Biomaterials Science An Introduction to Materials in Medicine.Elsevier.

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Course book

I Chapter I.2.3 - Metals: Basic PrinciplesI Chapter II.4.4 - Degradative Effects of the

Biological Environment on Metals and CeramicsI Chapter II.5.6 - Orthopedic ApplicationsI Chapter II.5.7 - Dental Implantation

Metals as biomaterials - Aalto · PDF fileMetals 1/ 55 Emilia Peltola, DSc Properties and challenges ... Can not be hardened via heat treatment and do not weld to a high standard.

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