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SAVIGNY Pascale & GIROUD Elodie 3 rd December 2002 Metallic Biomaterials Corrosion – Titanium, Stainless steels, Chromium Cobalt alloys, Amalgam – Orthopaedic & Dental Applications Metallic Biomaterials – december 2002 page 1
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Page 1: metallic biomaterials

SAVIGNY Pascale & GIROUD Elodie

3rd December 2002

Metallic Biomaterials

Corrosion – Titanium, Stainless steels, Chromium Cobalt alloys, Amalgam –

Orthopaedic & Dental Applications

Metallic Biomaterials – december 2002 page 1

Functional materials 4H1609

Course PM Version 1

Rolf Sandström

Page 2: metallic biomaterials

Summary

Introduction Page 3

1. Corrosion Page 5

1.1. Corrosion of metallic implant Page 5

1.2. Electrochemical aspects - mechanism Page 5

1.3. Pourbaix diagram Page 6

1.4. Rate of polarisation and polarization curves Page 7

1.5. Types of corrosion Page 7

1.6. Protection methods Page 9

1.7. Passivation of metallic materials Page 9

1.8. Conclusion on corrosion Page 10

2. Properties and applications of the most used metallic biomaterials Page 11

2.1. Stainless steels Page 11

2.2. CoCr Alloys Page 12

2.3. Ti Alloys Page 13

2.4. Dental metals Page 14

2.5. Other metals Page 14

2.6. Conclusion Page 15

3. Titanium and Ti alloys as Biomaterials Page 17

3.1. Background Page 17

3.2. Applications Page 21

References Page 26

Metallic Biomaterials – december 2002 page 2

Page 3: metallic biomaterials

Introduction

The definition of a biomaterial covers a broad area.  In fact, any natural or synthetic

material that interfaces with living tissue and/or biological fluids may be classified as a

biomaterial. 

However, certain physical, chemical, and mechanical characteristics render some materials

more desirable than others for biological application, depending on its intended use in the

body.  For example, the material for a bone implant must exhibit great compressive strength,

while the material for a ligament replacement must display far more flexibility and tensile

strength.  In all cases, however, a biomaterial must perform compatibly with the body.  In

other words, the biocompatibility and in some cases, bioactivity, of the material comprise key

factors in determining whether a new graft or implant succeeds in the body.

In order to define biocompatibility, it may be easier to define what it is not, rather than

what it is. A biocompatible material disrupts normal body functions as little as possible. 

Therefore, the material causes no toxic or allergic inflammatory response when the material is

placed in vivo.  The material must not stimulate changes in plasma proteins and enzymes or

cause an immunologic reaction, nor can it leads to carcinogenic or mutagenic effects.

Bioactive materials play a more aggressive role in the body.  While a biocompatible material

should affect the equilibrium of the body as little as possible, a bioactive material recruits

specific interactions between the material and surrounding tissue.  For example, a bioactive

material can encourage tissue integration to aid in the fixation of an implant in the body.  

Many total hip implants operations today rely partially on a porous coating of Hydroxyapatite

(HA), a normal component of bone, to help permanently stabilize the stem of the implant in

the bone.  The coating encourages the ingrowth from the surrounding tissue that interlocks

within the pores much like the pieces of a puzzle lock together.  Although many current

medical procedures call for inert biocompatible materials, the increasing understanding of

tissue interaction promises many more applications for aggressive bioactive materials.

The closely packed crystal structure and metallic bonding in metals or metal alloys

render them valuable as load bearing implants as well as internal fixation devices in large part

for orthopedic applications as well as dental implants.  316 L stainless steel, titanium alloys,

and cobalt alloys when processed suitably contribute to high tensile, fatigue and yield

Metallic Biomaterials – december 2002 page 3

Page 4: metallic biomaterials

strengths; low reactivity and good ductility to the stems of hip implant devices.   Their

properties depend on the processing method and purity of the metal, however, and the

selection of the material must be made appropriate to its intended use. 

Metallic biomaterials are normally considered to be highly corrosion resistance due to

the presence of an extremely thin passive oxide film that spontaneously forms on their

surfaces.

These films serve as a barrier to corrosion processes in alloy systems that would otherwise

experience very high corrosion rates. That is, in the absence of passive films, the driving force

for corrosion for typical implant alloys (e.g., titanium-based, cobalt chromium (CoCr)–based,

and stainless-steel alloys) is very high, and corrosion rates would also be high. The properties

of these passive oxide films depend to a large extent on their structure and chemistry, which

are themselves dependent on the substrate's prior thermal, mechanical, and electrochemical

history.

Metallic Biomaterials – december 2002 page 4

Page 5: metallic biomaterials

cathode V

- +

anode

electrolyte

cations

anions

Figure   1   : Electrochemical cell

Table   1   : S tandard Electrochemical Series

1. Corrosion

1.1. Corrosion of metallic implant

Corrosion is the unwanted chemical reaction of a metal with its environment, resulting

in its continued degradation to oxides, hydroxides or other compounds. Biological fluids in

the human body contains water, salt, dissolved oxygen, bacteria, proteins, and various ions

such as chloride and hydroxide. As a result, the human body is a very aggressive environment

for metals if we want to use them as biomaterials. Corrosion resistance of a metallic implant

material is consequently an important aspect of its biocompatibility.

1.2. Electrochemical aspects - mechanism

Corrosion occurs when a metal atom becomes ionized and goes into solution, or

combine with oxygen or other species in solution to form a compound which flakes off or

dissolves. The body environment is very aggressive in terms of corrosion since it contains

chloride ions and proteins and many chemical

reactions can occur. The electrolyte, which

contains ions in solution, serves to complete the

electrical circuit. Anions are negative ions that

migrate toward the anode, and cations are positive

ions that migrate toward the cathode. At the

anode, or positive electrode, the metal oxidizes by

losing valence electrons as in the following: M

Mn+ + ne-. So the anode is always the one which

corrodes and thus has to be protected.

The tendency of metals to corrosion is based on the Standard Electrochemical Series

of Nernst potentials, shown in the table 1, which are the potentials associated with the

ionization of metal when one electrode is the standard hydrogen electrode.

Metallic Biomaterials – december 2002 page 5

Reaction E 0 (volts)LiLi+ -3,05NaNa+ -2,71AlAl3+ -1,66TiTi3+ -1,63CrCr2+ -0,56FeFe2+ -0,44

Reaction E 0 (volts)CuCu2+ -0,34CoCo2+ -0,28NiNi2+ -0,23H22H+ 0AgAg+ +0,80AuAu+ +1,68

Page 6: metallic biomaterials

Nernst equation

1.3. Pourbaix diagram

The Pourbaix diagram is a plot of regions of corrosion, passivity and immunity as they

depend on electrode potential and pH. The Pourbaix diagrams are derivated from the Nernst

equation and from the solubility of the

degradation products and the equilibrium

constants of the reaction.

The corrosion region is set arbitrarily at the concentration of greater than 10 -6 molar.

Immunity, also called cathodic protection, is defined as equilibrium between metal and its

ions at less than 10-6 molar. In this region, the corrosion is energetically impossible. In the

passivity domain, the stable solid constituent is an oxide, hydroxide, hybrid, or slat of the

metal. Passivity is defined as equilibrium between metal and its reaction products at a

concentration less than 10-6 molar.

There are two diagonal lines in the diagram. The top oxygen line represents the upper limit of

the stability of water and is associated with oxygen rich solution or electrolytes near oxidizing

materials. In the region above this line, oxygen is evolved according to 2H2O O2 + 4H+ +

4e-. In the human body, saliva, intracellular fluid, and interstitial fluid occupy regions near the

oxygen line, since they are saturated with oxygen. The lower hydrogen diagonal line

represents the lower limit of the stability of water. Hydrogen gas is evolved according to

Metallic Biomaterials – december 2002 page 6

Figures   2 et   3   : Immunity, Passivity, corrosion diagram (left) and Pourbaix diagram of Fe (right)

Page 7: metallic biomaterials

Figure   4 : Potential current curves for different metals

2H3O+ + 2e- H2 + 2H2O. Aqueous corrosion occurs in the region between these diagonal

lines. In the human body, urine, bile, the lower gastrointestinal tract, and the secretions of

ductless glands, occupy a region somewhat above the hydrogen line.

Different parts of the body have different pH values and oxygen concentrations.

Consequently, a metal which performs well in one part of the body may suffer an

unacceptable amount of corrosion in another part.

1.4. Rate of polarisation and polarization curves

The regions in the Pourbaix

diagram specify whether corrosion will

take place, but they do not determine the

rate. The rate, expressed as an electric

current density, depends upon electrode

potential which can be seen in potential

current curves (figure 4). From those

curves, it is possible to calculate the

number of ions per unit time liberated

in the tissue, as well as the depth of

metal removed by corrosion in a given time. An alternative experiment is one in which the

weight loss of a specimen of metal due to corrosion is measured as a function of time.

The rate of corrosion also depends on the other factors such as mechanical stresses that are

applied on the material. The stressed alloy failures occur due to the propagation of cracks in

corrosive environments.

But the main idea is to remind that the corrosion rate depends largely on the pH.

1.5. Types of corrosion

We need distinguish two types of corrosion: endogenous (produced or growing from

within the material) and exogenous (developed from outside of the body, which means that

there is an external origin).

a. Endogenous corrosion

It is linked to the metal which is used and it can be either uniform (in case of quite

homogenous materials) or localised when the heterogeneities are sufficiently spread to give

rise to weak zones on the metal surface. We can give different types of corrosion:

Metallic Biomaterials – december 2002 page 7

Page 8: metallic biomaterials

- Crevice corrosion: a form of localized corrosion in which concentration gradients

around the pre-existing crevices in the material drive corrosion processes. Oxygen is

consumed at the surface, the pH decreases, which causes the passive film to break

down and then corrosion is even stronger.

- Pitting corrosion: a form of localized corrosion in which pits form on the metal

surface. The passivity is locally broken by the chlorides.

- Intergranular: It’s a local depassivation on the stainless steels grain boundaries

because chromium carbide precipitation has occurred and the Cr content can be locally

lower that a minimum value (9%) to be passive. It frequently occurs in the heat-

affected zone during welding, and the resultant corrosion is called weld decay.

Localised corrosion depends on the environment (O2 content, Cl- concentration, pH,

flow rate) but also on the material itself (segregation phenomenon, presence of different phase

with many grain boundaries or inclusions) and finally on the mechanical stresses that can be

applied to the material.

b. Exogenous corrosion

The causes are not related to the metal itself but to external factors. This corrosion

often appears with the existence of cathodic and anodic zones. The mains defaults are one the

one hand some accidents that can happen during the metal manufacturing (surface default,

local hammer-hardening, residual stresses), and in the other, some defaults in the structure

conception:

- Galvanic coupling: when two dissimilar metals with different electrochemical

potential are in proximity or in contact with each other. One is considered as the

anode, and the second one as the cathode. It results in galvanic corrosion which is the

dissolution of the less noble metal (anode). To reduce this kind of corrosion, we must

absolutely minimise the surface ratio between the cathode and the anode.

- Differential aeration: when two parts of a device are exposed to different amount of

oxygen, for example if the assembling is not waterproof, it can lead to a differential

aeration corrosion battery. The anodic zone which undergoes the corrosion, is the less

aerated.

Metallic Biomaterials – december 2002 page 8

Page 9: metallic biomaterials

1.6. Protection methods

Every attempt to fight against corrosion should start by knowing and classifying the

causes of corrosion, and then try to minimize the external causes; which means to be more

careful with the surface treatment and the fabrication methods.

To reduce corrosion we should choose to right material, but also the right design (no

corners in the device, no stagnant liquid). If we have the choice, we must use the best

environment as possible: it’s always better at lower the temperature and the oxygen content as

well as the chloride content… But of course it is not possible to change anything in the body,

and then we must use other techniques to prevent from corrosion, either kinetic or

thermodynamic:

- Use of inhibitors: they act directly on the reaction mechanism and modify the active

surfaces.

- Use of coatings or protective layers: they act like a physical barrier between the

aggressive milieu and the metal to be protected. There can be metallic coatings (Ni,

Cr, Zn, Al, stainless steels …) or non metallic coatings (paintings, varnishes, enamel,

glass, plastic materials … )

- Use of passivable metals and anodic protection: when a passive film is formed, this

causes a marked drop in current density due to the resistance of the film and its effect

as a barrier to diffusion

- Thermodynamic methods to place the material in his passivity domain.

1.7. Passivation of metallic materials

a. Passivation phenomenon

Passivation corresponds to the transformation of an active surface which is corroding

to a quasi inactive surface, by formation of a passivation layer. The first stage of the

formation of this layer is the adsorption of OH- ions. It leads to a compound which quickly

evolves either quickly (Al, Ti, Zr, Nb, Ta) or slowly (Cr, Fe, Co, Ni) to an oxide.

If we admit that the passivation layer is an oxide, the Pourbaix diagrams can define the

possible domains of passivity. However, this oxide is often considered different from a stable

compound and thus E-pH diagrams can not be considered as rigorous; nevertheless they allow

giving a general overview of metals passivity and corrosion properties.

Metallic Biomaterials – december 2002 page 9

Page 10: metallic biomaterials

To bring a metal in his passivation domain, we can impose a suitable value of potential to the

metal.

b. Stability of the passivation layers

Stability of the passivation layers and their auto-reparation possibility are the main

problems. They depend on pH, oxidant force in the milieu, presence of some ions (Cl -,Br-…),

etc.

1.8. Conclusion on corrosion

Corrosion of an implant in the clinical setting can result in symptoms such as local

pain and swelling in the region of the implant, with no evidence of infection; only cracking or

flaking of the implant (seen on x-rays films), and excretion of excess metal ions. At surgery,

grey or black discoloration of the surrounding tissues may be seen and flakes of metals may

be found in the tissue. Corrosion also plays a role in the mechanical failures of orthopaedic

implants. Most of these failures are due to fatigue, and the presence of a saline environment

certainly exacerbates fatigue. The extent to which corrosion influences fatigue in the body is

not precisely known.

When an implant is subjected to stress, the corrosion process could be accelerated due

to the mechanical energy. If the mechanical stress is repeated then fatigue stress corrosion

takes place as in the femoral stem of the hip joint and hip nails made of stainless steel.

However, other mechanisms of corrosion such as fretting may also be involved at point of

contact such as in the counter-sink of the hip nail or bone fracture for the screws.

Different parts of the body undergo different type and rate of corrosion. Wounds and

infections can significantly change pH. Corrosion and fatigue added together can have a very

bad effect on the organ. The general concern is now to assure that metals in screws and in the

plates are identical. The surgeons must be careful not to scratch metals or to leave them in

tissues.

Metallic Biomaterials – december 2002 page 10

Page 11: metallic biomaterials

2. Properties and applications of the most used metallic biomaterials

Because all biomaterials have to be biocompatible, the difference in corrosion

resistance and in mechanical properties permits to aim them toward different applications. We

can classify the metallic biomaterials into 4 main groups: stainless steels, CoCr alloys, Ti

alloys, dental metals and the others.

2.1. Stainless steels

Vanadium steel’s corrosion resistance in vivo being not enough high, the first stainless

steel utilized for implant fabrication was the 18-8 (302 in modern classification). Later some

Mo has been added to it to improve the corrosion resistance. It was called 18-8Mo and

became later the type 316 stainless steel. In the 1950s the carbon content of 316 stainless

steels was reduced from 0,08 to an amount of 0,03% in weight, to more increase the corrosion

resistance and to minimize the sensitisation. This stainless steel is known today as 316L

stainless steel and contains, more than carbon, 2% of manganese, 17-20% of chromium, 12-

14% of nickel, 2-4% of molybdenum and small amount of phosphorus, sulfur and silicon.

Stainless steels contain enough chromium to confer corrosion resistance by passivity. The

passive layer is not as robust as in the case of titanium or the cobalt chromium alloys. Only

the most corrosion resistant of the stainless steels are suitable for implants, even these types of

stainless steels are vulnerable to pitting and to crevice corrosion around screws.

The 316 and 316L stainless steels are austenitic and this phase can be influenced by

the amount of Ni and Cr, but enhances the corrosion resistance. They can be hardened by

cold-working. We can obtain by this way a wide range of properties as yield and ultimate

tensile strength or elongation. These two steels are widely used for implant fabrication but

because of their poor corrosion resistance in the highly stressed and oxygen-depleted regions,

there are suitable to use in temporary implant devices such as fracture plates, screws and hip

nails.

For the manufacturing of stainless steels, heat treatments are necessary before cold-

working. But it is also the occasion: to cause corrosion by the formation of chromium carbide

in the grain boundaries, to produce distortion of components that can be solved a perfect

control of the uniformity of heating, involve the formation of surface oxide scales which have

to be removed. After the scales are removed, the surface of the component is polished,

cleaned and passivated in nitric acid.

Metallic Biomaterials – december 2002 page 11

Page 12: metallic biomaterials

2.2. CoCr Alloys

There are two CoCr alloys extensively used in implant fabrications such as artificial

joints, or stems of prostheses for heavily loaded joints such as knee and hip: the castable

CoCrMo alloy and the CoNiCrMo alloy which is usually wrought by (hot) forging. The

castable CoCrMo has been used also for many decades in dentistry.

The two basic elements of the CoCr alloys form a sold solution of up 65% Co. The

molybdenum is added to produce finer grains, which results in higher strengths after casting

or forging. The chromium enhances the corrosion resistance as well as solid solution

strengthening of the alloy.

The abrasive wear properties of the wrought CoNiCrMo are similar to he cast

CoCrMo alloy (about 0,14 mm/year in joint simulation tests PEHD acetabular cup); however,

the former is not recommended for the bearing surfaces of joint prosthesis because of its poor

frictional properties with itself or other materials. The superior fatigue and ultimate tensile

strength of the wrought CoNiCrMo alloy make it suitable for the applications which require

long service life without fracture or stress fatigue, such in the case of stems of the hip joint

prostheses. As with the other alloys, the increased strength is accompanied by decreased

ductility. Both the cast and wrought alloys have excellent corrosion resistance. As a matter of

fact, cobalt chromium alloys are passive in the human body. They are widely used in

orthopaedic applications. They do not exhibit pitting corrosion. However the metallic

products released from the prosthesis because of wear, corrosion and fretting may impair

organs and local tissues, and moreover some alloys with certain amount of Co can be toxic in

the body. Low wear has been recognized as an advantage of metal-on-metal hip articulations

because of its hardness and toughness.

The CoCrMo alloy is particularly susceptible to work-hardening so the normal

fabrication procedure used with other metals cannot be employed. The fabrication method

consists of making a wax pattern of the desired element. The pattern is coated with a

refractory material, first by a thin coating with a slurry (suspension of silica in ethyl silicate

solution) followed by complete investing after drying:

- wax melted in furnace (100-150°C)

- mould heated at high temperature burning out any traces of wax

- molten alloy poured with gravitational or centrifugal force

- mould broken after cooled.

Metallic Biomaterials – december 2002 page 12

Page 13: metallic biomaterials

2.3. Ti Alloys

a. Pure Ti and Ti6Al4V

Attempts to use titanium for implant fabrication dates to the late 1930s. Titanium’s

lightness (4,5 g/cm3) and good mechano-chemical properties are salient features for implant

application. There are four grades of unalloyed commercially pure (cp) titanium for surgical

implant applications. The impurity contents separate them: oxygen, iron, and nitrogen should

be controlled carefully. Oxygen in particular has a great influence on the ductility and

strength. These alloys contain also hydrogen and carbon (respectively, 0,015 % and 0,1% in

weight) Titanium alloys can be strengthened and mechanical properties varied by controlled

composition and thermo mechanical processing techniques. Moreover the addition of certain

elements, such as aluminium or vanadium, enables it to have a wide range of properties.

In addition titanium alloys have an excellent corrosion resistance thanks to the formation of

an oxide layer on its surface.

Another kind of titanium alloys is appreciated for its capacity to “have the memory of

its shape”.

b. TiNi alloys

The titanium-nickel alloys show unusual properties i.e.: if it is deformed below the

transformation temperature, it reverts back to its original shape as the temperature is raised.

This phenomenon is called “shape memory shape”, which can be related to a diffusionless

martensitic phase transformation, which is also thermoelastic in nature, the thermoelasticity

being attributed to the ordering in the parent and martensitic phases. A widely known NiTi

alloy is 55-Nitinol. This alloy is composed by 55 weight % or 50 atomic % of Ni, Ti and

small amounts of Co, Cr, Mn and Fe. Some possible applications of shape memory alloys are

orthodontic dental archwire, intracranial aneurysm clip, contractile artificial muscle for an

artificial heart, vascular stent, catheter guide wire and orthopedic staple. 55-Nitinol exhibits

also others good properties as low temperature ductility, good fatigue properties, direct

conversion of heat energy into mechanical energy, good biocompatibility and corrosion

resistance in vivo. The mechanical properties of NiTi alloys are especially sensitive to the

stoichiometry of composition and the individual thermal and mechanical history.

Metallic Biomaterials – december 2002 page 13

Page 14: metallic biomaterials

Titanium is very reactive at high temperature and burns readily in the presence of

oxygen. Therefore, it requires an inert atmosphere for high temperature processing or is

processed by vacuum melting. Oxygen diffuses readily in titanium and the dissolved oxygen

embrittles the metal. As a result, any hot working or forging operation should be carried out

below 925°C. Machining at room temperature is not the solution to all the problems since the

material also tends to gall or seize the cutting tools. Very sharp tools with slow speeds and

large feeds are used to minimize this effect. Electrochemical machining is an attractive mean.

All these properties make titanium very used biomaterial and we will develop all these

properties in next chapter, where we will focus more our study on its applications.

2.4. Dental metals

Dental amalgam is an alloy made of liquid mercury and other solid metal particulate

alloys made of silver, tin, and copper…The solid alloy is mixed with liquid mercury in a

mechanical vibrating mixer and the resulting material is packed into the prepared cavity. The

final composition of dental amalgams typically contain 45 to 55% mercury, 35 to 45% silver,

and about 15% tin after fully set in about one day.

Amalgam often corrodes and is the most active material used in industry. Furthermore,

the use of mercury is forbidden because of the harmful effects of mercury on the human body.

Gold and gold alloys are useful metals in dentistry as a result of their durability, stability and

immunity to corrosion. Gold is widely used in dental restoration and in that setting it offers

superior performance and longevity. Gold is not, however, used in orthopaedic applications as

a result of its high density, insufficient strength and high cost. Gold alloys are used for cast

restorations, since they have mechanical properties, which are superior to those of pure gold.

Copper or platinum, alloyed with gold, increase its strength, while silver is only added for

compensation for the colour of copper.

2.5. Other metals

Several others metals have been used for a variety of specialized implant applications.

Tantalum has been subjected to animal implant studies and has been shown very

biocompatible. Due to its poor mechanical properties and its high density, it is restricted to

few applications such as wire sutures for plastic surgeons and a radioisotope for bladder

tumors.

Metallic Biomaterials – december 2002 page 14

Page 15: metallic biomaterials

Platinum group metals, such as Pt, Pd, Rh, Ir, Ru and Os are extremely corrosion

resistant but have poor mechanical properties. They are mainly used as alloys for electrodes

such as pacemaker tips because of their high resistance to corrosion and low threshold

potentials for electrical conductivity.

Thermoseeds made of 70% Ni and 30% Cu are used to deliver a constant hyperthermic

temperature extra corporally at any time and any duration, by applying an alternative

magnetic field.

2.6. Conclusion

We can conclude, according to the summary table of the principal properties bellow,

that mechanical properties and corrosion resistance of Ti alloys and CoCr alloys are quite

similar and are the best among all metallic biomaterials. However, the density is the important

parameter which difference them, and which explain why Ti may be more appreciated for

some applications. As a matter of fact its lightness is appreciated for orthopaedic implants…

That is why we will focus our study on titanium and titanium alloys.

Metallic Biomaterials – december 2002 page 15

Page 16: metallic biomaterials

MaterialsProperties

316L stainless steel

CoCrMo alloy

CoNiCrMo alloy

Grades TiTi6Al4V

alloyTantalu

m

Ten

sile

strength

(M

pa) 485-860 655 793-1793 240-550 860 207-517

Yie

ld stre

ngth

(0,2

% o

ffse

t) (M

pa)

172-690 450 240-1585 170-485 795 138-345

Elo

ng

atio

n

(%)

12-40 8 8-50 15-24 10 2-30

Reductio

n

of a

rea (%

)

- 8 35-65 25-30 25 -

Density

(g

/cm3)

7,9 8,3 9,2 4,5 4,5 16,6C

orro

sion

resista

nce

poor in highly stressed

excellent excellent excellent excellent good

 

and O2 depleted region

         

Table 2 : Summary table

Metallic Biomaterials – december 2002 page 16

Page 17: metallic biomaterials

3. Titanium and Ti alloys as Biomaterials

3.1. Background

Titanium and some of its alloys are used as biomaterials for dental and orthopaedic

applications. The most common grades used are commercially pure titanium and the Ti6Al4V

alloy, derived from aerospace applications.

a. Physiological Behaviour

These materials are classified as biologically inert biomaterials or bio inert. As such,

they remain essentially unchanged when implanted into human bodies. This is no doubt a

result of their excellent corrosion resistance. Titanium is a base metal in the context of the

electrochemical series; however, it forms a robust passivation layer and remains passive under

physiological conditions. Corrosion currents in normal saline are very low: 10-8 A.cm-2.

Titanium implants remain virtually unchanged in appearance. Ti offers superior corrosion

resistance but is not as stiff or strong as steels or Co-Cr alloys.

Passivation layer

Titanium derives its resistance to corrosion by the formation of a solid oxide layer to a

depth of 10 nm. Under in vivo conditions the oxide, TiO2 is the only stable reaction product.

The titanium implant surface consists so of a thin layer and the biological fluid of water

molecules dissolved ions, and biomolecules (proteins with surrounding water shell). The

microarchitecture (microgeometry, roughness…), of the surface and its chemical composition

are important due to the following reasons:

- Physical nature of the surface either at the atomic, molecular, or higher level relative

to the dimensions of the biological units may cause different contact areas with

biomolecules, cells, etc.

- Chemical composition of the surface may produce different types of bonding to the

biomolecules, which may then also affect their properties and functions.

The surface-tissue interaction is dynamic rather than static, i.e. it will develop into new

stages as time passes, especially during the initial period after implantation. The composition

of biofluid will then change continuously. Depending on the type of initial interaction, the

final results may be fibrous capsule formation or tissue integration.

Metallic Biomaterials – december 2002 page 17

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Hydroxyapatite – Ti alloys composites

There are more than 10000 artificial hip joint clinical applications a year. However,

because hip joints are sometimes damaged, broken and so on, there are many cases to be

operated again. Today, it is a subject for future improvement of its dependability.

Hydroxyapatite excels in affinity for a living body. Therefore it is expected to be

applied to artificial bones, dental roots and so on. Hydroxyapatite is a calcium phosphate,

whose chemical composition and properties are very closed to bones, and is moreover

biodegradable in the body. However, since the mechanical strength is low, apatite cannot be

used under a heavy load so that its application is limited. A high strength -rich - titanium

alloy, Ti-4,5Al-3V-2Fe-2Mo as example, has

remarkable superplastic formability at

temperature below 800 °C, more than 100 °C

lower than Ti-6Al-4V. By a superplastic

forming, titanium alloy can be formed easily into

complicated shapes. Then the application for

dental materials, such as dental base, has been

investigated. The method of implanting

hydroxyapatite into superplastic titanium alloy

substrate was examinated. Hydroxyapatite

granules are spread over the surface of titanium

alloy, and are heated at 750 °C in a vacuum.

There are then implanted into the alloy by giving

pressure of about 17 Mpa.

Figure 5 : SEM photographs for the pressed specimens under 17 Mpa at 750°C for 10 min.

Another method exists for the fabrication of these composites. Hydroxyapatite is

simply deposed on the surface of titanium alloy by plasma spraying. This method is besides

actually the only commercially accepted technique for deposing such coating.

Various processes have been used for manufacture hydroxyapatite coated implants for

biomedical applications, such as chemical, electrochemical or pulsed laser deposition sol–gel

Metallic Biomaterials – december 2002 page 18

Page 19: metallic biomaterials

technique, magnetron sputtering or ion implantation. The plasma spraying technique has

become the most frequently used technique to fabricate hydroxyapatite coatings.

Hydroxyapatite-titanium alloy

composites permit then a better mechanical

fixation facilitating the joint between the

prostheses and bones, because the

hydroxyapatite provides a bioactive

surface, i.e. it actively participates in bone

bonding. It involves an increase in the

long-term stability in the implant.

Figure 6 : REM picture of marrow bone cells on (a) a titanium

surface, (b) on the hydroxyapatite surface layer produced by ion implantation.

Moreover, fabrication of bioactive silica-based glasses – titanium alloys composite has

been reported. As a matter of fact a new family of potentially bioactive glasses displays good

physical compatibility with Ti. In order to fabricate dense coatings, glass powder is painted

over the metal substrates and the assemblies are fired to make the glass flow and adhere to the

metal.

Metallic Biomaterials – december 2002 page 19

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b. Mechanical Suitability

Titanium and its alloys possess suitable mechanical properties such as strength, bend

strength and fatigue resistance to be used in orthopaedics and dental applications. This is part

of the reason why they have been employed in load-bearing biomedical applications in stead

of materials such as hydroxyapatite, which displays bioactive behaviour.

Other specific properties that make it a desirable biomaterial are density and elastic

modulus. In terms of density, it has a significantly lower density (table 3) than other metallic

biomaterials, meaning that the implants will be lighter than similar items fabricated out of

stainless steel or cobalt chrome alloys.

Material Density Elastic Modulus E (GPa) Specific criteria: E /

Cortical Bone ~2.0 g.cm-3 7-30 ~3,5-15

Cobalt-Chrome

alloy

~8.5 g.cm-3 230 ~27

316L Stainless Steel 8.0 g.cm-3 200 25

CP Titanium 4.5 g.cm-3 110 24,4

Ti6Al4V 4.4 g.cm-3 106 24

Table 3. Densities of selected biomaterials and cortical bone.

Having a lower elastic modulus compared to the other metals is desirable as the metal

tends to behave a little bit more like bone itself, which is desirable from a biomechanical

perspective. This property means that the bone hosting the biomaterial is less likely to atrophy

and resorb.

Metallic Biomaterials – december 2002 page 20

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3.2. Applications

a. Orthopaedic Implants

Titanium is commonly used in orthopaedic implants such as joint replacements and

bone pins, plates and screws.

Figure 7 shows the various components of a total hip replacement. On the left is the

femoral stem made of a titanium alloy. The long round section fits down into the thigh bone

or femur. The white section is a hydroxyapatite coating to encourage bone bonding to the

implant. This section is also macro

textured to provide surface features for

the bone to mechanically interlock with.

The ball on top of the femoral stem is

called the femoral head. It is made of

zirconia ceramic and fits into the hip

joint in the pelvis.

The hemispherical item on

the right is the acetabular cup, also

made from titanium alloy. It is coated in a porous alumina ceramic, to allow bone ingrowth

for stabilisation. An ultra high molecular weight polyethylene (UHMWPE) liner fits inside the

acetabular cup and provides the articulating surface for the femoral

head.

Figure 8 shows prototype total knee replacement prosthesis,

similar in design to many commercial implants. It consists of

titanium alloy upper and lower structural components. A zirconia

wear surface has been fabricated for the upper section. Similar to the

hip prosthesis, this articulates against a UHMWPE insert on the lower section.

Other orthopaedic applications for titanium-based materials include bone pins, plates and

screws, used for repairing broken bones etc.

b. Ligament anchorages

Metallic Biomaterials – december 2002 page 21

Figure 7 : Implant components for a total hip replacement

Figure 8 : Total knee replacement prosthesis

Page 22: metallic biomaterials

Ti is also newly used in the anchorage screws (figure 9) in the knee (1), the shoulder

(2), the hand (3) and the ankle (4). A suture thread is attached to the screw, which is in turn

fixed to the bone. This Ti screw (figure 10) can easily be seen in the body by a radiography

control and it can also be removed from the tissue, if needed.

Metallic Biomaterials – december 2002 page 22

Figure 9   : Ligament anchorages

Figure 10   : Ti screw

Page 23: metallic biomaterials

c. Dental Applications

Titanium has been used for dental implants because of its excellent biocompatibility

and corrosion resistance, while application in general dentistry has been limited. Titanium

pins and posts are used to secure dental implants. They use threaded fixtures to secure them

into the jaw.

Titanium superstructures are now

being investigated as an alternative

to other metals such as gold for

implants such as polymer based

dentures (figure 11).

Shape Memory Alloys (SMAs)

SMAs have the ability to return to a predetermined shape when there are heated. When

the SMA is below its transition temperature, it has very low yield strength and can be

deformed quite easily into a new shape that it will retain. And when the material is heated

above its transition temperature, it undergoes a change in its crystal structure which makes it

return to its original shape. If the SMA encounters any resistance during its transformation, it

can generate extremely large forces.

The most common SMA is an alloy of nickel and titanium called Nitinol. Ti-Ni alloys

have thus special properties like shape memory effect, super elasticity and high wear

resistance.

Super elastic and thermal shape recovery alloys are used in orthodontic application.

Stainless steels have been employed as corrective measures for misaligned teeth for many

years. Owing to the limited “stretch” and tensile properties of these wires, considerable forces

were applied to the teeth, which caused a great discomfort. When the teeth succumb to the

Metallic Biomaterials – december 2002 page 23

Figure 11 : Side view of a super plastically-formed, titanium alloy, cantilevered

superstructure, attached to dental plaster analogues in a plaster model of a

patients jaw.

Page 24: metallic biomaterials

corrective forces applied, the stainless steel wire had to be re-tensioned and visits to the

orthodontist were maybe needed every three weeks in the beginning of the treatment.

But now super elastic wires are used for these corrective measures. Owing to their

elastic properties and extensibility, the level of discomfort can be reduced significantly as the

SMA applies a continuous, gentle pressure over a longer period. Visits to the orthodontist are

reduced to perhaps three times per year.

Metallic Biomaterials – december 2002 page 24

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d. Perspectives and conclusion

Researchers still want to improve metallic biomaterials, and find the best composition

of an alloy to optimise the biocompatibility, non toxicity and the corrosion resistance, but also

to lower the meting point and therefore to facilitate the manufacture of the material without

loosing the good mechanical properties of course!

In the field of orthopaedics applications, the present researches are about to find some

new alloys especially Vanadium-free Titanium alloys, which must replace the most popular

ones, TI-6Al-4V, because of the cytotoxic Vanadium, or Nickel-free shape memory and ultra-

elastic alloys because Nickel can have adverse physiological affects.

In Japan, the team of Professor Shinichi Nitta, from Tokyo University, has developed

artificial cardiac muscle to be attached on the outside of the heart and sandwiched by with

Shape Memory Alloy plates.

On the other way, conventional metallic porous materials are best suited for use as

coatings on implants since they do not readily have the required mechanical and processing

characteristics which would allow them to be used as bulk structural materials for implants,

bone augmentation, or substitutes for bone graft.  

A new porous biomaterial made of tantalum has recently been developed for potential

application in reconstructive orthopaedics. The material has an unusually high and

interconnecting porosity with a very regular pore shape and size. It can be made into complex

shapes and used either as bulk implant or as a surfacing coating. This porous tantalum

biomaterial has then desirable characteristics for bone ingrowth. Further studies are warranted

to ascertain its potential for clinical reconstructive orthopaedics.

Metallic Biomaterials – december 2002 page 25

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References

[1] Joseph D: BRONZINO, The biomedical engineering handbook, second edition, volume I.

[2] Jean BARRALIS & Gérard MAEDER, Précis de métallurgie, élaboration - structures –

propriétés – normalisation, AFNOR.

[3] www.abe.msstate.edu/classes/abe4523-6523/intro-history-metals-alloys.pdf (metallic

biomaterial)

[4] www.cp.umist.ac.uk/lecturenotes (corrosion)

[5] http://ttb.eng.wayne.edu/~grimm/BME5370/Lect2Out.html (biomaterial)

[6] http://www.materials.drexel.edu/LBTE%20website/biomaterials.html (biomaterial)

[7] http://www.azom.com/Details.asp?ArticleID=1520 (Titanium and titanium alloys as

biomaterials)

[8] http://www.choc.fr/index02.html (ligament anchorage)

[9] http://www.tekes.fi/julkaisut/BiomaterialResearchJapan.pdf (Study in Japan)

[10]http://orthonet.on.ca/emergingtrends/notes/A%20New%20Porous%20Tantalum

%20Biomaterial.htm (Study on Tantalum)

[11] The Biomedical Engineering Handbook – Second Edition – Volume 1 CRC Press

[12] Thin hydroxyapatite surface layers on titanium produced by ion implantation

H. Baumann, K. Bethge, G. Bilger, D. Jones, I. Symietz – 2002, Elsevier Science

[13] Implantation of hydroxyapatite granules into superplastic titanium alloy for biomaterials

T. Nonami, A. Kamiya, K. Naganuma, T. Kameyana - 1998, Elsevier Science

[14] http://www.csa.com/hottopics/bceram/biblio12.html (Silicate glass coating)

Metallic Biomaterials – december 2002 page 26