Metallic Bio Materials

METALLIC BIOMATERIALS

Contents1. Introduction and History2. Metallic Alloys3. Fabrication4. Drawbacks5. New Directions

2

History

•Preserved skeletons show that bone diseases have afflicted man since before recorded history. The use of materials as constituents of surgical implants is not new.

•One of the first used biomaterials were metallic biomaterials in the fabrication of surgical implants many decades ago.

• Substitutions of bone parts for repairing damaged portions of the human body have been recorded since the pre-Christian era.

• Bronze or copper were utilized in situations requiring the assembly of fractured bone parts.

500 BC 17th Century 1866 1893

Use of gold plates to fix teeth

Use of wire from iron, gold &

bronze for suture & wounds

First bone plate (steel with Ni-

coating)

Development of screws & plates from steel

1912

First bone plate of V-alloyed steel of high

strength

1920 1938 1946

Development of Cr-Ni and Cr-Ni-Mo steels

(stainless steels)

First prosthesis for total hip replacement

Commercial production of Ti and

Ti-alloys

What is a metallic biomaterial ?•Metallic biomaterials are metal implants

that are planted inside the body specially in bone replacement field due to their superior load bearing, mechanical & electrical properties.

•These favorable properties (good fracture resistance, electrical conductivity, formability) are related to the metallic inter atomic bonding that characterizes this class of material.

Examples of metallic biomaterials

Classification of Metallic Biomaterials

•Titanium based alloys•Stainless steels•Cobalt based alloys•Shape memory alloys•Combinations of metallic biomaterials

A small idea about how it works

Schematic illustration of a metallic hip implant component in the femur showing the direction and concentration of lines of

force acting during loading. The stiffer implant component acts to shield the bone.

So where are they in the body ?

•Metallic materials are used in biomedical devices for various parts of the human body like artificial joints, bone fixation devices, dental implants, orthodontic wires, crowns, and bridges, stents, clips, and artificial hearts.

Current Applications

Temporary

•! Surgical implants

•! Wires, plates, screws for fixation of broken bone

Permanent

•! Implants (teeth, spine cage)

•! Joint implants

•! Aneurysms clips

•! Aneurysms coils

•! Stents

•! Screws

Implanting Criteria1. Appropriate tissue-material interface 2. Non-toxic3. Non-corrosive4. Adequate fatigue life5. Proper design 6. Proper density7. Relatively inexpensive 8. Elastic and mechanical properties

comparable to those of bone

Design ConsiderationsMatching

mechanical properties of tissue

with mechanical properties of metal

Have to consider how the metal may fail inside the body

A material may fail inside the body by:

Corrosion

Fatigue

What got wrong ?•A major factor impeding success was the

accumulation of toxic copper ions in the liver, brain, and other body tissues once the implant had completely dissolved.

An example of a poisoned tissue

Metallic Alloys UsedAn Overview on A Few

Major Metallic Alloys

• Vanadium-Steel Alloys

• Stainless Steel Alloys

• Cobalt-Chromium Alloys

• Titanium Alloys

Vanadium-Steels

Vanadium-Steels

• The first metal produced specifically for human use

• Contained: up to 0.5% V, 0.4-0.5% C, 1.1-1.5% Cr.

• Was Used for Sherman plates (fracture plates)

Vanadium Steels

• Its corrosion resistance was inadequate in-vivo

• This caused the release of harmful material that may kill the surrounding tissue.

Stainless SteelsJust like old times!

Stainless Steels

• Replaced Vanadium Steels.

• Stainless Steels are:

Fe+low C%+≈17% Cr+≈11% Ni+≈2% Mo

• Have highest susceptibility for crevice corrosion (among all other alloys).

• Still used for fracture repairs (Fracture Screws, Plates, and Hip Nails).

Stainless Steel Alloys

• AISI 302

• AISI 316

• AISI 316L

AISI 302

• The first stainless steel used.

• Stronger and more resistant to corrosion than V-Steel.

• in 1948, it was considered ideal for orthopaedic applications

AISI 302

Element %C 0.15Cr 17-19Mn 2Ni 8-10P 0.045Si max 1

AISI 302

• Disadvantages:

- Too high C% for human body.

- Relatively low corrosion resistance to bio-fluids.

AISI 302Bye Bye!

AISI 316

• It had better corrosion resistance to salt water due to added Mo.

• It had lower carbon content.

• in the 1950’s, it was modified to give the AISI 316L.

AISI 316

Element %C 0.08Cr 16-18Mn 2Ni 10-14P 0.045Si max 1

Mo 2-3

AISI 316L

• Same as 316 but with lesser C% (0.03 instead of 0.08% max)

• Better corrosion resistance.

• Less probability for sensitization

Stainless Steels: Drawbacks

• Relatively low corrosion resistance (even the 316L can corrode in highly stressed, Oxygen-depleted regions such as under the fracture screws).

• Used only for Temporary fracture implants (may need 2nd surgery).

Cobalt-Chromium AlloysNice and Shiney!

Co-Cr Alloys

• Co-Cr solid solution of up to 65% Co.

• Higher corrosion resistance than StSt (due to Cr).

• Higher Young’s Modulus than StSt.

• Enhanced Strength (Solid Solution Strengthening due to Cr)

Co-Cr Alloys

• Cast Co.Cr.Mo alloy (ASTM F75)

• Wrought Co.Cr.W.Ni alloy (ASTM F90)

• Wrought Co.Ni.Cr.Mo alloy (ASTM F562)

• Wrought Co.Ni.Cr.Mo.W.Fe alloy (ASTM F563)

Co-Cr Alloys

Castable CoCrMo

• Most Clinically wear resistant metallic biomaterial (due to carbides formed during solidification).

• It’s Mo causes the grains to be finer.

• Excellent corrosion resistance.

• Used for decades in dentistry and relatively recently in artificial joints.

Cast CoCrMo alloy:Fixation

• Either:

- Using Acrylic Bone Cement;

- Sintering CoCrMo powders to the bone interface directly;

- Plasma Spray-Coating the surface with a bioactive material.

Wrought CoNiCrMo


• Superior fatigue and high tensile strength.

• Suitable for long service-life applications without fracture or stress fatigue.

• Used for stems of prosthesis for heavily-loaded joint (ex: hip and knee).

• It has poor frictional properties with itself and other materials.

Co-Cr Alloys: Drawbacks

• They have relatively high Young’s Modulus (which may cause stress-shielding).

• Extensive release of Ni and Co ions may have toxic effects on surrounding tissue and organs.

Interesting Point (IP)IP #1

Ni-release

The Wrought CoNiCrMo alloy releases Ni ions -----> so does StSt.

But: Although StSt has lower Ni%, it has a higher release rate!

Titanium AlloysThe Real Deal

Titanium Alloys

• Excellent in-vivo corrosion resistance. (owing to passive oxide layer that rapidly forms).

• Good fatigue resistance.

• Lower elastic moduli (100-110 compared with 200-225 GPa of other used alloys).

• Strong tendency for osseointegration. (wait for it)

Interesting PointIP #2

Metal-Tissue integration

Metal tissue integration or “osseointegration with metals” can occur. (Oxide layer is formed)

Metal-Tissue integration

But: this oxide layer may not be homogeneous and thus full osseointegration may not occur.

Otherwise: metals are biologically inert.

Titanium Alloys• Used for fracture fixation, joint implants, and

almost exclusively for endosseos dental implants.

Titanium Alloys

• Commercially Pure (CP) Ti Alloys.

• α & β Alloys:

- (α + β) Alloys

- β & Near-β Alloys

CP Ti (Alloys)

Not so pure!

CP Ti (Alloys)

• Used for endosseous dental implants where osseointegration occurs.

• Grades III & IV: used for osteosynthesis (fracture repairs and spinal fusion) but not joint replacement due to low fatigue strength and mechanical properties.

α & β Alloys

(α + β) Alloys

• Higher strength (yield, ultimate) than CP Ti.

• Superior corrosion-fatigue properties.

• Osseointegration tendency.

• Examples: Ti6Al4V; Ti6Al7Nb; Ti5Al2.5Fe.

(α + β) Alloys

• Ti6Al4V:

- Excellent corrosion resistance.

- Can be spray coated or sintered as powder on a substrate.

- A bioactive material (ex: Bioglass®) can be spray-coated on it to insure osseointegration.

β & Near-β Alloys

• If appropriately processed, can have very low elastic moduli (44-51 GPa for Ti13Nb13Zr, a near β-alloy) - increases by annealing.


• Good formability.

• High hardenability.

• better notch sensitivity than(α + β) Ti alloys.

Other Metallic Alloys

• Nickel-Titanium Alloys

• Zirconium-Niobium Alloys

• Tantalum

• Platinum and Platinum-Iridium Alloys

• Major Dental Restoration alloys

Nickel-Titanium AlloysShape Memory Awesomeness!

Ni-Ti Alloy: Nitinol®

• Nitinol® is a shape memory alloy.

• It has good corrosion resistance.

• It’s Martinsitic phase has elastic modulus ≈28-41 GPa.

• Ni release makes it a choice for temporary implants.

How can SMA’s be properly used in the

body?Glad you asked!

1 way of using SMA’s

The original shape is Austenite.

Deform or cool it ---> gives Martensite

Put it in body ---> heats up with body temp. to give Austenite with original shape again.

This original shape pulls or pushed bones or tissue together.

Nitinol®: Applications

• Orthopaedics: fracture fixation devices.

• Spinal rods for treatment of scoliosis

• Cages for use during spinal fusion and even self-locking joint replacement components.

• Orthodontics: large ‘working range’ wires during force application for tooth re-positioning.

• SMAs are also used for cardiovascular stents. (pseudo-elasticity)

Zirconium-Niobium Alloys

Zr-Nb Alloys

• Zr-Nb alloys, due to ZrO2 layer formed, have:

➡Corrosion Resistance

➡Wear Resistance

➡Hardness

➡High Reactivity

Zr-Nb Alloys

• Used for making orthopaedic components that are primarily intended for compressive loading and resisting wear (such as femoral hip implant and knee implant components)

To crack, or not to crack

Stabilised ZrO2 is a strong ceramic.

But: Zr-Nb alloys are better than regular ZrO2 ceramics due to less susceptibility for crack propagation.

To crack, or not to crack

Because: the brittle ZrO2 is reinforced by Zr-Nb alloy.

Tantalumfinally, a pure metal!

Tantalum

• It is considered biocompatible (due to Ta2O5 layer formed).

• Porous Tantalum structure is used for augmentation templates.

• It is weak but bone grows inside it and takes the load.

Platinum and Platinum-Iridium Alloys

Nobel Biomaterials

Pt and Pt-Ir Alloys

• Used for electrodes used in electrical stimulation (ex: neuromuscular stimulation or pace-makers) and sensing devices.

➡Superior corrosion resistance especially at extreme voltages.

Dental metallic implants

Types of alloys used depending on application

• Direct fillings in teeth (dental amalgams)

• Fabricating crowns and bridges (noble metal and base metal alloys)

• Partial denture framework (base metal alloys)

• Orthodontic wires and brackets (stainless steel types 302, 303, 304 and 305, CoCrNiMo alloys (Elgiloy), β-Ti, and Ni-Ti alloys), (because of their low elastic moduli, high strengths and consequently large working range, a desirable characteristic for this application).

• Dental implants (CP Ti and Ti6Al4V)

Dental amalgams

• Hg added to alloys containing Ag, Cu and Sn plus some other minor elemental additions

Amalgam types

• High (Cu >6 wt%)

• Low (Cu <6 wt%)

Advantages and drawbacks

Advantages

• Amalgams in situ formability to the desired shape

Drawbacks

• Hg toxicity that is still widely debated up till now while nothing yet has proven to be valid to prevent amalgam from being used.

Dental Casting Alloys – (Au-based, Co- and Ni-based, Ti-based)

Dental casting alloys are used for making

• dental bridges,

• crowns (with porcelain fused to a metal substrate),

• inlays,

• onlays,

Both noble and non-noble (base) metal alloys are used to form these often-complex shapes.

As they Require

• Sufficient strength,

• Toughness,

• Wear resistance,

• Corrosion resistance

• Biocompatibility

General Fabrication Steps

Metal Ore

Raw metal

Stock metal Shapes

Final metal device

General Fabrication

Steps

Heat Treatments

Testing

Purification

Alloying Separation

Solidification

Fabrication

ExampleMineral Ore

Metallic raw material in bulk form

Stock shapes e.g. sheet, tube, bar,plate,powder,wire

Preliminary implant device

Final implant device

Market

MiningOre separation / concentrationChemical extraction of metal RefiningAlloying

Casting, Rolling, Forging, PowderProduction, heat treating

Fabrication Investment casting Grinding Powder metallurgy

Surface preparationsNitriding Polishing

CleaningQuality control Packaging

Manufacture

Machining

Investment Casting

Finishing

Surface Treatments

Forging

Powder

Metallurgy

Manufacture of Metallic Implants

A variety of methods are required because not all implant alloys can be economically made in the same

way.

- Cobalt-based alloys are difficult to be machined so they are fabricated by casting or powder metallurgy.

- Titanium is difficult to cast so it is machined.

Example

They improve fixation of implants in bones by the application of macro or micro porous coatings on implants or the production of

certain degrees of surface roughness.

Surface Treatments!

Sinterng Flame

Spraying Plasma

Spraying

Surface Treatments!

Nitriding Ion Implantation

Surface Treatments!

These steps are important to the overall biological performance of the implant because they affect the

surface properties that come in direct contact with the blood and other tissues at the implant site.

Electrolytically controlled treatments

Chemical Cleaning

To remove machining chips or impurities

Finishing !

Material ASTM designation

Common trade names

Composition Wt%

Co-Cr-Mo F75VitalliumHaynes - Stellite 21Protasul – Micrograin - Zimaloy

58.9 – 69.5 Co27.0 – 30.0 Cr5.0 – 7.0 MoMax. 1.0 Mn, 1.0 Si, 2.5 Ni, 0.75 Fe, 0.35 C

Example

The fabrication of ASTM F75!

1. The alloy is melted at 1350-1450 0C

2. Poured or pressurized into ceramic molds of the desired shape through sprues, or pathways.

3. Once the metal has solidified into the shape of the mold, the ceramic mold is cracked away.

Investment Casting

This process can produce at least three microstructural that can influence negatively the implant properties:

A Cobalt rich matrix plus dendritic and grain boundary carbides , cored microstructure can develop due to nonequilbrium cooling.

Relatively large grain size results due to solidification. This decreases the yield strength.

Casting defects as inclusions, macro or micro porosity (due to metal shrinkage upon solidification) may arise.

Polished-etched view of a cast ASTM F75 femoral hip stem.

Note dendrites and large grains

In vivo fracture initiated from an inclusion formed during the

casting process

Powder Metallurgy

hot isostatic pressing

The fine powder is compacted and sintered

together under appropriate pressure and temperature (100 MPa,

1100 0C for 1 hour).Then forged to final shape.

Finer grain size and a finer distribution of

carbides can be obtained.

Drawbacks of metal

Major drawbacks• Nothing is more wonderful than inventing a new

material. Scientists always have been proud of inventing new materials.

• Any new material must have some drawbacks in the beginning of its invention.

• But the challenge her is to overcome this drawbacks and try to find a solution to it.

Wear

• Wear is a critical issue for prostheses, implants, and other medical devices

• Wear may lead to significant loss of material or failure of a medical device.

• Relatively small amount of wear can lead to significant degradation of function for some medical devices

• The most important two types of wear : adhesive wear and fatigue wear.

Wear• Adhesion wear: it originate from adhesion of two

surfaces that are in contact.

• The contact area is smaller than the appeared contact area.

• Fatigue wear: wear mechanism that occurs when a material undergoes cyclic loading.

• Fatigue wear can be observed in sliding, rolling, or impact wear processes.

Example on wear• Lets study joint prosthesis wear. There are four modes of

wear.

• Mode 1 wear in a total hip replacement prosthesis occurs between the femoral head and the ace tabular cup.

• Mode 2 wear may take place between the femoral head and the metal backing of the ace tabular cup.

• Interaction between metal debris, the femoral head, and the ace tabular cup is an example of three-body wear.

• Mode 4 is an articulation between two non-bearing secondary surfaces in prosthesis.

• Factors affecting wear rate:

Factors affecting wear rate

The type of motion.

The number of cycles of motion.

Clinical practice.

Design

• The implanted material is exposed to body fluids, such as intercellular fluid and blood depending on the surrounding tissue. Body fluid consists of inorganic ions, amino acids, proteins, and organic acids.

• The pH of body fluid is buffered at 7.15~7.35 but it decreases down to 5.2 during an inflammation reaction.

• Macrophages secrete active oxygen species, which chemically attack the surface oxide film of metallic materials, the influence of the presence of macrophages has been investigated.

Corrosion

• Macrophages secrete fibroblast growth factor, inducing fibroblasts, which produce collagen and polysaccharide for encapsulation of the implanted material.

• The thickness of capsular tissue depends on the biocompatibility and the size of material. With the cure of inflammation, the macrophage disappears within a few weeks and the cells originating from the surrounding tissue adhere to the surface of material.

• These mechanical degradations are often caused by fatigue, fretting fatigue, and wear accelerated by corrosion.

• Types of corrosions: useful corrosion and harmful one.

• Useful one that degrades when implanted to help formation of new bone that is capable of carrying the load alone instead of the implant like adding manganese alloys as WE43 and AE21 to bioadsorbable devices.

• Harmful one is the one that can “eat” in the implant and cause its decrease in volume which lead to its disfunction-ability by losing it function which have been implanted to.

• The reduction in bone density as a result of removal of normal stress from the bone by an implant.

• Wolff's law states that bone in a healthy person or animal will remodel in response to the loads it is placed under.

• Therefore, if the loading on a bone decreases, the bone will become less dense and weaker because there is no stimulus for continued remodeling that is required to maintain bone mass.

Stress shielding

• As we see here, the implant cares all the load by itself and the new bone that is formed doesn’t carry anything.

• The characteristics of the metal used here in found to be of high modulus. So we use a metal with low modulus to prevent this phenomena.

• Comparison between different metals:

MEMS & NANOCRYSTALS

The Story:

What are MEMS/bioMEMS?

operating principles often relies on:

•! Of mechanical and fluidic components that allow physical functions to be performed in conjunction with electrical functions.

“Microscopic Electro-Mechanical Systems” or MEMS.

MEMS and bioMEMS are miniaturized (sub-micrometer to millimeter) systems, often produced using batch fabrication techniques.

Moving

Rotating

Or

Vibrating

•!Are commonly employed to promote adhesion between underlying layers with the new layers or structures being subsequently built atop.

Ti or Cr

•!Are commonly used as electrical conductors. Cu, Au and Al

•! Is often used to provide a site for antibodies or single DNA strands to be immobilized. Au

•!Are a few of the numerous materials commonly plated in a very large number of applications for their electrical, magnetic or mechanical properties.

Cu, Au, Ni, Ni-Fe and platinum metals

(Pt, Rh, Pd etc.)

A variety of (PVD - Physical Vapor Deposition) systems is available for depositing thin film metals relevant for MEMS fabrication. Thin metal films enter the bioMEMS fabrication flow at various crucial moments.

When adequately patterned, such films can be used to produce conducting lines, electrodes, etc. Their hydrophilic or hydrophobic properties can also be used for biology applications

Stress can be controlled through varying the: Deposition rate (current density), The temperature, The agitation, The chemistry of the bath, etc.

The size of the plating cell can also

influence the reproducibility of the deposit characteristics.

Plated metals may be accompanied with various levels of in-plane and out-of-plane stress.

Ti in Biomedical application

Ti is often used in conventional biomedical application (e.g., for hip replacements) as it fulfils basic functions such as biocompatibility, bioactivity, resistance to corrosion and/or high resistance to abrasion.

Ti brazing is a very useful “conventional” encapsulation technology for pacemaker leads.

Unfortunately, Ti is somehow missing from the arsenal of MEMS materials with the exception of PVD thin films: no truly MEMS-friendly machining technique allows us to deposit and accurately pattern large amounts (tens to hundred of micrometers) of Ti.

Scientists are attempting to do so, using electrophoresis to deposit thick layers of titanium oxide for MEMS application.

Nanocrystalline

Over the past 20–30 years there have been increasing studies on the formation of NANOCRYSTALLINE (grain size <100 nm) and amorphous (glassy) metals for various applications.

Much higher yield strength, wear resistance and corrosion resistance has been shown for some pure metals formed with such fine microstructures.

Improved corrosion resistance has also been observed with nanocrystalline metals.

Nanocrystalline Production

Rapid melt cooling rates through atomization or melt

spinning methods, Extreme mechanical deformation to give heavy mechanical

deformation Followed by recrystallization anneals to yield high nucleation

rates with limited grain growth, Mechanical attrition or milling of gas atomized powders

followed by plasma spraying (for coatings), Pulsed electrodeposition and other PVD (Physical Vapor

Deposition) and CVD (Chemical Vapor Deposition) methods (for coatings and possibly bulk components).

Methods that have been reported for forming nanocrystalline and amorphous metals and coatings involve:

In addition to developing very fine grain structures, some of these approaches also offer the possibility of unique non-equilibrium phases forming at room temperature.

The determination of the properties of the novel compositions and structures possible by these methods (including their biocompatibility characteristics) presents an exciting area for future biomaterials research.

THANK YOU! (QUESTIONS)

Basamat SeifOmar AbdulRAHIMOmar AlaaSalah FathALLAHMohamad MehanyAhmad Waleed

Metallic Bio Materials

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