BIOMATERIALS...25 2. Ceramics as Biomaterials Bioactive Ceramics, Active Glasses and Glass-Ceramics • Bonding to bone was first demonstrated for a compositional range of bioactive

Ming Li, Ph.D.

Professor of Materials Science and Engineering

Central South University

E-mail:[email protected]

Office: Room 308, Chemistry Building, Main Campus

BIOMATERIALS

Lecture 4: Ceramics, Glasses, and Glass-

Ceramics

Sept. 18, 2019

2

Book: Biomaterials Science: An Introduction to Materials

in Medicine (3rd Edition, 2013)

• Two points for each error found

in the book

• Only count for the first person

who finds the error

NOTE

3

Last Lecture

• Properties and challenges

• Commonly used metals

Stainless steel

Titanium

Cobalt alloys

• Smart metal biomaterials

Shape memory metals

Magnetostrictive materials

4

Contents

• Properties of ceramics

• Ceramics as biomaterials

Inert ceramics

Porous ceramics

Bioactive Ceramics, Active

Glasses and Glass Ceramics

Biodegradable ceramics

5

Contents



Inert ceramics

Porous ceramics


Glasses and Glass Ceramics


6

Ceramic biomaterials

❑ Ceramics are refractory polycrystalline compounds;

• Inorganic

• Hard and brittle

• High compressive strength

❑ Applications:

• Orthopaedic load-bearing coatings

• Dental implants

• Bone graft substitutes

• Bone cements

1. Properties of ceramics

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1. Inert (Al2O3, ZrO2)

2. Nearly inert, porous (hydroxyapatite)

3. Bioactive (bioglass)

4. Resorbable/degradable (Ca3(PO4)2)

Types of ceramic biomaterials

Bioactive compound is a compound that has an effect on a

living organism, tissue or cell.

Bioresorbable compounds are designed to degrade safely

within the body.


8

Definitions

If the material is...

• toxic surrounding tissue dies

• nontoxic and biologically inactive (nearly inert)

fibrous tissue of variable thickness forms

• nontoxic and biologically active (bioactive): an

interfacial bond form

• nontoxic and dissolves: surrounding tissue

replaces it


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Pros:

1. inert or bioactive

2. high wear resistance

3. high stiffness and compression strength

4. aesthetic (dental)

Cons:

1. brittle

2. low tensile strength

3. low fatigue strength

Ceramic biomaterials


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Types of ceramic biomaterials

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Bioactivity spectra for

various bioceramic implants

(A) Relative rate of bioreactivity

(B) Time-dependence of

formation of bone bonding at

an implant interface.


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Ceramic Biomaterials


BIOLOX®* delta ceramic femoral head

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Natural hard tissues are ceramic-polymer composites

• Bones, Teeth, Shells

Hydroxyapatite Ca5(PO4)3OH gives teeth and bones their

hardness!

Hard tissues

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Bonding structure

The extent to which a ceramic lattice is ionic or covalent in

nature will depend on the electro-negativities of the atoms

involved

15

Contents



Inert ceramics

Porous ceramics

Bioactive Ceramics, Active Glasses

and Glass Ceramics


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Inert Ceramics

Alumina (Al2O3) and Zirconia (ZrO2)

2. Ceramics as biomaterials

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Used in total hip arthroplasty since 1970

• high-purity alumina (around 99.7%)

• MgO controls grain size during the

sintering process

Alumina used in orthopaedic applications

should have a grain size <7 μm

Alumina (Al2O3)

2. Ceramics as biomaterials: Inert Ceramics

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Crystal structure of alumina (○: aluminum; ●: oxygen)


Rhombohedral structure (a = 4.758 Å

and c = 12.991 Å)

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Production of Alumina

Three stages:

1. Extraction: The aluminum-bearing minerals in bauxite are

selectively extracted from the insoluble components by

dissolving them in a solution of sodium hydroxide.

Al(OH)3 + Na+ + OH− → Al(OH)4− + Na+

2. Precipitation: Crystalline aluminum trihydroxide is precipitated.

This is the reverse of the extraction process, except the

chemistries are well-controlled.

Al(OH)4− + Na+ → Al(OH)3 + Na+ + OH−

3. Calcination: Aluminum trihydroxide is calcined to form

alumina. The water is driven off to form alumina. This process

dictates the properties of the final product.

2Al(OH)3 → Al2O3 + 3H2O

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Total hip

replacement

implants

Total knee

replacement

implants Ceramic

Dental

ImplantsBIOLOX®* delta

ceramic femoral

head.

Alumina: Applications


21

Zirconia (ZrO2)

Yttrium stabilizes the tetragonal phase


22

Zirconia failures

Long term tension and moisture...

Monoclinic content from 1%→30% on the surface

→Surface roughness from 0.006 m up to 0.12 μm


23

Zirconia

Still widely used in dental applications


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• Good mechanical tissue-implant fixation

• Poor mechanical properties

Porous Ceramics

When pore sizes exceed 100 µm, bone will grow within the

interconnecting pore channels near the surface and maintain

its vascularity and long-term viability.

2. Ceramics as biomaterials

25

2. Ceramics as Biomaterials


Glasses and Glass-Ceramics

• Bonding to bone was first demonstrated for a compositional range of

bioactive glasses that contained SiO2, Na2O, CaO, and P2O5 in specific

proportions

• Many bioactive silica glasses are based upon the formula called 45S5,

signifying 45 wt.% SiO2 (S = the network former) and 5:1 ratio of CaO

to P2O5.

• Glasses with lower ratios of CaO to P2O5 do not bond to bone.

Substitutions in the 45S5 formula of 5–15wt.% B2O3 for SiO2 or 12.5

wt.% CaF2 for CaO or heat treating the bioactive glass compositions to

form glass-ceramics have no measurable effect on the ability of the

material to form a bone bond. However, adding as little as 3 wt.% Al2O3

to the 45S5 formula prevents bonding to bone.

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The compositional dependence of bone and soft tissue bonding

on the Na2O–CaO–P2O5–SiO2 glasses

Compositional dependence (in wt.%) of

bone bonding and soft tissue bonding of

bioactive glasses and glass-ceramics. All

compositions in region A have a constant

6 wt.% of P2O5. A-W glass ceramic has

higher P2O5 content. IB, Index of

bioactivity.

2. Ceramics as Biomaterials: Bioactive Ceramics,

Active Glasses and Glass-Ceramics

• All the glasses contain a constant 6 wt.% of

P2O5.

• Compositions in the middle of the diagram

(region A) form a bond with bone.

• Consequently, region A is termed the

bioactive bone bonding boundary.

• Silicate glasses within region B (e.g.,

window or bottle glass or microscope

slides) behave as nearly inert materials and

elicit a fibrous capsule at the implant–tissue

interface.

• Glasses within region C are resorbable and

disappear within 10 to 30 days of

implantation.

• Glasses within region D are not technically

practical, and therefore have not been

tested as implants

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Glasses and Glass-ceramics



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Types of silicate glass interfaces with

aqueous or physiological solutions

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30

Calcium Phosphate Ceramics• Bone typically consists by weight of 25% water, 15% organic materials,

and 60% mineral phases. The mineral phase consists primarily of calcium

and phosphate ions, with traces of magnesium, carbonate, hydroxyl,

chloride, fluoride, and citrate ions.

• Calcium phosphates occur naturally in the body

• But they also occur within nature as mineral rocks, and certain

compounds can be synthesized in the laboratory.

Mineral name, chemical name, and composition of various phases

of calcium phosphates



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Thermodynamic stability of calcium

phosphates in a water atmosphere

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Hydroxyapatite(Ca10(PO4)6(OH)2), HAP



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Optical properties

Raman spectrum of HAP samples

Hydroxyapatite

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960 cm-1

mouse cortical bone (785-nm laser)

Hydroxyapatite

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Raman FTIR

poorly crystalline HAP

bovine cortical bone

HAP ceramic

highly crystalline HAP

Hydroxyapatite

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Hydroxyapatite

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XRD patterns

Hydroxyapatite

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Hydroxyapatite

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Hydroxyapatite: Mechanical Properties

Hydroxyapatite

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Biodegradable bioceramics

Degradation Mechanism:

Degradation = chemical breakdown of the bulk materials

1. Physiochemical dissolution

2. Physical disintegration into small particles at grain

boundaries

3. Biological factors, such as phagocytosis

2. Ceramics as Biomaterials

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❑ All calcium phosphate ceramics biodegrade to varying degrees;

the rate of biodegradation increases as:

1. Decreasing Ca/P ratio

2. Surface area increases (powders > porous solid >dense solid);

3. Crystallinity decreases;

4. Crystal perfection decreases;

5. Crystal and grain size decrease;

6. There are ionic substitutions of CO32-, Mg2+, and Sr2+ in HA.

❑ Factors that tend to decrease the rate of biodegradation include:

(1) F− substitution in HA;

(2) Mg2+ substitution in β-TCP; and

(3) lower β-TCP/HA ratios in biphasic calcium phosphates.

2. Ceramics as Biomaterials: biodegradable bioceramics

42

Typical forms of calcium phosphates


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Degradation of ceramic biomaterials

Osteoclasts (bone-resorbing cells) mediate true degradation of

the ceramic component of bone

Takeway is that osteoclasts "eat"crystal fragments, which are

then metabolized intracellularly


44

α-TCP:

• dissolution speed limits usage

β-TCP:

• Osteoinductive degradation similar to

bone resorption

Tricalcium phosphates (TCP)


45

• preservation of strength

• degradation speed = regeneration speed

• degradation products

Challenges for degradable ceramics


46

Reading Materials:

Book: Biomaterials Science: An Introduction to Materials

in Medicine (3rd Edition, 2013)

• Ceramics, Glasses, and Glass-Ceramics: Basic Principles

47

Lecture 5: Carbon Biomaterials

On Tuesday, September 23, 2019

Next Lecture

BIOMATERIALS...25 2. Ceramics as Biomaterials Bioactive Ceramics, Active Glasses and Glass-Ceramics • Bonding to bone was first demonstrated for a compositional range of bioactive

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