Ming Li, Ph.D.

Professor of Materials Science and Engineering

Central South University

E-mail:liming0823@csu.edu.cn

Office: Room 308, Chemistry Building, Main Campus

BIOMATERIALS

Lecture 2: Properties of Materials

Sept. 11, 2019

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Last Lecture

• Introduction and overview

• Basic concepts and course requirements

• Covered topics

• History of biomaterials

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Contents

• Nature of matter and materials

• Bulk properties of materials

• Surface properties of materials

• Surface analysis techniques

• Role of water

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1. Nature of Matter and Materials

❑ Biomaterials are materials

❑ The key to understanding matter is to understand

attractive and interactive forces between atoms

What holds those atoms and molecules together to make a

strong nylon fiber or a cell membrane, or a hard, brittle

hydroxyapatite ceramic, or a sheet of gold, or a drop of

water?

In the early 18th century, Isaac Newton was pondering this

issue: “There are therefore Agents in Nature able to make

the Particles of Bodies stick together by very strong

Attractions.”

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1. Nature of Matter and Materials

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• Atom-atom interactions through electron cloud coupling

• Electrostatic interaction

• Hydrogen bond• Metallic bonding

• Covalent bonding

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Molecules

Supramolecules,

assemblies or

organs

Atoms

Hierarchical structure of Materials

Collagen protein

1. Nature of Matter and Materials

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2. Bulk Properties of Materials

❑ Physical properties

density, electrical, magnetic, optical, …

❑ Mechanical properties

Tensile strength, ductility, brittleness,

elasticity, plasticity, toughness, hardness, …

❑ Chemical properties

corrosion resistance, …

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Mechanical Properties of Materials

• elastic and shear modulus (E, G)

• strength (σy), ultimate tensile strength

(σB) and fracture strength (σf)

• ductility (δ)

• fracture toughness (KIC)

• fatigue strength (σNf)

• creep strength (σc)

2. Bulk Properties of Materials

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Load, stress and strain: deformation

𝜎𝑛 =𝐹

𝐴1

𝜀𝑛 =𝐼2 − 𝐼1𝐼1

=∆𝐼

𝐼1

nominal stress:

nominal strain:

true stress:

true strain:

𝜎𝑡 =𝐹

𝐴2

= 𝜎𝑛 𝜀𝑛 + 1

𝜀𝑡 1→2 = 𝑙𝑛𝐼2𝐼1= 𝑙𝑛 𝜀𝑛 + 1

2. Bulk Properties of Materials

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Typical nominal stress vs. nominal strain

2. Bulk Properties of Materials

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nominal stress vs. nominal strain

2. Bulk Properties of Materials

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1. Which is much stiffer?

Same stiffness - same slope

2. Which is stronger?

Red - holds a higher stress

before failure

3. Which is brittle?

Both are brittle because they only

exhibit elastic deformation. Red has

a higher strain-to-failure or total

elongation, but both are brittle strain

materials

2. Bulk Properties of Materials

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1. Which is much stiffer?

Green is much stiffer because it

has a steeper slope, therefore a

higher elastic modulus.

2. Which is stronger?

both have the same strength

because they hold the same

stress at failure.

3. Which is ductile?

Neither, both are brittle (only

elastic deformation)

2. Bulk Properties of Materials

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3. Surface Properties of Materials

Five Points About Surfaces

1. Surfaces have unique reactivity

2. The surface is inevitably different from the bulk

3. The mass of material that makes up the surface zone

is very small

4. Surfaces readily contaminate

5. Surface molecules can exhibit considerable mobility.

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❑ Reversal surface

structure when

transferred from air

to water

❑ Surface reactivity:

• Bonding orbitals of

atoms on the surface

• surface adsorption on

the medical device

surface

3. Surface Properties of Materials

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Surface characteristics

Rough, stepped

or smoothVarious surface

chemistries

Structurally or

compositionally

inhomogeneous

Depth

inhomogeneous

or overlayered

thin film

Highly crystalline or

disordered

Unreconstructed or

reconstructed surface

3. Surface Properties of Materials

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4. Surface Analysis Techniques

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Contact Angle

Contact angles directly measure surface wettability, and indirectly

probe surface energy, roughness, heterogeneity, contamination,

and molecular mobility.

𝛾𝑠𝑣 = 𝛾𝑠𝑙 + 𝛾𝑙𝑣 cos 𝜃

4. Surface Analysis Techniques

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4. Surface Analysis Techniques: Contact Angle

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Also called Electron Spectroscopy for Chemical Analysis (ESCA)

X-ray photoelectron spectroscopy

(XPS)

BE = ℎ𝑣 − KE

Binding

energyEnergy

of X-rays

Kinetic

energy

XPS analyzes to approximately 10 nm and gives information on

elements present, their concentrations and their bonding

environments.

4. Surface Analysis Techniques

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4. Surface Analysis Techniques: XPS

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Survey scan Detailed scan

C1s

4. Surface Analysis Techniques: XPS

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Secondary Ion Mass Spectrometry

(SIMS)

Static SIMS provides qualitative information on the atomic and

molecular composition in the outermost 1–2 nm of surface

with high analytical sensitivity and excellent x,y spatial

resolution.

4. Surface Analysis Techniques

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Other Surface Analysis Techniques

SEM provides a high resolution image of the surface. On

insulating materials, metallic coating is required and the image is

actually of the coating surface, not the underlying material.

• Scanning electron microscopy (SEM):

4. Surface Analysis Techniques

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• Attenuated total reflectance infrared (ATR-IR) spectrometry:

ATR-IR permits detailed molecular analysis of the outermost

1–5 microns of a sample.

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Raman spectroscopy and surface-enhanced Raman

spectroscopy will be introduced later

• Scanning Tunneling Microscopy (STM), Atomic Force

Microscopy (AFM), and the Scanning Probe Microscopies

(SPM).

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5. Role of Water

In the world there is nothing more submissive and weak

than water. Yet for attacking that which is hard and strong

nothing can surpass it.

Lao Tzu

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Five water molecule interactions

1. free water (similar to bulk water) (c)

2. tightly-bound water (more structured

and with limited mobility) (a)

3. intermediate water (b)

5. Role of Water

free water

tightly-

bound

water

intermediate

water

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Water: significance for biomaterials

When a protein or a cell approaches a biomaterial, it interacts

with the surface water first.

Lipid molecules with polar head

groups (white) and hydrophobic tails

(brown), when placed in water,

organize themselves to minimize

surface area contact between the

hydrophobic tail (typically comprised

of methylene units, -CH2-) and water.

This minimization of contact area,

depending on precise conditions,

can lead to micelles, liposomes or

bilayer sheets.

5. Role of Water

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Role of water molecules at the biointerface of

medical polymers

(a) Bio/blood-compatible

polymers.

(b) Non-bio/blood-compatible

polymers.

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Reading materials:

Book: Biomaterials Science: An Introduction to Materials in

Medicine (3rd Edition, 2013)

• Properties of Materials

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Lecture 3: Metals as Biomaterials

On Monday, Sept. 16 , 2019

Next Lecture