生物材料學 BIOMATERIALS
The Structure of Solids
Sheng-Yang Lee,
DDS, MS, PhD, FACD
Professor and Dean
School of Dentistry, Taipei Medical University
1
Purpose of the Class To develop in the students a
familiarity with the uses of
materials in medicine and with the
rational basis for these applications.
2
Properties of a material
‡
Properties of a material determined by its
1. Chemical composition
2. Structure
‡
Internal structural arrangement of the atoms
Æ Chemical behavior
‡
Levels of scale:
ƒ atomic or molecular (0.1 – 1 nm)
ƒ nanoscale or ultrastructural (1 nm - 1μm)
ƒ microstructural (1μm – 1 mm)
ƒ macrostructural (> 1 mm)
‡
Solid-liquid interface can affect dissolved species in the
surrounding fluid:
(i) Molecular level (3-15 Å) – chemical effect
(ii) Macromolecular level (15-500 Å) – mechanical nature
3
Atomic Bonding
4
Atomic Bonding
‡
All solids made of atoms
– held together with the interaction of the
outmost (valence) electrons
‡
Nature of the patterns:
1. Primary bond
(1) Metallic bonding – e- loosely held to the ions
Æ nondirectional bond
Æ plastic deformations
5
Atomic Bonding
(2) Ionic bonding
– formed by exchanging electrons between
metallic and nonmetallic atoms
Æ very directional bonds
* Strong
repulsive forces of like ions
Ælimited atomic arrangement
6
Atomic Bonding
(3) Covalent bonding
– formed when atoms share valence e- to
satisfy their partially filled electronic orbitals
* Overlap of valence orbitals↑Æ Bonds↑
(Bur limited by strong repulsive forces between nuclei)
-- highly directional and strong
* diamond – the hardest material known
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Atomic Bonding
2.
Secondary bonds
– can be a major factor contributing to material
properties
„
Two major secondary bonds:
(1) Hydrogen bond
– arising when H covalently bonded to an
electronegative atom (F, O, N) Æ H+
⇒ electrostatic force formation
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Atomic Bonding
(2) van der Waals bond
– arising through fluctuating dipole-dipole
interactions
- nondirectional bonds
- much weaker than hydrogen bonds
* when electrons are not distributed equally
among ions
Æ dipoles
* A fluctuating dipole moment of molecule or atom
Æ induce a moment in neighboring atoms
Æ weak electrostatic interaction of induced and
original moments
Æ an attraction force
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Atomic Bonding
Bond type
Substance
van der Waals
Hydrogen
N2
Phenol
HF
Na
Fe
NaCl
MgO
Diamond
SiO2
Metallic
Ionic
Covalent
Heat of
vaporization
(kJ/mol)
13
31
47
180
652
1062
1880
1180
2810
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Atomic Bonding
‡
The real materials may show some combinations of the
bonding characteristics.
e.g., silicon atoms Æ share electrons covalent bur a fraction
of electrons can be freed and permit limited conductivity
(semiconductivity) Æ Silicon has covalent & some
metallic bonding characteristics
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Crystal Structure 12
Crystal Structure
1.
Atoms of the Same Size
- Arrangement of atoms can be treated as an
arrangement of hard spheres in view of their
maintenance of characteristic equilibrium distances
(bond length)
Æ X rays (short wavelengths,
~ 1Å ≈ atomic radius)
measurement
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Crystal Structure
1.
Atoms of the Same Size
- Atoms arranged in a regular array Æ represented by a
unit cell Æ having a characteristic dimension, the
lattice constant, a.
* If extended into three dimensions Æ simple cubic
space lattice (one of three types of cubic crystals)
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Crystal Structure
‡
Another two cubic crystals:
(1) Face-centered cubic (fcc)
- Close-packed (or actually closest packed) in three dimensions
- Each atom touches 12 neighbors Æ coordination number (CN)
= 12 (rather than 6 in simple cubic) Æ most efficiently packed
structure (packing efficiency Æ 74%)
- represented by three layers of planes ABCABC…
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Crystal Structure
(2) Body-centered cubic (bcc)
- An atom located in the center of the cube
- lower packing efficiency (68%) than fcc
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Crystal Structure
‡
Noncubic crystals
Hexagonal close-packed (hcp)
- having the most efficient packed planes of atoms (as
fcc) with 12 CN (packing fraction Æ 74%)
- repeating layers every other plane as ABAB…
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Crystal Structure
‡
Others:
Orthorhombic
– Unit cell is a rectangular parallelepiped with
unequal sides
Monoclinic
– Unit cell is an oblique parallelepiped with one
oblique angle and unequal sides
Triclinic
– Unit cell has unequal sides and all oblique angels.
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Crystal Structure
Material
Cr
Co
Fe
Ferrite(α)
Austenite(γ)
Delta iron(δ)
Mo
Ni
Ti
Rock salt (NaCl)
Alumina (Al2O3)
Polyethylene
Polyisoprene
Crystal structure
bcc
hcp (below 417℃)
fcc (above 417℃)
bcc (below 912℃)
fcc (912-1394℃)
bcc (above 1394℃)
bcc
fcc
hcp (below 900℃)
bcc (above 900℃)
fcc
hcp
orthorhombic
orthorhombic
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Crystal Structure
2. Atoms of Different Size
„
„
„
Pure materials are seldom used for implants
Most of the materials used for implants made of more
than two elements.
Two or more different sizes of atoms mixed in a solid
Æ two factors must be considered:
(1) Type of site
(2) Number of sites occupied
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Crystal Structure
‡
At a certain radius ratio of the
host and interstitial atoms the
arrangement will be most stable
(i.e., the maximum
interaction between atoms)
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Imperfections In Crystalline Structures
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Imperfections In Crystalline Structures
‡
Imperfections in crystalline solids Æ called defects Æ
play a major role in determining physical properties
1. Point defects – lattice vacancies
– substitutional or interstitial atoms
Æ called alloying elements if put in
intentionally
Æ called impurities if they are
unintentional
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Imperfections In Crystalline Structures
2. Line defects (Dislocations) – created when an extra
plane of atoms is displaced or dislocated out of its
regular lattice space registry
* L’t
– Screw dislocation (Line parallel to shear direction);
R’t – Edge dislocation (Line ⊥ shear direction)
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Imperfections In Crystalline Structures
‡
↓ the strength of a solid crystal enormously
(∵ it takes much less energy to move or deform a
whole plane of atoms one atomic distance at a
time rather than all at once)
e.g., moving a carpet on the floor or a heavy
refrigerator
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Imperfections In Crystalline Structures
‡
If a lot of dislocations introduced in a solid
Æ strength ↑ considerably
(∵ the dislocations become entangled with each other
Æ impeding their movement)
e.g., Blacksmith heats a horseshoe red-hot and
hammers it repeatedly
Æ ↑ number of dislocations without breaking the
horseshoe
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Imperfections In Crystalline Structures
3. Planar defects – exist at grain boundary
* Two or more crystals mismatched (with different
orientation) at the boundaries (occurs during
crystallization)
Æ grain boundaries
(Grain: All of the atoms are in a lattice of one specific
orientation)
* Grain boundary is less dense than the bulk
Æ Most diffusion of gas or liquid takes place along the
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grain boundary
Imperfections In Crystalline Structures
* Grain boundary atoms possess higher energy than
the bulk Æ a more chemically reactive site at the
boundary (can be seen by polishing & etching of a
‘polycrystalline’ material)
* Grain size Æ affect physical properties
e.g., A fine-grained structure stronger than a coarse one
(∵ the former contains more grain boundaries
Æ interfere with the movement of atoms during
deforming Æ a stronger material)
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Long‐Chain Molecular Compounds (Polymers)
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Long‐Chain Molecular Compounds (Polymers)
‡
Polymers have very long-chain molecules formed
by covalent bonding along the balckbone chain
„
The long chains held together by
(1) secondary bonding forces
(e.g., van der Waals & hydrogen bonds)
or (2) primary covalent bonding forces
through cross-links between chains
„
The long chains are very flexible & tangled easily
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Long‐Chain Molecular Compounds (Polymers)
‡
Each chain can have side groups, branches, and
copolymeric chains or blocks
Æ interfere with the long-range ordering of chains
Æ Steric hindrance
Æ a more noncrystalline structure
•
Semicrystalline
– more commonly occurring structure for linear polymers:
Disordered noncrystalline regions ┼ Ordered crystalline regions
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Long‐Chain Molecular Compounds (Polymers)
‡
Degree of Polymerization (DP)
„
„
Defined as average number of mers or repeating
units per molecule (i.e., chain)
Each chain may have a small or large number of
mers depending on the condition of polymerization
Average molecular weight =
DP x molecular weight of mer
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Long‐Chain Molecular Compounds (Polymers)
‡
The longer molecular chains
(by progress of polymerization)
Æ relative mobility ↓
Æ physical properties of final polymer ↑
‡
The higher the molecular weight, the less the
mobility of chains
Æ the higher strength & greater thermal stability
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Long‐Chain Molecular Compounds (Polymers)
‡
The polymer chains can be arranged in three ways:
(1) linear
„
Polyvinyls, polyamides, polyesters, etc.
„
Much easier to crystallized than (2) & (3)
„
However, they cannot be crystallized 100% as metals
(2) branched
(3) cross-linked or three-dimensional network
„
e.g., (poly)phenolformaldehyde
„
Cannot be crystallized at all Æ noncrystalline,
amorphous polymers
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Long‐Chain Molecular Compounds (Polymers)
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Long‐Chain Molecular Compounds (Polymers)
‡
Linear polymers usually become semicrystalline:
„
Arrangement of chains in crystalline regions
Æ a combination of folded and extended chains
„
Chain folds Æ more difficult to form
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Long‐Chain Molecular Compounds (Polymers)
‡
Vinyl polymers
„
The most important linear polymer
„
Repeating unit: -CH2-CHRÆ R is some monovalent side group
R = H Æ PE (HDPE, LDPE, LLDPE)
R = CH3 Æ PP
R = C6H5 Æ PS
R = Cl Æ PVC
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Long‐Chain Molecular Compounds (Polymers)
‡
Three possible arrangements of side groups (R):
(1) Atactic
– Side groups randomly distributed
(2) Syndiotactic
– Side groups in alternating positions
(3) Isotactic
– Side groups in one side of the main chain
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Long‐Chain Molecular Compounds (Polymers)
‡
The isotactic and syndiotactic polymers usually
crystallize even when the side groups are large
39
Long‐Chain Molecular Compounds (Polymers)
‡
Copolymerization
Two or more homopolymers (one type of repeating unit
throughout its structure) chemically combined
Æ always disrupting the regularity
Æ promoting the formation of
noncrystalline structure
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Long‐Chain Molecular Compounds (Polymers)
‡
Addition of plasticizers
Æ preventing crystallization by keeping chains
separated from one another
Æ a noncrystalline version of a polymer
(that normally crystallizes)
e.g., (i) Celluloid – normally crystalline nitrocellulose
plasticized with camphor
(ii) Rigid noncrystalline polymer (like polyvinyl
chloride, PVC) + plasticizers
Æ more flexible solid
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Long‐Chain Molecular Compounds (Polymers)
‡
Elastomer or Rubber
„
Large stretchability at room T and can snap back to
their original dimensions when load released
Æ Recoverability
„
Noncrystalline (Amorphous) polymers that have an
intermediate structure consisting of long-chain
molecules in three-dimensional network
Æ ‘kink’ or ‘bend” in chains
Æ straighten when a load applied
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Long‐Chain Molecular Compounds (Polymers)
‡
Below glass transition temperature (Tg)
Æ natural rubber loses its compliance
Æ a glasslike material
„
Tg – Transition temperature between a
supercooled liquid and its rigid glassy solid
‡
To be flexible, all elastomers should have Tg
well below room temperature
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Supercooled And Network Solids
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Supercooled And Network Solids
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Glass
„
An amorphous solid below its transition temperature
„
Lacks long-range crystalline order, but normally has
short-range order
„
Usually supercooled from the liquid state and thus
retain a liquidlike molecular structure
Æ less density than that of the crystalline state of the
same material
Æ indicating inclusion of some voids (free volume)
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Supercooled And Network Solids
When a liquid cooled
Æ contracts rapidly and continuously (∵ decreased
thermal agitation
Æ atoms develop more efficient packing arrangements)
‡
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Supercooled And Network Solids
‡
In the absence of crystallization, the contraction continues
below Tm to the Tg Æ material becomes a rigid glass
‡
Below Tg, no further rearrangements occur. The only further
contraction is caused by reduced thermal vibrations of the
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atoms in their established locations
Supercooled And Network Solids
‡
Due to quasi-equilibrium state of the structure
Æ amorphous material tends to crystallize
‡
More brittle and less strong than crystalline
counterpart
‡
Very difficult to make metals amorphous,
∵ metal atoms are extremely mobile
‡
The ceramics and polymers can be made
amorphous because of the sluggish mobility
of their molecules
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Supercooled And Network Solids
‡
Network structure of a solid
Æ a three-dimensional, amorphous structure since the
restrictions on the bonds and rigidity of subunits
prevent them from crystallizing
Æ not flow at high temperature
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Composite Material Structure
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Composite Material Structure
‡
Consisting of two or more distinct parts
‡
Distinct phases separated on a scale larger than the
atomic, and properties (e.g., elastic modulus)
significantly altered in comparison with those of a
homogeneous material
‡
Bone and fiberglass Æ Composites
Brass, or metals (e.g., steel with carbide particles) Æ not
composites
‡
Although many engineering materials (including
biomaterials) are not composites virtually all natural
biological materials are composites
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Composite Material Structure
‡
The properties of a composite material depend
upon
1)
Shape of the inhomogeneities (second phase
material) e.g., particle, fiber, platelet or lamina)
2)
Volume fraction occupied by them
3)
Stiffness and integrity of the interface between the
constituents
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Reference
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自行編纂
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Summary
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Biomaterials
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Biocompatibility
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Biological Environment
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Swelling and Leaching
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Interfacial-Dependent Phenomena in Biomaterials
‡
The Structure of Solids
‡
Characterization of Materials
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