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Overview of Biomaterials
By:
Dr. Murtaza Najabat Ali (CEng MIMechE P.E.)
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Biomaterials can be divided into four major classes of materials
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• Polymers
o
Consists of repeating units strung together in long chains
o
Represent the largest class of biomaterials
o
Natural
o
Synthetic
o
Hydrophobic
o
Hydrophilic
o
Water swelling materials (Hydrogels)
o
Biostable
o
Biodegradable
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• Application of Polymers as Biomaterials
The hip joint is the largest load
bearing joint. A hip joint is lined
Polymers used as femoral head and
acetabular cup in total hip replacement
with a layer of cartilage that reduces
friction and acts as a shock absorber.
When the bone is exposed to
arthritis and injury, this protective
layer is damaged, causing extreme
pain.
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• Polymers
By using a Polymethylmethacrylate
(PMMA)
cement to adhere the metal to
the bone
By using a porous
metal surface to create
a
bone
ingrowth
interface
The acetabulam and the proximal
femur ca be also replaced, where the
femoral side is completely metal. The
acetabular side is composed of the
polyethylene bearing surface
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• Polymers
Bonding Time:
Cemented Vs. Cementless
Cemented
•Approximately 10min
Cementless
•One year or more for good ingrowth
Healing Time
Cemented
Cementless
Weight bearing
Next day with crutches
Next day depending on stability
Walk (crutches)
2 day after surgery
2 day after surgery
6-12 weeks depending on surgeon
Pain Free
5 days in some cases
Depends on stability
Normal Walk
2 weeks (6 weeks with
crutches is
recommended)
Couple of months to 1 year
depending on ingrowth rate
Everything good
Around six months for a
knee replacement
2 years onward
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• Polymers
PMMA and silicone elastomers are used as
Intraocular lenses to treat cataract
Hard contact lenses for
clear vision
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• Polymers
Polyethylene, Polytetrafluoroethylene
(PTFE) and silicone used as medical
tubing for drains and catheters
Ultra high molecular weight Polyethylene,
as Femoral head, acetabular cup or insert
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• Polymers
The healthy human knee joint is also
lined with articular cartilage. Arthritis
and injury can similarly damage this
protective layer of cartilage causing
extreme pain
Hyaluronic
Acid
(HA)
Injections restores lubrication
and fluid in the joint and
creates a shock absorber
between the bones
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• Polymers
PTFE and Polyurethane used
as Vascular and non-vascular
grafts
Pacemakers leads are insulated
by silicone
Polyvinylchloride (PVC) and
silicone used as Tubing, blood
storage
bags,
in
blood
transfusion, feeding and dialysis
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• Metals
•
Load bearing implants and internal fixation devices;
•
When processed suitably contribute high tensile, high fatigue and high yield strengths;
•
Properties depend on the processing method and purity of the metal.
•
When metallic biomaterials were first used in biomedical applications, the only requirement
was to achieve a suitable combination of physical properties to match those of the replaced
tissue with a minimal toxic response of the host
•
Few concepts were gradually introduced with time as requirements for metallic biomaterials
in the design of implantable devices, such as
 Foreign body reaction (particularly due to wear debris)
 Stress shielding
 Biocompatibility
 Bioactivity, and
 Osteoinduction
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• Metals
Although many metals and alloys are used for medical device applications, the most commonly
employed are
• Stainless steels
• Cobalt-base alloys
• Commercially pure Titanium and Titanium alloys
• Nickel- Titanium alloys, and some
• Noble metals (Au, Pt, Pd)
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• Metals
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• Metals
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• Metals
Ti alloy Pacemaker
case and metallic leads
encapsulated in silicone
tubing
Metal electrodes
In Phrenic stimulator for
respiratory control
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• Metals
Tooth metal crown
For securing removable denture
Ti alloy single teeth replacement
Metal wires and braces
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• Metals
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• Ceramics
• Bone bonds well to them
• Exhibit minimum foreign body reaction
• High stiffness
• Low friction and wear coefficients
But
• Low fracture toughness
Owing to its Brittle nature
• Low impact resistance
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• Ceramics
• Alumina (Al2O3)
• Zirconia
• Several porous ceramics (CaCO3)
Calcium Phosphates (CaPs)
Bioactive Glasses (BGs)
Glass Ceramics
• Hydroxyapatite Ca10(PO4)6(OH)2
• A range of suitable CaPs (amorphous CaP (ACP), dicalcium phosphate (DCP), tricalcium
phosphate (α-TCP, β-TCP), tetracalcium phosphate (TTCP) e.t.c.)
• Several BGs formulations containing e.g. SiO2 as network former and Na2O, CaO and P2O5 as
modifying oxides (Bioglass 45S5)
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• Ceramics
Alumina Femoral head
and Acetabular liner
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• Ceramics
Zirconia dental crown and bridges
Hydroxyapatite coated
metallic implants
Zirconia artificial tooth
root
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• Ceramics
Calcium Phosphate Scaffold
All-carbon mono-leaflet mechanical heart valves consist
of a rotatable housing ring within a compliant suture ring
HA scaffold (light purple colour)
with bone (yellow colour) that
has grown in the scaffold
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Example 6 :
Which of the following classes of biomaterials would be most appropriate
for use to fabricate an artificial tendon, a tissue that must sustain
substantial deformation at low forces and return rapidly to its original
dimensions upon release of the stress ? WHY ?
(a) Metals
(b) Ceramics
(c) Polymers
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The selection of the biomaterial and selection of the appropriate processing techniques for a
given application is determined primarily by
• Surface
• Bulk, and
• Degradative properties of the material
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What constitutes a surface?
An interface is the boundary region between two adjacent layers
We recognize (S/G), (S/L),
and (L/V) as surfaces
L
G
S
L
S
S
V
L
L
S
L = Liquid
G = Gas
S = Solid
V = Vapor
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Biomaterial surface
Protein adsorption
Proteins (red) adsorb differently to the different materials and are
depicted as elongated on metal and globular on polymer. Cells
(blue) interact with the materials via the adsorbed proteins and
conformation of the adsorbed proteins dictates how the cells will
respond (adhere, proliferate, differentiate, etc.).
The effect of protein size
on interaction with a
surface. Notice that the
larger protein composed
of more amino acids is
capable of making more
interactions
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 The surface of a material is defined to be the few atomic
layers on the exterior of the object
 Surface properties can be different from the bulk
properties
 Surface properties include both chemical and physical
characteristics
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 Surface Chemical Property
 Contact Angle
The contact angle is the angle at which a
liquid-vapor interface meets the solid-liquid
interface
• The contact angle is determined by the
resultant between Adhesive and
Cohesive forces
• As the tendency of a drop to spread out
over a flat solid surface increases, the
contact angle decreases
 Wetting
A shows a fluid with very little wetting, while C shows a
fluid with more wetting. A has a large contact angle, and
C has a small contact angle.
Wetting is the ability of a liquid to maintain
contact with a solid surface, resulting from
intermolecular interactions when the two
are brought together
Thus, the Contact angle provides an inverse measure of Wettability
Water bead on
a fabric that
has been made
hydrophobic
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 Surface Tension
• Surface tension is a measurement of the
cohesive energy present at an interface
• This forms a surface “Film” which makes it
more difficult to move an object through the
surface
 Hydrophilicity
• If the liquid is very strongly attracted to the
solid surface
• The droplet will completely spread out on the
solid surface
 Hydrophobicity
• If the solid surface is hydrophobic, the
contact angle will be larger than 900C
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 Surface Physical Property
 Surface Roughness
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Example 2.1:
Would a hydrophobic or hydrophilic polymer be a more appropriate choice for a Contact
lens application? WHY ? Would a melting temperature (Tm) of the polymer above 37oC or
below 370C be more appropriate for this application ? WHY ?
Example 2.2:
A 1ml droplet of distilled water is dropped onto each material “A” and “B” as shown
below. Which material is more hydrophilic ? Justify your answer.
Example 2.3:
Why do atoms at the surface of a crystalline material generally possess higher energy than
those inside of the crystal and what is the term for this heightened energy ?
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 Most important parameter
 They may play a less significant role in initial biological interaction/response, but they
have a large long-term impact
 They can be altered to allow the biomaterial to mimic the physicochemical properties of
the biological system
 Bulk characteristic of biomaterials include
• Mechanical Properties
• Physical Properties
• Chemical composition
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 Mechanical Properties
Such as,
• Strength
• Stiffness
• Anisotropy
• Fatigue properties/strength
Mechanical properties of materials are highly affected by their
physical and chemical characteristics
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 Physical Properties
Such as,
• Crystallinity
•
Thermal Transition Points
Melting Point (Tm)
Glass Transition Point (Tg)
 Chemical Composition
• It varies when chemicals are added or
subtracted, and when the ratio of
substances changes
• It determines the (bulk) properties of
the substance
• It also dictates other properties such
as chemistry at the surface
SEM Micrograph of steel
showing grain boundaries
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Factors that affect biomaterials’ degradation in vivo

The size and shape of the implant

Its location in the body

Chemical, physical and mechanical (both bulk and
surface) properties
Although the temperature and pH of body environment/fluids
are relatively mild, BUT !
•
Degradation can be undesired or desired
• The biocompatibility of degradation by-products is as important as the
biocompatibility of the intact material
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