CHAPTER
17
Biological Materials and
Biomaterials
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Biomaterials and Biological Materials
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Biomaterial: a systematically and pharmacologically inert
substance designed for implantation within or incorporation
with living systems.
 Examples: Orthopaedic implants, dental implants,
artificial heart valves, and joint replacements
• Biological materials: materials produced by biological
systems.
 Examples: bone, ligament, and cartilage tissues
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Biological Material: Bone
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Composition: Mixture of organic
and inorganic materials.
 Calcium and phosphate ions,
similar to Hydroxyapatite
(Ca10(PO4)6(OH)2) – 60-70% of
dry weight.
Inorganic portion
 Hydroxyapatite (HA): pallet
like, 20 to 80 nm long, and 2 to
5 nm thick, HCP.
 Collagen: fibrous, tough,
polymer
flexible, and highly inelastic;
provides bone with flexibility30% of dry weight.
From Basic Biomechanics, Susan J Hall, McGraw-Hill
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Bone
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Macrostructure:
 Cortical (outer shell, compact
 Trabecular (inner bone, cancellous)
Trabecular bone filled with bone marrow.
Have different properties.
Trabecular bone
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A longitudal section through an
adult femur
Cortical bone
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Mechanical Properties
• Cortical bone is stronger and stiffer than trabecular
bone.
• Bone density plays a critical role in mechanical properties.
• Trabecular bone has lower strength and high ductility.
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Strain-stress curves of cortical and trabecular bones.
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Mechanical Properties
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Bone is anisotropic.
It is generally stronger in the direction it is loaded.
Bone is weakest in the transverse direction.
Transversely isotropic or orthotropic models are used to study the
behaviour of bone.
• Bones are stronger in compression than tension.
Anisotrpic: exhibiting properties with
different values when measured in different
directions.
Tension: the force acting on the object is
always outward from the object.
Compression: the force acting on the
object is inward to the object.
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Bone fracture
• Bone supports various modes of loading such as tensile,
compressive, bending, torsional, shear, and combined.
Bending
Shear fractures
Tensile fractures
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Viscoelasticity of Bone
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Strain rate during walking is 0.001/s, during running it is
0.03/s, during impact trauma it is 1/s.
As strain rate increases, bone becomes stiffer and stronger
and more brittle.
Under high-energy fracture, the excess available energy
causes significant damage to the surrounding tissue.
Repeated loading can cause muscles to get tired.
Bone then carries a lot of load.
Microcracks propagate if enough time is not given for
healing.
This causes fatigue failure.
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Bone Remodelling
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Wolff’s law: Bone responds to applied stress by
remodelling.
Bone senses environmental stimuli and responds
accordingly.
It can change its alter its size, shape, and structure.
Principle: Optimize the content of the bone inside the body:
keep wherever it is needed, remove from the place it is not
needed.
Astronauts suffer bon loss due to weightlessness.
Moderate exercise with low weights reduces the bone loss
phenomenon in the aging population.
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Tendons and Ligaments
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Tendons and Ligaments
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Tendons connect muscles to bone: transfer forces generated
by muscle contractions to the bone.
Ligaments connect bone to bone: stabilizes the joint.
The functional loads for tendons are generally higher than
those for ligaments.
About 60% of the
total weight is water.
About 80% of the
dry weight is made
up of Type-I collagen.
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Microstructure
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Collagen molecules are secreted by fibroblasts.
These molecules form microfibrils, fibrils (20-150
nanometers) and bundles.
• Fibril is the primary load carrying member.
• Fibrils are crimped when unloaded.
SEM image of fibrils
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TEM image of fibrils.
(ligament sliced by
microtome and dehydrated
before imagery.)
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Mechanical Properties
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Primarily loaded in tension.
Stress strain curve is non-linear: Has a toe region, linear
region and a non-linear region.
• Fibrils are sequentially loaded until all the fibrils are
loaded, and then sequentially fail.
• ‘E’ is measured in “Region 2” (linear region).
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Tensile Test on Ligament
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Structure Function Relationship
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Wide variability, depending on the tissue.
The amount of collagen in the tissue, collagen fibril
density, and the extent of collagen cross-linking directly
influence the mechanical properties.
• Age, sex, activity level, all influence the collagen
content.
• Collagen fibril area fraction affects the ultimate stress.
• Higher collagen fibril density (fibrils/area) increases E.
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Constitutive Modelling
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Simple Hooke’s law is not useful due to nonlinearity.
Complex stress-strain relationships:
C, a, and b are parameters to be found empirically.
E=207 MPa
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Ligament and Tendon Injuries
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Muscles are primary stabilizers of joint.
If muscles fail to contract at the right time, ligaments take
the load and sometimes fail.
Tendon injuries take place due to aggressive contraction
of muscles.
Microscopic tears happen during everyday activities, but
they heal over time.
Ligament injuries do not heal easily if synovial fluid is
present.
Torn ligaments are often reconstructed with graft tissues.
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Articular Cartilage
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Articular Cartilage
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Joints of human body experience huge load.
Bone at the joints are covered with articular cartilage.
The articular cartilage reduces wear and friction at the
joint and distributes the load over a wide area.
1-6 mm thick, avascular, no nerve supply.
Made up of porous matrix,
water and ions.
70-90% of the wet
weight is water.
10-20% type II collagen,
4-6% proteoglycans.
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Microstructure
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Four zones of collagen fibril arrangement.
Collagen content is highest in the tangential region while the
proteoglycans content is lowest.
• In the deep region, the proteoglycans content is highest while
the water content is lowest.
• Deep zone anchors the
cartilage into the
subchondral bone.
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Mechanical Properties
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Highly viscoelastic, anisotropic and heterogeneous.
The mechanical behavior of the articular cartilage
is due to
i) intrinsic property of the matrix
ii) flow of the water within the matrix
iii) the effects of the presence of ions in the matrix.
Modulus of elasticity varies from 4-400 Mpa.
Usually loaded in compression and shear.
Equilibrium Aggregate modulus (static modulus of the
matrix) is measured by confined compression test.
Proteoglycan absorb water, develop strong repulsive forces
inside the matrix and thus resist compression (like a inflated
tire).
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Cartilage Degeneration
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Cartilage has limited capability to repair itself.
Repeated high stress loading and biochemical changes
lead to cartilage degeneration.
Prolonged abnormal joint stress distribution (due to
ligament injury) and single traumatic load can also cause
cartilage degeneration
If cartilage degenerates,
bone-to-bone contact takes
place at the joints, resulting in pain.
This is called osteoarthritis.
These joints will have to undergo
total reconstruction using metal
implants.
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Biomaterials
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Biomaterials: Biometals
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Biometals come in direct contact with human body fluids.
 Used to replace tissue
 Support damaged tissue while heeling
 Filler material
• Biocompatibility : Internal environment of human body is
highly corrosive
 Metals degrade and release harmful ions
 Chemical stability, corrosion resistance, non-carcinogenity and nontoxicity is called biocompatibility.
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High fatigue strength (50-100 million cycles) is desired.
Pt, Ti, Zr have good biocompatibility.
Co, Cu, Ni are toxic
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Stainless Steels as Biometals
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316 L stainless steel (cold worked, grain size of minimum
5) is used most often
 18Cr-14Ni-2.5Mo---F138
• Inexpensive, easily shaped
Fibula
• limited corrosion resistance
inside the body
 removed after healing
 Used as bone screws
Bone plate
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Spine plate
Intramedullary nail
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Stainless Steel
• First used in 1926
• Low carbon content (316L: 18 Cr, 14 Ni, 2.5 Mo)
– High carbon content causes corrosion of iron
- Cheap, easily formable
- grain size of 5 or finer
- 30% cold-worked state
• Chromium oxidizes to limit
corrosion (limited resistance)
• Suitable for short term use:
bone plates and fixation.
• Nickel released due to corrosion
can be toxic.
• Recently, nickel-free austenitic
steel has been developed.
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Cobalt Based Alloys
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Cr promotes long term
Co-28Cr-6Mo
Corrosion resistance by forming
Co-20Cr-15W-10Ni
passive layer.
Co-28Cr-6Mo-heat treated
Ni and W improve
Machinability and fabrication
Co-35Ni-20Cr-10Mo
• Initially hot worked and then cold finished
• Used in permanent fixation devices
Total knee replacement
prosthesis
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Cobalt Based Alloys
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Wide range, easily formable.
Ideal for joint replacement and fracture fixation.
High corrosion resistance (Chromium), fatigue resistance
and ductile.
• Difficult to form/machine, usually cast (lost wax
technique).
• Large grain size-less fatigue life.
• Wear causes metallic toxicity
 Co-Cr alloys have good wear resistance
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Titanium Alloys
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Easily formed, outstanding corrosion resistance
Forms TiO2 layer, very robust.
Low elastic modulus, highly biocompatible
Pure Ti is used in low strength applications
Alpha-beta alloys of Ti like Ti-6Al-4V (F1472) are strengthened
by solution heat treatment.
• Poor wear resistance and notch sensitivity but Ion implantation
improves wear resistance
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Issues in Orthopaedic Applications
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High yield strength, fatigue strength and
hardness of implants is desired.
 Implant should support healing
bone
• Low elastic modulus is desired
 Implant and bone should carry
proportionate amount of load
 Implant should not shield the bone
from load
 Stress shielding stops remodeling of
bone and weakens it.
 Elastic modulus of bone is only 17
GPa while most alloys have elastic
modulus greater than 100 GPa.
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Biopolymers
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Polymers are used in biomedical applications
 Cardiovascular, Ophthalmic and Orthopaedic implants
 Dental implants, dental cements and denture bases
• Low density, easily formed
and can be made biocompatible.
• Recent development – biodegradable polymers.
• Examples: Polyethylene (PET)
polyurethane, polycarbonate
polyetheretherketone (PEEK)
polybutylene terephthalate (PBT)
polymethyl methacrylate (PMMA)
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Cardiovascular Applications
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Heart valves can be stenotic or incompetent
Polymers are used to make artificial heart valves
Leaflets are made from biometals
Sewing ring made from PTFE or
PET
Connected to heart tissue
• Blood clogging is side effect
• PTFE is used as vascular graft to bypass clogged arteries.
• Blood oxygenators : Hydrophobic polymer membranes used
to oxygenate blood during bypass surgery
 Air flows on one side and blood on the other side and
oxygen diffuses into blood.
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Opthalmic Applications
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Eye glasses, contact lenses and Intraocular implants are
made of polymers.
• Hydrogel is used to make soft contact lenses
 Absorbs water and allows snug fit
 Oxygen permeable
 Made of poly-HEMA
• Hard lenses made from PMMA
 Not oxygen permeable
 Mixed with Siloxanylalkyl
Metacrylate and metacrylic
acid to make permeable and hydrophilic.
• Intraocular implants are made of PMMA
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Intraocular Lenses
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Before and after cataract surgery
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Orthopaedic Applications
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Bone cement: Fills space between implant and bone –
PMMA
 Centrifuging and vacuum techniques minimize
porosity
Used in joint prosthesis (Knee and Hip replacements).
centrifuging and vacuuming techniques used to reduce
microporosity.
UHMWPE (Ultra high molecular weight polyethylene is
used in bearing surface of implants.
Reduces friction and wear.
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Other Applications
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Drug delivery systems: polylactic acid (PLA) and
polyglycolic acid (PGA).
Polymer matrix with drug is implanted inside the body.
Drug is released as the polymer degrades.
Suture materials: high tensile strength and knot pull
strength.
Nonabsorbable sutures are generally made of
polypropylene, nylon, polyethylene tetraphthalate, or
polyethylene.
Absorbable sutures are made of PGA.
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Ceramics in Biomedical Applications
• First used in 1963, generating lot of interest
• Biocompatible: very inert, less toxic degeneration
products.
• Wear resistant, low friction but brittle.
• Some lose strength in contact with blood and water.
• Good bond with bone.
• very stiff, can cause some stress shielding.
• Orthopaedic implants and dental.
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Ceramic implants
• High purity alumina: excellent corrosion and wear
resistance, biocompatible.
• Purity must be 99.8% and grain size of 3 – 6
microns.
• Less friction, comparable to healthy hip (10 times
less wear debris).
• Can cause stress shielding.
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Ceramic Implants
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Dental implants: both for crown
and root.
• Crown is made up of porcelain,
which is also a ceramic.
• 4 types of response to biomaterial:
toxic (tissue dies), inactive (fibrous
tissue around implant), bioactive
(bond), resorption (implant
dissolves).
• Alumina is inert, forms fibrous
tissue, OK in dental implants.
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Tissue connectivity
• Porous alumina is used to serve as scaffold.
• The bone material grows into the pores available in the
ceramic, Osteoconductivity.
• Porosity reduces strength: minimal load bearing
applications only, provides moderate load-bearing
support.
• Bioactive ceramics develop adherent interface (glasses
with SiO2, Na2O, CaO, P2O5 show bioactivity.
• Resorbable ceramics degrade over a period of time and
replaced by bone.
• Ca3 (PO4)2 matches resorption rate with repair rate.
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Nanocrystalline Ceramics
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Brittleness of ceramics is not desired.
Nanocrystalline ceramics may improve on this inherent
weakness of these materials.
Examples: Calcium phosphate hydroxyapatite (HA),
calcium carbonate, and bioactive glasses.
Produced using standard powder metallurgy techniques.
Pressure assisted sintering is also used.
Nanocrystalline ceramics with exceptional levels of
strength, ductility, and therefore improved toughness.
Still in research stage.
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Composites in Biomedical Applications
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Composite materials offer combination of properties.
New materials to replace natural bone by combining highdensity PE (HDPE) with HA are found.
HA (20 to 40% vol) gives the material bioactivity while
the polymer gives it the fracture toughness.
Carbon fiber-reinforced thermoplastic composites can be
used to produce fracture-fixation devices flexible and
strong bone plates to avoid stress shielding.
Bone plates made up of poly-L-lactide (PLLA) reinforced
with raw u-HA particles have optimal degradation rate.
Dental implants using composites of SiC and carbon
fiber–reinforced carbon are currently researched.
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Composites in Biomedical Applications
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Composites are widely used in joint prosthesis.
Efforts are underway to substitute UHMWPE with PEEK
reinforced with carbon fibers to improve its wear
resistance.
This composite can also be used to produce a femoral stem
for hip implants.
Bioactive composite coatings have been produced by
combining bioglass with Ti–6 Al–4 V.
Bone cements are often reinforced with particulate HA to
improve bone attachment.
Glass fibers–reinforced PMMA and PC are used for fixed
dental bridges and removable dental prosthesis.
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Corrosion in Implants
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Highly corrosive environment, long time (life long).
Pitting corrosion and crevice corrosion are the most common
types of corrosion in biometals.
Pitting corrosion usually occurs on the underside of the screw
heads that secure the implant.
Crevice corrosion occurs when the metal surface is partially
shielded from the surrounding environment.
Crevice corrosion in the countersunk portion of the bone plate
is very common in stainless steel implants.
Galvanic corrosion occurs when two metals are in contact.
Fretting corrosion is also common due to repetitive loading
(in joint prosthesis).
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Corrosion in implants
• Titanium has superior
corrosion resistance. (form
a robust passive layer on
the outside )
• Cobalt-chromium alloys
also behave in the similar
manner, however, it is
moderately susceptible to
crevice corrosion.
• The passive layer formed
by stainless steel is not
very robust.
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Crevice corrosion in hip implant.
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Effects of Corrosion
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Mechanical integrity of the implants
might be compromised.
Corrosion products can result in
adverse tissue reaction.
Sometimes, swelling and pain in the
tissue surrounding the implant.
The corrosion debris can migrate
resulting in periprosthetic bone loss.
This results in loosening of the
implant.
This condition is known as osteolysis.
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Preventing Corrosion in Implants.
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Alloying, surface treatment and proper implant design can
minimize the corrosion in orthopaedic implants.
Nitriding the surface of Ti6Al4V implants reduces the chances
of fretting corrosion.
Resistance to pitting corrosion can be increased by the addition
of 2.5-3.5% molybdenum to the implant material.
Proper implant design to minimize the crevices can eliminate
crevice corrosion.
The surface of the implants can also be made passive prior to the
implantation through various chemical treatments.
Using the matched parts of the modular implant from the same
batch of the same variant of a given alloy reduces the chances of
galvanic corrosion.
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Wear in Biomedical Implants
• Joint prosthesis has parts that move.
• The consequence of having moving parts is friction and wear.
• Inflammatory response and also causes osteolysis.
• The shape changes due to wear affecting its normal function.
• Biotribology deals with the study of friction and wear in biomedical
implants.
• Friction and wear are the result of microsurface roughness of the
surfaces moving relative to each other.
• Irregularities on a ceramic artificial joint surface is 0.005 microns
while that on metal surface is 0.01 microns.
• The area of contact when these surfaces mate is relatively small (1%
of the total area).
• Adhesive Wear: Local contact stresses can exceed the yield strength
resulting in the bonding and debonding resulting in frictional
resistance and wear also causing wear debris.
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Wear in Implants
• Abrasive wear: When a harder surface rubs against a softer
surface, wear of the softer surface is produced by a
ploughing of the surface by the asperities in the harder
surface.
• 3 body wear: polyethylene cup with metal head and
debris.
• Sometimes transfer-film is formed which decreases wear
rate by increasing the contact area.
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Reducing wear
• Lubrication is necessary (synovial fluid).
• Boundary lubrication: a lubricant film adheres to the bearing
surfaces reducing the friction (significant asperity contact).
• Fluid film lubrication: fluid film forms between the bearing
surfaces completely separating them (no asperity contact).
• Mixed lubrication mechanism has the characteristics of both
fluid film and thick film lubrications.
• Both fluid film and boundary lubrication occurs in the joints
at different instances.
• Joint coefficient of friction is 0.001.
• Changes is biochemistry of synovial fluid can result in wear.
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Implant wear prevention
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Clearance between the mating surfaces are optimized to
promote fluid film lubrication.
• The surface of the implants is treated to make them harder.
- titanium implants are heated to about 11000F in the
presence of molecular nitrogen gas.
- This will result in solid solution of nitrogen in titanium on
the implant surface increasing the surface hardness.
• Coating the implant surfaces with very hard material.
• Coating the surfaces with amorphous carbon which has very
high hardness and low friction.
- Plasma assisted CVD is used.
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Future – Tissue Engineering
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Biomaterials lack the capacity to remodel.
They are not effective in long run.
Tissue Engineering: tissues can be grown outside the
body and re-implanted.
The mechanical properties and structure of these tissues
can be controlled using various techniques.
Regular structures called scaffolds are used to control the
structure of the tissues grown in-vitro.
These scaffolds are seeded with donor cells and placed in
a bioreactor where they are stimulated and supplied with
growth factors to promote proliferation.
Polylactic acid is a biomaterial that can be used for
scaffolds.
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Basic Principles of Tissue Engineering
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Example: Cartilage Tissue Engineering
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