1. EFFECT OF PHYSIOLOGICAL FLUIDS
Biocompatibility plays a very important role on deciding the life of biomaterials.
A completely "biocompatible" material would not
irritate the surrounding structures
provoke an inflammatory response
initiate allergic reactions
cause cancer
LECTURE 3
BIOMATERIALS

2. EFFECT OF PHYSIOLOGICAL FLUIDS
A "biocompatible" material should also not have its
properties degraded from an attack by the body's immune
system.
The term biocompatible suggests that the material
described displays good or harmonious behavior in contact
with tissue and body fluids.
Water constitutes a major portion of the fluids and these
react with the surface of the materials.
The interaction of water or in general other fluids affects the
properties of materials.
LECTURE 3
BIOMATERIALS

3. EFFECT OF PHYSIOLOGICAL FLUIDS
Water is the universal ether dissolving inorganic salts and
large organic macromolecules such as proteins.
Water suspends living cells as in blood and is the principal
constituent of all interstitial fluids.
It is believed that water is the first molecule to contact
biomaterials in any clinical application.
Due to water, the hydrophobic effect ,hydrophilic effect and
surface wetting effect occurs.
LECTURE 3
BIOMATERIALS

4. EFFECT OF PHYSIOLOGICAL FLUIDS
The hydrophobic effect is related to the insoloubility of
hydrocarbons in water and is the fundamental of lipids.
In other words, the hydrophobic effect is the property that
nonpolar molecules like to self-associate in the presence of
aqueous solution.
The hydrophobic effect is the fundamental life giving
phenomena attributed to water.
Hydrocarbons are sparingly soluble in water because of the
strong self association of water.
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BIOMATERIALS

5. EFFECT OF PHYSIOLOGICAL FLUIDS
The hydrophilic effect refers to a physical property of a
molecule that can transiently bond with water (H2O) through
hydrogen bonding.
This is thermodynamically favorable, and makes these
molecules soluble not only in water, but also in other polar
solvents.
The hydrophilic solutes exhibit Lewis acid or base strength
comparable to or exceeding that of water, so that it is
energetically favorable for water to donate electron density
to or accept electron density from hydrophilic solutes
instead of, or at least in competition with, other water
molecules.
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BIOMATERIALS

6. EFFECT OF PHYSIOLOGICAL FLUIDS
Generally speaking the free energies of hydrophilic
hydration are greater than that of hydrophobic hydration.
As in hydrophobic effect, size plays abig role in the
salvation of hydrophilic ions.
Small inorganic ions are completely ionized and lead to
separately hydrated ions.
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BIOMATERIALS

7. EFFECT OF PHYSIOLOGICAL FLUIDS
The interaction of water with the surfaces leads to surface
wetting effect.
The surface on which water spreads is called hydrophilic
and those on which water droplets form is called
hydrophobic.
Thus hydrophobic surfaces are distinguished from
hydrophilic by virtue of having no Lewis acid or base
functional groups available for water interaction.
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BIOMATERIALS

8. EFFECT OF PHYSIOLOGICAL FLUIDS
Structure and solvent properties of water in contact with
surfaces between these extremes must then exhibit some
kind of properties associated with the graded wettability
observed with contact angles.
If the surface region is composed of molecules that hydrate
then the surface can adsorb water and swell or dissolve.
At the extreme of water- surface interactions,surface acid or
base groups can abstract hydroxyls or protons from water
leading to water ionization on the surface.
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BIOMATERIALS

9. EFFECT OF PHYSIOLOGICAL FLUIDS
The surface energetics drives adsorption of water and then
in subsequent steps, proteins and cells interact with the
resulting hydrated surface.
Self association of water through hydrogen bonding is the
essential mechanism behind the water solvent properties.
As mentioned these interactions leads to the degradation of
the biomaterials.
It can be concluded that no theory explaining the biology of
materials can be complete with out accounting for the water
properties near surfaces.
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BIOMATERIALS

10. BIOLOGICAL RESPONSES
The Biological environment is surprisingly harsh and can
lead to rapid or gradual breakdown of many materials.
Superficially, one might think that the neutral pH, low salt
content, and modest temperature of the body would
constitute a mild environment.
However, many specialized mechanisms are brought to
bear on implants to break them down.
These are mechanisms that have evolved over millennia
specifically to rid the living organism of invading foreign
substances and they now attack our contemporary
biomaterials.
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BIOMATERIALS

11. BIOLOGICAL RESPONSES
The biological response can occur both in extravascular and
intravascular system.
The former deals with the changes outside the blood or lymph
vessel and the latter deals with in the blood vessels.
Let us consider that, along with the continuous or cyclic stress
many biomaterials are exposed to, abrasion and flexure may
also take place.
This occurs in an aqueous, ionic environment that can be
electrochemically, active to metals and plasticizing (softening)
to polymers.
LECTURE 3
BIOMATERIALS

12. BIOLOGICAL RESPONSES Then, specific biological mechanisms are invoked.
Proteins adsorb to the material and can enhance the
corrosion rate of metals.
Cells secrete powerful oxidizing agents and enzymes that
are directed at digesting the material.
The potent degradative agents are concentrated between
the cell and the material where they act undiluted by the
surrounding aqueous.
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BIOMATERIALS

13. BIOLOGICAL RESPONSES
To understand the biological degradation of implant
materials, synergistic pathways should be considered.
Swelling and water uptake can similarly increase the number
of site for reaction.
Degradation products can alter the local pH, stimulating
further reaction.
Hydroxyl polymers can generate more hydrophilic species,
leading to polymer swelling and entry of degrading species
into the bulk of the polymer.
Cracks might also serve as sites initiating calcification.
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BIOMATERIALS

14. BIOLOGICAL RESPONSES
Biodegradation is a term that is used in many contexts.
It can be engineered to happen at a specific time after
implantation, or it can be un unexpected long-term
consequent of the severity of the biological Degradation is
seen with metals, polymers, ceramics and composites.
Biodegradation as a subject is broad in scope and rightfully
should command considerable attention for the bio
materials scientist.
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BIOMATERIALS

15. BIOLOGICAL RESPONSES
Most biomaterials of potential clinical interest typically elicit
the foreign body reaction (FBR) a special form of non
specific inflammation.
The most prominent cells in the FBR are macrophages,
which attempt to phagocytose the material degradation are
often difficult.
The inflammatory cell products that are critical in killing
microorganisms can damage tissue adjacent to foreign
bodies.
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BIOMATERIALS

16. BIOLOGICAL RESPONSES Tissue interactions can be modified by,
changing the chemistry of the surface.
inducing roughness or porosity to enhance physical
binding to the surrounding tissues.
incorporating a surface-active agent to chemically bond
the tissue.
using a bioresorbable component to allow slow
replacement by tissue to simulate natural healing
properties .
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BIOMATERIALS

17. BIOLOGICAL RESPONSES
The nature of the reaction is largely dependent on the
chemical and physical characteristic of the Implant.
For most inert biomaterials, the late tissue reaction is
encapsulation by a rel­atively thin fibrous tissue capsule
(Composed of collagen and fibroblasts).
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BIOMATERIALS

18. CLASSIFICATION OF BIOMATERIALS
Biomaterials can be divided into three major classes of materials:
Polymers
Metals
Ceramics (including carbons, glass ceramics, and glasses).
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BIOMATERIALS

19. METALLIC IMPLANT MATERIALS
Metallic implants are used for two primary purposes.
Implants used as prostheses serve to replace a portion of
the body such as joints, long bones and skull plates.
Fixation devices are used to stabilize broken bones and
other tissues while the normal healing proceeds.
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BIOMATERIALS

20. METALLIC IMPLANT MATERIALS
Though many metallic implant materials are available commercially. The three main categories of metals which are used for orthopedic implants
Stainless steels
Cobalt-chromium alloys
Titanium alloys
will be discussed in detail.
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BIOMATERIALS

21. METALLIC IMPLANT MATERIALS
The Metallic implant materials that are used should have the following characteristic features:
Must be corrosion resistant
Mechanical properties must be appropriate for desired
application
Areas subjected to cyclic loading must have good fatigue
properties
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BIOMATERIALS

22. STAINLESS STEEL Stainless steel is the predominant implant alloy.
This is mainly due to its ease of fabrication any desirable variety of mechanical properties and corrosion behavior.
But, of the three most commonly used metallic implants namely
Stainless steel
Cobalt chromium alloys
Titanium alloys,
Stainless steel is least corrosion resistant.
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BIOMATERIALS

23. STAINLESS STEEL
The various developments which took place in the development of steel in metallic implants are discussed below.
Stainless steel (18Cr-8 Ni) was first introduced in surgery in
1926
In 1943, type 302 stainless steel had been recommended to
U.S. Army and Navy for bone fixation.Later 18-8sMo stainless
steel (316), which contains molybdenum to improve corrosion
resistance, was introduced.
In the 1950s, 316L stainless steel was developed by reduction
of maximum carbon content from 0.08% to 0.03% for better
corrosion resistance.
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BIOMATERIALS

24. CONSTITUENTS OF STEEL Type % C %Cr % Ni %Mn % other elements 301 0.15
16-18
6-8
2.0
1.0Si
304
0.07
17-19
8-11
1-Si
316, 18-8sMo
10-14
2-3 Mo, 1.0 Si
316L
0.03
2.3 Mo, 0.75Si
430F
0.08
1.5
1.0 Si, 0-6 Mo
CONSTITUENTS OF STEEL
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BIOMATERIALS

25. STAINLESS STEEL
The chromium content of stainless steels should be least
11.0% to enable them to resist corrosion.
Chromium is a reactive element.
Chromium oxide on the surface of steel provides excellent
corrosion resistance.
The AISI Group III austenitic steel especially type 316 and
316L cannot be hardened by heat treatment but can be
hardened by cold working.
This group of stainless steel is non-magnetic and
possesses better corrosion resistance than any of the
others.
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BIOMATERIALS

26. STAINLESS STEEL The inclusion of molybdenum in types 316 and 316L
enhances resistance to pitting corrosion.
Lowering the carbon content of type 316L stainless steels
makes them more corrosion resistant to physiological saline
in human body.
Therefore 316L is recommended rather than 316 for implant
fabrication.
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BIOMATERIALS

27. STAINLESS STEEL
The Stainless steels used in implants are generally of two types:
Wrought
Forged
Wrought alloy possesses a uniform microstructure with fine
grains.
In the annealed condition it possesses low mechanical
strength.Cold working can strengthen the alloy.
Stainless steels can be hot forged to shape rather easily
because of their high ductility.
They can also be cold forged to shape to obtain required
strength.
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BIOMATERIALS

28. APPLICATIONS OF SS STEEL
Devices
Alloy Type
Jewitt hip nails and plates
316 L
Intramedullary pins
Mandibular staple bone plates
316L
Heart valves
316
Stapedial Prosthesis
Mayfield clips (neurosurgery)
Schwartz clips (neurosurgery)
420
Cardiac pacemaker electrodes
304
APPLICATIONS OF SS STEEL
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BIOMATERIALS

29. STAINLESS STEEL
Electroplating has been shown to be generally superior to a
mechanical finish for increasing corrosion resistance which
can also be produced by other surface treatments such as
passivation with HNO3.
The reason why stainless steel implants failed , indicates a
variety of deficiency factors like
deficiency of molybdenum
the use of sensitized steel
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BIOMATERIALS

30. COBALT CHROMIUM ALLOYS
The two basic elements of Co-based alloys form a solid
solution of upto 65 wt % of CO and 35 wt % of Cr
To this Molybdenum is added to produce finer grains which
results in higher strength after casting or forging
Cobalt is a transition metal of atomic number 27 situated
between iron and nickel in the first long period of the
periodic table.
The chemical properties of cobalt are intermediate between
those of iron and nickel.
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BIOMATERIALS

31. COBALT CHROMIUM ALLOYS
The various milestones in the development of cobalt chromium alloys are discussed below.
Haynes developed a series of cobalt-chromium and cobalt-
chromium-tungsten alloys having good corrosion resistance.
During early 1930s an alloy called vitallium with a composition
30% chromium, 7% tungsten and 0.5% carbon in cobalt was
found.
Many of the alloys used in dentistry and surgery, based on the
Co-Cr system contain additional elements such as carbon,
molybdenum, nickel, tungsten
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BIOMATERIALS

32. COBALT CHROMIUM ALLOYS
Chromium has a body centered cubic (bcc) crystal structure
and cannot therefore have a stability of the phase of cobalt.
The solubility of the former in the latter increases rapidly as
the temperature is raised.
Metallic cobalt started to find some industrial use at the
beginning of this century but its pure form is not particularly
ductile or corrosion resistant.
The various milestones in the development of cobalt
chromium alloys are discussed below.
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BIOMATERIALS

33. COBALT CHROMIUM ALLOYS
Cobalt based alloys are used in one of three forms
Cast,
Wrought
Forged
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BIOMATERIALS

34. COBALT CHROMIUM ALLOYS
Cast alloy: The orthopedic implants Co-Cr alloy are made by investment casting.In an investment casting process,the various steps which are involved are
a wax model of the implant is made and ceramic shell is built
around the wax model
When wax is melted away, the ceramic mold has the shape
of the implant
The ceramic shell is not fired is obtained the required the
mold strength
Molten metal alloy is then poured in to the shell, cooling, the
shell is removed to obtain metal implant.
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BIOMATERIALS

35. COBALT CHROMIUM ALLOYS
Wrought alloy: The wrought alloy possess a uniform microstructure with fine grains. Wrought Co-Cr –Mo alloy can be further strengthened by cold work.
Forged Alloy: The Co-Cr forged alloy is produced from a hot forging process. The Forging of Co-Cr –Mo alloy requires sophisticated press and complicated tooling. These factors make it more expensive to fabricate a device from a Co-Cr-Mo forging than from a casting.
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BIOMATERIALS

36. COBALT CHROMIUM ALLOYS
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BIOMATERIALS

37. TITANIUM BASED ALLOYS
The advantage of using titanium based alloys as implant materials are
low density
good mechano-chemical properties
The major disadvantage being the relatively high cost and reactivity.
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BIOMATERIALS

38. TITANIUM BASED ALLOYS
Titanium is a light metal having a density of 4.505g/cm3 at
250C .
Since aluminum is a lighter element and vanadium barely
heavier than titanium, the density of Ti-6% Al-4% V alloy is
very similar to pure titanium.
The melting point of titanium is about 16650C although
variable data are reported in the literature due to the effect
of impurities.
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BIOMATERIALS

39. TITANIUM BASED ALLOYS Titanium exists in two allotropic forms,
the low temperature -form has a close-packed hexagonal
crystal structure with a c/a ratio of at room temperature
Above C -titanium having a body centered cubic
structure which is stable
The presence of vanadium in a titanium-aluminium alloy tends to form - two phase system at room temperature.
Ti-6 Al-4V alloy is generally used in one of three conditions wrought, forged or cast.
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BIOMATERIALS

40. TITANIUM BASED ALLOYS Wrought alloy
It is available in standard shapes and sizes and is annealed
at 7300C for 1-4 hours, furnace cooled to 6000C and air-
cooled to room temperature.
Forged alloy
The typical hot-forging temperature is between 900°C and
930°C.Hot forging produces a fine grained -structure with a
depression of varying  phase. A final annealing treatment
is often given to the alloy to obtain a stable microstructure
without significantly altering the properties of the alloy.
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BIOMATERIALS

41. TITANIUM BASED ALLOYS Cast alloy
To provide a metallurgical stable homogenous structure
castings are annealed at approximately 8400C .
Cast Ti-6 Al-4V alloy has slightly lower values for
mechanical properties than the wrought alloy.
Titanium and its alloys are widely used because they show
exceptional strength to weight ratio
good mechanical properties.
The lower modulus is of significance in orthopedic devices
since it implies greater flexibility.
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BIOMATERIALS

42. TITANIUM BASED ALLOYS
To improve tribiological properties of Titanium there are four general types of treatments made.
Firstly, the oxide layer may be enhanced by a suitable
oxidizing treatment such as anodizing
Secondly, the surface can be hardened by the diffusion
of interstitial atoms into surface layers
Thirdly, the flame spraying of metals or metal oxides
on to the surface may be employed
Finally, other metals may be electroplated onto the
surface
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BIOMATERIALS

43. TITANIUM BASED ALLOYS BONE SCREWS USED FOR IMPLANTATION LECTURE 3
BIOMATERIALS