EFFECT OF PHYSIOLOGICAL FLUIDS

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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

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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.

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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.

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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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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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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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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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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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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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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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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.

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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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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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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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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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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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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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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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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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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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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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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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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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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 2.0 1-Si
316, 18-8sMo 0.07 16-18 10-14 2.0 2-3 Mo, 1.0 Si
316L 0.03 16-18 10-14 2.0 2.3 Mo, 0.75Si
430F 0.08 16-18 1.0-1.5 1.5 1.0 Si, 0-6 Mo
CONSTITUENTS OF STEEL
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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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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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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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Devices Alloy Type
Jewitt hip nails and plates 316 L
Intramedullary pins 316 L
Mandibular staple bone plates 316L
Heart valves 316
Stapedial Prosthesis 316
Mayfield clips (neurosurgery) 316
Schwartz clips (neurosurgery) 420
Cardiac pacemaker electrodes 304
APPLICATIONS OF SS STEEL
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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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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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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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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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COBALT CHROMIUM ALLOYS

• Cobalt based alloys are used in one of three
  forms
• Cast,
• Wrought
• Forged

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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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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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COBALT CHROMIUM ALLOYS
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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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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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 1.587 at
  room temperature
• Above 882.50C ?-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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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
  900C and
• 930C.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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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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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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TITANIUM BASED ALLOYS
BONE SCREWS USED FOR IMPLANTATION