Dr. Alagiriswamy A A, (M.Sc, PhD, PDF)

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Dr. Alagiriswamy A A, (M.Sc, PhD, PDF) Asst.
Professor (Sr. Grade), Dept. of Physics,
SRM-University, Kattankulathur campus, Chennai
ABCs of Biomaterials
UNIT III Lecture 4
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CLASSIFICATION OF BIOMATERIALS

• Biomaterials can be divided into three major
  classes of materials
• Metals
• Polymers
• Ceramics (including carbons, glass ceramics, and
  glasses).

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Biological responses requirements

• Changing the chemistry at the surface
• Inducing roughness/porosity at the surface
• Incorporate surface reactive materials
  (bioresorbable helps in slow replacement by
  tissue)
• Should not secrete oxidizing agents
• Reduce corrosion rate of biomaterials

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METALLIC IMPLANT MATERIALS

• Stainless steel
• Cobalt-chromium alloys
• Titanium alloys

• Must be corrosion resistant
• Good fatigue properties
• Other compatible issues

• Metallic implants are used for two primary
  purposes.
• To replace a portion of the body such as joints,
  long bones and
• skull plates.
• Fixation devices are used to stabilize broken
  bones

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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 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
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Other features

• less chromium content should be utilized
  (because Cr is a highly reactive metal)
• Make use of austenite type steel (less magnetic
  properties)
• Lowered carbon content
• Inclusion of molybdenum helps corrosion
  resistance
• Electroplating technique (increases corrosion
  resistance)

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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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COBALT CHROMIUM ALLOYS

• Cobalt based alloys are used in one of three
  forms
• Cast as prepared
• Wrought (fine structure with low carbon contents
  pure forms)
• Forged

Cobalt based alloys are better than stainless
steel devices because of low corrosion resistance
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More details

• Cast alloy
• 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
• Molten metal alloy is then poured in to the
• shell, cooling, the shell is removed to obtain
  
• metal implant.

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• Wrought alloy
• possess a uniform microstructure with fine
  grains.
• Wrought Co-Cr Mo alloy can be further
  strengthened by cold work.
• Forged Alloy
• produced from a hot forging process.
• Forging of Co-Cr Mo alloy requires
• sophisticated press and complicated tooling.
• Factors make it more expensive to fabricate a
  device

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TITANIUM BASED ALLOYS

• The advantage of using titanium based alloys as
  implant materials are
• low density
• good mechano-chemical properties
• The major disadvantages
• relatively high cost
• reactivity.

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More details

• a light metal
• 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
• Ti-6 Al-4V alloy is generally used in one of
  three conditions
• wrought, forged or cast

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• THREE CLASSES OF CERAMICS (according to their
  reactivity)
• completely resorbable
• More reactive (Calcium phosphate) over a span
  of times
• Yielding mineralized bone growing from the
  implant surface
• surface reactive
• Bioglass ceramics Intermediate behavior
• Soft tissues/cell membranes
• nearly inert
• Less reactive (alumina/carbons) even after
  thousands of hours
• how minimal interfacial bonds with living
  tissues.

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DIFFERENT VARIETIES OF CARBON (NEARLY INERT
CERAMICS)

• Pyrolitic carbon
• Pyrolysis of hyrdocarbon gas (methane) 15000
  degrees
• Low temperature isotropic (LTI) phase
• Good bonding strength to metals (10 Mpa 35 Mpa)
• Inclusion of Si with C, wear resistance increases
  drastically
• Vitreous carbon (glassy carbon)
• controlled pyrolysis of a polymer such as phenol
  formaldehyde
• resin, rayon and polyacrylonitrile
• Low temperature isotropic phase
• Good biocompatibility, but strength and wear
  resistance are not good as LTI carbons
• Turbostratic carbon (Ultra low temperature
  isotropic carbons (ULTI))
• Carbon atoms are evaporated from heated carbon
  source and
• condensed into a cool substrate of ceramic,
  metal or polymer.
• Good biocompatibility

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Alumina (Aluminium oxide)

• Natural single crystal alumina known as sapphire
• High-density alumina prepared from purified
  alumina powder by isostatic pressing and
  subsequent firing at 1500-17000C.
• ?-alumina has a hcp crystal structure (a0.4758
  nm and c1.2999nm)
• load bearing hip prostheses and dental implants,
  hip and knee joints, tibial plate, femur shaft,
  shoulders, vertebra, and ankle joint prostheses

Alumina ceramic femoral component
Porous network SEM images

• high corrosion resistance
• wear resistance
• Surface finishing
• small grain size
• biomechanically correct design
• exact implantation technique

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• Glass Ceramics
• To achieve a controlled surface reactivity that
  will induce a direct chemical bond between the
  implant and the surrounding tissues.
• Also known as 45S5 glass. It is composed of SiO2,
  Na2O, CaO and P2O5.
• 45 wt. of SiO2 and 51 ratio of CaO to P2O5.
  Lower Ca/P ratios do not bond to bone.
• Bioglass and Ceravital fine-grained structure
  with excellent mechanical and thermal properties
• The composition of Ceravital is similar to
  bioglass in Sio2 content but differ in
  CaO,MgO,Na2O.
• Bioglass implants have several advantages like
• high mechanical properties
• surface biocompatible properties.

Bioglass
Ceravital
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• Resorbable Ceramics (first resorbable implant
  material-Plaster of Paris).
• Should not have variable resorption rates
• Should not have poor mechanical properties.
• Two types of orthophosphoric acid salt namely
  ?-tricalcium phosphate (TCP) and hydroxyapatite
  (HAP) (classified on the basis of Ca/P ratio).
• The apatite- Ca10 (PO4)6 (OH)2 crystallizes
  into the hexagonal rhombic system. The unit cell
  has dimensions of a 0.9432 mm and c 0.6881
  nm.
• The ideal Ca/P ratio of hydroxyapatite is 10/6
  and the calculated density is 3.219 g/ml.
• The substitution of OH- with F- gives a greater
  structural stability due to the fact that F- has
  a closer coordination than the hydroxyl, to the
  nearest calcium.

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POLYMERS

• Elastomers able to withstand large deformations
  and
• return to their original dimensions after
  releasing the
• stretching force.
• Plastics are more rigid
• materials
• Thermoplastic (can be
• reused, melted)
• Thermosetting (cant)

• Elastomers include, butyl rubber,
  chlorosulfonated polyethylene,
  epichlorohydrin,rubber, polyurethane,natural
  rubber and silicone rubber.
• Polymers toxicity
• Residual monomers due to incomplete
  polymerization/catalyst used for polymerization
  may cause irritations.

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Polymer Specific Properties Biomedical uses
Polyethylene Low cost, easy Possibility excellent electrical insulation properties, excellent chemical resistance, toughness and flexibility even at low temperatures Tubes for various catheters, hip joint, knee joint prostheses
Polypropylene Excellent chemical resistance, weak permeability to water vapors good transparency and surface reflection. Yarn for surgery, sutures
Tetrafluoroethylene Chemical inertness, exceptional weathering and heat resistance, nonadhesive, very low coefficient of friction Vascular and auditory prostheses, catheters tubes
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Polyethylene structures

• The first polyethylene PE,(-CH2-CH2-)n was
  made by reacting
• ethylene gas at high pressure in the presence of
  a peroxide catalyst for starting polymerization
  yielding low density polyethylene (LDPE).
• By using a Ziegler-Natta catalyst, high-density
  polyethylene (HDPE)
• can be produced at low pressure (first
  titanium-based catalysts)
• The crystallinity usually is 50-70 for low
  density PE and 70-80 or high density PE
• ultra high molecular weight polyethylene
  (UHMWPE) ??????

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ACRYLIC RESINS (organic glass)

• The most widely used polyacrylate is poly(methyl
• methacrylate, PMMA) The features of acrylic
  polymers
• high toughness/strength,
• good biocompatibility properties
• brittle in comparison with other polymers
• excellent light transparency
• high index of refraction.

Causes allergic reactions
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BONE CEMENT MIXING AND INJECTION

• PMMA powder MMA liquid mixed in a ratio of 21
  in a dough, to cure
• Injected in the femur (thigh bone)
• The monomer polymerizes and binds together the
  preexisting polymer particles.

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Hydrogels
Interaction with H2O, but not soluble
PHEMA absorbs 60 of Water, machinable when dry
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• Interesting features
• The soft, rubbery nature coupled with minimal
  mechanical/frictional irritation to the
  surrounding tissues.
• (2) Low or zero interfacial tension with
  surrounding biological fluids and tissues,
  thereby, minimizing the driving force for protein
  adsorption and cell adhesion
• (3) Hydrogels allow the permeating and diffusion
  of low
• molecular weight metabolities,waste products
  and salts as do living tissues.

HYDROGELS
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POLYURETHANES

• Polyther-urethanes block copolymers (variable
  length blocks that aggregate in phase domains)
• Good physical and mechanical characteristics
• Are hydrophilic in nature
• Good biocompatibility (blood compatibility)
• Hydrolytic heart assist devices
• Non-cytotoxic therapy

Consists of hard and soft segments
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POLYAMIDES (Nylons)

• Obtained through condensation of diamine and
  diacid derivative.
• Excellent fiber forming properties due to
  inter-chain hydrogen bonding and high degree of
  crystallinity, which increases the strength in
  the fiber direction.
• Hydrogen bonds play a major role
• As a catheter
• Hypodermic syringes

Diamino hexane adipic acid
November 4, 2013
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Biological responses requirements

• Changing the chemistry at the surface
• Inducing roughness/porosity at the surface
• Incorporate surface reactive materials
  (bioresorbable helps in slow replacement by
  tissue)
• Should not secrete oxidizing agents
• Reduce corrosion rate of biomaterials

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• Biosensors (in vitro/in vivo)
• analytical devices which convert biological
  response into a useful electrical signal
• to determine the concentration of substances
  either directly or indirectly
• areas of biochemistry, bioreactor science,
  physical chemistry, electrochemistry, electronics
  and software engineering, and others

http//www.lsbu.ac.uk/biology/enztech/
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Principle of biosensors (bio-recognition systems)
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WORKING PRINCIPLE OF BIOSENSOR

• biocatalyst (a) converts the substrate to
  product.
• This reaction is determined by the transducer
  (b)
• which converts it to an electrical signal.
• The output from the transducer is amplified (c),
• processed (d) and displayed (e).

output

• distribution of charges
• light-induced changes
• mass difference

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• Three so-called 'generations' of biosensors
• First generation normal product of the
  reaction diffuses to
• the transducer and causes the electrical
  response.
• Second generation involve specific 'mediators'
  between
• the reaction and the transducer in order to
  generate
• improved response.
• Third generation reaction itself causes the
  response and no
• product or mediator diffusion is directly
  involved.

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Brief applications of biosensor(s)

• Clinical diagnosis and biomedicine
• Farm, garden and veterinary analysis
• Process control fermentation control and
  analysis food and drink
• production and analysis
• Microbiology bacterial and viral analysis
• Pharmaceutical and drug analysis
• Industrial effluent control
• Pollution control and monitoring/Mining,
  industrial and toxic gases
• Military applications

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Tissue engineering (also referred to as
regenerative medicine)

• By restoring, maintaining, enhancing the tissue,
  and finally functionalize the organs
• Tissue can be grown inside or outside
• Finally to exploit the living cells in many ways

• To create products that improve tissue function
  or heal tissue defects.
• Replace diseased or damaged tissue
• Because
• Donor tissues and organs are in short supply
• We want to minimize immune system response by
  using our own cells or novel ways to protect
  transplant

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Tissue engineering

• Regenerate
• Identify the cues that allow for regeneration
  without scarring
• Like growing a new limb
• Repair
• Stimulate the tissue at a cell or molecular
  level, even at level of DNA, to repair itself.
• Replace
• A biological substitute is created in the lab
  that can be implanted to replace the tissue or
  organ of interest

• The cells themselves
• Non-soluble factors within the extracellular
  matrix (ECM) such as laminins,collagens,and other
  molecules
• Soluble factors such as cytokines, hormones,
  nutrients, vitamins, and minerals

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Normal strategies

• cell isolation
• cell culture
• scaffold material choice
• cell scaffold co-culture studies
• implantation in animals
• human trials

SUCCESSFULLY ENGINEERED TO SOME EXTENT
Skin Bone Cartilage Intestine
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Questions?