Bioceramics

1
Bioceramics
2
Ceramics in Medicine

• Historically common in medical industry glass
  beakers, slides, thermometers, eyeglasses, etc.
• Ceramic materials exist in the body
• Bone and teeth
• Thus, they are useful in devices and implants

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Ceramic vs Glass

• Ceramic an inorganic, nonmetallic, typically
  crystalline solid, prepared by application of
  heat and pressure to a powder
• Most ceramics are made up of two or more
  elements.
• Contain metallic and non-metallic elements, ionic
  and covalent bonds
• Glass (i) An inorganic product of fusion that
  has cooled to a rigid condition without
  crystallization (ii) An amorphous solid
• Glass-ceramic Product formed by the controlled
  crystallization (devitrification) of a
  glass-forming melt. Consists of two-phases
  crystals in a glass matrix.

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Other Definitions

• Amorphous
• Lacking detectable crystallinity
• Possessing only short-range atomic order also
  glassy or vitreous
• Bioactive material A material that elicits a
  specific biological response at the interface of
  the material, (usually) resulting in the
  formation of a bond between the tissues and the
  material.

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Crystalline vs Glassy (Amorphous) Ceramics

• Crystalline ceramics have long-range order, with
  components composed of many individually oriented
  grains.
• Glassy materials possess only short-range order,
  and generally do not form individual grains.
• The distinction is based on x-ray diffraction
  characteristics.
• Most of the structural ceramics are crystalline
  and oxides.

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Atomic Bonds

• Ionic
• Large differences in electronegativity
• Non directional strong bonds
• Covalent
• Small differences in electronegativity
• Strong, directional bonds
• All ionic, all covalent or covalent-ionic bonds
  possible

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Properties

• High melting temperature bond type
  (ionic-covalent)
• Low thermal conductivities and thermal expansion
  coefficients
• Strong ionic - covalent bonding
• Imperfections (grain boundaries, pores)
• High heat capacity and low heat conductance
  good thermal insulators
• Low density
• High strength, compressive strength usually ten
  times gt tensile
• Very high elastic modulus (stiffness greater than
  metals)
• Very high hardness
• Brittle due to ionic bonds
• Wear resistant because of high compressive
  strength and hardness
• Corrosion resistant and/or unreactive oxides do
  not oxidize further
• High melting point, chemical inertness, high
  hardness and low fracture strength can make it
  difficult to make ceramic components

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Ceramics as Biomaterials

• Advantages
• Inert in body (or bioactive in body) chemically
  inert in many environments
• High wear resistance (orthopedic dental
  applications)
• High modulus (stiffness) compressive strength
• Esthetic for dental applications
• Disadvantages
• Brittle (low fracture resistance, flaw tolerance)
• Low tensile strength (fibers are exception)
• Poor fatigue resistance (relates to flaw
  tolerance)

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Applications

• Orthopedics
• bone plates and screws
• total partial hip components (femoral head)
• coatings (of metal prostheses) for controlled
  implant/tissue interfacial response
• space filling of diseased bone
• vertebral prostheses, vertebra spacers, iliac
  crest prostheses
• Dentistry
• dental restorations (crown and bridge)
• implant applications (implants, implant coatings,
  ridge maintenance)
• orthodontics (brackets)
• glass ionomercements and adhesives
• Other
• inner ear implants (cochlear implants)
• drug delivery devices
• ocular implants
• heart valves

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Attachment

• Four types of ceramic-tissue attachment are
  related to the tissue response to a material
• Morphological fixation dense, inert, nonporous
  ceramics attach by bone (or tissue) growth into
  surface irregularities, by cementing the device
  into the tissues, or by press fitting into a
  defect
• Biological fixation porous, inert ceramics
  attach by bone ingrowth (into pores) resulting in
  mechanical attachment of bone to material
• Bioactive fixation dense, nonporous
  surface-reactive ceramics attach directly by
  chemical bonding with bone
• Resorbable dense, porous or nonporous
  resorbable ceramics are slowly replaced by bone

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Types of Ceramics

1. Nonporous, nearly inert materials are very strong
   and stiff
2. Porous, inert materials have lower strengths, but
   are useful as coatings for metallic implants
3. Nonporous, bioactive materials establish bonds
   with bone tissue
4. Resorbable materials may be porous or nonporous
   and degrade with time

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1. Nonporous, Nearly Inert Ceramics

• Alumina (Al2O3) and Zirconia (ZrO2)
• The two most commonly used structural
  bioceramics.
• Primarily used as modular heads on femoral stem
  hip components.
• Wear less than metal components, and the wear
  particles are generally better tolerated.
• Pyrolytic Carbon
• Coatings for heart valves, blood contacting
  applications

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Processing of Ceramics

• Compounding
• Mix and homogenize ingredients into a water based
  suspension slurry or, into a solid plastic
  material containing water called a clay
• Forming
• The clay or slurry is made into parts by pressing
  into mold (sintering). The fine particulates are
  often fine grained crystals.
• Drying
• The formed object is dried, usually at room
  temperature to the so-called "green" or leathery
  state.
• Firing
• Heat in furnace to drive off remaining water.
  Typically produces shrinkage, so producing parts
  that must have tight mechanical tolerance
  requires care.
• Porous parts are formed by adding a second phase
  that decomposes at high temperatures forming the
  porous structure.

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Solid State Sintering

• Sintering is a diffusional process that combines
  distinct powdered grains below the melting point
  into one cohesive material
• Powder particles are pressed together forming a
  compacted mass of powder particles
• Powders are milled or ground to produce a fine
  powder (d 0.5 5.0 mm)
• Smaller grain size greater strength
• Powder compact is then heated to allow diffusion
  to occur and the separated powder particles
  become fused together
• Usually T gt ½ Tm in Kelvin
• Higher temperature smaller pore size
• Final product consists of grains with boundaries
  containing a mixture
• of atoms from two separate particles
• Material also becomes denser as it is sintered

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Energy Minimization

• Sintering is driven by a reduction in surface
  energy
• Two surfaces are replaced by one grain boundary
• Atoms diffuse from the grain boundary to the void
  surface
• Fast diffusion occurs at grain boundary
• Voids are filled and the part is more
  dense with less surface energy

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Liquid Sintering

• Heat the compacted powder up just above the
  eutectic melting point
• Eutectic melting point is the minimum melting
  point of a combination of two or more materials
• On heating a small proportion of the ceramic
  material melts to form a highly viscous liquid
• Occurs at the periphery of the particles
• The liquid draws the ceramic particles together
• On cooling the viscous phase transforms to
  either
• Glass state (poor high-temp properties)
• Crystalline state (better high-temp properties)

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Alumina

• Al2O3
• Single crystal alumina referred to as Sapphire
• Most used in polycrystalline from
• Unique, complex crystal structure
• Strength increases with decreasing grain size
• Elastic modulus (E) 360-380 GPa
• Low friction and wear properties
• Good for joint bearings
• Grain size must be very small, lt 4 mm

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Zirconia

• ZrO2 (same compound as CZ, but a different
  crystal)
• Good mechanical properties
• Stronger than alumina (2-3 times stronger)
• Less stiff than alumina
• Surface of the zirconia can be made smoother than
  that of an alumina
• Zirconia-PE wear rates are ½ of alumia-PE wear
  rates
• Properties only good for tetragonal crystals
• Tetragonal form is unstable, may transform to
  other crystal structure with poor properties
• Must be stabilized to be useful, much to learn
  still

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Fabrication with Al2O3 and ZrO2

• Devices are produced by pressing and sintering
  fine powders at temperatures between 1600 to
  1700ºC.
• High purity alumina used in biomedical
  applications (gt99.5)
• Additives such as MgO added (lt0.5) to limit
  grain growth

a Alumina sintered 180 minutes at 1580 Cb
Zirconia sintered 120 minutes at 1400 C
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Alumina Zirconia Applications

• Orthopedics femoral head, bone screws and
  plates
• Alumina at a bone interface bone will grow right
  up to it, but will not grow in
• Ceramic-UHMWPE contact used in hip and knee
  replacements
• Ceramic-ceramics contact also used
• Problem with stiffness of alumina
• Dental restorations crowns, bridges, brackets
• Good mechanical and aesthetic properties

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Elemental Carbon

• Elemental, non-metal, many forms possible
• Properties depend on atomic structure
• Diamond, graphite, fullerenes, etc.
• Carbons generally have good biocompatibility
• Forms used in bio-applications
• Graphite lubricating properties
• Diamond-like carbon hard, wear-resistant
• Glassy carbon temp and chem resistant, low
  strength and poor wear resistance
• Pyrolytic carbon wear-resistant, fairly strong,
  brittle

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Pyrolytic Carbon

• Most successful and commonly used form
• pyrolysis thermal decomposition
• Occurs at high temp, with an inert gas (N or He)
• Instead of burning, the carbon polymerizes
  due to the absence of oxygen
• Often used as a coating material
• Preforms are coated, then machined
  and polished before assembly
• Diamond plated grinders and tools are
  needed because PyC is very hard
• Finish can be made very smooth

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Applications

• Very good blood-contacting properties
• Used to coat
• Heart valve components
• Stents
• Compatibility not perfect
• Anticoagulants needed
• Blood compatibility not completely understood
• Other applications
• Joint components
• solid PyC parts are possible

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2. Porous Ceramics

• Porous ceramics have very limited properties due
  to the porosity (reduced solid volume)
• Generally restricted to non-load bearing
  applications
• Coatings for metal or other materials
• Structural bridge for bone formation
• Increasing porosity results in
• Bone ingrowth to fix the component to tissue
• Decreased mechanical properties
• Increased surface area (more environmental
  effects)
• Pore size is critical to tissue growth
  angiogenesis
• Calcium hydroxyapatite is the most common
• Converted from coral or animal bones

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Calcium Hydroxyapatite (HA)

• Ca10(PO4)6(OH)2
• HA is the primary structural component of bone.
• consists of Ca2 ions surrounded by PO42 and OH
  ions.

HA microstructure
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HA

• Gained acceptance as bone substitute
• Repair of bony defects, repair of periodontal
  defects, maintenance or augmentation of alveolar
  ridge, ear implant, eye implant, spine fusion,
  adjuvant to uncoated implants.
• Properties
• Dense HA (properties are similar to enamel
  stiffer and stronger than bone)
• Elastic modulus 40 115 GPa
• Compressive Strength 290 MPa
• Flexure Strength 140 MPa
• Porous HA not suitable for high load bearing
  applications

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Bioceramic Coating

• Coatings of hydroxyapatite are often applied to
  metallic implants (most commonly
  titanium/titanium alloys and stainless steels) to
  alter the surface properties.
• In this manner the body sees hydroxyapatite-type
  material which it appears more willing to accept.
  
• Without the coating the body would see a foreign
  body and work in such a way as to isolate it from
  surrounding tissues.
• To date, the only commercially accepted method of
  applying hydroxyapatite coatings to metallic
  implants is plasma spraying.

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Bone Fillers

• Hydroxyapatite may be employed in forms such as
  powders, porous blocks or beads to fill bone
  defects or voids.
• These may arise when large sections of bone have
  had to be removed (e.g. bone cancers) or when
  bone augmentations are required (e.g
  maxillofacial reconstructions or dental
  applications).
• The bone filler will provide a scaffold and
  encourage the rapid filling of the void by
  naturally forming bone and provides an
  alternative to bone grafts.
• It will also become part of the bone structure
  and will reduce healing times

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3. Bioactive Ceramics

• Certain types of ceramics have been shown to bond
  to bone
• Bioactive glass
• Bioactive glass-ceramics
• Bioactive crystalline ceramics and bioactive
  composites exist also
• Have relatively high melt temperatures are (1300
  1450ºC)
• Can be cast into intricate shapes (in glass form)
• Can be ground into powders, sized, and used for
  packing material, etc.

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Glass

• Structure is isotropic, so the properties are
  uniform in all directions
• Brittle
• No planes of atoms to slip past each other
• No way to relieve stress
• Often more brittle than (crystalline) ceramics
• Typically good electrical and thermal insulators
• Transparent (amorphous)
• A supercooled liquid or a solid?
• Viscosity of water at room temp is 10-3 Poise
• Viscosity of a typical glass at room temp gtgt 1016
  P

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Glass Processing

• Completely melting ingredients to a homogeneous
  liquid and cooling to a homogeneous material.
• Glasses are most commonly made by rapidly
  quenching a melt
• Elements making up the glass material are unable
  to move into positions that allow them to become
  crystalline
• Result is a glass structure amorphous

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Glass Structure

• Glass-forming oxides
• e.g., SiO2 B2O3 P2O5 GeO2
• glass-forming network often the major component
• Glass-modifying oxides
• e.g., Na2O CaO Al2O3 TiO2
• modify glass network add positive
  ions to the structure and break up
  network
• minor to major component alter
  glass properties (e.g. softening pt)
• Even when molten, chains not
  free to move, very viscous

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Types of Glass

• Silicate glass (fused silica)
• SiO2
• Each silicon is covalently bonded to 4 oxygen
  atoms
• Soda-lime glass
• 70 wt SiO2 15 wt Na2O 10 wt CaO
• Window glass, bottles, etc.
• Borosilicate glass
• Some SiO2 replaced by B2O3
• 80 wt SiO2 15 wt B2O3 5 wt Na2O
• Pyrex glass cooking and chemical glass ware

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Glass-Ceramics

• Composite structure consisting of a matrix of
  glass in which fine crystals have formed
• Crystals can commonly be very fine (avg. size lt
  500 nm)
• Glass-ceramics are 50 to 99 crystalline
• The result is a mixture of glass-like and
  crystalline regions that
• Prevents thermal shock
• Lowers porosity
• Increases strength

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Glass-Ceramic Processing

• Glass with nucleating agent like TiO2 is formed
  into the desired shape
• Nucleating agents aide in the formation of the
  crystals
• Barely soluble in the glass
• Remain in solution at high temperatures
• Precipitate out at low temperature
• Act as nuclei for crystal growth at elevated
  temperatures
• Conversion takes part in two phases
• First glass is seeded with nuclei
• The formed material may be lowered to the
  nucleating temperature after forming OR
• It may be lowered to room temperature, then
  reheated to the nucleating temperature.
• Second crystals grow around the nuclei
• Following nucleation the temperature is then
  raised to the crystal growth temperature

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Bioactivity

• Bioactivity is very sensitive to composition
• Both in glasses and glass-ceramics
• Less than 60 mol SiO2
• High Na2O and CaO content
• High CaO/P2O5 ratio, minimum 51
• Composition makes the surface highly reactive
  when it is exposed to an aqueous environment

Bioactive glass implants (45S5) and matching
drill bits used to replace the roots of extracted
teeth
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Bioactive Ceramic Interfacial Reactions

• When the bioactive glass is immersed in body
  fluids sodium ions leach from the surface and are
  replaced by H through an ion exchange reaction.
• This produces a silica rich layer
• An amorphous calcium-phosphate layer is formed on
  the silica rich layer due to migration of the
  calcium and phosphate ions from the bulk of
  glass.
• Biological moieties such as blood proteins,
  growth factors and collagen are incorporated into
  the layer.
• The amorphous layer crystallizes into carbonate
  hydroxyapatite (equivalent to natural bone
  mineral).
• Bodys tissues are able to attach directly to the
  crystallized layer
• Layer grows to be approximately 100-150 mm in
  depth.
• Occurs within 12-24 hr
• Cells arrive within 24 to 72 hr and encounter a
  bonelike surface, complete with organic components

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Bioactive ApplicationsGlass and Glass-Ceramic

• A/W Solid Glass-Ceramics (Cerabone)
• Vertebral prostheses (for spinal fractures)
• Vertebral spacers (for lumbar instability)
• Iliac crest prostheses (restoration after bone
  graft removal)
• Solid Bioglass
• Douek cochlear implants (100 effective after 10
  years vs. 72 failure for metallic and polymeric
  implants of same type)
• Particulate Bioglass
• PerioGlas for treatment of periodontal disease
• NovaBone bone grafting material for orthopedics
  maxillofacial repair

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4. Resorbable Ceramics

• Degrade upon implantation in the host
• Rate of degradation varies from material to
  material
• rate needs to be equal to rate of tissue
  generation at specific site of application
• Almost all bioresorbable ceramics (except
  Biocoral and Plaster of Paris calcium sulfate
  dihydrate) are variations of calcium phosphate
• Uses of biodegradable bioceramics
• Drug-delivery devices
• Repair material for bone damaged by trauma or
  disease
• Space filling material for areas of bone loss
• Material for repair and fusion of spinal and
  lumbosacral vertebrae
• Repair material for herniated disks
• Repair material for maxillofacial and dental
  defects
• Ocular implants

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Calcium Phosphate

• Calcium phosphate compounds are abundant in
  nature and in living systems.
• Biologic apatites which constitute the principal
  inorganic phase in normal calcified tissues
  (e.g., enamel, dentin, bone) are carbonate
  hydroxyapatite, CHA.
• In some pathological calcifications (e.g.,
  urinary stones, dental tartar, calcified soft
  tissues heart, lung, joint cartilage)
• Form of calcium phosphate depends on CaP ratio
• Most stable form is crystalline hydroxyapatite
  Ca10(PO4)6(OH)2
• Ideal CaP ratio of 106
• Crystallizes into hexagonal rhombic prisms
• This apatite form of calcium phosphate is closely
  related to the mineral phase of bone and teeth
• Very low bulk solubility can be used as a
  structural biomaterial

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b-Tricalcium Phosphate (TCP)

• Another widely used form is ß-tricalcium
  phosphate ß-Ca3(PO4)2
• In aqueous environment surface reacts to form HA
• 4Ca3(PO4)2 2H2O ? Ca10(PO4)6(OH)2 2Ca2
  2HPO42-
• Often porous (partially sintered powders)

Tricalcium phosphate thin film (Osteologic) used
in orthopedic applications
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Stability

• Resorption caused by 3 factors
• Physiologic dissolution (depends on environment
  pH, type of CaP)
• Physical disintegration into small particles as a
  result of preferential chemical attack of grain
  boundaries (enhanced by porosity)
• Biological factors, such as phagocytosis, which
  causes a decrease in local pH concentration
• Apatite forms are the most stable
• high rate of dissolution ?
  low rate of dissolution
• TTCP gt a-TCP gt ß-TCP gt
  HA gt Fluorapatite
• Substitution of F- for OH- in HA greatly
  increases the chemical stability
• Get fluorapatite Ca10(PO4)6(F)2
• Found in dental enamel
• Principle is used in dental fluoride treatments
  ( 1 in 100 OH- replaced)

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Mechanical Properties

• Dense HA (properties are similar to enamel
  stiffer and stronger than bone)
• Elastic modulus 40 115 GPa
• Compressive Strength 290 MPa
• Flexure Strength 140 MPa
• Porous HA
• not suitable for high load bearing applications
• TCP
• Generally poor (more of a packing material)

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Summary

• 4 groups of ceramic for biomedical applications
• Nonporous, nearly inert structural components
• Porous, inert non-load bearing, coatings,
  fillers
• Nonporous, bioactive coatings, dental
  applications, strong attachment to bone
• Resorbable fillers, spinal/defect repair, drug
  delivery
• Function greatly affected by
• Composition bioactivity
• Structure (crystal and grains) mechanical
  properties
• Processing mechanical properties
• Porosity reactivity, degradation
• In vivo environment reactions with tissue/fluids