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Biomaterials
Chapter 7:
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Outline of Chapter 7
7-1Introduction to Biomaterials
7-2Bioceramics
7-3Polymeric Biomaterials
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What are Biomaterials?
A biomaterial can be defined as any material used to make
devices to replace a part or a function of the body in a safe,
reliable, economic, and physiologically acceptable manner.
A biological material is a material such as bone, skin, or artery
produced by a biological system.
The success of a biomaterial is highly dependent on three major
factors:
(1) the properties and biocompatibility of the implant,(2) the health condition of the recipient
(3) the competency of the surgeon who implants and monitors
its progress.
7.1 Introduction to Biomaterials
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Biocompatibility Requirements7.1 Introduction to Biomaterials
Acute systemic toxicity
Cytotoxicity
Hemolysis
Intravenous toxicity
Mutagenicity
Oral toxicity
Pyrogenicity
Sensitization
Schematic illustration of biocompatibility
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The Requirements for An Implant
7.1 Introduction to Biomaterials
Acceptance of the plate to the tissue surface, i.e., biocompatibility
Pharmacological acceptability (nontoxic, nonallergenic,nonimmunogenic, noncarcinogenic, etc.)
Chemically inert and stable (no time-dependent degradation)
Adequate mechanical strength
Adequate fatigue life
Sound engineering design
Proper weight and density
Relatively inexpensive, reproducible, and easy to fabricate and
process for large-scale production
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Class of Materials Used in the Body
7.1 Introduction to Biomaterials
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7.2 Bioceramics
What are bioceramics?
The class of ceramics used for repair and replacement of
diseased and damaged parts of the musculoskeletal system are
referred to as bioceramics.
The field of bioceramics is relatively new (1970s), but many
bioceramics are not new materials.
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Classification scheme for bioceramics
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Clinical uses of bioceramics
The uses go from head to toe and
include repairs to bones, joints,
and teeth. These repairs becomenecessary when the existing part
becomes diseased, damaged, or
just simply wears out.
Illustration of the head-to-toe
clinical uses for bioceramics
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Advantages and disadvantages of bioceramics
Advantages:
Biocompatible
Wear resistant
Lightweight (certain compositions)
Disadvantages: Low tensile strength
Difficult to fabricate
Low toughness
Not resiliant
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Ceramic Implants and the Structure of Bone
Basic criteria for choosing ceramic implant
The ceramic should be compatible with the physiological environment
Its mechanical properties should match those of the tissue beingreplaced
Most concern in the use of bioceramics: Cancellous (spongy bone)
Cortical (compact bone)
Longitudinal section showing the
structure of long bone
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Bioinert Ceramics Al2O3 and ZrO2
Bioinert ceramics:
maintain their physical and mechanical properties while in the host.
resist corrosion and wear, and have all the properties for bioceramics
Desired Properties of Implantable Bioceramics
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Al2O3 and ZrO2
Biomedical applications of Al2O3: There are many other applications of alumina as an implant material
including knee prostheses, ankle joints, elbows, shoulders, wrists, and
fingers
undergo little or no chemical change during long-term exposure to
body fluids
Medical grade alumina used as femoral balls in THP
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Al2O3 and ZrO2
Difference between Al2O3 and ZrO2
Al2O3 combine excellent biocompatibility and outstanding wear resistance
they have only moderate fl exural strength and low toughness
ZrO2 have higher fracture toughness, higher flexural strength, and lower
Youngs modulus than alumina
Some concerns with ZrO2
a slight decrease in fl exural strength and toughness of zirconia ceramics
exposed to bodily fluids
wear resistance of zirconia is inferior to that of alumina
may contain low concentrations of long half-life radioactive elements such
as Th and U, which are difficult and expensive to separate out
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Bioactive Ceramics
A bioactive material is one that elicits a specifi c biological
response at the interface of the material, which results in the
formation of a bond between tissues and the material.
Some types of bioactive ceramics:
Bioactive glasses
Bioactive glass-ceramics
Hydroxyapatite (HA)
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Bioactive glasses (BGs)
The first and most thoroughly studied bioactive glass is known as
BioglassR 45S5 and was developed at the University of Florida.
BioglassR 45S5 is a multicomponent oxide glass with the following
composition (in wt%): 45% SiO2, 24.5% Na2O, 24.4% CaO, and 6% P2O5
Fabrication of BGs:
BGs can be made using the processes developed for other silicate
glasses. The constituent oxides, or compounds that can be
decomposed to oxides, are mixed in the right proportions and melted at
high temperatures to produce a homogeneous melt. On cooling a glass
is produced. It is necessary to use high-purity starting materials and often the melting
is performed in Pt or Pt alloy crucibles to minimize contamination
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Bioactive glasses (BGs)
Advantages and disadvantages of BGs:
Advantages:
a relatively rapid surface reaction
the reaction rates and mechanisms have been determined
the bonding process (SiO2hydroxycarboapatite layer)
close to that of cortical bone.
Disadvantages:
mechanically weak
tensile bending strengths are typically 4060 MPa the fracture toughness is low
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Bioactive glasses (BGs)
(A) The middle ear cavity and the auditory ossicles
(B) Ear implants
Other applications of BGs:
fill the defect in the jaw (cone-shaped plugs)
repair the bone that supports the eye
used in the treatment of periodontal disease
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Bioactive glass-ceramics
Glass-ceramics are produced by ceramming a glass: converting
it to a largely crystalline form by heat treatment
Typical bioactive glass-ceramics:
CeraboneRA-W is a glass-ceramic containing oxyfluoroapatite (A) and
wollastonite (W).
CeravitalR is primarily now used in middle ear operations.
Bioverit IR is a class of bioactive machinable glass.
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Bioactive glass-ceramics
CeraboneR A-W
is produced by crystallization of a glass of the following composition
(in wt%): 4.6 MgO, 44.7 CaO, 34.0 SiO2, 6.2 P2O5, and 0.5 CaF2.
Oxyfluoroapatite [Ca10(PO4)6(O,F)2] as the A phase and
-wollastonite (CaOSiO2) as the W phase.
The applications include vertebral prostheses, vertebral
spacers, and iliac crest prostheses.
The composition of the residual glassy phase is (in wt%) 16.6
MgO, 24.2 CaO, and 59.2 SiO2.
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Bioactive glass-ceramics
CeravitalR
The composition of Ceravital is similar to that of Bioglass in SiO2content but differs somewhat in other components.
are clinically used is in the replacement of the ossicular chain in
the middle ear. In this application the mechanical properties of
the material are suffcient to support the minimal applied loads.
Bioverit IR
consists of two crystalline phases in a glass matrix: mica and
apatite.
several clinical applications : spacers in orthopedic surgery and
middle ear implants.
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Hydroxyapatite (HA)
The general formula A10(BO4)6X2. In HA, or more specifically calcium
hydroxyapatite, A = Ca, B = P, and X = OH.
Hydroxyapatite is chemically similar to the mineral component of
bones and hard tissues in mammals. it will support bone ingrowth and
osseointegration when used in orthopaedic, dental and maxillofacial
applications.
Natural bone is70% HA by weight and 50% HA by volume.
Background
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Hydroxyapatite (HA)
Crystal structure
Hydroxyapatite is hexagonal(space group is P63/m)
with a = 0.94132 nm and c =
0.6877 nm.
Substitutions in the HA
structure are possible.
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Hydroxyapatite (HA)
Key Properties of HA
The ability to integrate in bone structures and support bone
ingrowth, without breaking down or dissolving (i.e it is bioactive).
Hydroxyapatite is a thermally unstable compound, decomposing at
temperature from about 800-1200 oC depending on its
stoichiometry.
Generally speaking dense hydroxyapatite does not have the
mechanical strength to enable it to succeed in long term load
bearing applications
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Hydroxyapatite (HA)
Applications of HA
two forms for biomedical applications: either dense or porous
Bioceramic Coatings:
Coatings of hydroxyapatite are often applied to metallic implants
(most commonly titanium/titanium alloys and stainless steels) to alter
the surface properties.
Bone Fillers
Hydroxyapatite may be employed in forms such as powders, porous
blocks or beads to fill bone defects or voids.
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Bioceramics in Composite
main reason for forming composites is to improve the mechanical
properties, most often toughness, above that of the stand-alone
ceramic.
The first bioceramic composite was a stainless-steel
fiber/bioactive glass composite made of Bioglass 45S5 and AISI
316L stainless steel.
Other current bioceramic composites of interest:
Ti-fiber-reinforced bioactive glass
ZrO2-reinforced A-W glass
TCP-reinforced PE
HA-reinforced PE
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Bioceramics in Composite
Example of HA-reinforced PE
Effect of volume fraction of HA on and
strain to failure of HA-reinforced PEcomposites, in comparison to cortical bone
It shows how increasing thevolume fraction of HA to 0.5 in a
composite can be achieved with
in the range of that of cortical
bone. When the volume fraction
of HA in the composite isincreased above about 0.45 the
fracture mode changes from
ductile to brittle. For clinical
applications a volume fraction of0.4 has been found to be
optimum.
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Applications of Porous Bioceramics
Porous Bioceramics
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Applications of Porous Bioceramics in Cervical Vertebra
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Applications of Bioceramics in vertebral column
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Applications of Bioceramics in Limbs
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Applications of Porous Bioceramics in Stomatology
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Applications of Porous Bioceramics in Ophthalmology
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Example: Bioceramics for Femur Fracture Treatment
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Example: Bioceramics for Bone Fillers
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7.3 Polymeric Biomaterials
Advantages of polymeric biomaterials:
are ease of manufacturability to produce various shapes (latex, film,
sheet, fibers, etc.), ease of secondary processability, reasonablecost, and availability with desired mechanical and physical properties.
Required properties of polymeric biomaterials:
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Applications of polymeric biomaterials:
medical disposable supply
prosthetic materials
dental materials
implants
dressings
extracorporeal devices
encapsulants
polymeric drug delivery systems
tissue engineered products
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Polymerization and Basic Structure
Polymerization
Condensation or Step Reaction Polymerization
One major drawback of condensation
polymerization is the tendency for the
reaction to cease before the chains
grow to a sufficient length.
Natural polymers, such aspolysaccharides and proteins are also
made by condensation polymerization.
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Polymerization
Addition or Free Radical Polymerization
Ethylene
can be achieved by rearranging
the bonds within each monomer.
the breaking of a double bond
can be made with an initiator.
types of initiating species: free-radicals; cations, anions, and
coordination catalysts.
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Basic Structure
Polymers have very long chain molecules which are formed by covalent
bonding along the backbone chain.
Each chain can have side groups, branches and copolymeric chains or
blocks which can also interfere with the long-range ordering of chains.
MW of polymer = DP MW of mer (or repeating unit)
The relationship between molecular weight and degree of polymerization:
MW: molecular weight; DP: degree of polymerization
B i St t
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Basic Structure
Arrangement of polymer chains
Polyvinyls, Polyamides, Polyesters
Polyphenolformaldehyde
Dendritic polymers
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Polymers Used as Biomaterials
only ten to twenty polymers could be used as biomaterials.
mainly used in medical device fabrications from disposable to
long-term implants.
Polymers Used as Biomaterials
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Polymers Used as Biomaterials
Polyvinylchloride (PVC)
PVC is an amorphous, rigid polymer due to the large side group (Cl, chloride)
with a Tg of 75 to 105 oC.
high melt viscosity
PVC sheets and films are used in blood and solution storage bags and
surgical packaging. PVC tubing is commonly used in intravenous (IV)
administration, dialysis devices, catheters, and cannulae.
Polyethylene (PE)
five major grades: (1) high density (HDPE), (2) low density (LDPE),
(3) linear low density (LLDPE), (4) very low density (VLDPE), and (5) ultra
high molecular weight (UHMWPE). Pharmaceutical bottle, catheter, pouch, flexible container, and orthopedic
implants.
Polymers Used as Biomaterials
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Polymers Used as Biomaterials
Polypropylene (PP)
PP can be polymerized by a Ziegler-Natta stereospecific catalyst which
controls the isotactic position of the methyl group.
Thermal (Tg: 12C, Tm: 125167C and density: 0.850.98 g/cm3) and
physical properties of PP are similar to PE.
PP is used to make disposable hypothermic syringes, blood oxygenator
membrane, packaging for devices, solutions, and drugs, suture, artificial
vascular grafts, etc.
Polymethylmetacrylate (PMMA)
one of the most biocompatible polymers.
medical applications: blood pump and reservoir, an IV system, membranes
for blood dialyzer, and in in vitro diagnostics.
bone cement for joint prostheses fixation.
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Biodegradable Polymers
Polymers Used as Biomaterials
biodegradable polymers such as polylactide (PLA), polyglycolide (PGA),
poly(glycolideco-lactide) (PLGA), poly(dioxanone), poly(trimethylene carbonate),
poly(carbonate).
PLA, PGA, and PLGA are bioresorbable polyesters, and degrade by
nonspecific hydrolytic scission of their ester bonds.
The hydrolysis of PLA yields lactic acid which is a normal byproduct of
anaerobic metabolism in the human body and is incorporated in the tricarboxylic
acid (TCA) cycle to be finally excreted by the body as carbon dioxide and water
PGA biodegrades by a combination of hydrolytic scission and enzymatic(esterase) action producing glycolic acid which can either enter the TCA cycle
or is excreted in urine and can be eliminated as carbon dioxide and water.
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Sterilization
Sterilization techniques for biomedical polymers
dry heat, Steam sterilization (autoclaving), radiation, and ethylene oxide gas
In dry heat sterilization, the temperature varies between 160 and 190C.
Steam sterilization (autoclaving) is performed under high steam pressure at
relatively low temperature (125130C).
Radiation sterilization using the isotopic 60Co can also deteriorate polymers
since at high dosage the polymer chains can be dissociated or cross-linked
according to the characteristics of the chemical structures.
Chemical agents such as ethylene and propylene oxide gases, and phenolicand hypochloride solutions are widely used for sterilizing polymers since they
can be used at low temperatures.
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Surface Modifications for Improving Biocompatability
prevention of thrombus formation is important in clinical applications
where blood is in contact.
considerable platelet deposition and thrombus formation take place on the
artificial surfaces
For example: Heparin
one of the complex carbohydrates is currently used to prevent formation of
clots.
The major drawback of these surfaces is that they are not stable in the
blood environment.
S f M difi i f I i Bi bili
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Physical and Chemical Surface Modification Methods
Surface Modifications for Improving Biocompatability
PowerPoint
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Biodegradable Polymers
Biodegradable polymers are polymers that break down and lose their
initial integrity through the action of enzymes and/or chemical
deterioration associated with living organisms.
Biodegradable polymers were first introduced in 1980s.
natural biodegradable polymers
synthetic biodegradable polymers
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Typical Biodegradable Polymers
Polyglycolide (PGA): PGA is the simplest linear aliphatic polyester. It is
prepared by ring opening polymerization of a cyclic lactone, glycolide.
Polylactide (PLA): PLA is usually obtained from polycondensation of D- orL-lactic acid or from ring opening polymerization of lactide, a cyclic dimer
of lactic acid. Two optical forms exist: D-lactide and L-lactide. The natural
isomer is L-lactide and the synthetic blend is DL-lactide.
Poly(lactide-co-glycolide) (PLGA): L-lactide and DL-lactide (L) have been
used for copolymerization with glycolic acid monomers (G).
Polycaprolactone (PCL): -caprolactone is a relatively cheap cyclic
monomer. A semi-crystalline linear polymer is obtained from ring-openingpolymerization of -caprolactone in presence of tin octoate catalyst. PCL
is soluble in a wide range of solvents
Typical Biodegradable Polymers
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Typical Biodegradable Polymers
Polysaccharides from marine sourcesChitin: It is the second most abundant natural biopolymer. It is a linear
copolymer of N-acetylglucosamine and N-glucosamine with -1,4 linkage
Chitosan: Chitin is processed to chitosan by partial alkaline N-deacetylation
The applications of chitin and chitosan are limited because of their insolubility in
most solvents.
Polysaccharides from vegetal sources
Starch: it is a hydrocolloid biopolymer. It is a low cost polysaccharide,abundantly available and one of the cheapest biodegradable polymers.
Cellulose: it is another widely known polysaccharide produced by plants. It is
a linear polymer with very long macromolecular chains of one repeating unit,
cellobiose
Alginic acid or alginate: is another polysaccharide, present in brown algae. It
contains carboxyl groups in each constituent residue.
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Applications of Biodegradable Polymers
sutures
controlled drug release
tissue engineering
Required properties:
non-toxic
capable of maintaining good mechanical integrity until degraded
capable of controlled rates of degradation
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Chitin
Chitosan
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Amylose:
Amylopektin:
Cellulose: