a introducere in biomat

8/10/2019 A Introducere in Biomat

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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


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



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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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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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(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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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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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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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.



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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.



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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


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



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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:

a introducere in biomat

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