BIOMATERIALS ENT 311/4 Lecture 3 Polymeric Biomaterials.

BIOMATERIALSENT 311/4

Lecture 3 Polymeric Biomaterials

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Teaching Plan

POLYMERIC BIOMATERIALS

Review structures and properties of biopolymers.

Define & Describe the biomedical application of polymeric biomaterials

DELIVERYMODE

Lecture

Laboratory experiments

LEVEL OF COMPLEXITY

KnowledgeRepetition

COURSE OUTCOMECOVERED

Ability to describe the concept of biocompatibility & basic concepts of materials used in medical application

Ability to select biomaterials that can be used for different medical applications and explain the criteria that will lead to a successful implants

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1.0 Introduction

Application of synthetic polymers medical disposable supply prosthetic materials, dental materials implants dressings extracorporeal devices encapsulants polymeric drug delivery systems tissue engineered products orthodoses

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1.0 Introduction

Ease of manufacturability to produce various shapes

Ease of secondary processability reasonable cost Availability with desired mechanical and

physical properties.

Main Advantages

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1.0 Introduction

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2.0 Basic Structure

Polymers have very long chain molecules which are formed by covalent bonding along the backbone chain.

The long chains are held together by: primary covalent bonding forces thru crosslinks between

chains 2ndary bonding forces such as van derWaals & hydrogen

bonds

Each chain can have side groups, branches & copolymeric, chains or blocks

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2.0 Basic Structure

As the molecular chains become longer, their relative mobility decreases

The higher the molecular weight, the less the mobility of chains which results in higher strength & greater thermal stability

Polymer chains can be arranged in 4 ways: Linear Branched, Cross-linked Three-dimensional network

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2.0 Basic Structure

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2.0 Basic Structure

~Linear Polymers~Linear Polymers**The mer units are joined together end to end in The mer units are joined together end to end in single chains represents as a mass of spaghettisingle chains represents as a mass of spaghetti**May have extensive van der Waals & hydrogen May have extensive van der Waals & hydrogen bonding between chainsbonding between chains

~ Branched Polymers~ Branched Polymers*Side branch chains are connected to the backbone.*Side branch chains are connected to the backbone.*The branches resulted from side reactions that occur *The branches resulted from side reactions that occur during the polymer synthesis during the polymer synthesis *The chain packing efficiency is reduced by the *The chain packing efficiency is reduced by the formation of side branches which lowers the polymer formation of side branches which lowers the polymer densitydensity

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2.0 Basic Structure

Cross-linked Polymers

Crosslinking process is achieved either by synthesis or nonreversible chemical reaction carried out at elevated temperature

Is accomplished by additive atoms or molecules covalently bonded to the chains

Network Polymers

Trifunctional mer units, having 3 active covalent bonds to form 3-D networks

A polymer that is highly crosslinked maybe classified as a network polymer

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2.0 Basic Structure

Molecular Configuration

ISOTACTIC CONFIGURATION

All the side groups are situated on the same side of the chain

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2.0 Basic Structure

Syndiotactic Configuration

The side groups alternate sides of the chain

Atactic Configuration

Random positioning of side groups Conversion of one stereoisomer to another will involve

severing of bonds reformation after appropriate rotation

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2.0 Basic Structure

Copolymersa)Random copolymer-two different units are randomly dispersed along the chain

b)Alternating copolymer-the 2 mer units alternate chain positions

c) Block copolymer-the identical mers are clustered in blocks along the chain

d) Graft copolymer-homopolymer side branches of one type maybe grafted to homopolymer main chains that are composed of a different mer

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2.0 Basic Structure

Revision (Molecular Weight Calculation)

The no. average molecular weightMn=ΣxiMi

The weight average molecular weight

Mw=ΣwiMi

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3.0 Crystal & Amorphous Structure in Bioplymer

Crystallization is easier for polymer with shorter chain

Branched polymer in which side chains are attached to the main backbone chain at positions will not crystallize easily

Linear polymers are much easier to crystallize Partially crystallized structure (semicrystalline) is

commonly occur in linear polymers The cross-linked or 3-D network polymers cannot be

crystallized at all & they become amorphous polymers.

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Polymer with small side group are easy to crystallize Isotactic & syndiotactic polymers usually crystallize

even when the side groups are larger Copolymerization always disrupts the regularity of

polymer chains thus it is more amorphous Plasticizers can prevent crystallization by keeping

the chains separated from one another

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Classical “fringed-micelle” model which shows the amorphous and crystalline regions coexist

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4.0 Biopolymers Properties

Thermoplastic Polymers Usually have linear & branched structures, they

soften when heated & harden when cooled. The process reversible & can be repeated The reheating and reforming process did not

have significant change on the polymer properties

Mostly consist of a very long main chain of carbon atoms covalently bonded together

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Thermoplastic Polymers

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Thermosetting Polymers

The term implies that heat is required to permanently set the plastic

Thermosets polymer, once having hardened, will not soften upon heating, their structures are cross-linked & network.

They could be degrade or decompose if heated at very high temperature

Thermoset polymers are harder & stronger than thermoplastics

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Thermosetting Polymers

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Only ten to twenty polymers are mainly used in medical device fabrications from disposable to long-term implants

5.0 Polymeric Biomaterials

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Available commercially as high density (HDPE) low density (LDPE) linear low density (LLDPE) very low density (VLDPE) ultra high molecular weight (UHMWPE)

Clear to whitish translucent thermoplastic


Polyethylene (PE)

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Polyethylene (PE)…(continue)

• Low density

• High Density

• Linear low density

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HDPE -pharmaceutical bottles, nonwoven fabrics, & caps

LDPE - flexible container applications, nonwoven-disposable & laminated (or coextruded with paper) foil & polymers for packaging.

LLDPE - pouches & bags due to its excellent puncture resistance

VLDPE - extruded tubes.

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UHMWPE (MW >2×106 g/mol) has been used for orthopedic implant fabrications.

This orthopedic implant fabrications include load-bearing applications:

Acetabular cup of total hip Tibial plateau & Patellar surfaces of knee joints.

Specific Properties: Low cost, easy to process, excellent electrical insulator, excellent chemical resistance, tough & flexible even at low temperature

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PVC is amorphous, does not recrystallize due to the large side group (Cl, chloride)

It has a high melt viscosity hence it is difficult to process.

PVC homopolymer has high strength (7.5 to 9 psi) & brittle

PVC sheets & films – blood, solution storage bags & surgical packaging


Polyvinylchloride (PVC)

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PVC tubing-commonly used in intravenous (IV) administration, dialysis devices, catheters, & cannulae

Specific Properties: Excellent resistance to abrasion, good dimensional stability, high chemical resistance

Note: To prevent the thermal degradation of the polymer

(HCl could be released), thermal stabilizers -metallic soaps/salts are incorporated

Di-2-ethylhexylphthalate (DEHP or DOP) is used in medical PVC formulation.


Polyvinylchloride (PVC)…continue

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High melting (165-1770C) & heat deflection temperature

Additives for PP such as antioxidants, light stabilizer, nucleating agents, lubricants, mold release agents, antiblock, & slip agents are formulated to improve the physical properties & processability

PP has an exceptionally high flex life & excellent environment stress-cracking resistance, hence it had been tried for finger joint prostheses with an integrally molded hinge design [Park, 1984]


Polypropylene (PP)

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PP is used to make disposable hypothermic syringes, blood oxygenator membrane, packaging for devices, solutions, and drugs, suture, artificial vascular grafts, nonwoven fabrics, etc.

Specific Properties: Low density, good chemical resistance, moisture resistance & heat resistance

Good surface hardness & dimensional stability


Polypropylene (PP)…continue

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Commercial PMMA-amorphous material with good resistance to dilute alkalis & other inorganic solutions

Best known for exceptional light transparency (92% transmission), high refractive index (1.49), good weathering properties & as one of the most biocompatible polymers

Used broadly in medical applications: blood pump & reservoir, IV system, membranes for blood dialyzer

in vitro diagnostics.


Polymethylmetacrylate (PMMA)

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It is also found in contact lenses & implantable ocular lenses due to excellent optical properties

Dentures, & maxillofacial prostheses due to good physical & coloring properties

Bone cement for joint prostheses fixation

5.0 Biomedical Applications of Polymeric Biomaterials

Polymethylmetacrylate (PMMA )…continue

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PS has good transparency, lack of color, ease of fabrication, thermal stability, low specific gravity & relatively high modulus

Commonly used in tissue culture flasks, roller bottles, vacuum canisters & filterware

Acrylonitrile–butadiene–styrene (ABS) copolymers are produced by 3 monomers: acrylonitrile, butadiene & styrene

Resistant to common inorganic solutions, have good surface properties, and dimensional stability

For IV sets, clamps, blood dialyzers, diagnostic test kits

5.0 Biomedical Applications of Polymeric Biomaterials

Polystyrene (PS) and Its Copolymers

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Frequently found in medical applications due to their unique chemical & physical properties

PET (polyethyleneterephthalate) is so far the most important

Biomedical applications-as artificial vascular graft, sutures & meshes.

It is highly crystalline with high melting temperature, hydrophobic & resistant to hydrolysis in dilute acids

Polycaprolactone is crystalline & has a low melting temperature.

Soft matrix or coating for conventional polyester fibers. Tissue engineering


Polyesters

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Flexibility of carbon chain contributes to molecular flexibility, low melt viscosity and high lubricity

Nylons are hygroscopic and lose their strength in vivo when implanted

Poly (p-phenylene terephthalate) commonly known as Kevlar®

Very good mechanical properties, good thermal properties, good chemical resistance, permeable to gases

Tubes for intracardiac catheters,surgical sutures, dialysis devices components,heart mitral valves, sutures


Polyamides (Nylon)

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Commonly known as Teflon® The polymer is highly crystalline, high density, low

modulus of elasticity & tensile strength It also has a very low surface tension & friction

coefficient (0.1) Specific Properties: Chemical inertness, exceptional

weathering & heat resistance, nonadhesive, very low coefficient of friction

Application: Vascular & auditory prostheses, catheters, tubes


Polytetrafluoroethylene (PTFE)

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Rubbers have been used for the fabrication of implants

Natural rubber is compatible with blood in pure form Crosslinking by x-ray & organic peroxides produces

rubber with superior blood compatibility Silicone rubber developed for medical use Good thermal stability, resistance to atmospheric &

oxidative agents, physiological inertness Burn treatment, shunt, mammary prostheses,

maxillofacial implants


Rubbers

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Polyurethanes are usually thermosetting polymers: they are widely used to coat implants

polyurethane rubber is quite strong and has good resistance to oil and chemicals

Exceptional resistance to abrasion, resistance to breaking, very high elasticity modulus at compression traction & sheering remarkable

Adhesives, dental materials, blood pumps, artificial heart & skin


Polyurethanes

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Polyacetals & polysulfones are being tested as implant materials

Polycarbonates have found their applications in the heart/lung assist devices & food packaging

Polyacetals have reasonably high molecular weight & excellent mechanical properties

Excellent resistance to most chemicals & to water over wide temperature ranges

Hard tissue replacement


Polyacetal, Polysulfone & Polycarbonate

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Polysulfones have a high thermal stability due to the bulky side groups (therefore, they are amorphous) & rigid main backbone chains

Polycarbonates are tough, amorphous, & transparent polymers

Excellent mechanical & thermal properties, hydrophobicity & antioxidative properties


Polyacetal, Polysulfone & Polycarbonate

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Hydrolysis of PLA yields lactic acid which is a normal byproduct of anaerobic metabolism in the human body & is incorporated in the tricarboxylic acid (TCA) cycle to be finally excreted by the body as CO2 & water

PGA biodegrades by a combination of hydrolytic scission & enzymatic (esterase) action producing glycolic acid either enter the TCA cycle or is excreted in urine and can be eliminated as CO2 & water

PLGA can be controlled from weeks to over a year by varying the ratio of monomers & the processing conditions


Biodegradable Polymers…(continue)

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PLA-high tensile strength & low elongation resulting in a high modulus. Application:bone fracture fixation

PLGA-tissue engineered repair systems where cells are implanted within PLGA films or scaffolds

PLGA-drug delivery systems in which drugs are loaded within PLGA microspheres

Other-Poly-p-dioxanon:bioabsorbable polymer which can be fabricated into flexible monofilament surgical sutures


Biodegradable Polymers…(continue)

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Summary

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Extra: Biopolymers

Polymers are used in biomedical applications Cardiovascular, Opthalmic and

Orthopaedic implants Dental implants, dental cements and

denture bases

• Low density, easily formed and can be made biocompatible.

Recent development – biodegradable polymers.

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Extra: Cardiovascular Applications

Heart valves can be stenotic or incompetent Polymers are used to make artificial heart valves Leaflets are made from biometals • Sewing ring made from PTFE or PET

Connected to heart tissue• Blood clogging is side effect• PTFE is used as vascular graft to bypass clogged arteries.• Blood oxygenators : Hydrophobic polymer membranes

used to oxygenate blood during bypass surgery Air flows on one side and blood on the other side

and oxygen diffuses into blood.

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Extra: Opthalmic Applications

Eye glasses, contact lenses and Intraocular implants are made of polymers.

Hydrogel is used to make soft contact lenses Absorbs water and allows snug fit Oxygen permeable Made of poly-HEMA

• Hard lenses made from PMMA Not oxygen permeable Mixed with Siloxanylalkyl Metacrylate and metacrylic acid to make permeable and hydrophilic.

• Intraocular implants are made of PMMA

• Poly-HEMA-Poly(hydroxyethyl methacrylic) acid

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Extra: Orthopedic Applications

Bone cement: Fills space between implant and bone – PMMA

Centrifuging and vacuum techniques minimize porosity

• Used in joint prosthesis (Knee and Hip replacements)

• Other applications: Drug delivery systems: Polymer matrix with

drug implanted inside the body Struture materials: High tensile and knot

pull strength. Non-absorbable: Polypropylene, Nylon Absorbable : Polyglycolic acid.

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Extra: Tissue Engineering

Polymers can be synthesized and blend to suite the applications

Biodegradable polymers are used as scaffolding for generation of new tissues

In future, tissues can be generated in vivo or in vitro for repair or replacement.

BIOMATERIALS ENT 311/4 Lecture 3 Polymeric Biomaterials.

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