polymeric implant materials seminar 2003

8/22/2019 Polymeric Implant Materials Seminar 2003

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Polymeric Biomaterials2010

POLYMERIC BIOMATERIALSPOLYMERIC BIOMATERIALS

Polymers (from the Greek: polys, many; meros. part or unit) are large moleculesmade up by the repetition of small, simple chemical units termed monomers. Insome cases the repetition appears much as a chain is built up from its links. Inother cases the chains are branched are interconnected to form three-dimensionalnetworks.

Polymers have found applications in every specialty area and continue to be themost widely used materials in health care. Polymers can be classified in severaldifferent ways according to their structures, the type of reactions by which they are

prepared, their physical properties, or their technological use. The earliest and mostfrequent application of textile material for surgery is believed to be suturematerials, used to close wounds. As early as 4 thousand years ago linen was usedas a suture material. Later, natural fiber from the bark of trees, plaited horsehair,cottons and silk were also used. Due to the development of synthetic fibers likenylon, polyesters and polyolefins in the 1950s, synthetic fibers have graduallyreplaced natural fibers for wound closure purposes.

Basic Structure

Polymers have very long chain molecules which are formed by covalent bondingalong the backbone chain. The long chains are held together either by secondary

bonding forces such as van der Waals and hydrogen bonds or primary covalentbonding forces through crosslinks between chains. The long chains are veryflexible and can be tangled easily. In addition, each chain can have side groups,

branches and copolymeric chains or blocks which can also interfere with the long-range ordering of chains. For example, paraffin wax has the same chemicalformula as polyethylene (PE) [(CH2CH2)n], but will crystallize almost completely

because of its much shorter chain lengths. However, when the chains becomeextremely long {from 40 to 50 repeating units [CH2CH2] to several thousands asin linear PE} they cannot be crystallized completely (up to 80 to 90%crystallization is possible). Also, branched PE in which side chains are attached tothe main backbone chain at positions normally occupied by a hydrogen atom, willnot crystallize easily due to the steric hindrance of side chains resulting in a morenoncrystalline structure. The partially crystallized structure is called

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semicrystalline which is the most commonly occurring structure for linearpolymers. The semicrystalline structure is represented by disordered noncrystalline(amorphous) regions and ordered crystalline regions which may contain foldedchains as shown in figure below.

The polymer chains can be arranged in three ways; linear, branched, and acrosslinked (or three-dimensional) network as shown in figure below. Linear

polymers such as polyvinyls, polyamides, and polyesters are much easier tocrystallize than the cross-linked or branched polymers. However, they cannot becrystallized 100% as with metals. Instead they become semicrystalline polymers.

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Polymerization

In order to link the small molecules one has to force them to lose their electronsby the chemical processes of condensation and addition. By controlling thereaction temperature, pressure, and time in the presence of catalyst(s), the degree towhich repeating units are put together into chains can be manipulated.

The degree of polymerization (DP) is defined as an average number of mers, orrepeating units, per molecule, i.e., chain. Each chain may have a different numberof monomers depending on the condition of polymerization. Also, the length ofeach chain may be different. Therefore, it is assumed there is an average degree of

polymerization or average molecular weight (MW)

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The following factors influence the mechanical properties of polymers.

1. Composition.2. Molecular weight,3. Amount of unreacted monomer in the polymer,4. Morphology (Structure),

5. Crystallinity.6. Configurational structure7. Additives.

Polymers Used as Biomaterials

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1. polyolefin

A polyolefin is a polymer produced from a simple olefin (also called analkene with the general formula CnH2n) as a monomer. For example, polyethylene

is the polyolefin produced by polymerizing the olefin ethylene. An equivalent termis polyalkene; this is a more modern term, although polyolefin is still used in the

petrochemical industry.

Properties

Polyolefins are impossible to join by solvent cementing because they haveexcellent chemical resistance and can only be adhesively bonded after surfacetreatment because they have very low surface energies. They are also extremelyinert chemically and exhibit decreased strength at lower temperatures.

A more specific type of olefin is a poly-alpha-olefin (or poly--olefin,sometimes abbreviated as PAO), a polymer made by polymerizing an alpha-olefin.Many poly-alpha-olefins have flexible alkyl branching groups on every othercarbon of their polymer backbone chain. These alkyl groups, which can shapethemselves in numerous conformations, make it very difficult for the polymermolecules to line themselves up side-by-side in an orderly way. Therefore, many

poly-alpha-olefins do not crystallize or solidify easily and are able to remain oily,viscous liquids even at lower temperatures.

2. polyamide

A polyamide is apolymercontaining monomers ofamides joined bypeptidebonds. They can occur both naturally, examples being proteins, such as wool andsilk, and can be made artificially through step-growth polymerization, examples

being nylons, aramids, and sodium poly(aspartate).Ex:

Production from monomers

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Polyamides are polymers where the repeating units are held together byamide links. The amide link is produced from the condensation reaction of anamino group and a carboxylic acid or acid chloride group. A small molecule,usually water, or hydrogen chloride, is eliminated. The amino group and the

carboxylic acid group can be on the same monomer, or the polymer can beconstituted of two different bifunctional monomers, one with two amino groups,the other with two carboxylic acid or acid chloride groups.

Amino acids can be taken as examples of single monomer (if the differencebetween R groups is ignored) reacting with identical molecules to form apolyamide:

The reaction of two amino acids. Many of these reactions produce longchain proteins Aramid (pictured below) is made from two different monomers

which continuously alternate to form the polymer and is an aromatic polyamide:

The reaction of 1,4-phenyl-diamine (para-phenylenediamine) andterephthaloyl chloride to produce Aramid.

3. Fluorocarbons

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Fluorocarbons, sometimes referred to as perfluorocarbons, are organofluorinecompounds that contain only carbon and fluorine bonded together in strongcarbonfluorine bonds. The electron withdrawing nature, or electronegativity, offluorine results in many of the unique characteristics of fluorocarbons. Forexample, the electronegativity of fluorine makes single bonds to carbonremarkably strong. Resultingly, fluoroalkanes are more chemically and thermallystable than alkanes. However, fluorocarbons with double bonds (fluoroalkenes)

and especially triple bonds (fluoroalkynes) are more reactive than correspondinghydrocarbons. Also, the electronegativity of fluorine also reduces the cohesiveintermolecular forces of fluorocarbons by mitigating the effect of the Londondispersion force. Fluoroalkanes can serve as oil-repellant/water-repellantfluoropolymers, solvents, liquid breathing research agents, and powerfulgreenhouse gases. Unsaturated fluorocarbons tend to be used as reactants, asfluorocarbons with double and triple bonds are not as stable

4. Silicone rubber

Silicone rubber is apolymerthat has a "backbone" ofsilicon-oxygen linkages,the same bond that is found in quartz, glass and sand. Normally, heat is required tovulcanise (set) the silicone rubber; this is normally carried out in a two stage

process at the point of manufacture into the desired shape, and then in a prolongedpost-cure process. It can also be injection molded.

Silicone rubber offers good resistance to extreme temperatures, being able tooperate normally from -55C to +300C. At the extreme temperatures, the tensilestrength, elongation, tear strength and compression set can be far superior toconventional rubbers although still low relative to other materials. Organic rubberhas a carbon to carbon backbone which can leave them susceptible to ozone, UV,heat and other ageing factors that silicone rubber can withstand well. This makes itone of the elastomers of choice in many extreme environments.

Compared to other organic rubbers, however, silicone rubber has a very lowtensile strength. For this reason, care is needed in designing products to withstandeven low imposed loads. Silicone rubber is a highly inert material and does not

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react with most chemicals. Due to its inertness, it is used in many medicalapplications and in medical implants. However, typical medical products like

breast implants and catheters have failed because of poor design.

Structure

Silicone rubber chain

Polysiloxanes differ from other polymers in that their backbones consist of Si-O-Si units unlike many other polymers that contain carbon backbones. Oneinteresting characteristic is an extremely low glass transition temperature of about-127C (Fitzpatrick 1999:428). Polysiloxane is very flexible due to large bondangles and bond lengths when compared to those found in more basic polymerssuch aspolyethylene. For example, a C-C backbone unit has a bond length of 1.54 and abond angle of 112, where as the siloxane backbone unit Si-O has a bondlength of 1.63 and a bond angle of 130.

5. Polyvinylchloride (PVC)

PVC is an amorphous, rigid polymer due to the large side group (Cl, chloride)with a Tg of 75~105C. It has a high melt viscosity hence it is difficult to process.To prevent the thermal degradation of the polymer (HCl could be released),thermal stabilizers such as metallic soaps or salts are incorporated. Lubricants areformulated on PVC compounds to prevent adhesion to metal surfaces and facilitatethe melt flow during processing. Plasticizers are used in the range of 10 to 100

parts per 100 parts of PVC resin to make it flexible. Di-2-ethylhexylphthalate(DEHP or DOP) is used in medical PVC formulation. However, the plasticizers oftrioctyltrimellitate (TOTM), polyester, azelate, and phosphate ester are also used to

prevent extraction by blood, aqueous solution, and hot water during autoclavingsterilization. PVC sheets and films are used in blood and solution storage bags and

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surgical packaging. PVC tubing is commonly used in intravenous (IV)administration, dialysis devices, catheters, and cannulae.

6. Polyethylene (PE)

PE is available commercially in 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). HDPE is polymerizedin a low temperature (60~80C), and at a low pressure (~10 kg/cm 2) using metalcatalysts. A highly crystalline, linear polymer with a density ranging from 0.94 to0.965 g/cm3 is obtained. LDPE is derived from a high temperature (150~300C)and pressures (1,000~3,000 kg/cm2) using free radical initiators. A highly

branched polymer with lower crystallinity and densities ranging from 0.915 to0.935 g/cm3 is obtained. LLDPE (density: 0.91~0.94 g/cm3) and VLDPE (density:0.88~0.89 g/cm3), which are linear polymers, are polymerized under low pressuresand temperatures using metal catalysts with comonomers such as 1-butene, 1-hexene, or 1-octene to obtain the desired physical properties and density ranges.

UHMWPE (MW > 2 106 g/mol) has been used for orthopedic implantfabrications, especially for load-bearing applications such as an acetabular cup oftotal hip and the tibial plateau and patellar surfaces of knee joints.Biocompatability tests for PE are given by ASTM standards in F981, F639, and

F755.

7. Polypropylene (PP)

PP can be polymerized by a Ziegler-Natta stereospecific catalyst which controlsthe isotactic position of the methyl group. Thermal (Tg:-12C, Tm:125~167C anddensity: 0.85~0.98 g/cm3) and physical properties of PP are similar to PE. The

average molecular weight of commercial PP ranges from 2.2~7.0 105 g/mol andhas a wide molecular weight distribution (polydispersity) which is from 2.6 to 12.Additives for PP such as antioxidants, light stabilizer, nucleating agents, lubricants,mold release agents, antiblock, and slip agents are formulated to improve the

physical properties and processability. PP has an exceptionally high flex life andexcellent environment stress-cracking resistance, hence it had been tried for finger

joint prostheses with an integrally molded hinge design [Park, 1984]. The gas and

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water vapor permeability of PP are in-between those of LDPE and HDPE. PP isused to make disposable hypothermic syringes, blood oxygenator membrane,

packaging for devices, solutions, and drugs, suture, artificial vascular grafts,nonwoven fabrics, etc.

8. Polymethylmetacrylate (PMMA)

Commercial PMMA is an amorphous (Tg:105C and density:1.15~1.195g/cm3) material with good resistance to dilute alkalis and other inorganic solutions.PMMA is best known for its exceptional light transparency (92% transmission),high refractive index (1.49), good weathering properties, and as one of the most

biocompatible polymers. PMMA can be easily machined with conventional tools,

molded, surface coated, and plasma etched with glow or corona discharge. PMMAis used broadly in medical applications such as a blood pump and reservoir, an IVsystem, membranes for blood dialyzer, and in in vitro diagnostics. It is also foundin contact lenses due to excellent optical properties, dentures, and maxillofacial

prostheses due to good physical and coloring properties, and bone cement for jointprostheses fixation (ASTM standard F451).

Another acrylic polymer such as polymethylacrylate (PMA), polyhydroxyethyl-methacrylate (PHEMA), and polyacrylamide (PAAm) are also used in medicalapplications. PHEMA and PAAm are hydrogels, lightly cross-linked by

ethyleneglycoldimethylacrylate (EGDM) to increase their mechanical strength.The extended wear soft contact lenses are synthesized from PMMA and N-vinylpyrollidone or PHEMA which have high water content (above 70%) and ahigh oxygen permeability.

9. Polystyrene (PS) and its Co-Polymers

PS is polymerized by free radical polymerization and is usually atactic. Three

grades are available; unmodified general purpose PS (GPPS, Tg:100C), high

impact PS (HIPS), and PS foam. GPPS has good transparency, lack of color, easeof fabrication, thermal stability, low specific gravity (1.04~1.12 g/cm3), andrelatively high modulus. HIPS contains a rubbery modifier which forms chemical

bonding with the growing PS chains. Hence the ductility and impact strength areincreased and the resistance to environmental stress-cracking is also improved. PSis mainly processed by injection molding at 180~250C. To improve processabilityadditives such as stabilizers, lubricants, and mold releasing agents are formulated.

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GPPS is commonly used in tissue culture flasks, roller bottles, vacuum canisters,and filterware. Acrylonitrile-butadiene-styrene (ABS) copolymers are produced bythree monomers; acrylonitrile, butadiene, and styrene. The desired physical andchemical properties of ABS polymers with a wide range of functional

characteristics can be controlled by changing the ratio of these monomers. Theyare resistant to the common inorganic solutions, have good surface properties, anddimensional stability. ABS is used for IV sets, clamps, blood dialyzers, diagnostictest kits, and so on.

10. Polyesters

Polyesters such as polyethyleneterephthalate (PET) are frequently found in

medical applications due to their unique chemical and physical properties. PET isso far the most important of this group of polymers in terms of biomedicalapplications such as artificial vascular graft, sutures, and meshes. It is highlycrystalline with a high melting temperature (Tm: 265C), hydrophobic andresistant to hydrolysis in dilute acids. In addition, PET can be converted byconventional techniques into molded articles such as luer filters, check valves, andcatheter housings. Polycaprolactone is crystalline and has a low meltingtemperature (Tm: 64C). Its use as a soft matrix or coating for conventional

polyester fibers was proposed by recent investigation.

11. Polyurethanes

Polyurethanes are usually thermosetting polymers: they are widely used to coatimplants. Polyurethane rubbers are produced by reacting a prepared prepolymerchain with an aromatic di-isocyanate to make very long chains possessing activeisocyanate groups for cross-linking. The polyurethane rubber is quite strong andhas good resistance to oil and chemicals.

12. Polyacetal, Polysulfone, and Polycarbonate

These polymers have excellent mechanical, thermal, and chemical propertiesdue to their stiffened main backbone chains. Polyacetals and polysulfones are

being tested as implant materials, while polycarbonates have found theirapplications in the heart/lung assist devices, food packaging, etc. Polyacetals are

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produced by reacting formaldehyde. These are also sometimes calledpolyoxymethylene (POM) and known widely as Delrin (DuPont). Thesepolymers have a reasonably high molecular weight (>2 104 g/mol) and haveexcellent mechanical properties. More importantly, they display an excellent

resistance to most chemicals and to water over wide temperature ranges.Polysulfones were developed by Union Carbide in the 1960s. These polymers havea high thermal stability due to the bulky side groups (therefore, they areamorphous) and rigid main backbone chains.

They are also highly stable to most chemicals but are not so stable in thepresence of polar organic solvents such as ketones and chlorinated hydrocarbons.Polycarbonates are tough, amorphous, and transparent polymers made by reacting

bisphenol A and diphenyl carbonate. It is noted for its excellent mechanical and

thermal properties (high Tg:150C), hydrophobicity, and antioxidative properties.

Viscoelastic Behavior of Polymers

Viscoelasticity is the property of materials that exhibit both viscous and elastic

characteristics when undergoing deformation. Elastic materials straininstantaneously when stretched and just as quickly return to their original stateonce the stress is removed. Viscoelastic materials have elements of both of these

properties and, as such, exhibit time dependent strain. Whereas elasticity is usually

the result of bond stretching along crystallographic planes in an ordered solid,viscoelasticity is the result of the diffusion of atoms or molecules inside of anamorphous material.

All materials exhibit some viscoelastic response. Synthetic polymers, wood, andhuman tissue as well as metals at high temperature display significant viscoelasticeffects. In some applications, even a small viscoelastic response can be significant.To be complete, an analysis or design involving such materials must incorporatetheir viscoelastic behavior. The behavior of complex fluids, such as polymer

blends and multiblock copolymers (e.g., Estane) is intermediate between that ofsolids and fluids. For short times, the response is elastic and the stress isproportional to the applied strain. The effect of viscoelasticity on polymermorphology becomes even more important as the dynamical asymmetry betweenthe polymer components increases. The behavior of the slow and fastcomponents leads to deformation and phase separation properties that are quiteunlike those observed for symmetric systems.

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The two-fluid model, in which separate velocities are needed for the

monomeric components to describe viscous drag effects and the viscoelastic natureof the polymeric chains, has been reasonably successful in understanding blends

and homopolymers. Many of the mesoscale properties of the system relax ontimescales shorter than thermodynamic timescales and are effectively in localequilibrium on hydrodynamic timescales, which allows us to employ SCFTtechniques to calculate their properties.

The following demonstration illustrates the logarithmic nature of creep

Creep and recovery. Stress and strain vs. time t

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Fig.2 Regions of creep behavior. Strain vs time t, for different load levels.

Creep curves may exhibit three regions, primary creep in which the curve isconcave down, secondary creep in which deformation is proportional to time, andtertiary creep in which deformation accelerates until creep rupture occurs. Tertiarycreep is always a manifestation of nonlinear viscoelasticity, and secondary creep isusually nonlinear as well. Although secondary creep is represented by a straightline in a plot of strain vs. time, that straight line has nothing whatever to do withlinear viscoelasticity. Linear response involves a linear relationship between cause

and effect: stress and strain at a given time in the case of creep.

Stress relaxation is the gradual decrease of stress when the material is held atconstant strain. If we suppose the strain history to be a step function beginning attime zero: (t) = 0H(t),

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The stress (t) in a viscoelastic material will decrease as shown in Figure.3.

The ratio is E(t) = ( (t) / 0)

is called the relaxation modulus. In linear materials, it is independent of strainlevel, so E(t) is a function of time alone.

Stress relaxation describes how polymers relieve stress under constant strain.Because they are viscoelastic, polymers behave in a nonlinear, non-Hookeanfashion.[1] This nonlinearity is described by both stress relaxation and a

phenomenon known as creep, which describes how polymers strain under constantstress.

Polymeric Biomaterials Applications

Wound management (synthetic suture material) Clips Adhesives Surgical meshes Orthopedic devices

Ophthalmic (intraocular lens) Pins (spilli) Rods (barre) Screws (viti) Tacks (chiodini) Ligaments Tissue engineering Dental applications

Guided tissue regeneration Membrane Void filler following tooth extraction Cardiovascular applications (coronary arteries and the vessels) Stents Intestinal applications

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Anastomosis rings Drug delivery system Covering of permanent Implants

Advantages

Good biocompatibility. Possibility of changing in composition and in physical-mechanical

properties. Low coefficients of friction. Easy processing and workability. Ability to change surface chemically and physically. Ability to immobilize cells or biomolecules within them or on the surface

(Drug Eluting Stent). Good biocompatibility. Possibility of changing in composition and in physical-mechanical

properties. Low coefficients of friction Easy processing and workability Ability to change surface chemically and physically Ability to immobilize cells or biomolecules within them or on the surface

(Drug Eluting Stent)

Disadvantages

Presence of substances that may be issued in the body [ monomers (toxic),catalysts, additives ] after degradation.

Ease of water and biomolecules absorption from surrounding. Low mechanical properties. In some cases, difficult sterilization.

Polymers advantage over Metals

Considerations in the selection strength.

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Overall time and rate of degradation/corrosion (a very high degradation ratecan be associated with inflammations).

Biocompatibility.

Lack of toxicity.

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polymeric implant materials seminar 2003

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