POLYMERS

Polymers - long chain molecules of high molecular weight

-(CH2)n-

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n State Use

1-4 gas burned for energy

5-11 liquid gasoline

9-16 med. visc.liquid

kerosene

16-25 hi visc. liq. oil, grease

25-50 solid paraffin wax

1000-3000

tough plastic PE bottles, containers

Common Polymer Biomaterials

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Polymers In Specific Applications

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application properties and design requirements polymers used

dental •stability and corrosion resistance, plasticity•strength and fatigue resistance, coating activity•good adhesion/integration with tissue•low allergenicity

PMMA-based resins for fillings/prosthesispolyamidespoly(Zn acrylates)

ophthalmic •gel or film forming ability, hydrophilicity•oxygen permeability

polyacrylamide gelsPHEMA and copolymers

orthopedic •strength and resistance to mechanical restraints and fatigue•good integration with bones and muscles

PE, PMMA PL, PG, PLG

cardiovascular •fatigue resistance, lubricity, sterilizability•lack of thrombus, emboli formation•lack of chronic inflammatory response

silicones, Teflon, poly(urethanes), PEO

drug delivery •appropriate drug release profile •compatibility with drug, biodegradability

PLG, EVA, silicones, HEMA, PCPP-SA

sutures •good tensile strength, strength retention•flexibility, knot retention, low tissue drag

silk, catgut, PLG, PTMC-GPP, nylon,PB-TE

The Bulk State : Solid

Polymers can be either amorphous or semi-crystalline, or can exist in a glassy state.

amorphous glassy state hard, brittle no melting point

semi-crystalline glassy state hard, brittle crystal formation when cooled exhibit a melting point

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Glass transition temperature, Tg

related to chain mobility increased flexibility, lower Tg

factors : flexible links in backbone

size of pendent groups

interaction between chains

• plasticizers interfere with bonding, increase chain movement, decrease Tg

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Effect of Temperaturesemi-crystalline

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Liquid ViscousLiquid

Rubber

tough plastic

semi-crystalline plastic

crystalline solid

molecular weight (g/mol)

T

10 1000 100000 1000000

Tm

Tg

Crosslinked Networkscrosslinks

covalent; H-bonding; entanglements

crosslinking increased molecular weight swell in solvents

• organogel• hydrogel

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Thermal Properties

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Polymer Tg (ºC) Tm (ºC)

Nylon 6,6 45 267

UHMWPE -125 140

Silicone -123 -29

poly(urethane) 0-90 125-225

poly(methylmethacrylate) 105 160

poly(D,L-lactide) 50 amorphous

poly(-caprolactone) - 60 57

poly(glycolic acid) 35 210

Mechanical Properties

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Diffusion in PolymersPolymers can also act as solvents for low molecular weight compounds. The diffusion of small molecular weight components in polymers is important in a number of fields :

purification of gases by membrane separation dialysis prevention of moisture loss in food and drugs (packaging) controlled drug delivery (transdermal patches, Ocusert) polymer degradation

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Leaching - Undesirable polymers often contain contaminants as a result of their

synthesis/manufacturing procedure/equipment

may also contain plasticizers, antioxidants and so on

these contaminants are a frequent cause of a polymer’s observed incompatibility

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Drug Delivery

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Ocusert

TD - Scopolamine

In Vivo Degradation of Polymers no polymer is impervious to chemical and physical actions of

the body

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Physical Chemical

sorption/swelling hydrolysis

softening oxidation

dissolution enzymatic

stress cracking

fatigue cracking

Mechanisms causing degradation

Hydrolytic Degradationhydrolysis

the scission of chemical functional groups by reaction with water

there are a variety of hydrolyzable polymeric materials:

esters

amides

anhydrides

carbonates

urethanes

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Hydrolytic Degradationdegradation rate dependent on

hydrophobicity crystallinity Tg

impurities initial molecular weight, polydispersity degree of crosslinking manufacturing procedure geometry site of implantation

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Hydrolytic Degradationbulk erosion (homogeneous)

uniform degradation throughout polymer

process random hydrolytic cleavage (auto-catalytic)

diffusion of oligomers and fragmentation of device

surface erosion (heterogeneous) polymer degrades only at polymer-water interface

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Polyesters

Oxidative Degradationusually involves the abstraction of an H to yield an ion or a radical

direct oxidation by host and/or device• release of superoxide anion and hydrogen peroxide by

neutrophils and macrophages• catalyzed by presence of metal ions from corrosion

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Enzymatic DegradationNatural polymers degrade primarily via enzyme action

collagen by collagenases, lysozyme glycosaminoglycans by hyaluronidase, lysozyme

There is also evidence that degradation of synthetic polymers is due to or enhanced by enzymes.

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-20.0

0.0

20.0

40.0

60.0

80.0

-5 0 5 10 15 20 25

in vitro

in vivo

% w

eigh

t los

s

time (weeks)

C.G. Pitt et al., J. Control. Rel. 1(1984) 3-14

Z Gan et al., Polymer 40 (1999) 2859