polymer slides
DESCRIPTION
biomedical engineeringTRANSCRIPT
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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
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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
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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
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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
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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
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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
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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
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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