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Modern Materials I - Polymers and Biomaterials

• Penn State has a rich traditionin materials research.

• First school to offer a materialscurriculum in 1960.

• Developed the firstinterdisciplinary materialslaboratory in 1962.

• Penn State is ranked #1 in theUS in materials research

Materials You Know

Material: Zeolites for watersoftening.

Na+ ions in aluminosilicatecages are easily exchangedfor Ca2+

Sodium Zeolite A

Material: Kevlar for bulletproof vests.

Hydrogen bonding betweenpolymer chains produces astrong material.

Ceramics (inorganic, nonmetalic solids)

Polymers

Kevlar

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Materials You Know

Material: Light EmittingDiodes in traffic lights.

A diode is a semiconducor devicethat only allows current to flow inone direction only.

LEDs are diodes

The chemical makeup of the LEDdetermines the wavelength (color) ofthe photon.

Semiconductors

Materials You KnowMr. McGuire: Are you listening?

Ben: Yes I am.

Mr. McGuire: “Plastics.”

Ben: Exactly how do you mean?

Mr. McGuire: There's a great future in plastics. Thinkabout it. Will you think about it?

Ben: Yes I will.

The Graduate 1967

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Monomer = repeating unitPolymer = macromolecule made by joining monomers

Polymer SynthesisCondensation polymerspolymers form by eliminating a small molecule (H2O,

HCl, CO2) from monomersExamples: Biological Polymers

ProteinsDNAcarbohydrates

Addition polymersMonomer contains a double bondPolymers form by addition reaction

POLYMERS

Condensation Reactions2 molecules join by eliminating a small molecule

(such as water)

Esters and Amides are formed by condensation reactions:

carboxylic + alcohol → ester + wateracid

+ HOR’ → + H2O

carboxylic + amine → amide + wateracid

+ → + H2O

C

O

R OH

C

O

R OH

C

O

R OR'

C

O

R NHR'

N H

R

H

4

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Structure of Proteins

Secondary structure ofproteins is maintainedby H-bonding

Tertiary structure (proteinfolding) is even morecomplex. Thisstructure is importantin protein activity.

DNA

monomer

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ADDITION POLYMERSADDITION POLYMERS

ethylene Poly(ethylene) (lunch bags)

Teflon

Poly(styrene) (styrofoam)styrene

TFE

Cross-LinkingBonds formed between polymerchains make the polymer stiffer.

• Natural rubber is too soft and chemicallyreactive to make a useful material.

• By vulcanizing the rubber (crosslinking thechains with sulfur) useful materials are made.

isoprene Natural rubber (gummy)

Cross-linked rubber (tough elastomer)

S8

heat

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Poly(vinylalcohol): An Addition Polymer

C=CH

H

H

OHvinyl alcohol

catalystheat

CHCH

OH

2( )xCH

OH

CH2

poly(vinyl alcohol)

linear chain polymerx = 10 - 103 4

cross linker: borax (B O )742-

Resulting material is viscoelastic

Properties of Polymers•Polymer chains tend to be flexible and easily entangled orfolded. Crystalline alignment of chains makes a polymerstiffer.

•Stretching or extruding a polymer can increasecrystallinity.

•Degree of crystallinity determined by chemical structureand polymer chain length:

low density polyethylene (LDPE) has an average molecular weight of 104 amu (used in plastic wrap)

high density polyethylene (HDPE) has an averagemolecular weight of 106 amu (used in milk cartons).

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BiomaterialsBiomaterialsCharacteristics of Biomaterials

Biomaterials are any materials that havebiomedical applications.For example, the materials that are used to fillteeth are biomaterials.

The biomaterials must be biocompatible:The body’s immune system must not attack thebiomaterial.

BiomaterialsBiomaterials

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BiomaterialsBiomaterialsCharacteristics of Biomaterials

Physical requirements:Biomaterials must be created for a specificenvironment.Artificial heart valves must open and close 70to 80 times per minute.

Chemical requirements:Biomaterials must be of medical grade.Polymers are very important biomaterials: bewareof fillers, stabilizers, etc.

BiomaterialsBiomaterials

Polymeric Biomaterials

The degree to which the body toleratesforeign materials depends on the natureof the atomic groups in the material.

Naturally occurring biomaterials arepolymers of sugars (polysaccharides),nucleotides (RNA, DNA) and amino acids(proteins, enzymes, etc.).

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Types of Polymers

Plastic: materials that can be formed into shapes.

Thermoplastic: materials that can be shaped more thanonce. (Used as replacements for blood vessels.)

Thermosetting: materials that can only be shaped once(Used in dental devices, and orthopedics such as hipreplacements.)

Elastomer: material that is elastic. If moderately deformed,the elastomer will return to its original shape.Used as catheters, and for covering leads on implantedelectronics, like pacemakers.

BiomaterialsBiomaterials

BiomaterialsBiomaterialsExamples of Biomaterial Applications

Heart Replacement and Repairs:

A heart that fails completely must be replaced by a donororgan.

About 60,000 people in the US suffer heart failure and only2,500 donor hearts are available.

About 250,000 heart valve replacements are made each year.

About 45 % of these valve replacements occur with amechanical valve.

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BiomaterialsBiomaterialsExamples of Biomaterial Applications

The replacement valve must be smooth to preventthe destruction of blood vessels.

The valve must also be anchored to the inside ofthe heart.

Polyethylene terephthalate, called Dacron™, isused in the artificial heart valves.

Dacron™ is used because tissue will growthrough a polymer mesh.

BiomaterialsBiomaterialsExamples of Biomaterial Applications

Vascular grafts:

A vascular graft is the replacement for a piece ofblood vessel.

Dacron™ is used for large arteries.

Polytetrafluoroethylene,-[(CF2CF2)n-]-, is used for smaller vascular grafts.

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BiomaterialsBiomaterialsExamples of Biomaterial Applications

Artificial Tissue:Artificial skin, which is grown in the laboratory, isused to treat patients with extensive skin loss.

The challenge with growing artificial skin isgetting the cells to align properly.

Therefore a scaffold must be used for the cells.

The most successful scaffold is lactic acid-glycolic acid copolymer.

BiomaterialsBiomaterialsExamples of Biomaterial Applications

Hip Replacements:

About 200,000 hip replacements are performed each year.

A metal ball, a cobalt chromium alloy, is often used in ahip replacement.

This alloy is attached to a titanium alloy and cementedusing a tough thermoset polymer.

The acetabulum, which accommodates the femur, is linedwith a polyethylene layer.