Download - Notes Biomaterials I
Biomaterials - IBME 379/385, CHE 379, Spring 2003 (Schmidt)
Goals:By the end of this lecture, you should be able to:• Describe the key pros/cons of different materials• Describe different mechanical tests & interpret data• Describe differences between metals, ceramics, polymers• Identify condensation & addition polymerization reactions• Define thermoset & thermoplastic polymers• Calculate average molecular weight of a polymer• Calculate degree of polymerization for a polymer• Discuss the properties that affect polymer degradation• Describe different polymers processing techniques
Outline:I. Introduction & General Classification of MaterialsII. Analysis of Material PropertiesIII. Polymer Basics
A. ClassificationB. Polymerization ReactionsC. Copolymers
IV. Polymer PropertiesA. Desired Polymer PropertiesB. Thermoset & Thermoplastic BehaviorC. Elastomer BehaviorD. Hydrogels
V. Polymer Degradation & Biodegradable PolymersVI. Polymers Processing for Tissue Engineering
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I. Introduction & General Classification of Materials
Define "biomaterial":
List key pros/cons & common biomedical uses of current materials:
• Metals
• Ceramics
• Polymers
• Composites
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Metals:
Titanium
Gold
Ceramics:
Hydroxy apatite
Pyrolytic carbonLTI pyrolytic carbon
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Polymers:
Natural polymers
Synthetic polymers (non-degradable)
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Synthetic polymers (degradable)
II. Analysis of Material Properties
BiomechanicsFailure analyses (tensile fracture, compression, shear stress,fatigue, wear,...)
Structure & GeometryImaging techniques (photography, microCT, histology)(not discussed in class)
Biocompatibility and Cell Response(discussed in later lectures)
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BiomechanicsA common property measured of most materials is tensile strength:
To construct this tensile stress-strain plot, a rod or "dog-bone"shaped material specimen is stretched using a mechanical testmachine (instron). Force (Newtons) is applied to the specimen, anddeformation of the specimen is measured (mm). Stress, σ (N/m2 orPascals), is calculated as force divided by the original cross-sectionalarea. Strain, ε (%), is calculated as the change in length divided bythe original length.
For the plot above:Region A =Region B =Point 1 =Point 2 =Point 3 =Young's modulus (E) or stiffness =
B
A
3
Stress
Strain
1
2
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Example Problem: You are to design a cable that must support an elevator cab thatweighs 10,000 lb. The cable is made from the aluminum alloy, whose data is presented inthe figure below. Calculate the minimum diameter of the cable required to support thecab without permanent deformation.
Stress-Strain for Alluminum Alloy
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
45,000
0.00 0.02 0.04 0.06 0.08 0.10 0.12
Strain (in/in)
Str
ess (
psi)
Expanded View
0
5000
10000
15000
20000
25000
30000
35000
40000
0 0.001 0.002 0.003 0.004
Strain (in/in)
Str
ess (
psi)
Stress-strain curves can also provide information on brittleness vs.ductility….
Which curve above represents the behavior for a brittle material?
A ductile material?
Strain
Stress
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Other biomechanical tests:Compression
Fatigue Wear
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III. Polymer Basics
Polymers can be defined as:
Polymer advantages over metals and ceramics:
1. 2. 3. 4. 5.
Polymer disadvantages compared to metals and ceramics:
1. 2.
Why are polymers typically used in Tissue Engineering applications?
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A. Classification
Polymers can be classified according to:
1. Polymerization mechanismCondensation polymerizationAddition (free radical) polymerization
2. Polymer structure
Linear
Branched
A' (A)n Y (A)n Y
(A)n (A)n
Crosslinked (networks)
3. Polymer behavior
Thermoplastic –
Thermosetting –
A' (A)X-2 A"
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B. Polymerization Reactions
1. Condensation Polymerization
R-NH2 + R'COOH --> R'CONHR + H20 (amine) (carboxylic acid) (amide)
Most natural polymers (polysaccharides, proteins) are made by condensationpolymerization
2. Addition (Free Radical) Polymerization
H
H
H
H
H
H
H
H
H
H
H
H
C C C C C C
n
The breaking of a double bond usually occurs using an initiator (e.g.,free radical such as benzoyl peroxide).
The free radicals (R•) can react with monomers:
RCH2 C•
H
X
This free radical can then react with another monomer in a processcalled propagation:
R• + M --> RM•
RM• + M --> RMM•
The propagation process can be terminated by combining two freeradicals or by transfer.
R• + CH2=CHX ---->
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Common condensation and free radical polymers:
Degree of polymerization (DP):
DP related to molecular weight: M w = (DP) x (M.W. of mer)
Polydispersity:M wM n
= (Wi• MWi)/ Wi∑∑
(Xi• MWi)/ Xi∑∑
EXAMPLE: Calculate the degree of polymerization if polyethylene(C2H4)n has a molecular weight of 100,000 g/gmol.(How will this change for a condensation polymer?)
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Molecular weight affects polymer properties:
C. Copolymers
Definition:
Types of copolymers:
--AABBABAABBBABAABAAABBABA—
--ABABABABABABABABABABABAB—
--AAAAAA--BBBBBB--AAAAAA--BBBBBB—
--AAAAAA--AAAAAA--AAAAAA--B B BB B BB B B
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IV. Polymer Properties
A. Desired Polymer Properties
Some of the properties that should be considered for the selection ofa polymer for a particular biomedical use are:(this is what the medical doctor and engineer would specify)
The macroscopic properties of the biomaterial (above) will depend onthe following fundamental characteristics of the polymer:(this is what the polymer chemist would control)
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B. Thermoset & Thermoplastic Behavior
Examples of plastics (thermoplastic and thermoset) and elastomers:
Thermoplastics, elastomers & hydrogels (not shown above) are mostimportant in BME. See handout on hydrogels.
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Thermoplastic Behavior:
What are some properties and processing conditions that affectcrystallinity?
How does MW of a thermo-plastic polymer affects itsstrength & thermal stability?
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C. Elastomer Behavior
Elastomers are highly elastic over a range of temperatures. Whatprovides this elastic property?
Are these materials amorphous or crystalline?
What happens at temperatures above Tm? Does the polymer liquefy?Why or why not?
D. Hydrogels
Hydrogels are a unique form of polymers for implantation.
Definition:
Hydrogels can be up to 90% water (by weight).Examples: agarose gels, gelatin, collagen gels, ...
Pros and Cons:
Example = contact lenses
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V. Polymer Degradation & Biodegradable Polymers
Definition:
Biodegradable polymers are also referred to as:
Biodegradable polymers are used as scaffolds for tissue engineeringapplications for many reasons:
Degradation occurs via hydrolysis, enzymatic action, .
Possible concerns with degradable polymers (with respect to TissueEngineering):
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Common Biodegradable Synthetic Polymers:
There are several biodegradable polymers that already exist and thatare being developed for tissue engineering applications. Two of themore common biodegradable polymers are PGA and PLA. Thesematerials are commercially available and are already FDA-approvedfor surgical procedures (e.g., biodegradable sutures).
Polyesters:Polyglycolic Acid (PGA)
C
O
C O
H
H
C
O
OC
H
H
O
Polylactic Acid (PLA)
C
O
C O
H
C
O
OC
CH3
H
O
CH3
Which polymer is likely more crystalline? Why?
What properties of the above polymers will affect degradation rates?
One can tailor polymer properties (degradation rate) by makingcopolymers of PGA & PLA --> PLGA or poly(lactic-co-glycolicacid).
Polyesters commonly used as suture material, adhesives, and in TEapplications (breakdown products are natural).
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Degradation of Biodegradable Polymers:
Factors that will affect degradation:
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Degradation (Hydrolysis) of PLGA:
These degradation products, although natural to the body, are acidic -- too fastof a degradation rate can be detrimental to cells (pH ).
PLGA tends to degrade by bulk degradation. More hydrophobic polymers,such as polyanhydride, tend to degrade by surface erosion.
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VI. Poymers Processing for Tissue Engineering
Polymer Foams (solvent casting & particulate leaching)
Fiber extrusion and fiber bonding
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Larger device extrusion (e.g., conduits)
Phase separation
Solid Freeform Fabrication (SFF) and 3-D Printing(see article by Griffith; below is new development by Chen at UT)
Laser
Lens
BeamShutter
XYZController
CADStation
Platform
Liquid Polymer andContainer
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Simpler Methods for 3-D Polymer Processing
Chemical/Biomolecule Modification