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

BME 379/385, CHE 379: Biomaterials I -- 2

I. Introduction & General Classification of Materials

Define "biomaterial":

List key pros/cons & common biomedical uses of current materials:

• Metals

• Ceramics

• Polymers

• Composites


Metals:

Titanium

Gold

Ceramics:

Hydroxy apatite

Pyrolytic carbonLTI pyrolytic carbon


Polymers:

Natural polymers

Synthetic polymers (non-degradable)


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)


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


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


Other biomechanical tests:Compression

Fatigue Wear


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?


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"


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 ---->


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?)


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


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)


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.


Thermoplastic Behavior:

What are some properties and processing conditions that affectcrystallinity?

How does MW of a thermo-plastic polymer affects itsstrength & thermal stability?


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


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):


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).


Degradation of Biodegradable Polymers:

Factors that will affect degradation:


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.


VI. Poymers Processing for Tissue Engineering

Polymer Foams (solvent casting & particulate leaching)

Fiber extrusion and fiber bonding


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


Simpler Methods for 3-D Polymer Processing

Chemical/Biomolecule Modification

Notes Biomaterials I

Documents

Transcript of Notes Biomaterials I