BIOM 573 - Exam 1: Intro and Polymers

3 Tenants of Ethics

1. place service before profit;

2. honor of the profession before personal advantage;

3. public welfare above all other considerations

Subjects Integral to Biomaterial Science - What to Consider

toxicology, biocompatibility, inflammation and healing, functional tissue structure and pathobiology, anatomical sites of implantation, mechanical and performance requirements, industry (manufacturing, sterilization, distribution, sales, etc.), ethics, regulation

Chemistry

Ion

atom has + or - charge

Chemistry

Isotope

atoms of a particular element that have different number of neutrons

Interatomic Forces

Primary bonds (weaker to stronger)

metallic, covalent, ionic

Interatomic Forces

Secondary bonds (weaker to stronger)

Van der Waals, Hydrogen bonds

Bond Stiffness

F = delta*S (S=dF/dr=d2U/dr2)

How does Bond Stiffness relate to Modulus of Elasticity?

E=S/L0

Types of Mechanical Loading

1. Normal

   1. compression

   2. tension

      1. bending

2. Shear

   1. torsion

   2. transverse shear

Strain-Rate Dependency

Tissue/Bone Reaction

slow => ductile, fast => brittle

Poisson’s Ratio

lateral strain/longitudinal strain

Modulus of Rigidity

G = E/(2(1+pois); more elastic material the larger the G

Composite Materials

formed from two or more dissimilar materials; unlike metallic alloys, materials remain distinct from each other at a microscopic level

Creep

deformation increases with constant force with respect to time

Stress Relaxation

stress decreases with constant deformation (with time) (i.e. stretching muscles)

Hysteresis

difference in stress values during loading and unloading

Viscoelasticity Model

viscous component (damper => Newton’s Law, liquid) and elastic component (spring => Hooke’s Law, solid)

Toughness

ability of a material to absorb energy/loading; energy to failure

Resilience

ability of a material to absorb energy when it is deformed elastically

Polymers

long, repeating units (monomers) creating chains of molecules; have unique properties depending on types of molecules and types of bonding

Polymers

Hydrocarbons

composed of H and C with covalent bonds; polymers consist of hydrocarbon backbones

Isomers

compounds with different atomic arrangements

Copolymer

containing more than one chemically distinct repeat unit (monomer)

Polymers

Molecular Weight

based on fraction of molecules within various size ranges => MW is a distribution; because not all polymer chains grow to same length (free radical/self-assemble polymerization)

Polymers

Molecular Shape

each molecular chain has the ability to bend, coil, or kink leading to intertwining and entanglement of neighboring chain molecules => mechanical and thermal characteristics are a function of the ability of chain segments to rotate in response to stresses

Polymers

Structure Types for Repeating Units

Isotactic: same side of chain;

Syndiotactic: alternate sides of the chain;

Atactic: random configuration

Polymers

Thermoplastics

can be remolded when heated; lacks crosslinks; mostly linear polymers with some branches and flexible chain

Polymers

Thermosets

cannot be remolded; has covalent crosslinks => resist vibrational and rotational chain motions

Polymers

Crystallinity

degree of structure order; depends on polymer chain configuration and rate of cooling during solidification (fast => amorphous, slow => crystalline)

Polymers

Semicrystalline Polymers

has small crystalline regions (crystallites), with precise alignment forming a multilayered structure

Polymer Defects

Point Defect

in crystalline regions; vacancies, interstitial atoms, and ions

Polymer Defects

Chain Ends

chemically different than normal chain units

Polymer Defects

Chain Segments

can leave the crystal and reenter it at another point creating a loop or act as a tie molecule

Polymers

Diffusion

permeability and absorption characteristics relate to the degree to which foreign substances diffuse into the material; can lead to swelling and chemical reactions => degradation of materials mechanical and physical properties

Polymers

Mechanisms of Plastic Deformation

elongation of amorphous regions (tie chains) → crystalline regions (block chains) elongate → tilting of lamellar chain folds → separation of chain block segments → reorientation of block segments

Polymers

Viscoelasticity of Amorphous Polymers - Temperature-Dependent

behave like glass at low temperatures, rubbery solid at intermediate temperatures, viscous liquid at high temperatures

Polymers

Standard Linear Solid Model

E1 in parallel with E2+damper; good model for creep and stress relaxation of tissues

Polymers

Relaxation modulus

magnitude is dependent on time (strain rate, hysteresis) and temperature; Er(t) = stress/maintained strain

Polymers

Creep Modulus

Ec(t) = constant stress/strain

Polymers

Factors that Influence Mechanical Properties

1. increasing temperature or lowering strain rate => decreases tensile/compressive strength and modulus, increases ductility;

2. increasing bond strength and chain alignment => modulus rises;

3. increasing crystallinity => increases strength, more brittle

Polymers

Biodegredation

cleavage of bonds as a consequence of a biological agent (hydrolysis)

Polymers

Bioerosion

mass loss of a material in a biological setting

Polymers

Resorbable

process of eliminating products from degredation

Polymers

Surface Erosion

erosion restricted to the surface of the material

Polymers

Bulk Erosion

erosion that occurs throughout the sample, causing the whole material to degrade

Polymers

Three Mechanisms of Chemical Degradation

1. cleavage of crosslinks

2. cleavage of side chains

3. cleavage of backbone

Smart Polymers

stimulus response, self healing, shape memory

Smart Polymers

Self-Healing through Exhaustion of Healing Agents

release agent in response to stimulus (mechanical, temperature, chemical); Limitations: # of healing agents, uncontrollability, not efficient or effective if correct # doesn’t break

Smart Polymers

Self-Healing System with Reversible Chemical Bonds

bonds break in response to stimulus and reattach in different order allowing it to be continuous; Limitations: altering structure, energy input, decrease overall properties to make specific bonds weaker