Learn: Biomaterials: Exam 1

Biomaterial

A material intended to interface with biological systems to evaluate, treat, augment, or replace any tissue, organ, or function of the body

Biomaterial science

study of biomaterials and their interactions with the biological environment

Biocompatability

the ability of a material to perform with an appropriate host response in a specific application

metals

inorganic materials possessing non-directional metallic bond (highly mobile electrons)

characteristics of metals

load bearing implants and internal fixation devices; potential high tensile strength, high fatigue and high yield strengths

ceramics

inorganic compounds that contain metallic and non-metallic elements; for which inter-atomic bonding is ionic or covalent, and which are generally formed at high temperatures

glass

an inorganic product of fusion that has cooled to a rigid condition without crystallization; an amorphous solid

glass-ceramic

Polycrystalline solids prepared by the controlled crystallization (devitrification) of glasses.

Bioactive material

A material that elicits a specific biological response at the interface of the material, resulting in the formation of a bond between the tissues and the material.

Polymers

Organic materials; long chains that are held by directional covalent bonds.

elastomers

Sustain substantial deformation at low stresses and return rapidly to their initial dimensions upon release of stress.

Hydrogels

Ability to swell in water and to retain a significant fraction of water within their structures without completely dissolving.

composites

Consists of two or more chemically distinct components, one of which often a polymer.

synthetic polymers

Can be easily mass produced and sterilized; physical, chemical and mechanical properties can be tailored to specific applications.

naturally derived polymers

Have chemical composition similar to the tissue they are replacing; low mechanical properties.

hydrophobic

water-fearing

hydrophilic

water-loving

anisotrophy

Mechanical properties in different directions.

engineering stress

σ = F/A₀

engineering strain

ε = (li − lo)/lo

shear stress

T = F/A₀

shear strain

γ = tan q

hooke's law (tension)

σ = Eε

modulus of elasticity (youngs modulus)

slope of stress/strain curve (stiffness)

Hooke's Law (Shear)

τ = Gγ

shear modulus

G in hookes law

yielding

onset of plastic deformation

Stress-Strain Curve

fractures in metals

Occurs when noticeable necking starts.

fracture in polymers

Occurs when polymer backbone chains are aligned and about to break.

ductility

Calculated as % elongation or % area reduction.

((Lf-Lo)/Lo) x 100

hardness

resistance to permanently indenting the surface

toughness

total area under the stress/strain curve

large hardness means

Resistance to plastic deformation or cracking in compression; better wear properties.

thermal stress

σ = Eαl(T₀ − Tf) = EαlΔT

brittle fracture

no warning

ductille fracture

warning/necking before fracture

Griffith Theory

Crack propagation occurs when the released elastic strain energy is at least equal to the energy required to generate new crack.

fatigue fracture

Occurs suddenly after the material has been subjected to many cycles of altering stress or strain.

fatigue failure steps

Crack initiation

Crack propagation

Final failure

fatigue limit

Endurance limit, where S-N graph becomes horizontal.

fatigue strength

stress level that will cause failure after a given number of cycles.

fatigue life

Number of cycles required to cause fatigue fracture at a specified stress

creep

Deformation of a sample under constant load over time.

creep test

Performed by exerting a constant (usually tensile) load on the specimen, at a fixed temperature.

stage 1; primary creep

Creep (strain) rate decreases as strain increases with time; due to repositioning within the material in response to loading

stage 2; secondary creep

Equilibrium within material substructure; balance between recovery = strain hardening; strain rate minimum

stage 3; tertiary creep

Strain rate increases; defects appear inside the material.

nabarro-herring creep

Atomic diffusion is in the opposite direction resulting in an elongation of the grain along the line of applied stress.

coble creep

Vacancies migrate along the grain boundaries rather than through the bulk of the grain.

stress relaxation

Time dependent mechanical property, is the decrease in stress seen over time under constant strain.

below Tg (glass transition)

Chains in amorphous region are not able to rotate or slide, no time-dependent deformation

above Tg (glass transition)

Polymer chain can move past each other, time-dependent deformation indicative of viscous flow

viscoelastic material

Exhibit both viscous and elastic properties

Maxwell Model

Proposes a spring and dashpot in series

Maxwell Model Strength

Correctly predicts exponential decrease in stress over time during stress relaxation.

Maxwell Model Limitation

Incorrectly predicts Newtonian flow and linear increase in strain during creep

Voigt Model

Uses same component as the Maxwell model, but in different order (parallel)

Voigt Model Strength

Correctly predicts exponential strain increase over time during creep.

Voigt Model Limitation

Does not predict decrease in stress during stress relaxation.

elastic material

Material returns to its original shape when the external force is removed, given that the material does not exceed the elastic limit

viscous material

Material viscosity is the resistance of a fluid to deform

structural properties

Stiffness and maximum force; energy and elongation

Material (Mechanical) Properties

Modulus and maximum stress; strain energy density (SED) and strain

Time-Dependent Properties Cause Tissues To

Creep or elongate under constant or cyclic load; relax or reduce load under constant or cyclic elongation; show hysteresis when cycled in displacement control.

Ligament and Tendon Viscoelastic Properties

Show very little creep under subfailure loads; show ~30% force relaxation from initial peak load under subfailure elongations; show large hysteresis for first cycle that decreases with repeated cycling.

preconditioning

Required to remove this first cycle effect.

cytotoxicity

Lysis of cells, inhibition of cell growth

sensitization

Model to estimate potential for contact sensitization.

irritation

Choose test with relevant body entry route, duration of exposure, contact time

Intracutaneous Activity

Localized tissue reaction

systemic toxicity

Potential harmful effects with

genotoxicity

Determine gene mutations, changes in chromosome structure and number, DNA or gene toxicities caused by medical devices, materials or extracts.

implantation

Local pathological effects on living tissue of a sample of material implanted to a relevant tissue.

Haemocompatibility

Effects of blood-contacting devices on blood or blood components.

chronic toxicity

Effect on body of exposures from devices and materials for >10% of the life-span of the test animal.

Carcinogenicity

Tumorigenic potential.

Biodegradation

Absorption, distribution, biotransformation, elimination of leachables and degradation products

Reproductive and Developmental Toxicity

Reproductive function, embryonic development, prenatal and early postnatal development

surface roughness

Affects the properties of cells such as cell adhesion, cell selection, and cellular integration

rough surfaces

Fixation of the device by providing friction and mechanical interlocking.

smooth surfaces

blood flow

AFM (Atomic Force Microscopy)

Measurement of roughness parameter

wettability (surface energy)

whether the surface is hydrophilic or hydrophobic

hydrophilic surface

Encourages formation of a film over the material to discourage the adhesion of gas bubbles and bacteria

contact angle measurement

A drop of water is placed on the surface

Hydrophobic Surface

Causes the water to bead up on the surface producing a higher angle of contact

chemical composition evaluation

Surface contamination; coverage of surface treatment; analysis of chemical function

surface charge

Influence the percent cell adhesion and retention under flow conditions

crystallinity

Determine specific responses to thermal and mechanical properties; stimulate certain cell activities

porosity

Indicate the capacity of the material to support tissue ingrowth

high porosity

Desirable to aid in implant fixation; however this can compromise the mechanical strength of the device

surface modification

Can be used to alter how the material is viewed in vivo without affecting mechanical properties

Plasma Glow Discharge

A low pressure, ionized gas deposits a film over the surface of the material or etches the surface

radiation grafting

Break chemical bonds, which form free radicals or other reactive groups.