Metals Flashcards
Why are metallic biomaterials used?
primarily for load bearing applications due to their high strength.
What is an alloy?
alloys are metals containing 2 or more elements. They usually have a greater corrosion resistance/ strength than exclusively the element.
What are the applications of metals for biomaterial applications?
Bone/Joint Replacement
Dental Implants
Cardiovascular devices (stents/ artificial valves)
Aneurysm Clips
External Prostheses
Surgical Equipment
Defibrillators and Pacemakers.
What are the generic advantages of metals ?
Well developed tech due to long existence.
Can be moulded into complex shapes.
Can be modified.
High load bearing ability.
Wide ranging fabrication techniques (e.g. casting or forging )
Fracture Resistance
Electrical Conductivity
What are the generic disadvantages of metals?
Major disadvantage is corrosion
Stress shielding phenomenons are also common when using metals (reduced stress on a bone due to an implant which decreases the density of bone)
What are the most common metals used for biomaterial purposes and give an example of what they may be used for ?
Stainless Steel – implants.
Cobalt Chromium Molybdenum – implants.
Titanium – dental implants / surgical tools.
What are stress , strain and what is hookes law?
stress - force/area
strain - extension/ original length
hookes law says that stress and strain are directly proportional.
How is a tensile test done and why is it done ?
Done to produce a stress strain curve to gather information about the material. A circular rod is held at one end of the tensile testing machine and load is applied at the other end.
What is the proportional limit on a stress/strain graph ?
Portion which obeys hookes law. Straight line which helps calculate the Youngs Modulus.
What is the elastic limit on a stress/strain graph ?
The final point where the material will return back to its original length when the load is removed.
What is the yield point on a stress/strain graph ?
the point where the material begins to deform plastically which causes permanent, irreversible damage. upper and lower yield points.
What is the ultimate stress point on a stress/strain graph ?
maximum stress a material can handle before failure. noted as the peak of the graph.
What is the fracture /breaking point on a stress/strain graph ?
the point where material breaks. The final point on the graph.
What are the physical properties of metals ?
Shiny
Hard
High density
High tensile strength
High melting and boiling points
Good conductors of electricity and heat.
Ductile
Malleable.
What are the chemical properties of metals ?
Easily lose electrons (surface charge).
Surface Reactive (surface change)
loss of mass ( via corrosion which produces oxides.)
What is creep ?
Time dependency of metal properties decreasing the mechanical strength.
Constant Load at high temps will undergo an increase in length which is time dependent.
Why are the surface properties of the metal important ?
as almost all interactions between cells/ tissues and metals occur at the implant surface.
What can happen to metals in human body condition?
immersion can lead to corrosion and rough surfaces can harm humans.
The ions released from corrosion can be cytotoxic and may reduce the biocompatibility of the metal.
What are the basic criteria for implanted materials ?
biocompatibility with living tissue and bio functional force transfer. Should have specific bulk and surface properties as well as chemical, physical or biological properties.
What is the Youngs Modulus of bone value and why is it important for the implant metal to be similar ?
Youngs Modulus of bone is 18GPa.
Implant should be similar to more uniformly distribute stress and therefore both the bone and the implant will deform at similar rates.
What is hardness and toughness ?
hardness is a materials resistance to permanent surface indentation and increased hardness = less wear for an implant.
toughness is the amount of energy required to cause a fracture and a high toughness is desirable for implantables.
What is biocompatibility and what determines if a material is biocompatible ?
biocompatibility is the ability of the material to perform with an appropriate biological response in a specific application. Dependent on corrosion resistance and cytotoxicity of biproducts.
What are the types of corrosion ?
stress corrosion
fretting corrosion
crevice corrosion
pitting corrosion
galvanic corrosion
What is corrosion resistance ?
chemical or electrochemical degradation of metals due to their reaction with their environment. destroys objects left in open and moist air.
What is stress corrosion ?
failure by cracking caused by increased stress.
stress corrosion cracking is cracking caused by environment and stress applied.
What is fretting corrosion ?
fretting corrosion gradual wear caused by micro motion/ rubbing or gnawing between 2 materials surface.
combines fretting with environmental conditions
What is crevice corrosion ?
crevice corrosion is a localised attack on a metal surface at a gap joining 2 surfaces. Crevice can be either between 2 metals or a bone an metal.
What is pitting corrosion ?
cavities or holes are produced in the material. more dangerous than uniform as it harder to predict, detect and design against.
What is galvanic corrosion ?
caused when 2 dissimilar metals are coupled together in a corrosive electrolyte.
What does the cytotoxicity of the corrosion products depend on ?
amount of material dissolved by corrosion per unit time.
amount of corroded material removed by metabolic processes per unit time.
amount of corrosion particles deposited in the tissue.
What is osseointegration ?
direct contact or interaction with bone. largely dependent on the surface composition of biomaterial and the biocompatibility.
What are the advantages and disadvantages of hydroxyapatite on metals ?
helps with rapid osseointegration.
can cause bio integration within 4 weeks.
can be unstable as it is inorganic and susceptible to bacteria.
used in dental applications and bone fillings.
How is titanium manufactured ?
gold standard of bio metals.
high melting point makes casting difficult.
often done through controlled machining such as lathing, threading or milling.
very brittle as it readily absorbs nitrogen, hydrogen or oxygen from the air.
What are the advantageous properties of titanium ?
biocompatibility – excellent corrosion resistance.
oxide layer makes it bio inert.
low level of charge transfer compared to other metals.
high yield strength and tensile strength.
low density and ductile.
allows bone growth adjacent to oxide surface.
What are the disadvantageous properties of titanium ?
can cause titanium metallosis and can accumulate in lungs, liver and spleen and trace levels
What is used in titanium alloys and what are the properties of titanium alloys ?
Aluminium and Vanadium can be applied to make it an alloy.
excellent corrosion resistance.
resistant to charge transfer.
stronger than titanium.
exhibits osseointegration properties.
What are the properties of cobalt, chromium and molybdenum alloys ?
high mechanical strength
low ductility
high corrosion resistance.
easily castable and low cost.
What are the properties of Iron, Chromium and nickel based alloys ?
high mechanical strength
high ductility
surgical steel alloys.
can be prone to pitting and crevice corrosion as well as hypersensitivity when in presence of nickel.
used in dental and surgical equipment applications.
What are the properties of precious metals such as gold or platinum ?
unaffected by air, moisture or heat.
high cost per unit weight
low mechanical strength
high ductility.
