Week 12 - Biomaterials as Engineering Materials Flashcards
What are the four key implant material characteristics and their associated tissue response?
Toxic - surrounding tissue dies
Bioinert - fibrous tissue of variable thickness forms
Bioactive - interfacial bond forms
Bioresorbable - surrounding tissue replaces metal
Where does the failure of biomaterials almost always occur?
Biomaterial-tissue interface
Name 5 highly bioinert materials
Alumina Zirconia Silicon nitride Gold Platinum
Name 1 somewhat bioinert material
Passivated titanium
Name 3 weakly bioinert materials.
Vitallium
Stainless steel
Nitinol
How does the tissue response vary between highly bioinert and weakly bioinert materials?
Highly bioinert - thin fibrous capsule forms
Weakly bioinert - thicker fibrous capsule forms
How and why is HA coating done?
It is a bioactive ceramic.
Plasma spraying or vapour methods
What are the five key properties related to selecting a biomaterial as an engineering material?
Corrosion Diffusion Tribology (wear) Mechanical properties Stress shielding/fatigue
How do you minimise corrosion? (4)
- Careful selection and coupling of metals
- Careful handling to minimise crevices & cracks
- Modified surface layer leading to passive alloy
- Minimised tensile stresses (which can cause corrosion)
Why must selected materials be corrosion resistant?
The body is a highly corrosive environment, and corrosion of metals leads to metals ions being released into the body = not good mate!!
What is galvanic corrosion?
An electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte.
How do metals leach?
Corrosion
What are the two cases where leaching is a good thing?
Drug eluting implants
Bioglass as it stimulates osteogenesis
What is the difference between thermoplastic polymers and thermosetting polymers?
Thermoplastics can be remelted back into a liquid, thermosetting always remain in a solid state
Do thermoplastic polymers leach?
Not usually, only when they have additives (like plasticisers)
Do thermosetting polymers leach?
Yes as they usually contain traces of unreacted monomers.
PMMA leaching can cause extreme low blood pressure, even DEATH WOW
What are the 3 key things to consider with moving parts?
Friction
Wear
Lubrication
What is the friction coefficient?
Friction coefficient = sliding force/normal load
What are typical friction coefficient values?
- 5-1 for metal on metal
- 5-1 for polymer on polymer
- 5-0.8 for metal on polymer
- 05 for alumina-alumina
What are the 3 categories of wear?
- Wear rate
- Abrasive & adhesive wear
- Fatigue & corrosion wear
When will abrasive wear occur?
If one material is softer than the other
What are the four main material combinations for joint prostheses? Rank them in terms of wear rate (worst to best)
Metal on polyethylene
Metal on metal
Ceramic on polyethylene
Ceramic on ceramic
What is titanium nitride used for?
Stem in joint replacements
Nitride is added to make the titanium wear resistant.
Why do metal wear particles cause less inflammation?
They are not antigens (won’t be targeted by antibodies)
What is toughness?
Resistance to fracture propagation. Measured as the work done to create unit area of new surface.
Name the 3 crack failure modes.
Mode I - opening
Mode II - sliding
Mode III - out of plane tearing
What is the theoretical strength of ceramics? What is the actual strength?
Theoretical: E/5 to E/10
Actual: E/100, E/1000
What are the two key issues of importance for ceramic strength?
Why is there such a broad range in strengths for different ceramics?
Why is their actual strength E/1000?
What is stress concentration caused from?
Atomically sharp surface flaws (scratches, pores)
Why is fibreglass an exception to the E/1000 rule?
It is coated in a polymer film which protects its surface
What is the Griffith Crack Theory?
Phenomenon of stress concentration at the tip of a flaw
Maximised for deepest flaw and sharpest radius
What does ZTA stand for? When was it first used?
Zirconia toughened alumina
2001
Really really popular in orthopaedics
Why is ZTA so good?
Combines the toughness of zirconia with the superior wear resistance of alumina.
