Biodegradable Metals Intro Study Guide Flashcards
What is stent restenoises and describe the current technology in place for preventing it?
-Restenoises is re-narrowing of an artery at the stent.
- Technology to prevent it:
- Drug eluting coating
- Drug eluting stents contain a time releasing drug that prevent restenosis of the plaque over the stent.
Name three metals that have been used
- SS 316L
- Ti Alloy
- Co-Cr Alloys (Wrought)
Name the metal that is considered the gold standard for use as a cardiovascular stent.
-Stainless steel (namely 316L steel)
State two challenges of a permanent metallic stent
- Permanent fixture
- Need to continuously take blood thinners to prevent blood coagulation and blood clot formation
- Difficulties in inserting a second stent within the same artery
State two examples of internal fracture fixation devices
- Bone Plates
- Nails/Rods
- Pins
- Screws
Which fracture has a part of the bone twisted?
A. Transverse
B. Spiral
C. Incomplete Fracture
D. Closed Fracture-B. Spiral
What is fracture? Differentiate between open and closed fracture.
- A fracture is a partial or complete break in the bone. When a fracture occurs, it is classified as either open or closed:
- Open fracture (Also called compound fracture)- the bone exist and is visible through the skin, or a deep wound that exposes the bone through the skin.
- Closed fracture (Also called simple fracture)-the bone is broken but the skin is intact.
State the rationale for biodegradable materials. Name two metals that are currently being investigated as a biodegradable metal material?
- Metals are more suitable than polymers for applications that require high strength to bulk ratio such as bone fixation screws / pins and for coronary stents
- Mg, Fe and Zn based alloys are some classes of metals that are being investigated
State two challenges of permanent metallic fracture fixation devices.
- Stress shielding in orthopedic implants resulting in implant loosening and repeat surgeries
- Weakens new bone tissue
- Implant infection
- Metal ion dissolution
Name and describe three types of fractures
- Incomplete Fracture – the broken bone is not completely separated.
- Transverse- The break is in a straight line (right angle) across the bone.
- Spiral – A fracture where at least one part of the bone has been twisted.
- Oblique - fracture diagonal to bone axis.
- Comminuted – the bone has broken in to number of pieces.
What are some of the advantages and disadvantages of the use of Magnesium for implants?
-Main Advantages
– Biodegradable
– Biocompatibility: Mg is present in the body
– Lightweight
– Stimulatory effects on the growth of new bone tissue
– Exhibit good mechanical properties
– Elastic modulus of pure Mg is similar to human bone.
-Main Disadvantages
–Low corrosion resistance
–Degradation occurs before the end of healing process
–Fast hydrogen evolution
Why are Mg Alloys being researched for biomedical applications instead of pure Mg?
-Mg alloys have shown osteoconductive and osteoinductive properties, thus offering less invasive repair and temporary support during tissue recovery.
Explain the ASTM system of classifying magnesium alloys, and give an example.
- First letter is stands for the metal with the highest amount in the alloy
- Second letter stands for the metal with the second highest amount in the alloy
- First number is the wt% of the the metal with the highest amount (the first letter one)
- Second Number is the wt% of the metal with the second highest amount (the second letter one)
Ex. AZ91D
-9 wt% aluminium, 1 wt% zinc and it is in the D modification
What is the main challenge of using pure Mg in the body?
- Low corrosion resistance
- Degradation occurs before the end of healing process
- Fast hydrogen evolution
Describe the corrosion behavior of unprotected Mg alloys, and how does this behavior affect its potential as a biodegradable metal?
- Attacked by most acids; chloride solutions usually found in the form of seawater are most damaging.
- This corrosion behavior makes it attractive as a potential biodegradable metal material
- In-vivo conditions contain high chloride content
Describe general characteristics of Mg and state its advantages for aerospace applications.
- Characteristics of Mg:
- It is lightweight
- It is abundant in the world
- Has a lower Specific Heat
- Uses lesser energy in recycling - Toughness is greater than ceramics
- Elastic modulus and compressive yield strength closer to natural bone compared to other implants.
- Magnesium is essential to human metabolism and is naturally found in bone tissue.
- Mg alloys have shown osteoconductive and osteoinductive properties, thus offering less invasive repair and temporary support during tissue recovery.
- The in vivo corrosion of magnesium based implant involves the formation of a soluble, non-toxic oxide that is harmlessly excreted in the urine.
- In addition, due to functional roles and presence in bone tissue, magnesium may actually have stimulatory effects on the growth of new bone tissue.
- Magnesium Alloys in Aerospace
- Application of magnesium for ground and air transportation vehicles reduces their total weight, reduces fuel consumption, and increases recyclability.
What are the side effects of low Mg levels in the body?
-Low levels of Mg can cause asthma, diabetes, osteoporosis
What biomedical applications are Mg alloys being considered for?
-Potential applications: Stents, fracture fixation devices
What does AZ91D stand for?
-9 wt% aluminium, 1 wt% zinc and it is in the D modification
What are some of the challenges associated with Mg use in the aerospace and automobile industries?
- Flammability of Magnesium Pure
- Magnesium metal and its alloys are highly flammable when in the molten state or powdered form. Burning or molten magnesium metal reacts violently with water Highly Flammable.
- Corrosion Resistance
- Magnesium occupies the highest anodic position on the galvanic series, and, as such, there is always the strong potential for corrosion as shown in the example of figure.
- Lack of High Strength Magnesium Alloys
- Compared to high strength aluminum alloys, magnesium alloys are not as strong (tensile strength) and have a lower modulus of elasticity.
State representative automotive applications where Mg is being explored for its potential use.
- Inner door (VW)
- Hybrid Mg/Al engine bonnet (VW)
- Prototype Opel G90 with Mg sheet roof
- Sandwich interior panel
- Prototype front fender
