Metallic Biomaterials

Question 1

Present one reason why you would choose a metallic material over polymers or ceramics for use in biomedical devices (1 mark)

Answer 1

Mechanical Properties:

• stronger structural strength
• superior utility in load-bearing applications
• more resistant to fracture
• higher hardness or wear resistance
• stiffer (but can be a negative in certain applications)

Electrical Properties:

• excellent conductivity
• e.g. applicable for stimulants such as pacemakers and in deep brain stimulation

Functional Properties:

• ease of manufacture into complex shapes
• can achieve a variety of surface finishes depending on application
• easy to sterilise
• for repetitive loading: polymers/ceramics are weaker and/or more brittle compared to metallic materials

Question 2

True or False? Presence of ions in solution accelerates the corrosion degradation of most metals. (1 mark)

Answer 2

True

Corrosion is a redox reaction and the flow of ions in solution stimulates and enhances the corrosion reaction between the metal and the solution.

Question 3

Some metals and metal alloys are able to develop a thin passive oxide layer on its surface. Briefly explain why this is important in the context of biomaterials (2 marks)

Answer 3

This layer impedes any further degradation by oxidation (corrosion) as it coats the surface of the biomaterial.

In the reactive environment of the body, in addition to preventing oxidation this layer also may minimise the immune response by e.g. preventing the leaching of metallic ions from the biomaterial.

Question 4

Name one metallic element that is conducive to the formation of a passive oxide layer to provide corrosion resistance. (1 mark)

Answer 4

Chromium

Titanium

Question 5

Name one example of a metallic ion with a known biological function and describe its
biological role in humans. (2 marks)

Answer 5

• Ca²⁺: part of bone/teeth structure
• Cr³⁺: trace amount in sugar regulation
• Na²⁺ & Ca²⁺ : are responsible for muscle contraction by triggering impulse
• Potassium: nerve signalling and muscle contraction
• Magnesium: maintains normal nerve and muscle formations. Supports a healthy immune system, helps bones stay strong.

Question 6

Choose the correct statement to complete the sentence. For the formation of an interstitial alloy, the atomic size of the alloying element must be:

(A) significantly smaller than the atomic size of the matrix element.

(B) similar to the atomic size of the matrix element.

(C) significantly larger than the atomic size of the matrix element.

(D) carbon.

Answer 6

(A) significantly smaller than the atomic size of the matrix element

Question 7

Which element is responsible for the formation of a passive layer in stainless steel SS316?

(A) Chromium

(B) Molybdenum

(C) Cobalt

(D) Nickel

Answer 7

(A) Chromium

Question 8

What is the minimum amount of chromium (wt%) needed to impart the ability to form a passive layer for corrosion resistance?

(A) 1%

(B) 11%

(C) 21%

(D) 16%

Answer 8

(B) 11%

Question 9

Describe why nickel is used as an alloying element in stainless steel 316L (2 marks)

Answer 9

Nickel stabilises austenite phase which enables stability at room temperature.

Austenite SS316L with a FCC crystal structure is much tougher and ductile than the martensite BCC phase, therefore making austenite a desirable phase for SS316L. However, austenite SS316L itself is not very stable under 727 degrees C. Nickel is therefore added as a stabiliser of the austenite phase so it maintains desirable properties at room temperature.

Question 10

Explain why some medical device companies would decide to use stainless steel 316L (SS316L) implants over other materials, despite SS316L’s inferior corrosion resistance properties to Co- and Ti-alloys. (2 marks)

Answer 10

Stainless steel is a cheap material and can be used for temporary implants or biomedical
instruments. It also has good mechanical properties and good processability.

It is much cheaper than Cobalt and Titanium alloys and is mainly used in temporary fixation devices and surgical instruments that don’t have long term contact with the body’s internal environment.

Question 11

Which of the following elements are used to alloy cobalt-chrome systems to enhance strength and corrosion resistance? (more than one answer, need all answers correct to get 1 mark)

(A) Mo
(B) Ni
(C) Hf
(D) Zn

Answer 11

A = Mo: Molybdenum increases corrosion resistance, and strength
B = Ni: Nickel increases castability, as well as corrosion resistance and strength

Question 12

True or False? Pure titanium needs to be alloyed to allow for passive layer formation for corrosion resistance. (1 mark)

Answer 12

False

Question 13

Choose the correct statement to complete the sentence (1 mark). The more impurities present in pure titanium,

(A) both strength and ductility are enhanced.

(B) strength is enhanced, but ductility is reduced.

(C) strength is reduced, but ductility is enhanced.

(D) both strength and ductility are reduced.

Answer 13

(B) strength is enhanced, but ductility is reduced.

The more pure titanium, the weaker it is. But it’s more ductile.

Question 14

Choose the correct statement to complete the sentence (1 mark). By introducing both α- and β- stabilizers to form α-β alloys such as Ti-6Al-4V,

(A) both strength and ductility are enhanced in comparison to commercially pure Ti (cpTi)

(B) strength is enhanced, but ductility is reduced in comparison to cpTi.

(C) strength is reduced, but ductility is enhanced in comparison to cpTi.

(D) both strength and ductility are reduced in comparison to cpTi.

Answer 14

(B) strength is enhanced, but ductility is reduced in comparison to cpTi.

(UTS increases but %elongation decreases)

Question 15

15. What do the numbers in Ti-6Al-4V signify? (1 mark)

(A) weight% of the element after the number (i.e. right-hand side)

(B) weight% of the element before the number (i.e. left-hand side)

(C) atomic% of the element after the number (i.e. right-hand side)

(D) atomic% of the element before the number (i.e. left-hand side)

Answer 15

(A) weight% of the element after the number (i.e. right-hand side)

6% aluminium and 4% vanadium (the rest is titanium)

Question 16

For the following three materials (stainless steel 316L, cobalt-chrome alloy Co-28Cr-6Mo, and titanium alloy Ti-6Al-4V), rank from best/highest = 1 to worst/lowest = 3. (half mark each, 4 marks total)

Material:

• Stainless steel 316L
• Cobalt-chrome alloy (typically Co-28Cr-6Mo)
• Titanium alloy (typically Ti6Al4V)

Property:

• Young’s modulus (*highest GPa value = 1 lowest GPa value = 3)
• Ultimate tensile strength
• Shear modulus
• Fatigue strength
• Density
• Wear resistance
• Biocompatibility
• Corrosion resistance

(*Keep in mind that a high GPa for Young’s modulus isn’t necessarily the best for many biomedical implants.)

Answer 16

see table

Question 17

Which of the following elements is a β-phase stabilising element for titanium alloys? (1 mark)

(A) aluminium

(B) vanadium

(C) germanium

(D) gallium

Answer 17

B)Vanadium

• beta phase 4%wt

NOTE: Aluminium - alpha phase 6%wt

Question 18

Which of the following Ti alloys generally have the lowest Young’s modulus? (1 mark)

(A) commercially pure Ti

(B) α-titanium alloys

(C) α-β titanium alloys

(D) β titanium alloys

Answer 18

D) β titanium alloys have Young’s modulus very close to bone

Question 19

Nitinol is a nickel-titanium alloy with which of the following proportions stable at room temperature? (1 mark)

(A) Ni 50%, Ti 50% (wt%)

(B) Ni 50%, Ti 50% (atomic%)

(C) Ni 55%, Ti 45% (atomic%)

(D) Ni 45%, Ti 55% (wt%)

Answer 19

(B) Ni 50%, Ti 50% (atomic%)

Question 20

True or False?

NiTi alloys generally have better corrosion resistance than cobalt chrome and stainless steel alloys. (1 mark)

Answer 20

True

Question 21

Despite having considerably more nickel in the structure of nitinol, the rate of nickel release is significantly lower from nitinol materials than stainless steel 316L. Briefly describe the reason why this is the case. (2 marks)

Answer 21

The passive oxidised layer (TiO₂) in Nitinol significantly prevents and lowers the nickel ion release in comparison to the less effective chromium oxide (Cr₂O₃) passive layer found in SS316L. There is also much more titanium in nitinol (i.e. ~45wt%) that can impart corrosion resistance than the amount of chromium (<20wt%) in stainless steel 316, so there is more oxide material available for passive layers.

Question 22

Describe in your own words how nitinol alloys’ shape memory effect works, with references to the changes in the underlying crystal structure. You may use particular examples. You may also use drawings to aid description. (3 marks)

Answer 22

Nitinol has two stable phases about a phase transition temperature. This includes a stronger austenite phase with a Face-centred cubic (FCC) crystal structure, and a weaker martensite phase with a Body-centred tetrahedral (BCT) crystal structure.

For nitinol’s shape memory to take effect, the initial shape of the implant occurs in its FCC phase. Then the temperature is cooled below the phase transition temperature, forcing the implant to convert to BCT phase, where the implant is shaped into desired shape.

An example would be nitinol cardiovascular stent, where the expanded stent state would occur in FCC phase. To allow easy delivery, the stent would be crimped in the BCT state. Therefore, upon facing an increase in temperature in the body (37C vs 24C), the stent would expand in vessel as it reverts back to FCC structure.

Question 23

Name one characteristic an ideal material for implantable electrodes should possess. (1 mark)

Answer 23

• Low impedance between electrode and tissue (low current needed) by minimizing fibrous capsule formation.
• Balance of stiffness and flexible at the same time to withstand both insertion and cyclic loading.
• Needs to allow reversible charge transfer.

Question 24

Which of the following characteristics does NOT apply to magnesium metal alloys? (1 mark)

(A) Mg metal alloys are biodegradable.

(B) Mg metal alloys have one of the lowest Young’s modulus compared to other metal materials used for implants.

(C) Mg metal alloys have one of the lowest tensile strengths compared to other metal materials used for implants.

(D) Mg metal alloys are corrosion resistant.

Answer 24

D) Mg alloys in body react with water to generate hydrogen gas, therefore is not really corrosion resistive.

Question 25

Name ONE issue that is hindering the progression of magnesium alloys to widespread clinical use. (1 mark)

Answer 25

Magnesium degradation (‘corrosion’ by reaction with water) leads to hydrogen gas production which can hinder tissue repair and regeneration.

Its rate of degradation has not been shown to match the rate of tissue/bone remodelling in all cases.

Question 26

Name ONE additive manufacturing technique/method used to fabricate intricate titanium alloy structures. (1 mark)

Answer 26

● Hot isostatic pressing

● Selective laser melting

● Electron beam melting

● 3D metal printing

Question 27

Name ONE medical device that has been either FDA approved or cleared that is made of 3D-printed titanium. (1 mark)

Answer 27

Spinal cage implant

Cranial/Craniofacial implants