Homework (Exam 1) Flashcards
What is the main difference between the old definitions for biomaterials and those from more recent years?
- Main difference is the tunability of new generation of biomaterial to form a material-tissue bonding and release drugs.
- Previous generation of biomaterials are inert or degradable to avoid material-tissue interface.
Is it preferable to make implants bioactive or inert? Why?
- It is preferable to make bioactive implants to facilitate biological bonding of material tissued interface.
Which types of intermolecular forces dominate the assembly of atoms and molecules into matter for each of these types of materials?
(a) stainless steel
(b) platinum
(c) polyethylene
(d) wood
(e) sapphire
(f) hydrogel
(g) alumina
- Stainless steel: Metallic boding
- Platinum: metallic bonding
- Polyethylene: Covalent and Van der Waals
- Wood: Covalent
- Sapphire: Ionic
- Hydrogel: Covalent and Van der Waals
- Alumina: ionic
List desirable properties of biomaterials giving examples of each property.
- Biocompatibility: all in vivo implants
- Mechanical strength: load bearing orthopedic and orthodontic implants
- corrosion resistance: Metallic plates and screws passivity
- Electrical conductivity: pace makers, neural stimulators
- Enzyme/target specific: drug scaffolds
The bond-energy curves for two engineering materials are shown in the Figure below. Your task is to select the better material (A or B) for use in each application as described below. Please provide a rationale for each of your selections.
a) A beam that shows little deflection under moderate loads
b) A crucible to be used at high service temperature
c) A device designed to sense changes in temperature by changing its dimensions
- a) A beam that shows little deflection under moderate loads: higher modulus of elasticity –> steeper curve - Curve B
- b) A crucible to be used at high service temperature: better molecular integrity low separation distance - deeper well - Curve B
- c) A device designed to sense changes in temperature by changing its dimensions: Sensitivity to small changes - easy separation of molecules at lower energy - Curve A
The figure below shows stress-strain curves for three materials:
- Which material has the highest modulus and why
- Which materials has the highest ductility and why
- What material has the highest toughness and why
- Which material does not exhibit any significant plastic deformation prior to fracture and why
- Each of the three curves represent the typical stress response to strain of three classes of materials previously studied, which material is I, II and III?
- Which material has the highest modulus and why? steapest curve - Material I
- Which materials has the highest ductility and why? largest plastic region - Material III
- What material has the highest toughness and why? Largest area under elastic and plastic of the stress strain curve - Material II
- Which material does not exhibit any significant plastic deformation prior to fracture and why? A linear curve - Material I
- Each of the three curves represent the typical stress response to strain of three classes of materials previously studied, which material is I, II and III? Material I = Ceramics; Material II = metal; Material III = polymer
You have two metal specimens both of which are cylindrical with a diameter of 20 mm and a length of 200 mm. One is made of titanium alloy (E = 100 GPa) and the other of stainless steel (E = 200GPa). Both of them are subjected to a tensile force of 500N. Which of them will have a higher stress? Which will develop a higher strain? Given the information provided can you tell which of the specimens will be stronger?
Diameter of Ti alloy and Stainless steel = 20 mm
Legth of Ti alloy and Stainless steel = 200 mm
Tensil Force on Ti alloy and Stainless steel = 500 N
- Which of them have higher stress?
σ = F/A
Force and Cross section dimensions are the same for both Titanium and stainless steel. Hence same amount of stress will be exerted.
- Which of the speciment will develop higher strain? In the linear region/elastic region, Titanium will develop higher strain due to lower elastic modulus.
- which of the specimen will be stronger? Stainless steel with higher modulus of elasticity will be stronger in the linear regions.
- Note: ultimate tensil strength and subsequent fracture point are very important to determine max. stress, max strain and strength.
- Since there was no information base on the plastic region, the ansers fot this questions are base on the linear elastic region information.
-
_Elastic region: _
- E = σn / €n E = 100 ; €yield = 0.008
- 100 = σn / 0.008 —> σn = 0.8 GPa
-
Plastic region:
- σtrue = C(€true)n
- C = 1600 MPa n = 0.2
- Break point or €break= 0.6
- Toughness = Intg (0-break) σ d€
- Toughness = [Intg (0-€yield) σ d€] + [Intg (€yield-€break) σ d€]
- Toughness = [Intg (0 - 0.008) Area under the linear region ] + [Intg (0.008 - 0.6) C(€true) n d€]
- = (1/2) x 0.008 x 0.8 + 320 [Intg (0.008 - 0.6) d€]
- = 3.2 MPa + 57.5896
- Toughness = 60.789 MPa
If you were to design a metallic cardiovascular stent would you treat the surface to be more hydrophilic or hydrophobic and why?
- Very hydrophyllic – affect blood flow, as blood is hydrophylic
- Very hydrophobic – may attract hydrocarbon fat which may clog blood vessels.
- Hence an accurate ratio of hydrophyllic and hydrophobic is requiered.
Nitinol
- List properties
- List processing
- Examples of curent implants
- Shape memory
- Process: Temperature dependent. Austenitic-Mantensite transformation
- Ex: vascular stents for blood clots. Orthodontics
Stainless steel
- List properties
- List processing
- Examples of curent implants
- Increasing corrosion resistance by adjusting carbon component
- Process: only cold treatment not heat treatment
- Ex: orthopedict. In fracture fixation devices
CpTi
- List properties
- List processing
- Examples of curent implants
- Light, excellent corrosion resistance, different grades based on O2 content
- Process: ** machined** because casting is difficult. alpha-Ti no heat treatable but weldable. Beta-Ti are heat and cold treatable.
- Ex: orthopedic implants and dental implants
Ti6Al4V
- List properties
- List processing
- Examples of curent implants
- Alpha & Beta stabilizers. High tensile strength and good formability.
- Process: 1) Heat treatable, 2) polishing, 3) blasting, 4) plasma spraying
- Ex: bone contact interfaces. hip stem
CoCrMo
- List properties
- List processing
- Examples of curent implants
- Hardest, strongest. Excellent wear properties.
- Process: Only cold working. Difficult to machine. Investment casting and powder metallurgy
- Name 2 metallic medical device applications in which textured or porous materials/coatings are used.
- Describe how the surface were rendered textured/porous
- For each of the medical device applications listed, why are these porous/textured materials/coatings used (i.e., what is the desired response to the roughened surface)?
- Orthopedic hip femoral stem
- Sand and grit blasting, acid etching, plasma spraying, HA coating
- Osseointegration and tissue growth, biochemical integration.
- CoCr acetabular cup
- Sintered CoCr beads
- Porous CoCr acetabular cup to prevent loosening.
Bulk vs. surface of biomaterials
- Indicate 2 methods for impovement of the bulk properties of metals
- Indicate 2 methods for improvement of the surface of metals
- Bulk:
- Annealing: heating and slow cooling to produce refined grain structure. Reduces hardness, remove residual stress, improve material toughtness and ductility restoration.
- Normalizing: uniformity in grain size. Other methods (hardening, quenching, tempering)
- Surface:
- Blasting and acid etching
- Surface anodization. Other methods: plasma spraying, fluoride surface treatment, laser etching and micro arc oxidation
