Biomaterials Flashcards
1
Q
What is Biomaterials?
A
A material intended to interface with biological systems to evaluate, treat, augment, or replace any tissue, organ, or function of the body.
2
Q
What is an important attribute that a biomaterial should have from the point-of-view: A manufacturer, a clinician, and a patient.
A
Biocompatibility, and covered by insurance is shared by all point-of-views.
3
Q
Definition of Biocompatibility?
A
The ability of a material to perform with an appropriate host response.
4
Q
Examples of Biocompatibility?
A
Resistant to blood clotting, resistant of infection, “uncomplicated” healing”, ** must do patient no harm**
5
Q
Key considerations of Biocompatibility?
A
- Application (*e.g. implanted vs. non-implanted, needs will differ per application)
- Material (Properties: mechanical (bulk property), surface, chemical etc.)
- Processing (smooth vs rough)
- Time/”Life” (degradable cs. non-degradable)
6
Q
Testing Biomaterials
A
- Application Specific
- Tissue Specific
Dominated by Surface Characterization though, because this is where the most interaction occurs with biological systems
7
Q
Types of Chemical Bonds (Primary Bonds)
A
- Covalent
- Ionic
- Metallic
- Ionic/Covalent Mixed
8
Q
Types of Chemical Bonds (Secondary Bonds)
A
- Polar Bonds
2. Non-Polar
9
Q
What is a covalent bond?
A
Share electrons to achieve octet rule.
Mostly occurs in polymers and other organics
Poor Thermal and electrical conductivity
Single bond allows rotation of atoms and thus result sin felixible and deformable material.
10
Q
What is an Ionic bond?
A
Transfer of electrons that generate ions
Bonding due to electrostatic attraction
Atoms arrange in crystal lattice
Mostly found in ceramics
High strength and stiffness but brittle due to atoms inability to move in response to external forces.
Electrons not available for charge transfer = Bad conductors
11
Q
What is a metallic bond?
A
Metals atoms are good donors of electrons. Tightly packed positive ions surrounded by electrons.
Good charge transfer
12
Q
What determines whether two elements will bond covalently, ionically or netallically?
A
Electronegativity: the measure of how strongly an atom wants electrons.
13
Q
What is a dipole?
A
A molecule with a spatial separation between the negative and positive charge (ex. H2O)
14
Q
Which of the following materials would you expect to be strongest?
A. Iron
B. Diamond
C. Magnesium
D. Ice
A
A. iron
15
Q
Which of the following materials would you expect to have the highest thermal conductivity? A. Iron B. Diamond C. Magnesium D. Ice
A
A Iron
16
Q
What are material properties?
A
Are quantifiable attributes of a material that do not depend on the amount of the material present.
17
Q
What is the most basic mechanical test procedure for measuring load-deformation?
A
Uniaxial Tensile Testing
18
Q
What do you measure in tensile test?
A
- Amount of elongation
2. Amount of force required o produce deformation
19
Q
Process of Uniaxial Tensile Test?
A
- Test specimen loaded and fixated to bottem clamp first then top.
- Top clamp moves away from the bottom and measures the force required
- Force vs. deformation data produced.
20
Q
Is force vs. deformation a mechanical property?
A
No. This relationship depends on the size of specimen, That’s why there are standards.
21
Q
Engineering Stress Equation?
A
Stress = Force/Area
22
Q
Engineering Strain
A
Strain = change of length/ original length
23
Q
When calculating the yield strength what is the offset percantage from Young’s modulus?
A
0.2%
24
Q
What measurement(s) do you need to take before testing?
A
Thickness, Width of Gage, Length of gage
25
How do you find toughness in the Stress-Strain curve?
The whole area under the curve.q
26
How do you find resilience in the stress-strain curve?
The area under the the linear region of the stress-strain curve.
27
Which of the following would be considered be made from a biomaterial according to the modern definition for biomaterials?
(a) Pacemaker for the heart
(b) walking stick
(c) stethoscope
(d) bascular graft
(e) totthbrush
(f) suture
(a) pacemaker for the heart
(d) Vascular graft
(f) suture
28
Of ceramics, polymers, and metal, which have low resistance to crack propagation and fracture?
Ceramics = lowest resistance to crack propagation due to inability to undergo ductile deformation. High stresses at crack-tips can exceed the failure strength of the material and cause local failure that will cause the crack to grow (propagate).
29
If you had to produce an arterial artery, what are the essential properties you will incorporate in your design? Which type of material would you first consider? why?
The artificial artery properties:
Leak proof - prevent blood loss
Flexible - be easily positioned
elastic (resilient) - for changes in blood pressure without undergoing failure
non-thrombogenic - don't caude blood clots
non-immunogenic - not elicit an immune or adverse response
Material - Polymer
30
Why are the surface properties of a biomaterial important?
The majority of body-device interactions occur on the surface of the material. The surface properties dictate much of how a biomaterial behaves in relation to the body.
31
Cold welding
Joining of two pieces of metal join without fusion/heating at the interface of the two parts to be welded.
32
What makes cold welding possible w/ metal?
In the absence of a reactive atmosphere (mostly Oxygen), metals will not undergo surface oxidation. In addition, the electron-cloud config. of metals have no bonding directionality, and so when placed in perfect contact with another piece, the atoms have no way of knowing that they are separate pieces and thus will bond.
33
Do you expect other material, such as ceramics or polymers to be able to be cold welded?
I do not expect this. Ceramics have ionic bonds which are the most stable config, to minimise surface energy
34
What is preventing two separate pieces of gold from instantly bonding when touched together?
Oxygen will almost always react with the surface of any material even gold. So when the thin oxide layer is rubbed out this only creates more surface area to react and only a small portion cold welds.
35
Behaviour of material in the linear (elastic) region?
Stress = E* Strain
Just like hooke's Law.
Elastic modulus is the constant that characterizes the elastic properties
36
What is yield stress?
The stress which causes the onset of permanent deformation.
37
What is the proportional limit?
The highest stress at which stress is linearly proportional to the strain. Like a spring.
38
What is the maximum stress that material can undergo before fracture called?
Ultimate stress (strength)
39
What is it called when the stress levels continues to increase after the yield stress is reached?
Strain Hardening
40
What is strain hardening?
This resistance is caused by the decreased mobility of atomic planes within the material due to the interaction of multiple dislocations.
41
Why does the stress drop after ultimate strength has been reached?
The material's cross-section decreases called "Necking." As area decreases, the force required to produce constant deformation rate also decreases
42
What is uniform plastic deformation?
Occurs before ultimate strength point reaches. The entire specimen stretches.
43
What is Local plastic deformation?
Occurs after ultimate strength point. Preferential stretching near necking region.
44
What is Strain Energy Density?
The amount of work required to deform a material per unit volume.
45
What are the types of material fractures?
Brittle and Ductile fractures
46
What kind of loads can a material undergo?
```
Tensile
Compression
Bending
Twisting
Shearing
```
47
What is fracture?
When the cohesive strength between adjoining atoms is exceeded by applied stress.
48
Poisson's effect?
When the under tension the material contracts. Generally speaking.
49
Define failure.
Definition of of failure depends on design contraints.
50
Which mode fracture occurs in depends on:
1) Material properties
2) Temperature
3) Rate of Loading
51
What happens in a Ductile fracture?
First, happens after plastic deformation.
1) Microdefects (voids) grow and join to form cracks
2) Crack propagate 45 degrees to direction of applied load (Along the plane of max shear stress)
3) When crack attains certain length it becomes
critical" and grows rapidly.
52
What happens in brittle fracture?
No plastic deformation.
Cleavage of atomic bonds along crystallographic planes
1) Crack propagates 90 degrees to the largest principal stress
53
What is corrosion?
The loss of material due to oxidation. Loss enters into body as metal ions.
54
Driving force for Corrosion?
Usually associated w/ metal.
Greater thermodynamic stability in oxides than in metallic state.
Greater thermodynamic stability in dissolved species than solid state.
Potential Difference!!!
55
Types of Corrosion?
```
Uniform Attack
Galvanic Corrosion
Crevice corrosion
Pitting corrosion
intergranular corrosion
selective leaching
stress corrosion
erosion-corrosion
Fretting
```
56
Why does Fatigue failure happen?
Micro-cracks or other imperfections/defects (dislocations,voids, cavities, etc.) which result in stress concentrations. These stress risers produce local failures which grow over many loading cycles.
57
What determines fatigue life of a material?
```
MAterial composition
Surface processing/treatment
geometry
hot/cold rolling
heat treatment
temperature
type of load
ec.
```
58
How to test for fatigue life?
Requires cyclic testing to failure of a lot of samples
59
Example of fatigue testing?
Rotating Beam Fatigue Test
Applies bending load
Outersurface oscillate from tension to compression (open and close cracks)
60
What are S-N Diagrams?
Is a maximum stress per cycle vs. Cycles to failure diagram.
Used to characterize fatigue behaviour.
61
Limitations to S-N diagrams
Underestimates fatigue life.
62
The line on the S-N curve means?
Means there is 50% probability of failure at the given stress value.
63
Uniform attack?
Corrosion uniformly over the surface exposed to a solution which makes oxidative and reduction reactions.
ex. Rust
64
Crevice Corrosion
Inside the crevice it is the anode giving electrons to the cathode side. By doing this metal ions are formed in the crevice and thus grows the crevice.
65
Intergranular Corrosion
The boundaries between grains have different composition rendering them more reactive.
66
Leaching
Happens in alloys, where one of the constituent is selectively dissolutioned.
67
Erosion-Corrosion
Accelerated corrosion due to relative fluid motion stripping away oxide layer and exposing new metal surfaces.
68
Fretting Corrosion
Similar to Erosion-corrosion but happens due to mechanical wear exposing tnew metal surfaces.
69
Hardness
A surface property: The ability to resist local plactic deformation
70
Limitations to Wettability quantification.
Assumes perfectly clean and flat surface.
71
What are the most commonly used metals for biomedical implants and devices?
Titanium and its allows
Stainless steel
Cobalt-Chromium
72
Metals must be:
Highly corrosion resistant
Biocompatible
Wear resistant
Excellent material properties to application
73
What is Steel?
Iron (Fe) and Carbon (C)
| C = [0.3 -1%]
74
Effect of Carbon in Steel?
More Carbon means its stronger but more brittle. Carbon immobilizes dislocations further.
Also, can produce different phases by adjusting carbon content and Temperature.
75
What is Stainless steel?
Iron-based allow with at least 10.5% chromium and Carbon
76
Corrosion resistance in stainless steel?
Attributed to the formation of chromium oxide on its surface.
Corrosion resistance further improved by adding more chromium
Addition of molybdenum increases pitting corrosion resistance
Addition of nitrogen, increases mechanical strength and pitting corrosion resistance
77
Stainless Steel alloys categorised based on micro structure?
Martensitic stainless steel
ferritic stainless steel
austenitic stainless steel
duplex stainless steel
78
Metallic Elements
Generally pure.
Seldomly used
Commercially-pure titanium
(cp-Ti)
79
Alloys
Combo. of two or more constituents in a solid solution (mixture)
single-phase or multi-phase
e.g. SS, Co-Cr
80
Intermetallic Compounds
Combo. of two or more constituents with regularly-repeating patterns
e,g, Nitinol
81
Amalgams
Alloys ot mercury with other metals
82
Fe_3C: Iron Carbide (a.k.a. cementite
Intermetallic compound
| precipitates out of solution when C% too high.
83
Ferritic Stainless Steel
```
11-30% chromium
BCC
Magentic
Not heat-treatable
Rarely cold worked (since decreases ductility)
```
BME applications"
Very few- instrument handles, medical guide pins
84
Martensitic Stainless Steel
```
10.5 - 18% Chromium
Body-centred-Tetragonal (BCT)
Created by rapid cooling (quenching)
High carbon content (>1.2%)
Magentic
```
BME applications:
Dental instruments, scalpel & surgical instruments
85
Austenitic Stainless steel
```
15-20% Cr content
FCC
Not hardenable by heat treatment
Highly ductile - cold worked to harden
Non-magnetic
```
BME applications:
Implants, wires, devices
86
316L Stainless Steel
(<0.03%) Carbon content
Carbon must be low to prevent formation of chromium carbide which takes away Cr to make protective oxide layer
requires Ni to stabilize austenitic phase
87
Processing technique for 316L SS
Cold Working = decrease grain size
Annealing = increase hardness
Solidification = Increase strength
88
Implant-Grade Austenitic 316L SS Benefits:
Corrosion resistant
good formability
Nitric acid passivation
relatively inexpensive
89
New Developments in SS:
Nitrogen Strengthening
"Electroslag remelting" = increase N content in SS for strength and corrosion resistance
90
Titanium Properties
High Strength
Lightweight
Corrosion resistant
Low stiffness, high ductility
91
cp-Ti Allotropic means?
Means it can exist in two crystal structures in the same state
Above 882.5 C = beta (BCC)
Below 882.5 C = alpha (HCP)
92
Alpha Ti alloys?
Alpha stabilizers: Al, Ga, Sn
Bot heat treatable, but weldable
used for cryogenic app.
relatively low strength
93
Beta Ti alloys?
Beta stabilizers: Mo, Ta, V, Nb, W, Cr, Fe, Co, Ni, Cum Mn
More biocompatible (w/ Ta + Mo)
Heat treatable
cold formable
94
alpha + beta Alloys (Ti -6Al-4V)
Vanadium allergen!
Strong and heat treatable
Not weldable
95
Applications of Ti alloys
Orthopaedic implants
dental root implant
surgical slips
VA pumping casing
96
Nickel-Titanium
Intermetallic compound
Equal content of Ni and Ti
Shape memory
Superelastic
97
Co-Cr Processing & Fabrication
Casted = artificial joint interfaces, dental implants
Wrought Co-Cr alloys = High load applications= Hip stems, artificial knees
Difficult to machine b/c high hardness
98
Tantalum
Cp-Ta= excellent corrosion reistance, excellent biocompatability
App.
Suture wires for skin closure, tendon & nerve repair
Foils and sheets for nerve anastomoses
porous Ta for tissue ingrowth
99
Zircornium
Excellent biocompatability
Forms an adherent, protective surface oxide film
excellent corrosion resistance
Biocermaic!!!
100
Oxinium
For knee and hip implants
Alloy used: Zr-2.5Nb
Surface treated w/ oxygen to develop ZrO_2 layer - providing improved abrasion resistance
101
Metals for medical electrodes
Noble metals = Pt, Au, Rh, Pd, Ir
Why? Elevated conductivity = can be made smaller without losing signal
resistant to corrosion
102
Why add Ir to Pt?
Pt is very soft
Solid strengthening
103
Sterilization process - Key requirements
Kill as many og.
Minimize probability of non-sterile unit
Be usable on the device in the final packaging
Have no damaging effect on any of the device components or packaging
Process high volumes
Contain cost per unit
104
Sterility Assurance Level (ASL)
Max. acceptable SAL is less than one in a million product can be non-sterile.
105
Terminal Sterilization
Products are sterilized in their final packaging
BME devices must indergo this.
106
Techniques for Sterilization
Steam, Radiation, EtO, Gas
107
Steam Sterilization
Autoclave
Process: Expose to Saturated stean @ 15 psi and 121 Celsius for at least 15 min
Performed for:
Implants prior surgery
Resusable tools
108
Radiation-based Sterilization (GAMMA Ray)
Nuclear decay of unstable Cobalt-60 isotope emits gamma rays (High energy photons)
High penetration distance
Very effective
Can be used on a wide range of materials (even polymers)
Note: long exposure time necessary
Many polymers(e.g. UHMWPE) require inert-gas atmosphere to prevent oxidation due to radiation
Biodegradable polymers degrade under radiation
109
Radiation-based Sterilization (Electron Beam)
(High energy electrons)
Short exposure times
Can be used on a wide range of materials (even polymers)
Note:
Shorter penetrating distance
Polymers may degrade under higher-energy levels and larger doses
110
X-Ray sterilization
Operates similar to e-Beam
111
Considerations for radiation
Dose uniformity
Actual dose received by product in package depends on:
radiation source
product density
Box/pallet size (packaging)
config. of the sterilizatio unit
112
Ceramic processing
1) Plasma spray coating
2) Slow melt processing
3) liquid-phase sintering (vitrification)
4) solid-state sinteting
5) syntheis of glass-ceramics
113
Alumina
Mostly used for load bearing application
high wettability for lubrication in knee joint
114
Alumina: production and processing
Natural: either from bauxite ore or native corundum using Bayer process
Synthetic:
Highe rpurity but more expensive
Processing
Solid-state sintering @ 1600 celsius
0.5% MgO added to limit grain growth
115
Zirconia
Inert due to thermodynamically stable
Harder than alumina
116
Zirconia:Processing
During cooling, it expands due to phase transformation which produces 3-5% volume increase
117
Yttria-Stabilized Zirconia
Alleviate volume expansion
The Yttria grains produce compressive force on the expanding zirconia grains = Transition toughening
Therefore, closing cracks maybe
118
Yttria-stabilized Zirconia applications
Hip and knee replacements
issue: YSZ the tetragonal phase is metastable
Recall due to improper heat treatment.
119
Limitation to alumina and Zirconia
Restricted to compressive loading
Avoid impact loading
120
What is a ceramic?
Inorganic non-metallic material comprised of two ore more metallic & non-metallic elements
121
Silicon dioxide crystal struct:
Quartz
Cristobalite
Tridymite
122
How to produce ceramics?
Powder => Add water + binders =>Heat=> Cool
123
Bioceramics: Types of tissue attachment
1) Mechanical fixation - complex geometry
2) Biological fixation - porous
3) Bioactive fixation - elicit specific biological response
4) dissolution/replaced slowly by natural tissue
124
Dense HydroxyApatite app.
middle ear implant, alveolar ridge reconstruction
| mandibular augmentation
