1
Q
why is it easier to heal bone than cartilage
A
bone has a higher metabolism than cartilage (it has a higher turnover rate)
2
Q
ceramics bond type?
A
ionic bonds
3
Q
why are ceramics used for osseointegration
A
they have calcium phosphate in them which allows it to integrate into the bone
4
Q
metal bond type?
A
metallic bonds
5
Q
metallic bond characteristics?
A
highly mobile electrons
6
Q
hydrogel characteristics
A
swell many times when put in water but won’t dissolve
7
Q
why don’t hydrogels dissolve when put in water
A
crosslinked
8
Q
physical hydrogels vs chemical hydrogels
A
physical –> held together with entanglements
chemical –> held together with covalent bonds
9
Q
FDA doesn’t approve __
A
it doesn’t approve materials, it approves devices
10
Q
synthetic vs. natural debate
A
- natural polymers have similar chemical properties to the tissues they are replacing
- natural are more easily integrated
- natural has lower mech. properties
- difficult to remove pathogens from chemical
- sythetic do not interact in an active manner
- synthetic can be specifically tailored
11
Q
distance of surface properties
A
1nm
12
Q
surface properties dictate __
A
biological response; where proteins will attach
13
Q
anisotropy meaning
A
different mechanical properties in different directions
14
Q
chemical vs. physical surface properties
A
chemical –> hydrophobicity
physical –> roughness
15
Q
bonding in metal is __
A
nondirectional
16
Q
coordination number
A
number of nearest neighboring atoms
17
Q
FCC
A
APF = .74
4 atoms in each unit
18
Q
BCC
A
APF = 0.68
2 atoms in each cell
19
Q
HPC
A
APF .70
20
Q
in Titanium, which is the more ductile crystal structure
A
BCC is more ductile than HCP
21
Q
describe process for miller indicies
A
- determine where it crosses the xyz axis
- tape reciprocal of intercepts
- multiple by integers to clear fractions
- write integers in parentheses, no commas (h k l)
22
Q
dimension of point defects
A
zero dimensional
23
Q
examples of point defects
A
vacancies and interstitial
24
Q
both vacancies and interstitial cause __
A
lattice strain
25
vacancies and interstitial are caused by __
entropy during formation
26
types of impurities
interstitial and sustitutional
27
hume-Rothery rules for substitutional solids
1. want size of replacement atom to be same as original
2. electronegativities are similar
3. valence charge are similar
4. same crystal structure
28
types of solid state diffusion
vacancy diffusion and interstitial diffusion
29
crystal structure of ceramics
AmXp crystal structure
30
why do individual point defects not occur in ceramics
it would affect the electoneutrality of the material
31
two types of defect in ceramics
schottky defects -> vacancies in both cation and anions
| frenkel defects -> vacancy/interstitial pair
32
why do frenkel defects only occur with cations
frenkel defects --> vacancy/ interstitial pair
anions are too big to reside in interstitial space
33
_ materials have the longest burgers vectors
ceramics
34
what is degree of polymerization
number of repeat units in a polymer
35
configuration vs. conformation
configuration --> break bonds
| conformation --> bond rotation
36
is configuration or conformation harder to change
configuration is harder to change
37
isotactic syntactic atactic
isotactic --> R groups on the same side
syntactic --> R groups on alternating sides
atactic --> R groups are random
38
crosslinking __ crystallinity
crosslinking decreasing crystallinity
| it prevents the alignment fo chains so it is harder to form lamellae
39
different types of polymer synthesis methods
Addition polymerization and condensation polymerization
40
addition polymerization?
initiation -> activation of monomer using radical
propagation -> monomers join the polymer chain
termination -> end of polymerization
41
condensation polymerization
polymerization occurs through the elimination of water
42
molecular reasoning for decreasing crystallinity
anything that prevents chain alignment will prevent crystallinity
43
things the influence crystallinity
```
SBTR
1. Side groups
2. Branching
3. tacticity
4.Reguality of copolymers
...... more randomness leads to less crystallinity
```
44
Longer polymer chains have __
higher mechanical properties
45
__ is more indicative of its final properties than __
Mw than Mn
46
Polydispersity Index
Mw/Mn
| smallest possible value is 1
47
what is IR spectroscopy used for
to determine what kind of bonds are in a material
48
As wavelength __ energy __
as wavelength increases, energy decreases
49
IR radiation interacts with what kinds of bond
dipoles
50
how does IR radiation work
IR radiation hits the bonds, which increases their amplitude (not frequency) which results in absorption of IR at that frequency
51
Fourier transform IR spectroscopy
allow the sample to be bombarded with many wavelengths simultaneously; allows for more scans in a small time
52
How does liquid chromotography work
separation by size
53
gel permeation chromatography
mobile phase is non-polar/organic.
| Stationary phase is hydrophobic
54
what are the one-dimensional defects
Edge, screw, mixed
55
burger's vector in edge dislocations
burgers vector is perpendicular to the dislocation line
56
__ is the driving force for deformation
lattice strain
57
burger's vector in screw dislocation
burger vector is parallel to dislocation line
58
what is the defining characteristic of mixed dislocations
the burger's vector is neither perpendicular or parallel to the dislocation line
59
__ develops as a result of dislocations
lattice strain
60
__ does not change for a given defect
burger's vector
61
a dislocation must end on__
on the crystal surface, on itself, or on another dislocation
62
plastic deformation occurs due to __
dislocation glide
63
dislocation allow __ deformation
plastic deformation
64
how does dislocation glide work
a dislocation can glide one atomic spacing at a time until it exists the crystal by forming and breaking one bond at a time
65
dislocation glide occurs on ___ becaue __
planes of highest atomic density becuase less energy is required to break and form each bond
66
plane of highest atomic density is the same as _
slip plane
67
slip only occurs if__
slip only occurs if the crystal's dislocation plane(defined by the burger's vector) coincides with the plane of highest atomic density (slip plane)
68
ductile materials have a higher number of __
slip systems
69
why are ceramics brittle
they have a low number of slip planes
| linear defects are limited due to electoneutrality
70
in polymers, __ do not play a large role in mechanical properties
linear dislocations
71
what is surface tension
atoms at surface are not bonded to the max number of atoms, so they have higher energy
72
surface tension is considered a __
planar defect
73
__ are examples of two dimensional defects
(planar defect)
surface tension
grain boundaries
74
grain boundaries deal with the __, not the surface
the bulk, not the surface
75
how are grain boundaries similar to surface tension
atoms in grains do not have optimal coordination number, so they have higher energy
76
why does corrosion happen at grain boundaries
atom at grain boundaries have higher energy because they are not bound to the man number of atoms
77
tilt boundaries are composed of __
edge dislocations
78
twist boundaries are composed of
screw dislocations
79
examples of three dimensional defects
voids and precipitates
80
what are precipitates
they are clusters of substitutional or interstitial impurities
81
what are voids
they are aggregates of vacancies
82
why can pores be useful
they allow the exchange of fluids and gasses within the material
83
what are 2 examples of porogens
salts --> disslove in water
| wax --> melt away at high temperature
84
what is the chain folded model
polymer chains fold back on themselves at the faces
85
why are polymers not 100% crystalline
the face of the lamella is amorphous
chain folded model
86
what are spherulites
they are 3D aggregates of lamellae
87
why are spherulites more prone to chemical attacks
they are like grain boundaries (metals and ceramics)
88
burger vectors of linear defects are much longer in
polymers due to the size of the unit cell
89
sliding in polymers usually takes place __
along the axis of polymer chains due to the covalent bonds within the chain
90
what is viscous flow
deformation in non-crystalline materials
91
what is melting point
temp above which atomic movement is big enough to break the highly ordered structure
92
there is not distinct __ in amorphous ceramics (glasses)
melting temperature
93
amorphous ceramics are also known as
glasses
94
polymer have a distinct __
melting temperature
95
factors the influence polymer melting temperature
1. branching -> decrease
2. increase MW -> increase
3. bulky side groups -->increase glass transition because they reduce movement around the backbone
96
cold crystallization is a __ process
exothermic
97
why does cold crystallization increase crytallinity
the increased temp allows the polymer chains to move into a more ordered state
98
what influences melting temperature
anything that affects its ability to crystallize
| i.e. branching
99
why does increasing MW increase Tm
per volume, higher MW have fewer chain ends. the chain ends are the first to vibrate since they are free
100
more energy under DSC curve mean_
higher crystallinity
101
Tg is lower if __
there is easier rotation around the backbone
102
differential scanning calorimetry tells us what_
the percent crystallinity
103
power compensated DSC
sample and reference are heated to the same temp and the power needed to maintain that temp is recorded
104
what are the instrumentation for tensile testing
1. grips
2. load cell
3. extensometer - records elongation
4. computer- generates stress stain curve
105
units of strain
dimensionless
106
why is the force in compression testing negative
because it is in the opposite direction of tensile testing
107
besides force, __ is also negative for compressive testing
strain
108
in tensile testing, force is applied __ to the cross-section
perpendicular
109
slope of stress strain curve is __
modulus of elasticity
110
larger slope of stress strain curve mean __
siffer material
111
the linear region of a stress stain curve represents __
elastic deformation
112
why do materials with stronger bonds deform less
they require more energy to deform
113
values of E are greater for __ than __
E is greater for ceramics than metals
114
mechanical properties in polymeric materials are highly __
direction dependent
115
why are mechanical properties highly directional dependant
along the chain axis, there are primary (covalent) bonds
in other directions, there are secondary bonds
116
plastic deformation occurs after the __
elastic region
117
plastic deformation is the point where stress strain curve __
doesn't follow hooke's law
118
yield strength
stress at the end of the elastic region
119
yield point strain
strain at the end of the elastic region
120
after yielding, there is a __ in stress required to continue plastic deformation
increase in stress
121
the max stress is known as
tensile strength
122
after max stress, __ occurs
necking occurs
123
__ is used as a key design parameter, since by the time it has reach its tensile strength, it has become too deformed
yield stress
124
brittle materials have __ ductility
low
125
how is plastic deformation different in ceramics than metals
necking occurs in both metals and polymers, but when necking occurs in polymers, the chains orient with the load
126
just before fracture in polymers, why is there an increase in stress
more strength is needed to overcome the primary bonds
127
how is necking different in polymers and metals
in metals, elongation is confined to the original necked region
in polymers elongation occurs by growth in length of the necked region
128
metals and crystalline ceramics undergo deformation due to
dislocation glide along a slip plane
129
why is plastic deformation favorable
it is thermodynamically favorable to have dislocation glide along defects all the way through the crystal structure
130
what is the critical resolved shear stress
stress needed for slip to occur
131
need __ to be oriented in the proper direction for slip to occur
plane of highest atomic density
132
why is BCC more ductile than HCP
because it has more slip systems
133
why are ceramics brittle
they don't have many slip systems because of electroneutrality
134
why are polycrystalline materials strong
the presence of grain boundaries hinder slip
135
in polymers, increasing __ or decreasing __ causes a reduction in E
increasing temp or decreasing strain rate
136
below Tg, there is no __
rotation around the backbone
137
why are polymers brittle at high strain rates
at high strain rates, the polymers don't have enough time to reorient with the axis of loading
138
in semi-crytalline polymers, spherulites contain __ regions that radiate from the center
lamellar regions
139
what are the stages of semi-crystalline deformation
1. tie chains expane
2. lamella reorient
3. blocks of lamella separate from each other
4. blocks and tie molecules orient along the loading axis
140
on a molecular level, how do you strength polymers
prevent chain sliding
141
strength a polymer also means to increase __
yield strength
142
why does increasing percent crystallinity in polymers strengthen it?
it reduces amorphous regions where most of the chain sliding occurs
143
what are the four ways to strengthen polymers
1. thermal treatment
2. Increasing MW
3. crosslinking polymers
4. adding filler material
144
how does cold crystallization increase polymer strength
heating up the polymer allows the chains to reorient which increases percent crystallinity
145
why does cooling from melt increase polymer strenght
at high temps, spherulites are formed
spherulites are crystalline regions
146
why does cooling a polymer quickly make it weaker
spherulites are formed at higher temperature, so cooling it quickly means less time for spherulites to develop which leads to less percent crystallinity
147
why does increasing MW strength polymers
more entanglement
makes sliding harder
148
why does crosslinking polymers make it stronger
adjacent chains are covalently bonded to each other
this requires more energy for sliding to occur
149
why does adding fillers increase polymer strength
fillers form secondary bonds with the polymer which prevents sliding
150
elastomers have large __ at low __
large strain at low stress
151
why don't elastomers undergo plastic deformation
coiled chains are crosslinked which prevents them from slipping past each other
152
why do elastomers return to their coiled state
it is thermodynamically favorable to be in the more disordered state
153
what is creep
plastic deformation of a sample under constant load over time
154
what is primary creep
increase in strain with time
155
why does creep rate decrease over time
it is due to the repositioning of dislocations over time
156
what is secondary creep
linear relationship between creep strain and time
157
the slope of the secondary creep is __
steady state creep
158
what is tertiary creep
defects appear within the material and leads to failure
159
creep rate increases with __ temperature and stress
increasing temp and stress
160
higher creep rate = __ time to rupture
lower time to rupture
161
what are the molecular causes of creep in metals
grain boundaries sliding or migration of vacancies
162
what is stress induced vacancy diffusion
cause of creep in metals
vacancies move to the face parallel to the stress
163
dislocation climb
cause of creep
dislocation move one atomic spacing
164
why are ceramics resistant to creep
electroneutrality constrictions
165
what is the molecular causes of creep in polymers
creep is due to viscous flow of chains in amorphous regions
166
what influences creep in polymers
crstyallinity and temperature
167
creep __ as crystallinity increases in polymers
decreases
168
why is there no creep in polymers below Tg
there is no rotation of flow of chains
169
what is stress relaxation
decrease in stress over time when under constant strain
170
what is an example of stress relaxation
a rubber band will relax over time when put around a fat stack of bills
171
what are the molecular causes for stress relaxatoin
movement of chains in the amorphous regions of the polymer
172
what is ductile fracture
if a material undergoes plastic deformation before breaking
173
why is ductile fracture the preferred mode of fracture
because there is visible warning in the form of plastic deformation before failure
174
what is brittle fracture
there is no plastic deformation before breaking
175
why do ceramics undergo brittle fracture
because they have less slip
176
what is a stress concentrator
stress can be amplified at the tip of a flaw
177
why is the presence of a stress concentrator more significant in brittle than ductile materials
plastic deformation reduces the localized stress in the area around the flaw
there is not plastic deformation in brittle materials
178
why is more energy need to ductile fracutre
the crack initiation can be stabilized by the ductile material, meaning you need more energy to propagate the fracture
179
why can fatigue occurs at significantly lower loads than predicted by tensile testing
during fatigue testing, repeated stress increases the number of dislocation and creates more imperfections in the structure
180
stages of fatigue failure
1. initiation
2. propagation
3. failure
181
why does strengthen a material make it more brittle
you are reducing plastic deformation
182
what is the molecular cause of strengthening metals and ceramics
reduce the movement of dislocations
183
why do smaller grains increase strength
smaller grains have more grain boundaries
grain boundaries discourage dislocation
184
fatigue failure is always
brittle
185
fatigue life is measured at a specific __ value
stress
186
equation for fatigue life
Nf = Ni + Np
```
Nf = cycles til failure
Ni = cycles to inititation
Np = cycles to propagate
```
187
three things that affect fatigue life (Nf)
1. stress risers
2. biodegradable materials
3. environment
188
cracks will start (initiate) at
stress risers
189
stress riser do what__
amplify the stress locally
190
how does biodegradable materials affect fatigue life
flaws form as degradation occurs
susceptible to failure at end of degradation
191
how does the environment affect fatigue life
salts and proteins can interact with the material
192
a reason why the environment can lead to a shorter fatigue life
corrosion fatigue
193
what is corrosion fatigue
corrosion due to cyclic stress and chemical attack
194
__ is specifically the degradation of metals in the body
corrosion
195
why can materials fail even though the general conditions in the body are not hard
inflammation can lead to the release of acids and oxidizing agents around the implant
196
what causes corrosion on a molecular level
the presence of anodic and cathodic regions
197
corrosion is an electrochemical process that involves the transfer of __
electrons
198
oxidation occurs at the __
anode
199
reduction occurs at the
cathode
200
in corrosion, the material will dissolve at the __
anode
201
on a molecular level, how do you prevent corrosion
prevent transfer of electrons from anode to cathode
202
how can you slow degredation
1. slow the rate of either the oxidation or reduction
| 2. add an insulative layer
203
what does the nernst Equation tell
how active a material is a particular pH
204
what is galvanic corrosion
when two different types of metals are placed in the body, the body acts like a wire connecting the two electrodes
205
the __ material acts as the anode and dissolve
the more active material
206
what does the Pourbaix diagram tell
regions of non-corrosion as a function of cell potential and pH
207
what are the 3 regions of a pourbaix diagram
immunity, passivation, corrosion
208
pourbaix diagram cannot be used to tell __
the rate of a reaction
209
in a pourbaix diagram, anything not in the __ or __ regions is susceptible to corrosion
immunity and passivation region
210
what is meant when a material is in the passivation region of a pourbaix diagram
the material has an insulating layer due to surface oxidation
211
corrosion in the passive layer is also dependent on __
pH
212
that dashed lines on a pourbaix diagram represent _
the stability of water
213
if a material is not within the dashed lines on a pourbaix diagram, why do we not care about it
outside of the dashed lines do not represent body-like conditions
214
what is the potential reason behind crevice corrsion
no oxygen in the crevice sets up an anode region
215
what is pitting corrosion
small defects on the surface of the material acts as anodes
216
what is intergranular corrosion
corrosion due to grain boundaries
grain atoms have more energy and are susceptible to chemical attack
217
how do you prevent intergrandular corrosion
create larger grain sizes
but this will make the material weaker
218
why is a material under stress more susceptible to corrosion
it is in a higher energy state
219
what is stress corrosion cracking
(different than fatigue corrosion)??
corrosion that occurs due to metal under stress and corrosive environments?
220
what is fatigue corrosion
continual loading cna remove the passive layer from a material
221
why can protein attachment to the surface of a material lead to corrosion
proteins can remove the passivating layer
222
what are 2 things than can change the equilibrium on a pourbaix diagram
protein attachment
bacteria
223
what are the two ways polymers degrade
swelling and chain scission
224
what is swelling
the polymer will absorb water and become more ductile by reducing secondary bonds between the chains
225
unlike swelling, chain scission breaks __ bonds
primary
226
what are the two types of chain scission
hydrolysis and oxidation
227
how does chain scission by hydrolysis work
water cleaves primary bonds
228
rate of scission by hydrolysis increases due to __
1. reactivity of the groups
2. lower number of bonds to break
3. water can penetrate faster
229
how to you slow scission by hydrolysis
1. increase crystallinity makes it harder for water to penetrate
2. increase hydrophobicity
230
what is chain scission by oxidation
radicals attack and break covalent bonds
231
because chain scission by oxidation involves radicals, it involves
initiation, propagation, termination
232
chain scission by oxidation is caused by __ after an implant is placed in the body
inflammation
233
how to you speed up the rate of chain scission by oxidation
1. increase the amount of bonds susceptible to chain scission
2. lower MW -> less bonds to break
234
how to you prevent oxidation when you have to use radicals to crosslink a polymer
you heat the polymer to allow radicals to combine and dissipate
235
how can pores increase degredation
1. they acts are stress raisers
| 2. they increase surface area which allows them to be cleaved by environmental factors
236
why do polycrystalline ceramics degrade faster than ceramics made from a single crystal?
they have grain boundaries
237
what do biodegradable mean?
chemical breakdown of material mediated by bioenvironment
238
__ materials degrade by hydrolysis whereas __ materials degrade by __
synthetic materials degrade by hydrolysis
natural materials degrade by enzymes
239
why is hydrolysis a more consistent degredation
enzymes can vary between patients
240
why are natural materials used for localized drug delivery
enzymes can be localized, which means the natural material will only degrade where the enzyme is present
241
bulk degradation vs. surface degradation
bulk degration --> water enters the polymer very fast
surface degradation --> degrades just the surface
242
why don't you want implants to have bulk degradation
bulk degradation causes the implant to collapse and reduce mechanical properties
243
in surface degradation, __ is not lost
mechanical properties
BMED 4751
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