Course F - Biomaterials Flashcards
define biomaterials
“A biomaterial is any material, natural or man-made that comprises whole or part of a living structure, or biomedical device which performs, augments or replaces a natural function”
do a brief comparison between mad-made an biomaterials
Man-made:
- often large energy inputs needed
- can be stronger
- regularly metallic
- wide range of materials
Biomaterials:
- synthesized at RTP from CO2, H2O, O2
- sustainable, recyclable, biodegradable
- generally made from C, H, O, N with some others
- usually organic polymers/minerals
what is the elastic behaviour of a standard linear elastic solid
- springs instantaneously back into shape
- described by σ = Eε and τ = Gγ
what is the ‘elastic behaviour’ of liquid flow
- does not recover original shape
- response is time dependent
- described by τ = η dγ/dt
η = viscosity
- this is the response of a “Newtonian liquid”
what is the continuous spectrum of the two extremes of elastic behaviours (liquid flow and linear elastic solid) called, what properties do most materials have
- the spectrum is called viscoelasticity
- most materials have a mixture of the two properties/ lie somewhere on the spectrum
what two ‘components’ can we use to help model the elastic behaviour of (bio)materials, what are their responses
1) springs
- instantaneous response
- stress and strain in phase
σ = kε
2) dashpots
- leaky cylinder
- delayed response
- stress and strain in antiphase
σ = η dε/dt
what is the Maxwell model for elastic behaviour (using the two components)
we add a dashpot in series with a spring
this gives
dashpot response:
dε/dt = σ/η
spring response:
ε = σ/k
the components are in series so the stresses are equal, we add the strains giving:
ε =εd + εs
dε/dt = σ/η + (1/k)(dσ/dt)
this can be rearranged to a differential equation and solved with oscillating stress and strain terms of
σ = σo exp(iωt) and ε = εo exp(iωt - φ)
E = kω^2 (Tr)^2 / (1+ω^2 (Tr)^2))
Tr = response time
tanφ = 1/ωTr
what are the three main cases that we can consider for the Maxwell model (regarding frequency)
1) High freq. ωTr»1
- no time for viscous flow, we only see elastic response of spring, E–>k, φ–>0
2) Low Freq. ωTr«1
- plenty of time for viscous flow, elastic ext. of spring negligible compared to displacement of dashpot
E—> kω^2(Tr)^2 —> small
φ–>π/2
3) intermediate freq. ωTr approx. = 1
- movement of spring and dashpot are similar in magnitude
- Energy dissipated but strain is recovered
what are two alternative models to the maxwell model
1) Voigt Model:
- spring and dashpot in parallel
- better at describing response at const. stress
2) Standard Linear solid model
- spring in parallel with a series arrangement of a spring and dashpot
- Better than (1) at high freq.
in every materials selection/merit index question, what are the two types of variable
- Fixed Variables, these are constants for the purposes of the question
- Free variables, these can be varied accordingly
what is the general method for finding a merit index to select materials
1) construct two eq’s for the properties of interest (e.g. density, ρ and failure strength, σf) using fixed and free variables
2) rearrange to eliminate the free variables
3) rearrange for a ratio of the variables of interest e.g. σf/ρ, this is the merit index
4) work out whether the merit index should be maximised or minimised for the best material properties for the purpose of the question, state which
5) it is often useful to take logs (as the materials maps are log-log graphs) to determine if the y-intercept should be greater or lower for best properties
what are the two key properties for strength, what do we want them to be
1) failure strength, σf
2) density, ρ
Ideally we want max σf and min ρ
what are the two key properties for stiffness, what do we want them to be
1) Young Modulus, E
2) Density, ρ
we want max E, min ρ
what are the 3 ways to select for toughness
1) Impact resistance
2) stress-limited failure
3) strain-limited failure
how would you select for toughness by maximising impact resistance
simply maximise Gc
- go to Gc against E map
- draw horizontal line
- maximise y-int
how would you select for toughness by maximising stress-limited failure
maximise the static tensile load that can be supported without crack propagation
we know
σf = sqrt(EGc / πc)
so maximise
sqrt(EGc)
- on a graph of Gc against E, this means grad of -1 and maximise y-int
how would you select for toughness by maximising strain-limited failure
Maximise “Stretchiness” = amount of tensile strain without crack propagation
we know
εf = σf/E = sqrt(Gc / πcE)
so we maximise
sqrt(Gc/E)
- for a Gc against E graph this means draw a line of grad 1 and maximise the y-int
what is natural rubber
- a polymer of the monomer isoprene
- extracted from indigenous amazon trees
what are the 2 conditions for a polymer to have large recoverable strains
1) enough energy for the chain backbone shape to change
- depends on Ea for bond rotation and the thermal energy present
2) Chains must uncoil but not slide past each other as this would lead to plastic deformation
What are the elastic properties of natural rubber, how can they be improved
- Normally natural rubber has poor elastic qualities, it is brittle when cold and deforms easily when hot
- this can be improved by Vulcanization = adding sulphur to make cross-links and prevent plastic deformation
what is the driving force for a polymer chain to recoil i.e. undergo elastic not plastic mechanism
- tensile forces = extension = polymer chain extends, lower entropy
- elastic energy is stored due to the change in ENTROPY not enthalpy
- there is a driving force for the chain to recoil back to a higher ENTROPY form
how are proteins formed (brief explanation)
- condensation polymerisation of amino acids, forms peptide bonds
give the two examples of protein rubbers, briefly explain their structure and properties
- resilin and elastin
- both are randomly coiled and contain cross-links
Resilin:
- ideal density of cross-links, very good elastic properties (better than any mad made)
- found in insect wings
Elastin:
- similar to resilin
- found in neck ligaments
what are the two main folded protein structures, give examples of where each are found
1) α-helix, found in Keratin
2) β-sheet, found in spider silk
describe the properties/structure of Keratin and some places it is found
- when dry it’s in α-helix form, when wet the secondary bonds are easily broken and it can be stretched to form β-sheets
- Keratin is tough
- Keratin forms hair, nails, hoof etc., in this form it is heavily sulphur cross-linked α-helix
- fairly strong/stiff
what are the two types of spider silk, what are their compositions, what are their properties
Dragline silk:
- stiff, E = 10 GPa
- 20-25% β-sheet miscelles
- σf = 1400 MPa = V high
Viscid silk:
- extensible
- E = 0.1GPa
- V little β-sheet
- very low coefficient of restitution, this means it absorbs energy efficiently and dissipates it, it has a slow response
describe what Collagen is and explain its hierarchical structure
Collagen is a protein fibre ONLY found in animals
1) polypeptide chains form left-handed coils (different to right handed α-helix)
2) 3 of these chains coil in a right-handed sense to form a collagen “molecule”
3) several collagen “molecules” arrange themselves into ‘fibrils’ with covalent cross-linking bonds
4) these fibrils are then densely packed
- this results in a stiff/strong material
explain the structure/materials of skin and summarise its properties
- skin is a composite mostly formed of Collagen ‘fibres’ in an elastin ‘matrix’
- collagen fibres are short and aligned in 3D, not uniaxially but in preferred directions
- anisotropic properties, good stretchiness
