Biomaterials (Week 3) Flashcards
Irreversible deformation is known as
plastic deformation
- Ductility:
- Ductile materials
- Brittle materials
- measure of degree of plastic deformation that has been sustained at fracture.
- • Ductile materials can undergo significant plastic deformation before fracture.
- • Brittle materials can tolerate only very small plastic deformation.
- Ductile materials
- Brittle materials
- Ductile materials can undergo significant plastic deformation before fracture.
- Brittle materials can tolerate only very small plastic deformation.
Yield strength (σy):
the strength required to produce a very slight yet specified amount of plastic deformation.
Bulk mechanical properties derived from stress-strain curves: (8)
- Elastic/plastic deformation
- Poisson’s ratio
- Yield strength
- Ductility
- Ultimate tensile strength
- Hardness
- Resilience
- Toughness
Plasticity
The plasticity of a material is its ability to undergo some degree of _permanent deformation without failure. _
Elasticity
Elasticity of a material is its power of coming back to its original position after deformation when the stress or load is removed. Elasticity is a tensile property of its material.
Ductility:
amount of plastic strain required to break the material.
Yield strength (or hardeness)
- is a measure of resistance to plastic deformation.
- a point where permanent deformation occurs. ( If it is passed, the material will no longer return to its original length.)
- stress at which noticeable plastic strain occurs. Indicates failure when deformation is not permitted. Examples?? Ceramics don’t undergo plastic deformation.
Plastic deformation
It occurs commonly in _________ and _________ and rarely in ___________
irreversible deformation.
It occurs commonly in metals and polymers, and rarely in ceramics
UTS: (Ultimate tensil strength)
- This is the highest value of stress on the stress-strain curve.
- It is the maximum stress** which the material can support **without breaking
- Both hardness and tensile strength are indicators of a metal’s resistance to plastic deformation.
for materials in which permanent deformation is acceptable, failure is deemed to occur when a noticeable peak develops. The onset of necking corresponds to a maximum in the nominal stress versus nominal strain defining the ultimate tensile strength of the material.
Failure:
yield stress exceed.
Hardness:
- Hardness _provides a measure of how successful a material resists plastic deformation. _(a small dent or scratch).
- Both hardness and tensile strength are indicators of a metal’s resistance to plastic deformation.
Resilience:
measure of the elastic energy** that can be **stored in** a unit of volume of **stressed material.
- The area under only the elastic region of the stress-strain curve is a measure of the ability of the material to store elastic energy (the way a compressed spring does).
Toughness
measure of the energy required to deform a unit volume of material to its braking point.
- The entire area under the stress-strain curve is a measure of the energy required to fracture the material
Other properties, beside mechanicals, taken into account during material selection are (3):
thermal properties, optical properties and magnetic properties.
Thermal properties: (example)
becomes a significant consideration if the implanted material contributes to an unnatural flow of heat through the surrounding tissue.
ex: patient feels colder than normal due to heat loss to a metal rod; heat conduction through a metal filling can be a source of discomfort.
Optical properties: (3)
the most significant optical properties are
- color,
- refractive index and
- transparency.
Magnetic properties:
non-magnetic alloys - magnetic resonance (MR) safe – increasing regulations.
**An interface is **
**the boundary region between two adjacent bulk phases **
Biomaterial surface:
region of an interface with properties + characteristics different from the bulk:
Stress (σ)
Strain (εn)
Shear Stress (τ)
τ = *force (F) */ original cross sectional area (A)
Shear Strain (γ)
γ = tan θ
comparing the curves, which of those materials:
- *(a) is stronger?
(b) is more ductile?
(c) will absorb more energy prior to fracture?**
a) Metal because the area under the curve is larger. Stronger = Thoughness
b) Polymers are more ductile they can elongate or neck a lot before braking.
c) Metals absorb more energy prior to fracture making it very though. It has the highest area under the curve.
5 important things about the Physics of Surfaces (5)
- Surfaces have unique reactivities.
- The surface is inevitably different from the bulk
- The mass of material that makes up the surface zone is very small
- Surfaces readily contaminate
- Surface molecules can exhibit considerable mobility
Heterogeneity in physical structure:(5)
- Material dependant: Metals vs. Polymers vs. Ceramics vs. Gels
- Chemistry: Polar vs. Apolar, Charge, Reactivity, Patterned
- Morphology: Smooth, Rough, Stepped, Patterned, Diffuse
- Order: Crystalline, Amorphous, Semi-Crystalline, Phases
- Environment: Hydration, Solvent Quality
An interface is :
the boundary region between two adjacent bulk phases
Events at biomaterials surface includes: (5)
- Hydration
- Adsorption
- Degradation
- Electrical behavior
- Other biochemical reactions
Surface energy and tension in liquids:
Such processes occur to minimize interfacial energy.** This means systems move toward l_owering their free energy. _**
The work (w) required to create a new surface is proportional to
the # molecules at the surface, hence the area (A):
δ*w = * γ . δA
γ is the proportionality constant defined as the specific surface free energy (force/unit length, N/m) or (energy/unit area, mJ/m2).
γ acts as a restoring force to resist any increase in area, for liquids it is numerically equal to the surface tension. (is a measure fo the cohesive forces holding molecules together.)
(γ is the energy that must be supplied to increase the surface area by one unit)
degree of hydrophilicity or hydrophobicity.
dependent on surface contact angle – indicates interaction with surface –
interaction that will minimize energy of the system
Σ F (forces) = 0
- **Hydrophilic: **
- ** Hydrophobic:**
- Hydrophilic: water droplet spreads out on the surface = more wetting.
- **Hydrophobic: water droplet balls up. Oil in Teflon pan for example. **
**. **
γ: is the proportionality constant defined as
the specific surface free energy (force/unit length, N/m) or (energy/unit area, mJ/m2).
acts as a restoring force** to **resist any increase in area, for liquids it is _numerically equal to the surface tension. _
Surface tension acts to
decrease the free energy of the system, hence some observed effects:
- liquid droplets form spheres
- Meniscus effects in capillaries
**The surface tension (γ) must be **
**the opposing force acting along the interface **
Role of water in biomaterials
- Biomaterial will first see water before proteins or cells ever diffuse to the surface.
- The nature of this surface water layer may be the most important event during bio- interactions at interfaces.
- Water has the second highest surface tension Υ=72.8 dyne/cm.
- Water organization is altered close to a surface compared to the bulk.
- **Hydrophilic **surfaces have a significant effect on water organization.
- Hydrophobic surfaces have a low water density zone.
- Hydrophobic effect: decrease in water entropy, hydrophobic substance will minimize its interfacial energy by coalescing into a **shape of minimum surface area for a given volume. **
Electrical nature of a metal surface (4)
- Excess charge on a surface
- Presence of electrochemical reactions (oxidation-reduction —> corrosion)
- Corrosion will lead to surface and bulk degradation resulting in metal ion and particle generation.
- Surface may become charged by
- Adsoprtion of ionic species present in solution or differential adsorption of OH-
- Ionization of -COHH or NH2 groups
Physical, chemical and thermal characterization of biomaterials: (11)
- Microstructural characaterization (optical and electron microscopy)
- Scanning probe microscopy
- X-ray diffraction and scattering methods
- FT-IR spectroscopy
- DLS techniques
- Contact angle measurements
- Mercury intrusion porosimetry
- Gas Adsorption methods
- Differential scanning calorimetry (DSC) o Thermal gravimetric analyzer (TGA)
- Dynamic mechanical analyzer (DMA)
Considerations when characterizing of biomaterials
- Surfaces are different, therefore samples for analysis have to be prepared in such a way they resemble the surface of the implant/application of final use.
- Consider the potential for surface alteration during analysis (charging). Because of potential artifacts consider at least 2 characterization techniques.
- Consider contamination: inorganic materials are more prone to contamination than polymeric materials because of their high surface energy. On the other hand, polymeric materials are more susceptible to charging during analysis and therefore analysis can get distorted.
- Electric conductive metals and carbons will often be easier to characterize than insulators using electron, x-ray, and ion interaction methods.
What to consider when choosing a biomaterial
Fabrication —-> microstructure is the bulk (types of bonds) —-> properties —-> performance
Why porosity on a biomaterial is good?
beause cells migrate to the pores of the surface and reproduce or growth of tissue gives stability to the implant
Most common metallic alloys used:
- Titanium and its alloys
- stainless steels
- cobalt-chromium alloys
- Others (Nitinol, Tantalum, magnesium)
- Why the femoral part of a knee replacement has to be very polish and strong?
- What material is used for the femoral part, for the tibial part and for the spacer?
- why cannot we use 2 soft materials rubbing together?
- It has to be very polish to be able to roll and glide against the polyethylene. Strong enough to take weightbearing loads, flexible enough to bear stress without breaking, and able to move smoothly against each other as required.
- femoral part: cobalt-chrome alloy, tibial part: titanium, spacer: polyethylene.
- Becuase of the interlocking due to porosity.
The oxide layer formed on biomaterial surfaces protects the metal against 1) _______ assist with 2) ______________________ and protects the surface against accelerated 3) ____________ ________ _____________
- corrosion
- osseointegration
- metal ion dissolution
**in bone- contacting interfaces the metal selected to augment a bone should ideally have elastic modulus equivalent to that of the tissue so to avoid **
Stress-shielding
CoCr are difficult to machine so what methods are used? (2)
- Investing casting
- powder metallurgy
Titanium alloys are difficult to cast so they are:
**Machined **into the final implant geometry.
What happens to the proteins at the surface of materials
Surfaces can be adsorbed or denatured at the surface
Segregation of species to the surface is due to
Surface forces
Deforming forces that a material can be subjected to?
- Tensil
- Compressive
- Torsional
- Combination of forces
**4 Factors that play a role in changing the response of a material **
- Kind of force applied
- the rate of application of force
- the temperature of the material
- **the surrounding environment **
What is the difference between Nominal and True stresses?
- Nominal stress ussumes a constant area when a tensile stress or shear stress is applied.
- True stress takes into account the change in transverse area when a material is subjected to tensile stress or shear stress.
**Surface energy: **
**is the force that arises as pressure develops. **
2 special properties of water
- The dipole combined with the bent shape of the molecule and
- its small size
Benefits of biomaterials research can only be appreciated when these materials are characterized at both:
** the material level and device level following regulatory guidelines **
** Different regions of implants present particular surface features**
** (rough/ polished) to maximize mechanical stability and minimize wear **
**Surface forces will lead to the movement of **
**charges to the surface – minimize interfacial energy **
Most metallic biomaterials have form on their surface :
** a stable and passive surface oxide layer that enhances their corrosion resistance properties. **
**What is the key to the biocompatibility of metals in-vivo **
**The presence of a stable surface oxide layer is the key to the biocompatibility of metals in-vivo **
Biocompatibility
An ability to minimize adverse host reactions.
Oxide layer will protect the metal surface against what?
** accelerated metal ion dissolution in presence of physiological fluids **
The oxide layer protects the metal surface against accelerated metal ion dissolution and assist with?
osseointegration
- Metal wear in-vivo can trigger or lead to 2 things.
- This is concerning with what type of material?
- ** inflammatory reaction due to the toxicity associated with some metal ions.**
- And it can lead to osteolysis due to bone cells resorption.
- **This is particularly concerning with CoCr alloys **
**CoCr alloys are extremely difficult to machine into the complicated shapes of some implants (e.g. knee implant) by conventional machining methods. Therefore, many Co-based alloys are shaped into implants by **
- investment casting or by
- **powder metallurgy. **
Titanium alloys are very difficult to cast so they are typically:
** machined into the final implant geometry. **
