Things to remember Flashcards
What is the element Co?
What is the element Mo
Cobalt - alloys used where high wear resistance is needed
Molybdenum
What is the effect of cooling on casts on grain boundaries
Cooled quickly – small grains, thin boundaries.
smaller grains are better.
Cooled slowly – large grains, thicker boundaries.
What is bad about titanium?
Bad wear resistance
What are the effects of casting?
Casting has the following effects:
Large grain size
Sensitisation at grain boundary
Reduced yield and fatigue strength
These can be alleviated by:
Annealing (heat treatment)
Hot/cold work (forging)
Or prevented by:
Alternative methods such as hot isostatic pressing
What are the advantages and disadvantages of electrochemical machining?
Advantages
Produces stress free, burr free surfaces with no burning
or thermal damage to work piece surfaces. Better
corrosion resistance than with mechanical finishing. SS
surfaces R 0.1 to 0.4 mm
Disadvantages
Low machining accuracy, problems with clear cuts and
sharp corners.
What are the most suitable methods for primary fabrication?
What are the most suitable methods for finishing?
What are the most suitable methods for surface finishing?
Focusing primarily on orthopaedic devices:
Most suitable method for primary fabrication?
Casting
Forging
Finishing Drilling Milling Turning Grinding
Surface finish
Polishing
Shot-peening
Why use polymers? compared to materials like metals
• Easier to produce • Biocompatibility • Often cheaper • Designed to mimic • Replacement to old practices • Designed to prevent additional surgery/trauma to patient
Polyethylene
Low cost, easy to process, excellent electrical insulator, excellent chemical resistance, tough & flexible even at low temperature
Tubes for various
catheters, hip joint, knee
joint prostheses
Ultra High Molecular Weight Poly (Ethylene) is used to fabricate acetabular cups in
artificial hips, bearing surface of some knee prostheses, blood contacting tube
Polypropylene
Excellent chemical resistance,
weak permeability to water
vapors, good transparency &
surface reflection
Yarn for sutures, surgery
Polypropylene (Prolene ®) sutures are widely used clinically
Polytetrafluoroethylene (PTFE)
Chemical inertness, exceptional
weathering & heat resistance,
non-adhesive, very low
coefficient of friction
Vascular & auditory
prostheses, catheters,
tubes
For a heart valve, it serves as a sewing ring / receptor for sutures
Other application- shunts to carry cerebral spinal fluid from hydrocephalic
patient
• Middle ear drain tubes, sutures
polyvinyl carbonate (PVC)
Excellent resistance to abrasion,
good dimensional stability, high
chemical resistance
Flexible or semiflexible medical tubes,
catheter, inner tubes, components of
dialysis installation & temporary blood
storage device
Polyacetals
Stiffness, fatigue endurance,
resistance to creep, excellent
resistance to humidity, gas & solvent
action
Hard tissue replacement
PMMA - poly (methyl methacrylate)
Optical properties, exceptional
transparency, easy thermoformation
& welding
Bone cement, intraocular lenses, contact
lenses, fixation of articular prostheses,
dentures
maxillofacial prostheses
Polycarbonate
Rigidity & toughness up to 140 degrees,
transparency, good electrical
insulator, physiological inertness
Syringes, arterial tubules, hard tissue
replacement
Polyethyleneterephtalate (PET)
Transparency, good resistance
to traction & tearing, resistance
to oils, fats, organic solvent
Vascular, laryngeal, esophageal
prostheses, surgical sutures, knitted
vascular prostheses
Polyamide
Very good mechanical properties, good thermal properties, good chemical resistance, permeable to gases
Tubes for intracardiac catheters,
surgical sutures, dialysis devices
components, heart mitral valves,
sutures
What natural polymers are there?
▪ Natural polymers ▪ Fibrin ▪ Collagen ▪ Chitosan ▪ Gelatin ▪ Hyaluronan
Bioresorbable Polymers
Poly(-caprolactone) (PCL) Polylactic acid (PLA) PGA (poly glycolic acid) PLGA (polylactic-co-glycolic acid) Lactide/glycolide copolymers Poly-L-lactic acid (PLLA)
Poly(-caprolactone) (PCL): •Biodegradable polymer •Semi-crystalline •Modulus ≈ 0.5 GPa, Strength ≈ 16 MPa •Low melting point (60 °C)
Polylactic acid (PLA): •Biodegradable polymer •Two forms: semi-crystalline P-L-LA •amorphous P-DL-LA •Modulus ≈ 1.8 GPa, Strength ≈ 50 MPa
- PGA (poly glycolic acid) - relatively very fast resorbing polymer
- PLGA (polylactic-co-glycolic acid) is one of the most widely investigated biodegradable polymers for drug delivery
- Lactide/glycolide copolymers - have been subjected to extensive animal and human trials without any significant harmful side effects
- PLLA is also an excellent biomaterial and safe for in vivo studies and use (lactic acid contains an asymmetric αcarbon atom with three different isomers as D-, L- and DL-lactic acid)
What are the types of polymer synthesis?
Chain reaction - this type of polymerisation (also known as addition reaction) is a three-step process involving two chemical entities (monomer and catalyst)
Step-reaction -
or condensation polymerisation, E.g. polymerisation reaction involves: terephthalic acid and ethylene glycol (both of which are bifunctional)
Polymer Chemical Structures
- Linear polymers are made up of one long continuous chain
- Branched polymers have a chain structure that consists of one main chain with smaller molecular chains branching from it.
- Cross-linking in polymers occurs when primary valence bonds are formed between separate polymer chain molecules.
Polymer Physical Structure
Crystalline or amorphous
Degradation Mechanisms
Surface erosion (poly(ortho)esters and polyanhydrides) ▪ Sample is eroded from the surface ▪ Mass loss is faster than the ingress of water into the bulk
Bulk degradation (PLA,PGA,PLGA, PCL)
▪ Degradation takes place throughout the whole of the sample
▪ Ingress of water is faster than the rate of degradation
What are the Advantages of bioresorbable polymers?
- Eliminated from the body and replaced by host tissue
- No need for second surgical procedure to remove implant
- Avoid complications of metal implants – stress shielding, corrosion, release of metal ions
- Allow transfer of loads to healing bone
- Revision surgery not complicated by presence of implant
- Compatible with MRI imaging
- Can be used to deliver bioactive agents etc
Bioresorbable polymers
- Poly--hydroxyacids (PLA, PGA)
- Poly(hydroxybutyrate) (PHB)
- Poly(hydroxyalkonates) (PHA)
- Polyorthoesters
- Polyanhydrides
- Polyphosphazenes
- Poly(pseudo-amino acids)
- Poly(ester-anhydrides)
- Polyoxalates
- Polyurethanes
- Polysaccharides
Degradation Pathway for PLA and PGA
Polyglycolic acid -> Glycloic acid -> glyoxylate -> glycine -> serine -> pyruvate -> acetyl-coa -> H2O and CO2
Polylatic acid -> lactic acid -> pyruvate -> acetyl-coa -> h2O and CO2
Factors affecting degradation rate
Material Properties - Effect on degradation rate
Water uptake (hydrophilicity) ↑
Glycolide/lactide ratio ↑
Crystallinity ↓
Molecular weight ↓
Residual monomer ↑
Additives/fillers ↑↓
Environmental Factors - Effect on degradation rate
Temperature ↑
pH ↑↓
Implantation site – fluid flow,
cellular activity, enzymes ↑↓
Degradation of PLA/PGA Polymers
• Current products based on poly(lactic
acid), poly(glycolic acid) & co-polymers
• Degrade from inside out (autocatalysis)
• Release acidic degradation products – in
an “acid burst”
• PLLA-based materials slow to be resorbed
(3-5 years)
Opportunities for improving bioresorbables
Biocompatibility and Degradation Profile
• Acidic breakdown products
• Not consistently replaced by bone
• Non-optimum degradation profile
Strength/Stiffness
• Polymers not as strong/stiff as the
metals we are replacing
Added functionality
• Delivery of drugs/actives
• Shape memory
Biocompatibility and Long-Term Response
Foreign body reactions
• Osteolysis
• Extra-articular soft tissue reaction
• Intra-articular synovial reaction
PGA thought to be worse than PLA • degrades more quickly, higher acidity/toxicity of products • But: PGA response seen within 8-16 weeks, PLA degrades in 1.5 – 3/5 years so response may go unreported
Reported incidences vary
• depends on implant, anatomical location, etc
• even PGA can be very low
High strength-solid phase deformation
▪ Development of process to produce high
performance fibres of PLA, PGA and copolymers
▪ Polymer is extruded into unoriented fibre
▪ High strength fibre is produced by drawing
extruded fibre using custom drawing frame
Future Developments
Bocompatibility and degradation rate
- Erodible polymers (no bursting effect)
- Less acidic breakdown products
- Materials having ‘responsive’ degradation - degraded by enzymes/cells during healing
High strength materials
- Advanced fibre composite materials
- Degradable Liquid Crystal Polymers
- Nanocomposites
- Resorbable metals
Added functionality
- Release of drugs/actives
- Shape memory devices
HAPEX
HA (hyprdroxyapatite) and polyethylene composite
HAPEX
• HAPEX is an artificial bone analogue composite
made from HA (hydroxyapatite) and polyethylene
• HAPEX is used for orthopaedic implants like
tympanic (middle ear) bones
Some examples of Biomedical Composites (non-resorbable)
Dental
• Bis-GMA / inorganicparticles
• PMMA/KF
Bone replacement / substitute
• PE/HA particles (HAPEX)
Tendons and ligaments
• Hydrogels/PET
• Polyolefins/UHMWP
• E fibres
Prosthetic limbs
• Epoxy/CF, GF, KF
Bone filling, regeneration • Poly(propylene fumarate)/TCP • PEG-PBT/HA • PLGA/HA fibres • P(DLLACL)/HA • Starch/HA
Matrix Systems
• Polymeric Matrices (bioresorbable and non-bioresorbable) • Mostly thermoplastics • Polysulfone • Poly-ether-ether-ketone (PEEK) • Polyethylenes (UHMWPE and HDPE) • Poly-tetra-fluoro-ethylene (PTFE) • Poly(methylmethacrylate)(PMMA) • Polylactic acid (PLA) • Poly(lactic-co-glycolic acid) (PLGA) • Polycaprolactone (PCL) • Hydrogels
UHMWPE
Ultra high molecular weight polyethylene
Rule of Mixtures
E1 = EfVf + Em(1− Vf)
