Things to remember Flashcards by Max Harding

What is the element Co?

What is the element Mo

Cobalt - alloys used where high wear resistance is needed

Molybdenum

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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.

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What is bad about titanium?

Bad wear resistance

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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

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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.

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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

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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

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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

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Polypropylene

Excellent chemical resistance,
weak permeability to water
vapors, good transparency &
surface reflection

Yarn for sutures, surgery

Polypropylene (Prolene ®) sutures are widely used clinically

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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

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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

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Polyacetals

Stiffness, fatigue endurance,
resistance to creep, excellent
resistance to humidity, gas & solvent
action

Hard tissue replacement

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PMMA - poly (methyl methacrylate)

Optical properties, exceptional
transparency, easy thermoformation
& welding

Bone cement, intraocular lenses, contact
lenses, fixation of articular prostheses,
dentures

maxillofacial prostheses

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Polycarbonate

Rigidity & toughness up to 140 degrees,
transparency, good electrical
insulator, physiological inertness

Syringes, arterial tubules, hard tissue
replacement

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Polyethyleneterephtalate (PET)

Transparency, good resistance
to traction & tearing, resistance
to oils, fats, organic solvent

Vascular, laryngeal, esophageal
prostheses, surgical sutures, knitted
vascular prostheses

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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 = Ef*Vf + Em*(1− Vf)