Things to remember

Question 1

What is the element Co?

What is the element Mo

Answer 1

Cobalt - alloys used where high wear resistance is needed

Molybdenum

Question 2

What is the effect of cooling on casts on grain boundaries

Answer 2

Cooled quickly – small grains, thin boundaries.
smaller grains are better.

Cooled slowly – large grains, thicker boundaries.

Question 3

What is bad about titanium?

Answer 3

Bad wear resistance

Question 4

What are the effects of casting?

Answer 4

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

Question 5

What are the advantages and disadvantages of electrochemical machining?

Answer 5

 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.

Question 6

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?

Answer 6

Focusing primarily on orthopaedic devices:
Most suitable method for primary fabrication?
 Casting
 Forging

Finishing
 Drilling
 Milling
 Turning
 Grinding

Surface finish
 Polishing
 Shot-peening

Question 7

Why use polymers? compared to materials like metals

Answer 7

• Easier to produce
• Biocompatibility
• Often cheaper
• Designed to mimic
• Replacement to old practices
• Designed to prevent additional
surgery/trauma to patient

Question 8

Polyethylene

Answer 8

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

Question 9

Polypropylene

Answer 9

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

Yarn for sutures, surgery

Polypropylene (Prolene ®) sutures are widely used clinically

Question 10

Polytetrafluoroethylene (PTFE)

Answer 10

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

Question 11

polyvinyl carbonate (PVC)

Answer 11

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

Question 12

Polyacetals

Answer 12

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

Hard tissue replacement

Question 13

PMMA - poly (methyl methacrylate)

Answer 13

Optical properties, exceptional
transparency, easy thermoformation
& welding

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

maxillofacial prostheses

Question 14

Polycarbonate

Answer 14

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

Syringes, arterial tubules, hard tissue
replacement

Question 15

Polyethyleneterephtalate (PET)

Answer 15

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

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

Question 16

Polyamide

Answer 16

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

Question 17

What natural polymers are there?

Answer 17

▪ Natural polymers
▪ Fibrin
▪ Collagen
▪ Chitosan
▪ Gelatin
▪ Hyaluronan

Question 18

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)

Answer 18

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)

Question 19

What are the types of polymer synthesis?

Answer 19

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)

Question 20

Polymer Chemical Structures

Answer 20

• 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.

Question 21

Polymer Physical Structure

Answer 21

Crystalline or amorphous

Question 22

Degradation Mechanisms

Answer 22

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

Question 23

What are the Advantages of bioresorbable polymers?

Answer 23

• 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

Question 24

Bioresorbable polymers

Answer 24

• Poly--hydroxyacids (PLA, PGA)
• Poly(hydroxybutyrate) (PHB)
• Poly(hydroxyalkonates) (PHA)
• Polyorthoesters
• Polyanhydrides
• Polyphosphazenes
• Poly(pseudo-amino acids)
• Poly(ester-anhydrides)
• Polyoxalates
• Polyurethanes
• Polysaccharides

Question 25

Degradation Pathway for PLA and PGA

Answer 25

Polyglycolic acid -> Glycloic acid -> glyoxylate -> glycine -> serine -> pyruvate -> acetyl-coa -> H2O and CO2

Polylatic acid -> lactic acid -> pyruvate -> acetyl-coa -> h2O and CO2

Question 26

Factors affecting degradation rate

Answer 26

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 ↑↓

Question 27

Degradation of PLA/PGA Polymers

Answer 27

• 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)

Question 28

Opportunities for improving bioresorbables

Answer 28

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

Question 29

Biocompatibility and Long-Term Response

Answer 29

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

Question 30

High strength-solid phase deformation

Answer 30

▪ 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

Question 31

Future Developments

Answer 31

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

Question 32

HAPEX

HA (hyprdroxyapatite) and polyethylene composite

Answer 32

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

Question 33

Some examples of Biomedical Composites (non-resorbable)

Answer 33

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

Question 34

Matrix Systems

Answer 34

• 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

Question 35

UHMWPE

Answer 35

Ultra high molecular weight polyethylene

Question 36

Rule of Mixtures

Answer 36

E1 = EfVf + Em(1− Vf)