lectures 1-5

What was the first advancement in biomaterials?

The spare parts man

However, materials were very hydrophobic and rigid

Name some examples of biomedical applications for polymers in bones and joints

Name some examples of biomedical applications for polymers in vascular prosthesis

Name some examples of biomedical applications for polymers in soft tissue

Name some examples of biomedical applications for polymers in ducts and canals

Name some examples of biomedical applications for polymers in tendons and ligaments

Sketch and label a hip joint prosthesis

Why is there a wire wrapped around the plastic cup of a hip joint prosthesis

so it can be located on an x-ray

What materials are use to make hip joint prothesis and why?

Polyethylene is the main polymer used for the acetabular cup.

Ultra high molecular weight polyethylene is used because it has an excellent combination of water resistance and low friction

Why isnt LDPE used?

It is highly branched and very difficult to pack together

How is methyl methacrylate polymerised using redox reactions?

With the use of fentons reagent

Fe²⁺ catalyses the breaking of the O-O bond of hydrogen peroxide

Fe²⁺ + H₂O₂ —> Fe³⁺ + OH^. + OH^-

What is fentons reagent used for

It is used to produce PMMA cement in situ

A mixture of PMMA and methyl methacrylate monomer makes a paste

It is divided into 2 parts with one containing DMPT and another containing benzoyl peroxide

When these are mixed together, they generate radicals and the monomer polymerises

What are the major ways in which a hip joint prosthesis fails?

Femoral stem shifts and loosens: This is vey painful. it also introduces a site for bacterial to grow and cause infections. infections in this region of the body are very difficult to treat

PMMA cement comes out of alignment: PMMA cement becomes polished due to abrasion against bone and results in a release of cement debris

Wear of polyethylene cup

What else can polyethylene be used for?

It can be used in dentistry for fake teeth

Draw the different structured of polyethylene

What were dumbetons studies

He examined 40+ polymers to see if any of them wear better than polyethylene

He applied a load ontop of a polymer and slid it across a smooth stainless steel substrate

He did this under wet and dry conditions

What were the outcomes of dumbletoms studies

Under dry conditions, many polymers were superior to polyethylene

Under wet conditions, the wear resistance of polyethylene was superior to others

this is because polyethylene does not have many polar groups for water to bond to

What did duncan dowson do?

He developed a hip joint simulator

What equation enabled lab wear to be compared with clinical tests

V= kWx

V= volume of material removed by wear (mm³)

k= wear factor(mm³/Nm)

W= applied load (N)

x= sliding distance (m)

Why did the explanted acetabular cups show more wear than lab experiments predicted?

because of the effect of surface roughness on wear

Sketch a graph of wear factor vs counterface roughness for wet and dry conditions

What are the advantages of using ceramic components in hip joint prosthesis

What are hydrogels?

A unique family of polymers that contain water as a part of its structure.

They are lightly crosslinked, water-swollen polymer networks of either natural or synthetic origin

What is the old definition of biocompatibility

the device should not adversely affect the environment in which it is placed, and should not itself be adversely affected by that environment

What is the new definition of biocompatibility

the device shall perform in the environment in which it is placed, with an appropriate host response.

This definition allows for biointegration and biodegradability

What are the desired properties of a biomaterial

Exposed hydrophilic groups

low interfacial tension

reversible and weakly bound

this forms a native protein layer (mainly adsorbed alubumin)

What are the undesired properties of a biomaterial

Exposed hydrophobic groups

High interfacial tension

irriversible and strongly bound

this forms a denatured protien layer(mainly adsorbed fibrinogen)

What are the response steps when a biomaterial interacts with blood

Describe how fibrinogen is used for clotting

The flowchart is a schematic picture of thrombus formation

Fibrinogen is converted to a fibrin monomer in the presence of thrombin.

Then a fibrin dimer

Then a fibrin polymer for clotting

How are platelets involved in fibrin formation?

When there is a vascular lesion on smooth muscle cells, there is an adhesion of platelets

platelets begin to aggregate and form fibrin

How can we make blood compatible materials

Choose or design polymers with minimum platelet aggregation
Attach thrombin inhibitors to surface
Choose or design polymers with minimal protein (esp fibrinogen) adsorption

How can we minimise fibrinogen deposition when making biomaterials

we need biomaterials with low interfacial tension in biological fluids

Draw and label a droplet of liquid on a solid surface

How do you calculate interfacial tension?

3 key points to remember about interfacial tension

Surface Tensions γ are the sum of polar and dispersive components

Lowest values of Interfacial Tension arise when polar and dispersive
components of the two phases match (i.e. are equal)

If polar component of one phase is zero the whole polar product term
2 (γ1p γ2p) ½ becomes zero leading to a high interfacial tension

Who invented hydrogels?

Otto wichterle

What is the most important hydrogel to remember?

2-hydroxyethyl methacrylate (HEMA)

PolyHEMA hydrogel has an equilibrium water content (EWC) of 40% AND IS STABLE

What is the equilibrium water content (EWC)

The amount of water in hydrogels

Why is the equilibrium water content (EWC): such an important property ?

WATER IN HYDROGELS Acts as:
- A transport medium for dissolved species
- A surface energy 'bridge' between the polymer and the body
- A plasticiser-giving the materials flexibility
- A lubricant-reducing the coefficient of friction at the surface

How can we control the water content of hydrogels ?

We see a progressive increase in water content as we move from methyl methacrylate through 2-hydroxyethyl methacrylate to N-vinyl pyrrolidone

sketch a graph to show the water content of HEMA-MMA, and HEMA-NVP

does the water have the desired effect on the polar component of surface energy ? Sketch a graph to show this

What did wichterle say ocular biomaterials should have?

• match the mechanical properties of the eye,
• allow diffusion of metabolites and oxygen
• be wettable by tears like natural tissue
• have compatibility with the host structure

What is biomimesis

1) examine the natural system

2) attempt to build a synthetic analogue

What soft tissues do we have that need to be kept hydrated?

eyes, mouth, lungs, joints

To what extent to hydrogels resemble natures molecules?

How is oxygen permeability affected as a function of water content?

as EWC increases, oxygen permeability increases. It also increases with temperature

What are the mechanical properties of hydrogels in tension and compression with an EWC of 40%

They have elastic behaviour and rapid recovery

What are the mechanical properties of hydrogels in tension and compression with an EWC of 20%

They have plastic behaviour and a slow recovery

How do hydrogels freeze

What are interpenetrating networks?

usually consist of a linear reinforcing polymer and an entwined cross-linked hydrogel copolymer.

A similar principle governs the function of collagen which reinforces the hydrophilic matrix of natural hydrogels

This technology allows polymers to be synthesised which have
water contents similar to conventional hydrogels but which are
mechanically tougher and stronger

Why are N-vinyl pyrrolidone (NVP), N,N-Dimethyl acrylamide (N,N-DMA), and N-acryloyl morpholine good nitrogen containing monomers?

monomers have a double bond

in the presence of radicals they form an end chain

conjugation increases the rate of polymerisation and can give alternative sequences

What other type of polymers are good for hydrogels

Polyurethanes are an important nitrogen containing monomer

Name a monomer that can be added for additional solvent power

THFMA. It is a hydrophobic monomer used for additional solvent power

What is the effect of ratio of hydrophilic monomer to hydrophobic (polyurethane plus methacrylate) components on EWC

Effect of ratio of hydrophilic monomer to hydrophobic (polyurethane plus methacrylate) components on Elastic Modulus

Effect of Hydrophilic Monomer on EWC and Freezing Water Content

Compare oxygen permeabilities of conventional hydrogel copolymers and silicone hydrogel copolymers

How are macroporous hydrogels made?

They can be made by polymerising around ice crystals

Describe the structure of the eye

THE CORNEA IS AVASCULAR & TRANSPARENT - ca 80% WATER

The Front Surface Of The Cornea Is Covered By The Tear Film (composed of proteins, lipids, mucins and electrolytes)

The aqueous flow arises from the lachrymal gland

Several glands are located in and around the eyelids, which give rise predominantly to lipids (meibomium glands) and mucinc (goblet cells)

Describe the structure of the tear film

What happens when you open and close your eye

WHEN YOU CLOSE YOUR EYELID IT COMPRESSES THE OILY LIPID
LAYER (such as fatty acid, triglyceride, or cholesterol)
WHEN YOU OPEN YOUR EYE THE LIPID RACES ACROSS THE SURFACE
TO STOP THE TEARS EVAPORATING

Four Major Types Of Ophthalmic Biomaterial

Intra-ocular lens

total artificial cornea

contact lens

synthetic tears

create a good design for contact lenses