Bioinspired Materials

Question 1

List some unique properties of biomaterials

Answer 1

• Self cleaning
• Self healing
• Super Adhesive
• Fracture Resistant
• Stimuli responsive

Question 2

What makes these properties specific to biomaterials so special despite us being able to replicate them with enhancement?

Answer 2

Natural biomaterials are made from abundant and inexpensive chemicals at room temperature and under low pressures.
Natural biology is incredibly complex and can get around these barriers that we cannot

Question 3

On an ashby plot showing strength vs toughness what material is top right? and what about biomaterials?

Answer 3

Metallic glass composites
biomaterials are significantly lower in both axis

Question 4

Explain how Mollusc shells have adapted to be highly fracture resistant

Answer 4

A mollusc shell consists of 2 components, calcium carbonate ceramic platelets and polymer glues.
The microstructure is highly ordered consisting of ceramic platelets stacked in a brick and mortar formation, the mortar being polymers
This pattern dissipates stress and causes cracks to deflect out rather than propagate through the shell.
Additionally the polymers are flexible and allow for slight movement to absorb the stresses

Question 5

Give an example of a man made material based off this mollusc shell biomaterial

Answer 5

• Alumina and PMMA composite which is highly fracture resistant

Question 6

Along side the brick and mortar formation, there are other ways of increasing fracture resistance. List 3 of these

Answer 6

1) Waviness and dovetailing in sheet layers
2) Asperity or anchoring using specific polymer structures between ceramic layers
3) Twisting into a helical structure > the added hydrogen bonds improve strength

Question 7

What is unique about Sea cucumber skin?

Answer 7

It is soft, however when mechanical pressure is applied it becomes hard

Question 8

Explain how sea cucumber skin becomes hard when mechanical pressure is applied?

Answer 8

• The dermal layer is composed of a soft matrix of connective tissues and primed stiff collagen fibers, that can easily flow
• When mechanical pressure is applied the nervous system releases proteins called stiparin which cross link collagen fibers to form a stiff rigid structure

Question 9

Based off sea cucumber skins ability to harden, what biomaterial have engineers been able to produce?

Answer 9

PVA/Cellulose nanocomposite - water stimuli
PNIPAm - temperature stimuli
Anionic and Cationic Hydrogels - pH responsive

Question 10

Explain why PVA/cellulose nanocomposite material structure becomes soft?

Answer 10

This structure is a solid matrix of PVA polymer chains crosslinked with cellulose nanofibers
Both of these polymers are hydrophilic
When water is absorbed it acts like a plasticiser, interacting with the polymers reducing the intermolecular forces holding the chains in place making the material soft

Question 11

Explain how PNIPAm works and why it responds to temperature changes

Answer 11

This is a thermo-responsive polymer gel
At low temps this polymer is a hydrophilic and water soluble
When heated above the critical temperature, the polymer coils into a globule exposing only the hydrophobic regions becoming insoluble and solid.

Question 12

How can PNIPAm be utilised in drug delivery?

Answer 12

1) PNIPAm can be used to encase the drug in its hydrophobic regions while it is ina swollen hydrophilic state
2) When the temperature is increased above the critical temperature, the polymers coil into hydrophobic state, this expels the water s drug out of the polymer matrix

Question 13

What is a cationic hydrogel?

Answer 13

These are hydrogels that swell at a low pH becoming more hydrophilic in acidic conditions

Question 14

How can cationic and anionic hydrogels be used for drug delivery to specific regions?

Answer 14

Cationic hydrogels can be loaded with a drug payload
Once they reach an acidic environment like the stomach they swell taking in water. This expands the polymer mesh allowing for drug release

Question 15

How do self healing materials tend to work?

Answer 15

• So long as they are not damaged too badly, they can repair
• The reformation or hydrogen bonds in mobile polymer chains

Question 16

Explain the mechanism behind the self healing of hydrgels

Answer 16

The polymer chains are split
A series of reactive chain ends are exposed
If the chain ends are mobile enough a they can reconnect and reform the lost bond

Question 17

Give an example of a self healing polymer

Answer 17

PAA-PVPON copolymer

Question 18

Explain in detail how PAA-PVPON copolymer heals by itself?

Answer 18

• The cut ends are exposed to water for 45oC for x amount of time
• Due to the hydrophilic nature of these polymers the water is absorbed causing swelling, which makes the polymer chains more mobile and free to move
• This added mobility allows the polymer chains to reconfigure and align, forming new bonds hence healing

Question 19

Explain how geckos are able to stick to any surface?

Answer 19

• They have millions of hairs called setae each covered in micro hairs called spatulae
• These are able to conform to any surface increasing contact area and forming millions of van der waals bonds
• The culmination of all these weak bonds enables the gecko to stick

Question 20

explain a material that engineers have been able to create based off gecko adhesive properties

Answer 20

• A flexible setae-like coated material, that mimics the mushroom suction shaped hairs
• When a voltage is applied the material curls up to retract from the contacting surface unsticking

Question 21

Why is the prevention of biofilm formation necessary?

Answer 21

Biofilm formation costs the health service millions per year
it also has signifcant impacts on lives

Question 22

Why are antibiotics not an optimal solution

Answer 22

antibiotic resistance

Question 23

What are nanopillars and how are they a solution to biofilm formation

Answer 23

These a natural mechano-bactericidal surfaces that effectively stop bacteria infection by penetrating cells walls, rupturing them

Question 24

What is a human engineered material based off these nanopillars?

Answer 24

Black Silicon, which has seen as much effectiveness as dragonfly wings

Question 25

Explain how we measure for hydrophobicity and the angles associated?

Answer 25

The internal angle from ground to water droplet

Question 26

List a few materials that are super hydrophobic

Answer 26

Rose petals
Butterfly wings
gecko skin

Question 27

When water first lands on a surface, there is a layer of air trapped underneath in the microstructure, what is this called and what different surface structures are there?

Answer 27

• Cassie Baxter state
• Wenzel State if the liquid displaces the air
  the different structures are a microstructure a nanostructure and a hierarchal (combo of both)

Question 28

Is the surface more ore less hydrophobic if the liquid displaces the air becoming wenzel state?

Answer 28

• More hydrophilic and an increased wettability angle

Question 29

How can the surface adhesion of a liquid be measured?

Answer 29

1) Tilt the surface with the droplet
2) Measure the advancing angle and receding contact angle
3) The difference between these 2 is the contact angle hysteresis (CAH)
4) The larger the CAH the more adhesive the surface

Question 30

In a confocal image, what is meant by red dots and green dots?

Answer 30

Green are living red are dead cells

Question 31

How are pocket and spear type nanostructures created out of titanium? And what are their uses?

Answer 31

• Chemical etching and thermal treatment
• Orthopaedic implants

Question 32

What is more effective at killing bacteria and stopping biofilm formation out of pocket and spear type nanopillars?

Answer 32

Pocket type kills more bacteria and also reduces level of biofilm formation over a prolonged period.
This is because they are deeper and so bacteria cannot form a layer of dead cells as a blanket over the nanopillars for protection
Despite this spear type offers a smaller surface area so less bacteria initially adhere but less are killed and biofilm formation is relatively easy

Question 33

Describe what is going on in this diagram

Answer 33

• Pocket and spear type nanopillar surfaces
• Spear type: Less bacteria successfully adhere at the start but they form a layer of protective dead cells, forming a protective blanket layer for cells to adhere onto and form a biofilm
• Pocket types initially more adhere due to larger surface area, however the pockets mean a protective layer cannot form

Question 34

What is special about Nepenthes Pitcher plants?

Answer 34

These plants rely on insects slipping into their digestive tract and being unable to climb out.
It does this by having a porous surface, infused with a lubricant which makes a super slippery hydrophobic film

Question 35

Engineers have copied this in 2 different ways, explain what they are

Answer 35

1) Epoxy nanopillars infused with silicone oil
2) Porous PDMS structures and also infusing them with oil

Question 36

What are some challenges and limitations to using this oil coated surface principle to make a superhydrophobic surface?

Answer 36

In the body the constant flow of fluids over time would deplete and wash away the oily lubricating layer.
Studies show after 2 days it can mostly be lost in-vivo.

Question 37

Explain how a liquid-like solid surface works?

Answer 37

This is a specialised superhydrophobic coating that is covalently bound to the surface, so it cannot be washed away under in-vivo flow conditions.

Question 38

Why are liquid-like solid surfaces effective at preventing biofilm formation?

Answer 38

• They are superhydrophobic so bacteria struggle to adhere initially
• The coating also repels the EPS glue-like matrix that bacteria produce to stick better. This means that an EPS overcoat cannot form producing a biofilm layer