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Chemistry - Year One > Biomaterials > Flashcards

Biomaterials Flashcards

1
Q

How does having loose cross-links impact the functionality of the biomaterial?

A

It makes the material softer

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2
Q

How does having dense cross-links impact the functionality of the biomaterial?

A

It makes the material stiffer

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3
Q

What are hydrogels?

A

They are a 3D ‘solid-like’ network that can hold large amounts of water in a swollen scaffold

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4
Q

What is the typical polymer content of a hydrogel?

A

0.1 - 10%

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5
Q

Why is the highly porous network of hydrogels ideal for growing tissues and cells?

A

It allows for the diffusion of nutrients and oxygen into the structure

It allows for the diffusion of carbon dioxide, metabolites and toxins out of the structure

Enables cell infiltration, proliferation and connectivity

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6
Q

What is chemical cross-linking?

A

Covalent bonds are formed as cross-links

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7
Q

What properties does chemical cross-linking lead to?

A

It makes them resistant to strain, and hence an elastic behaviour

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8
Q

What is physical cross-linking?

A

Non-covalent interactions e.g. van der Waals, H-bonding, ionic, entanglement etc… hold the cross-links together

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9
Q

What properties does physical cross-linking lead to?

A

It leads to visco-elastic properties; becomes liquid-like at higher stress

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10
Q

What are three common synthetic polymers used in hydrogel formation?

A

PEG (Poly(ethylene glycol))

PHEMA (Poly(2-hydroxyethyl methacrylate))

PVA (Poly(vinylalcohol))

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11
Q

Why is PEG used in hydrogels?

A

Chemically and biologically inert

Provides precise control over cell interactions and behaviour - ‘blank slate’ material

Easily functionalised, giving highly modular and tunable gel properties

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12
Q

Why is PHEMA used in hydrogels?

A

High mechanical strength due to cross-links

Highly biocompatible and bioinert

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13
Q

Why does PHEMA have a high mechanical strength?

A

Monomer precursors are often contaminated with a difunctional monomer that leads to the formation of chemically cross-linked networks

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14
Q

Why do we need to ensure we remove all of the monomer from PHEMA?

A

The monomer is highly toxic

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15
Q

Why is PVA used in hydrogels?

A

High elasticity, but mechanically weak when physically cross-linked

Highly biocompatible and bioinert

Alcohol groups are easily functionalised

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16
Q

What are the common themes for synthetic polymers?

A

Bioinert (need functionalisation to interact with cells)

Low immunogenicity

Non-degradable

Easily functionalised (versatile)

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17
Q

What are three common natural polymers used in hydrogels?

A

Collagen

Alginate

Hyaluronic acid

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18
Q

Why is collagen used in hydrogels?

A

A major component of the ECM so it is biocompatible

High mechanical strength due to the formation of self-assembled fibres

Bioactivity (no need to functionalise)

Naturally cell adhesive

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19
Q

How does collagen form the self-assembled fibres?

A

3 collagen strands come together to form a right-handed triple helix

The helices come together to form a non-covalent bundle

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20
Q

What is the primary structure of collagen?

A

A repeating glycine, proline and hydroxyproline backbone

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21
Q

What are some drawbacks of using collagen for hydrogels?

A

Potential contamination can lead to an immune response

Collagen must be processed to form hydrogels, leading to a loss of mechanical strength (we lose the secondary and tertiary structures)

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22
Q

Where is alginate extracted from?

A

Seaweed and algae

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23
Q

Why is alginate used in hydrogels?

A

Bioinert

Can be made by 3D printing in the presence of calcium ions due to fast gelation speed (ionic crosslinks)

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24
Q

What is the disadvantage of using alginate in hydrogels?

A

Not cell adhesive; we need to functionalise the COOH groups with bioactive groups

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25
Q

Why is hyaluronic acid used in hydrogels?

A

Already present in the ECM; biocompatible and bioactive

Easily functionalised

Hydrophilic

High charge density leads to gels with a high water content

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26
Q

What are common themes for natural polymers?

A

Biocompatible (but may be contaminated)

Usually cell adhesive and biodegradable

Inherently bioactive

Less controlled (heterogenous)

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27
Q

What are the two mechanisms of gelation?

A

A+B strategy

AB strategy

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27
Q

What is the A+B strategy?

A

A mechanism of gelation where a complementary group has to be added to form a covalent cross-links

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28
Q

What is the AB strategy?

A

A method of gelation where the polymer already has both functional groups needed to form covalent cross-links

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29
Q

What is needed for ideal cross-linking chemistry?

A

Gels quickly

Minimises damage to cells and tissues (non-toxic)

Ensures no side reactions with any biomolecules present (selectivity)

Avoids complex chemistry

1:1 ratio of reactive groups

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30
Q

What are the disadvantages of an amide coupling reaction?

A

Slow

Low selectivity

Can hydrolyse easily in aqueous conditions

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31
Q

Why are thiol-‘ene’ reactions better than amide couplings?

A

More selective

Faster

It can be non-toxic

32
Q

What is the mechanism of a thiol-‘ene’ reaction?

A

A conjugate addition with a thiol

33
Q

What is a disadvantage of performing a photoactivated reaction between a thiol and an alkene?

A

Involves radicals, and produces a hydroxy radical which damages cells

34
Q

What are the benefits of the cycloaddition reaction between an azide and an alkyne?

A

Stable functional groups that do not occur in nature

Ring strain promotes the reaction

Very selective and non-toxic

35
Q

What are the disadvantages of the cycloaddition between an azide and an alkyne?

A

Slow reaction

The cyclooctyne is the hydrophobic

The cyclooctyne is difficult to synthesise

36
Q

How can we make dynamic or self-healing gels?

A

If the cross-linking reaction is reversible

This gives us the opportunity to create materials that undergo a change over time, or self heal after damage

37
Q

What is an example of a cross-link that can form self-healing gels?

A

Imines (reaction between an aldehyde and an amine)

Hydrazone (reaction between a hydrazine and an aldehyde)

38
Q

Why are imines poor self-healing gels?

A

The equilibrium lies towards the aldehyde and not the imine (hydrazones are less prone to this hydrolysis)

39
Q

What is the purpose of the ECM?

A

To provide physical scaffolding, as well as key biomechanical and biochemical signals

40
Q

What method can we use to create fibrous materials?

A

Electrospinning

41
Q

What is electrospinning?

A

A solution of polymer is ejected through a needle at a high voltage (5000-10000 V), charging the liquid and forming a ‘Taylor Cone’. This is because the electrostatic repulsion overcomes the surface tension, leading to a very fine jet of liquid that, when dries, whips backwards and forward due to electrostatic repulsion onto a plate where it can be collected

42
Q

What are two methods of 3D printing?

A

Extrusion printing (continuous flow)

Inkjet printing (droplets)

43
Q

How does the solidification rate of 3D printing impact the resolution?

A

The higher the solidification rate, the greater the resolution

This is because there is less chance for it to diffuse

44
Q

What techniques can be used to increase the solidification rate?

A

We can apply UV light and heat, but this is damaging to cells

45
Q

What method, other than 3D printing, can be used to create solid polymer material?

A

Stereolithography

46
Q

How does stereolithography work?

A

UV light is used to convert liquid polymer into solid material

Any excess liquid can be washed away to give the desired printing material

The resolution is very good

47
Q

What are the two disadvantages of stereolithography?

A

Potential toxicity from the UV light

There is a limited choice of ‘inks’

48
Q

What polymers are generally used for electrospinning and 3D printing?

A

PCL (Poly(caprolactone))

Poly(lactic acid)

Poly(glycolic acid)

49
Q

What makes PCL a good polymer to create fibres out of?

A

High biocompatibility with good mechanical strength

High crystallinity (hydrophobic), with strong non-covalent interactions holding the individual polymer chains together

Slow breakdown (approximately 2 years)

50
Q

What makes poly(lactic acid) and poly(glycolic acid) different to PCL?

A

They are less hydrophobic hence less crystalline

This leads to a lower strength, and so a faster breakdown (5-6 weeks)

51
Q

How does the breakdown of the polyesters impact inflammation?

A

They breakdown into carboxylic acids, which increases tissue pH and hence causes inflammation

The faster the rate of breakdown, the increase in the level of inflammation

52
Q

How can we tune polyester degradation rates?

A

We can create mixtures or co-polymers of varying polyesters

53
Q

What is the formula for Young’s Modulus?

A

Young’s Modulus = Stress / Strain

53
Q

What can rheometry tell us about our gel?

A

It can tell us about the kinetics of gel formation during cross-linking

It can tell us about the properties by comparing the values of G’ and G’’

54
Q

Why is Cryo EM better than EM?

A

Dehydrating can induce ‘artefacts’

55
Q

What can Cryo EM tell us about our fibrous material?

A

Cross-link density from the size of the pores

Yields information about the orientation and diameter of fibrous materials

56
Q

How can we bind synthetic materials to the ECM?

A

We can functionalise them with specific peptide sequences (e.g. RGD)

57
Q

How can we attach growth factors and other proteins to biomaterials?

A

We can simplify the protein structure to a short synthetic peptide mimic, greatly reducing the chemoselective and regioselective issues

This is cheaper, easier and more efficient to do

58
Q

How can we generate SH on the protein surface for conjugating to a protein?

A

We can genetically engineer a cysteine residue on the protein surface

59
Q

How can we create responsive biomaterials?

A

We can incorporate peptide-based cross-linkers that cells can degrade

60
Q

What are responsive biomaterials?

A

Biomaterials that change when a specific stimulus is applied

61
Q

What are MMPs?

A

Matrix Metalloproteinases (MMPs) are proteases released by cells to breakdown and remodel their extracellular environment - this can cause our synthetic cross-links to degrade if we incorporate a short MMP-sensitive peptides onto our cross-links

62
Q

How can light be used to create responsive biomaterials?

A

By photo-caging or photo-cleavage

63
Q

What is photo-caging?

A

UV light is shone on the component, and this causes loss of an aromatic unit to produce a thiol

64
Q

What is photo-cleavage?

A

The scaffold is degraded by shining UV light through our biomaterial, selectively breaking cross-links and removing aromatic molecules

65
Q

What is protein fouling?

A

When a biomolecule enters the body, it undergoes rapid protein adsorption on to the surface

66
Q

What is the Vroman effect?

A

Initially, small and mobile proteins bind onto our biomaterial first, but are then replaced with larger proteins due to them having more contact with our surface

67
Q

What disruptive effects can protein fouling occur?

A

It can block desired functionality (steric effect)

It can highlight material as an immune threat

It can activate undesired signalling pathways by making hidden binding sites visible

68
Q

How can we reduce protein fouling?

A

We can use ‘stealth’ materials e.g. PEG, PVA, or zwitterionic chains

Charged surfaces can also be used to ‘tune;’ the protein layer to resist certain proteins

69
Q

What effect does minimising protein fouling have on our biomaterial?

A

It minimises interactions with other cells, decreasing the effectiveness of the biomaterial

A compromise is needed between the level of protein fouling and biomaterial functionality

70
Q

How can we prevent soluble growth factors from being destroyed by endocytosis?

A

We need to bind them to our biomaterial, which applies its signalling effect

This can be done by covalently ‘tethering’ a protein to our biomaterial surface

71
Q

What is the positive feedback loop that amplifies the formation of growth factors?

A

Growth factors encourage cell growth

The growth factors are sequestered by the biomaterial

The cells released additional growth factors

72
Q

What are the five effects that occur when a biomolecule enters the body?

A

Protein adsorption (protein fouling)

Immune system activation

Macrophage invasion

Chronic inflammation and macrophage fusion

Fibrous encapsulation

73
Q

What happens when our immune system is activated?

A

It provides signalling for an immune response to occur

Neutrophils arrive and initiate acute inflammation

74
Q

What occurs in the macrophage invasion step?

A

Inflammation is increased

Enzymes and acids are released in an attempt to degrade the material

Macrophages try to engulf the material - it is too small to do this, and so enters frustrated phagocytosis

75
Q

What occurs in the chronic inflammation and macrophage fusion step?

A

Macrophages join together to form a ‘foreign body giant cells’

Pro-healing cytokines are released, leading to the recruitment of fibroblasts

76
Q

What occurs in the fibrous encapsulation step?

A

Fibroblasts deposit collagen to surround the impact, effectively encapsulating it

This may prevent the function from occurring

Chemistry - Year One (16 decks)

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