Biomaterials Flashcards
How does having loose cross-links impact the functionality of the biomaterial?
It makes the material softer
How does having dense cross-links impact the functionality of the biomaterial?
It makes the material stiffer
What are hydrogels?
They are a 3D ‘solid-like’ network that can hold large amounts of water in a swollen scaffold
What is the typical polymer content of a hydrogel?
0.1 - 10%
Why is the highly porous network of hydrogels ideal for growing tissues and cells?
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
What is chemical cross-linking?
Covalent bonds are formed as cross-links
What properties does chemical cross-linking lead to?
It makes them resistant to strain, and hence an elastic behaviour
What is physical cross-linking?
Non-covalent interactions e.g. van der Waals, H-bonding, ionic, entanglement etc… hold the cross-links together
What properties does physical cross-linking lead to?
It leads to visco-elastic properties; becomes liquid-like at higher stress
What are three common synthetic polymers used in hydrogel formation?
PEG (Poly(ethylene glycol))
PHEMA (Poly(2-hydroxyethyl methacrylate))
PVA (Poly(vinylalcohol))
Why is PEG used in hydrogels?
Chemically and biologically inert
Provides precise control over cell interactions and behaviour - ‘blank slate’ material
Easily functionalised, giving highly modular and tunable gel properties
Why is PHEMA used in hydrogels?
High mechanical strength due to cross-links
Highly biocompatible and bioinert
Why does PHEMA have a high mechanical strength?
Monomer precursors are often contaminated with a difunctional monomer that leads to the formation of chemically cross-linked networks
Why do we need to ensure we remove all of the monomer from PHEMA?
The monomer is highly toxic
Why is PVA used in hydrogels?
High elasticity, but mechanically weak when physically cross-linked
Highly biocompatible and bioinert
Alcohol groups are easily functionalised
What are the common themes for synthetic polymers?
Bioinert (need functionalisation to interact with cells)
Low immunogenicity
Non-degradable
Easily functionalised (versatile)
What are three common natural polymers used in hydrogels?
Collagen
Alginate
Hyaluronic acid
Why is collagen used in hydrogels?
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
How does collagen form the self-assembled fibres?
3 collagen strands come together to form a right-handed triple helix
The helices come together to form a non-covalent bundle
What is the primary structure of collagen?
A repeating glycine, proline and hydroxyproline backbone
What are some drawbacks of using collagen for hydrogels?
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)
Where is alginate extracted from?
Seaweed and algae
Why is alginate used in hydrogels?
Bioinert
Can be made by 3D printing in the presence of calcium ions due to fast gelation speed (ionic crosslinks)
What is the disadvantage of using alginate in hydrogels?
Not cell adhesive; we need to functionalise the COOH groups with bioactive groups
Why is hyaluronic acid used in hydrogels?
Already present in the ECM; biocompatible and bioactive
Easily functionalised
Hydrophilic
High charge density leads to gels with a high water content
What are common themes for natural polymers?
Biocompatible (but may be contaminated)
Usually cell adhesive and biodegradable
Inherently bioactive
Less controlled (heterogenous)
What are the two mechanisms of gelation?
A+B strategy
AB strategy
What is the A+B strategy?
A mechanism of gelation where a complementary group has to be added to form a covalent cross-links
What is the AB strategy?
A method of gelation where the polymer already has both functional groups needed to form covalent cross-links
What is needed for ideal cross-linking chemistry?
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
What are the disadvantages of an amide coupling reaction?
Slow
Low selectivity
Can hydrolyse easily in aqueous conditions
Why are thiol-‘ene’ reactions better than amide couplings?
More selective
Faster
It can be non-toxic
What is the mechanism of a thiol-‘ene’ reaction?
A conjugate addition with a thiol
What is a disadvantage of performing a photoactivated reaction between a thiol and an alkene?
Involves radicals, and produces a hydroxy radical which damages cells
What are the benefits of the cycloaddition reaction between an azide and an alkyne?
Stable functional groups that do not occur in nature
Ring strain promotes the reaction
Very selective and non-toxic
What are the disadvantages of the cycloaddition between an azide and an alkyne?
Slow reaction
The cyclooctyne is the hydrophobic
The cyclooctyne is difficult to synthesise
How can we make dynamic or self-healing gels?
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
What is an example of a cross-link that can form self-healing gels?
Imines (reaction between an aldehyde and an amine)
Hydrazone (reaction between a hydrazine and an aldehyde)
Why are imines poor self-healing gels?
The equilibrium lies towards the aldehyde and not the imine (hydrazones are less prone to this hydrolysis)
What is the purpose of the ECM?
To provide physical scaffolding, as well as key biomechanical and biochemical signals
What method can we use to create fibrous materials?
Electrospinning
What is electrospinning?
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
What are two methods of 3D printing?
Extrusion printing (continuous flow)
Inkjet printing (droplets)
How does the solidification rate of 3D printing impact the resolution?
The higher the solidification rate, the greater the resolution
This is because there is less chance for it to diffuse
What techniques can be used to increase the solidification rate?
We can apply UV light and heat, but this is damaging to cells
What method, other than 3D printing, can be used to create solid polymer material?
Stereolithography
How does stereolithography work?
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
What are the two disadvantages of stereolithography?
Potential toxicity from the UV light
There is a limited choice of ‘inks’
What polymers are generally used for electrospinning and 3D printing?
PCL (Poly(caprolactone))
Poly(lactic acid)
Poly(glycolic acid)
What makes PCL a good polymer to create fibres out of?
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)
What makes poly(lactic acid) and poly(glycolic acid) different to PCL?
They are less hydrophobic hence less crystalline
This leads to a lower strength, and so a faster breakdown (5-6 weeks)
How does the breakdown of the polyesters impact inflammation?
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
How can we tune polyester degradation rates?
We can create mixtures or co-polymers of varying polyesters
What is the formula for Young’s Modulus?
Young’s Modulus = Stress / Strain
What can rheometry tell us about our gel?
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’’
Why is Cryo EM better than EM?
Dehydrating can induce ‘artefacts’
What can Cryo EM tell us about our fibrous material?
Cross-link density from the size of the pores
Yields information about the orientation and diameter of fibrous materials
How can we bind synthetic materials to the ECM?
We can functionalise them with specific peptide sequences (e.g. RGD)
How can we attach growth factors and other proteins to biomaterials?
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
How can we generate SH on the protein surface for conjugating to a protein?
We can genetically engineer a cysteine residue on the protein surface
How can we create responsive biomaterials?
We can incorporate peptide-based cross-linkers that cells can degrade
What are responsive biomaterials?
Biomaterials that change when a specific stimulus is applied
What are MMPs?
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
How can light be used to create responsive biomaterials?
By photo-caging or photo-cleavage
What is photo-caging?
UV light is shone on the component, and this causes loss of an aromatic unit to produce a thiol
What is photo-cleavage?
The scaffold is degraded by shining UV light through our biomaterial, selectively breaking cross-links and removing aromatic molecules
What is protein fouling?
When a biomolecule enters the body, it undergoes rapid protein adsorption on to the surface
What is the Vroman effect?
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
What disruptive effects can protein fouling occur?
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
How can we reduce protein fouling?
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
What effect does minimising protein fouling have on our biomaterial?
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
How can we prevent soluble growth factors from being destroyed by endocytosis?
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
What is the positive feedback loop that amplifies the formation of growth factors?
Growth factors encourage cell growth
The growth factors are sequestered by the biomaterial
The cells released additional growth factors
What are the five effects that occur when a biomolecule enters the body?
Protein adsorption (protein fouling)
Immune system activation
Macrophage invasion
Chronic inflammation and macrophage fusion
Fibrous encapsulation
What happens when our immune system is activated?
It provides signalling for an immune response to occur
Neutrophils arrive and initiate acute inflammation
What occurs in the macrophage invasion step?
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
What occurs in the chronic inflammation and macrophage fusion step?
Macrophages join together to form a ‘foreign body giant cells’
Pro-healing cytokines are released, leading to the recruitment of fibroblasts
What occurs in the fibrous encapsulation step?
Fibroblasts deposit collagen to surround the impact, effectively encapsulating it
This may prevent the function from occurring
