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