Hydrogels Flashcards
What is a hydrogel built up of?
Polymer network of polymers with cross-linkings solvated in water. 60-99% water
Give example of applications of hydrogels.
Drug delivery, diapers, cosmetics, contact lenses, wound dressing
Define LCST (lower critical solution temp.) of a hydrogel compared to UCST for a normal polymer.
When the temp. increases two normal polymers mix more easily, the energy barrier at 50/50 can be overcome. But hydrogels mix up to some temp. LCST when they demix and have two phases instead of one –> the hydrogel collapses.
Explain the interactions of hydrogels in swollen and collapsed state.
In the swollen state is the water-monomer interaction stronger than monomer-monomer interaction, i.e. hydrogen bonds are stronger and polymer is hydrophilic.
In the collapsed state are the monomer-monomer interactions dominant and therefore is the polymer hydrophobic, it will cluster to try and stay away from water. Which means it is more dense and there is less water in between –> collapse.
Explain the three different types of water in the hydrogel; primary and secondary bound water and free excess water.
The primary bound water is bound to the sidegroups of the polymer and the secondary bound water to the chain. Free bulk water comes into the network because of osmotic pressure, because the concentration of the polymers are higher and bulk water tries to fill the spaces in between the chains.
Explain co-non-solvency.
When have two solvents in a hydrogel. The second solvent is called a co-non-solvent.
Water and ethanol for example mixes for some conc. and for some not, this can make the hydrogel collapse at a lower temp. than normally. Between 10-50% methanol does the hydrogel collapse for all temp.
Explain cooperative hydration, which explains why cononsolvency can change the LCST of a hydrogel.
Along the polymer there are intervals with more methanol or water and the monomers in between without hydrogen bonds cluster. Because they tend to cluster the composition with minimum coverage corresponds to the minimum radius of gyration.
Explain the concept of stimuli responsive hydrogels.
As a reaction a stimuli, usually a change in pH or temp., the hydrogel either collapses or swells more.
Other stimuli are: ionic strength, magnetic/electric fields, light.
Explain how drug release by hydrogel works. How can slow or fast release be done?
When the hydrogel collapses from a change in temp. or pH the drug contained in the network is pushed out of the hydrogel –> fast delivery.
When the temp. is cooled down on the other hand, further below LCST, it swells more and by diffusion the drug escapes the hydrogel –> slow delivery.
How does more carbon in the sidegroups affect the LCST?
When the sidegroups contains more carbon does the hydrogel have larger tendency to collapse i.e. at lower temp. because the polymer is more hydrophobic, which increases the LCST interactions.
Explain how negative thermosensitive drug release can be realised. That is when temp. is lowered.
By grafting hydrogel onto a micelle containing a drug. When the temp. is high the hydrogel is collapsed and won’t let anything pass through the membrane. When it is cooled down the hydrogel swells and lets the drug pass out.
Explain how a pH sensitive hydrogel works.
Anionic: a chain with a carboxylic sidegroup -COOH, when the pH rises meaning there are more OH-ions and the hydrogen of the carboxylic group will react with the OH-ions forming water. –> the chain is more charged and repelling each other, there is more space for water and it is also more hydrophilic –> swollen state
Cationic: with a aminogroup -N, when the pH decreases, there are more hydrogen ions, the nitrogen binds to these forming a positive group -NH, the chains repel each other and more space for water.
Explain what multiresponsive behavior of a hydrogel means.
A hydrogel that is both responsive to changes in temp. and pH, by changing the pH can the LCST of the hydrogel either be lowered or raise, eg. lower pH gives lower LCST. In the same way can the pH be kept constant and the temp. changed to make the hydrogel swell/deswell.
