L6: Gels Flashcards
How are drugs incorporated into gels?
They can be SUSPENDED in the matrix, or DISSOLVED in the liquid
A pharmaceutical gel is a semisolid system of at least two constituents, what are they?
A condensed mass, enclosed and interpenetrated by a liquid.
Explain what happens when the particles of drug are added in to the gel.
They link together to form INTERLACED NETWORK, imparting the RIGIDITY to the structure, continuous phase is held between the meshes.
In gels, when the coherent matrix is rich in liquid, what is the product called?
Jelly.
In gels, if the liquid is removed, and only gel framework remains, what is this called and give an example?
Xerogel.
Gelatin sheets, tragacanth flakes
Explain what a single phase gel/system is.
Contain linear/branched polymer MACROMOLECULES, that dissolve in water, have no definite boundaries with the dispensing medium.
What are macromolecules classed as?
Natural polymers –> tragacanth
Semisynthetic cellulose derivatives –> methyl cellulose
Synthetic polymers –> carbomer polymers
Why do macromolecules not form true solutions?
Because of their size.
Form colloidal dispersions instead (sol).
What’s different about a macromolecule (colloidal dispersion) even if it is liquid and appears to be a solution?
Sometimes cloudy.
More viscous than base solvent.
What do two phase gels contain rather than macromolecules?
Floccules of small particles.
The structure of a two phase gel is not always stable, on shaking, what happens to it?
It becomes a liquid.
What is the two phase gel referred to as, if the particles are large?
Magma.
Give examples of two phase systems.
Aluminium hydroxide gel
Magnesia magma
What are gels that contain water called? And how are they formed?
Hydrophilic gels. (Hydrogels).
Formed by lyophilic sols.
Aqueous gels can be divided into 2 groups, depending on what?
On the nature of the bonds between the chains of the network.
Explain what ‘type 1 gels’ are.
Irreversible systems
With 3D network, Formed by COVALENT bonds between the macromolecules.
Type 1 gels are ‘irreversible systems with a 3D network’ formed by what?
By COVALENT bonds between the MACROMOLECULES.
How is the network made in type 1 gels?
May be made by POLYMERISING water soluble polymers with a CROSS-LINKING agent.
If the polymers in type 1 gels contain little water, they are said to be…
Rigid.
What can happen when the polymers in type 1 gels absorb water?
Can swell and become more flexible.
(Type 1 gels) The network polymers can swell in water, but cannot dissolve, why?
Due to the stable cross linking structure.
The network polymers in type1 gels have been used in…..
…the fabrication of expanding implants.
….the prolonged drug release system.
Explain what type 2 gels are.
3D network, held together by weaker bonds. E.g. Hydrogen bonds, dipole dipole attraction.
Heat reversible.
type 2 gels are heat reversible, what does this mean?
They can change from ‘sol’ to gel on either heating, or cooling.
Type 2 gels can be used….
Topically.
What is usually the base in lyophobic gels?
Base is usually OILY.
Liquid paraffin
Vegetable oil
Aqueous gels are often prepared using what type of gelling agent?
Organic gelling agent.
Lyophobic gels usually use what type of gelling agent?
Inorganic gelling agent.
Usually based on a silicate compound.
What type of ‘phase’ is a lyophobic gel?
Two phase.
Name one pharmaceutical application of gels. T
Topical application.
What’s different about a gel compared to creams/ointments? (TA)
Gels are non greasy formulations.
Whats one pharmaceutical area where gels are useful topically?
For hairy regions of the body.
E.g. Synalar gel for scalp use.
Why are gels useful as lubricants? (Topically)
As they spread easily over skin.
Name one pharmaceutical application of gels. (O)
Ophthalmic application.
Why are gels better than ointments? (Ophthalmic)
Less visual distortion after application.
Explain how (ophthalmic gel) works
Forms a protective film over the cornea.
Well tolerated.
Why are gels better than solutions? (Ophthalmic)
Increased retention time
May result in less frequent administration.
Name an ophthalmic gel
Viscotears 0.2%
Name an anaesthetic gel (topical).
Lidocaine gel
Name one pharmaceutical application of gels. (Bio)
Bioadhesives
What are bioadhesives used for?
To treat oral lesions
Name one pharmaceutical application of gels. (Bu)
Buccal application
Name one pharmaceutical application of gels. (W)
Wound management.
Name one consideration of using a gelling agent.
Dispersing medium
If it is added to dispersing medium in a haphazard manner, there’s a tendency for the agent to CLUMP.
Name one consideration of using a gelling agent
Temperature.
Some are more soluble in cold water than hot. Methyl cellulose
Some more soluble in hot than cold water. Gelatin
Name one consideration of using a gelling agent
PH
Some require a neutraliser, or pH adjusting chemical to create the gel.
Name one consideration of using a gelling agent
Time.
Most require 24-48 hours to completely hydrate, and reach max viscosity and clarity.
Name one consideration of using a gelling agent
Drug
Easier to add active drug BEFORE gel is formed.
Only if drug doesn’t interfere with gel formation.
Name one consideration of using a gelling agent
Mouth
Only a few can be administrated orally.
Name another additive for gels. (H)
Humectants.
Name another additive for gels. (P)
Preservatives.
What does a humectant do in gels ?
Can minimise water loss from a gel.
Example of a humectant?
Glycerol
