SL22107: Formulation of liposomes and gels

What is a gel

A viscoelastic solid-like material

Made of cross-linked network and a solvent put together by physical or chemical forces

What causes the appearance of a gel

entrapment and adhesion of a liquid in the large surface area of solid 3D matrix

How is a gel classified, give the 5 factors

Hydrophile-lipophile balance of polymer (HLB)

regularity of structure

polymer solvent interaction

molecular weight of polymer

flexibility of polymer chain

How can we describe gel characteristics

Large increase in viscosity above gel point

Appearance of rubber like elasticity

Gels typically retain their shape, until high stress = deformation

Describe wet soft solid gels

• soap

• shampoo

• toothpaste

• hair gel

• contact lenses

ALL MADE FROM POLYMERIC COMPOUNDS

Describe what is meant by supramolecular gels and some uses

Low molecular mass compounds

Function: energy transfer, hybrid materials, tissue engineering, regenerative medicine

Describe what is meant by macromolecular gels and some uses

Large molecular mass compounds formed by hydrogen bond cross linking

Uses: Gelatin, collagen, agar, starch, gellan gum

What is a smart gel

Gels which respond to their environment e.g. pH change, difference in light

Describe the classification of gels

• Natural or Artificial

• Supramolecular or Macromolecular

• Physical or Chemical

Physical vs chemical gels

Physical gels are reversible. Chemical gels irreversible

What are hydrogels

Water molecules trapped within 3D network

e.g. gelatin

What are some characteristics of hydrogels

• Can retain significant amounts of water

• Water insoluble

• Used in topical drug delivery, wound healing, soft contact lenses, implant coating

Organic liquid gels: organogels

Organic liquids trapped in 3D matrix

e.g. petroleum

Jelly gel

When coherent matrix is rich in liquid

e.g. ephedrine sulphate jelly

Xerogel/Aerogel

When the liquid from the coherent matrix is removed and only the 3D network remains

e.g. gelatin sheet

How does diffusion in hydrogel occur

If a gel is highly hydrated it occurs through the pores

If a gel is low hydrated it occurs through dissolving in the polymer and is transported between the chains

What effect does cross linking have on diffusion rate

Increases hydrophobicity of a gel

Decreases the diffusion rate of the drug

What happens when hydrogels swell

Swelling characteristic of polymeric gel can be changed by heat, pH or application of electrical current

swollen at low temperature allows for drug delivery

shrunken at high temperature doesn’t allows for drug delivery

How are macromolecules formed

From irreversible thermal chemical reactions e.g. strong chemical bonds

From reversible physical reactions e.g. weak non-covalent interactions

Type 1 gels

Irreversible systems

3D network formed by covalent bonds between macromolecules

Formed by polymerisation of monomers of water soluble polymers in presence of X-linker

Type 2 gels

Held together by intermolecular bonds

Form a gel by cooling below gel point (T)

Very viscous

Why are type 2 gels suitable for topical skin applications

Has gelling properties

Dries rapidly

leaves a plastic film with the drug after contact with the skin

What happens when water soluble polymer chains are covalently cross linked into a 3D structure

A gel forms when the dry material interacts with water

How do cross linked polymeric gels behave in water

They swell but cannot dissolve due to cross links

Used for expanding implants and antibiotic-loaded gels for treating middle ear infections

What are SAFIN’s formed from (Self-Assembled Fibrillar Network)

Combination of non-covalent interactions

• H bonds

• PI-PI stacking

• donor acceptor interaction

• metal coordination

• van der waals

these interactions are weak so they can be transformed = thermally reversible

How are supramolecular gels made

1. LMMC heated to isotropic supersaturated solution then condense

2. Highly ordered aggregation give rise to crystals = crystallisation

3. Random aggregation results in amorphous precipitate

4. An intermediate between crystallisation and amorphous precipitate creates a gel

What are some uses of supramolecular gels

• cosmetic formulation

• media for tissue engineering as its similar composition to body

• controlled release of molecules

• model antigens for immunogenicity studies

• Wound dressing