Gels Prepared by Ph D Halina Falfushynska Gels

Gels Prepared by Ph. D Halina Falfushynska

Gels as semisolid systems Gels consisting of small inorganic particles or large organic molecules enclosing and interpenetrated by a liquid. Some gel systems are as clear as water in appearance and others are turbid, since the ingredients may form aggregates, which disperse light. The concentration of the gelling agents is mostly less than 10%, usually in 0. 5 to 2. 0% range, with some exceptions. 2

Transmission Electron Micrograph (TEM) of a Cadmium Sulfide Aerogel

The majority of gels are formed by aggregation of colloidal sol particles. The continuous phase is held within the meshes. A gel rich in liquid may be called a jelly; if the liquid is removed and only the gel framework remains this is termed a xerogel. Sheet gelatin, acacia tears and tragacanth flakes are all xerogels. 4

Gels 5

Gel structures Aggregates of spherical particles Physical gel with crystalline junctions Framework of Rod-like particles 6 Chemical gel -covalent junctions

Structure of the polysaccharide • Change temperature • Change solvent quality • Change ionic environment It’s what happens to amylose 7

Syneresis is when the interaction between particles of the dispersed phase becomes so great that on standing, the dispersing medium is squeezed out in droplets and the gel shrinks. Syneresis is a form of instability in aqueous and nonaqueous gels. Thixotropy is a reversible gel-sol formation with no change in volume or temperature 12/2/2020 8

Gelation of lyophobic sols Gels may be flocculated lyophobic sols where the gel can be looked upon as a continuous floccule. Examples are: aluminium hydroxide and magnesium hydroxide gels. Gel structure, Flocculated lyophobic sol, e. g. aluminium hydroxide.

Types of Hydrogels • Classification – Method of preparation • Homo-polymer, Copolymer, Multipolymer, Interpenetrating polymeric – Ionic charge • Neutral, Catatonic, Anionic, Ampholytic – Physical structure • Amorphous, Semi-crystalline, Hydrogen-bonded

Amorphous Gel Hydrosorb Gel in the dosing syringe can be applied easily, accurately and economically. breakdown of coatings and necrotic tissue deep in the wound

Hydrogen-bonded Gel

Types of Hydrogels Collagen • Natural Polymers – Dextran, Chitosan, Collagen, Dextran Sulfate – Advantages • Generally have high biocompatibility • Intrinsic cellular interactions • Biodegradable • Cell controlled degradability • Low toxicity byproducts – Disadvantages • Mechanical Strength • Batch variation • Animal derived materials may pass on viruses

Up to 30% of all protein in the body is collagen!

Properties of Hydrogels • Swelling properties influenced by changes in the environment – p. H, temperature, ionic strength, solvent composition, pressure, and electrical potential • Can be biodegradable, bioerodible, and bioabsorbable • Can degrade in controlled fashion

Properties of Hydrogels • Pore Size • Fabrication techniques • Shape and surface/volume ratio • H 2 O content • Strength • Swelling activation

Why Hydrogels ? : Tissue Engineering/Cell Culture Systems • Scaffold provides extracellular matrix: – Cell adhesion sites – Control of tissue form and thus function – Diffusion of growth factors, metabolites, and nutrients • Build it, Shape it, and Seed it with cells and nutrients

Why Hydrogels ? : Tissue Engineering • • Biocompatible H 2 O content Sterilizibilty Ease of use High mechanical Strength Surface to volume ratio Good cell adhesion High nutrient transport

Opthalmic use : The artificial cornea is made of a dual network hydrogel with a clear center and peripheral pores Hydrogel biophysical and biochemical characteristics can be altered to control angiogenic growth factor delivery

Gelation of lyophilic sols Gels formed by lyophilic sols can be divided into two groups depending on the nature of the bonds between the chains of the network. Gels of type I are irreversible systems with a threedimensional network formed by covalent bonds between the macromolecules. Typical examples are the swollen networks that have been formed by the polymerization of monomers of water-soluble polymers in the presence of a crosslinking agent. For example, poly (2 -hydroxyethylmethacrylate), [poly (HEMA)], crosslinked with ethylene glycol dimethacrylate, [EGDMA], forms a three-dimensional structure, that swells in water but cannot dissolve because the crosslinks are stable. 12/2/2020 22

Liesegang rings • Undergoing a precipitation reaction, under certain conditions of concentration and in the absence of convection

Liesegang rings

Liesegang rings in minerals Malachite Agate Turmaline Onix

Liesegang rings Liesegang-like rings (LRs) in slightly dilated breast ducts associated with lactational Cholelith, distribution of bilirubin and proteins

Many gel-forming substances are available for preparing pharmaceutical gels and jellies. Although these substances share some common physical characteristics, the intended use may require gelling attributes of a certain substance or blend of substances. For example, Pharmaceutical Nasal Gels must be: 1. Nasal adherent 2. Odorless 3. Non-irritating 4. Water soluble 12/2/2020 27

And Pharmaceutical Ophthalmic Gels must be: 1. Optically clear 2. Sterile 3. Mucomimetic 4. Lubricating 5. Demulcent 6. Non-irritating or non-sensitizing 7. Water soluble or miscible Some simple gel formulations are shown as following: 28

Practical Example on Medicated Gels: Benzocaine Gel Inside the condom contains a lubricant benzocaine 5% to prolong his climax • Benzocaine is a local anesthetic commonly used as a topical pain reliever. It is the active ingredient in many over-the-counter anesthetic ointments. It is also combined with antipyrine to form Anti-Biotic Otic Drops, to relieve ear pain and remove earwax. 29

Medical application of gels