Hydrogels
A hydrogel is a natural or synthetic material that is able to absorb and swell with more than 10% by weight of water without breaking up or dissolving. This ability to absorb and swell with water means that hydrogels have many diverse applications including as biomaterials, wound dressings, medical devices, lubricious (slippy) and hydrophilic coatings, membranes, controlled drug delivery, sensors and in horticulture and agriculture.
Hydrogels are different from water absorbent sponges and other similar materials in that the absorbed water in a hydrogel becomes an intrinsic part of its chemical and physical structure - the water cannot simply be squeezed out. Many hydrogels are extremely biocompatible and they support the diffusional transport of water and aqueous solutes.
Hydrogel Membranes: Diffusion and Osmosis
The spontaneous diffusional flow of water molecules, through a semi-permeable membrane, from a region of higher water concentration to a region of lower water concentration is known as osmosis. Osmosis is normally facilitated by the diffusional flow of aqueous solutes in the opposite direction; higher salt concentration to lower salt concentation. The spontaneous diffusional flow of water and solute molecules will continue until an equilibrium has been reached and the relative concentrations on either side of the membrane are balanced.
Water-swollen hydrogels are highly permeable to water and aqueous solutes and, in thin film form, are often used as semi-permeable membranes. Typical applications using the osmotic and diffusional properties of hydrogels include controlled drug delivery and membrane separation processes such as reverse osmosis and haemodialysis.