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A hydrogel is a biphasic material, a mixture of porous and permeable solids and at least 10% of water or other interstitial fluid. [1] [2] The solid phase is a water insoluble three dimensional network of polymers, having absorbed a large amount of water or biological fluids.
The formulation of an accurate theory of gel formation that correctly predicts gelation parameters (such as time, rate, and structure) of a broad range of materials is highly sought after for both commercial and intellectual reasons. As noted earlier, researchers often judge gel theories based upon their ability to accurately predict gel points.
Hydrogels have the remarkable ability to swell in water and aqueous solvents. During the process of swelling, surface instability can occur. This instability depends on the thickness of the hydrogel layers and the surface tension. [7] A higher surface tension stabilizes the flat surface of the hydrogel, which is the outer-most layer.
Hydrogels can be used as drug delivery vehicles, for transdermal application, ophthalmic drug delivery, [11] cancer treatment [12] or for wound dressing. [7] [13] As a type of water based formulation, hydrogels are generally less greasy and are easier to be removed than oil-based formulations like organogels. [6]
The Ramberg–Osgood equation was created to describe the nonlinear relationship between stress and strain—that is, the stress–strain curve—in materials near their yield points. It is especially applicable to metals that harden with plastic deformation (see work hardening ), showing a smooth elastic-plastic transition.
Specifically, hydrogels can be designed to release drugs or other agents in response to physical characteristics of the environment like temperature and pH. [12] The responsiveness of hydrogels is a result of their molecular structure and polymer networks. [12] Hydrogel nanoparticles have a promising future in the drug delivery field.
There are also fluids whose strain rate is a function of time. Fluids that require a gradually increasing shear stress to maintain a constant strain rate are referred to as rheopectic. An opposite case of this is a fluid that thins out with time and requires a decreasing stress to maintain a constant strain rate (thixotropic).
An upturned vial of hair gel Silica gel. A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. [1] [2] Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady state, although the liquid phase may still diffuse through this system.