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A hydrogel is a network of crosslinked polymer chains that are hydrophilic, sometimes found as a colloidal gel in which water is the dispersion medium. A three-dimensional solid results from the hydrophilic polymer chains being held together by cross-links. The crosslinks which bond the polymers of a hydrogel fall under two general categories: physical and chemical. Physical crosslinks consist of hydrogen bonds, hydrophobic interactions, and chain entanglements (among others). Because of the inherent cross-links, the structural integrity of the hydrogel network does not dissolve from the high concentration of water.[1] Hydrogels are highly absorbent (they can contain over 90% water) natural or synthetic polymeric networks.

The first appearance of the term 'hydrogel' in the literature was in 1894.[2]

Photo of the same short-peptide-based hydrogel, held in forceps to demonstrate its stiffness and transparency.

Mechanical properties

Hydrogels possess a vast range of mechanical properties, which is one of the primary reasons why they have recently been investigated for a wide spread of applications. By modifying the polymer concentration of a hydrogel (or conversely, the water concentration), the Young’s Modulus, Shear Modulus, and Hydrogels possess a vast range of mechanical properties, which is one of the primary reasons why they have recently been investigated for a wide spread of applications. By modifying the polymer concentration of a hydrogel (or conversely, the water concentration), the Young’s Modulus, Shear Modulus, and Storage Modulus can vary from 10 Pa to 3 MPa, a range of about five orders of magnitude.[19] A similar effect can be seen by altering the crosslinking concentration.[19] This much variability of the mechanical stiffness is why hydrogels are so appealing for biomedical applications, where it is vital for implants to match the mechanical properties of the surrounding tissues.[20]

Hydrogels have two main regimes of mechanical properties: rubber elasticity and viscoelasticity:

Rubber elasticityrubber elasticity and viscoelasticity:

In the unswollen state, hydrogels can be modelled as highly crosslinked chemical gels, in which the system can be described as one continuous polymer network. In this case:

where G is the shear modulus, k is the Boltzmann constant, T is temperature, Np is the number of polymer chains per unit volume, ρ is the density, R is the ideal gas constant, and  is the (number) average molecular weight between two adjacent cross-linking points. can be calculated from the swell ratio, Q, which is relatively easy to test and measure.[19]

For the swollen state, a perfect gel network can be modeled as:[19]

In a simple uniaxial extension or compression test, the true stress, , and engineering stress, , can be calculated as:

where  is the stretch.[19]

In order to describe the time-dependent creep and stress-relaxation behavior of hydrogel, a variety of physical lumped parameter models can be used.[19] These modeling methods vary greatly and are extremely complex, so the empirical Prony Series description is commonly used to describe the viscoelastic behavior in hydrogels.[19]

Environmental response

Natural hydrogel materials are being investigated for tissue engineering; these materials include agarose, methylcellulose, hyaluronan, Elastin like polypeptides and other naturally derived polymers. Hydrogels show promise for use in agriculture, as they can release agrochemicals including pesticides and phosphate fertiliser slowly, increasing efficiency and reducing runoff, and at the same time improve the water retention of drier soils such as sandy loams.[27]

In the 2000 there has been an increase in research on the use of hydrogels for drug delivery. Polymeric drug delivery systems have overcome challenge due to their biodegradability, biocompatibility and anti-toxicity.[28]

In the 2000 there has been an increase in research on the use of hydrogels for drug delivery. Polymeric drug delivery systems have overcome challenge due to their biodegradability, biocompatibility and anti-toxicity.[28] Recent advances have fueled the formulation and synthesis of hydrogels that provide strong backbone for efficient component for drug delivery systems.[29] Materials such as collagen, chitosan, cellulose and poly (lactic-co-glycolic acid) all have been implemented extensively for drug delivery to various important organs in the human body such as: the eye,[30] nose, kidneys,[31] lungs,[32] intestines,[33] skin[34] and the brain. Future work is focused on better anti-toxicity of hydrogels, varying assembly techniques for hydrogels making them more biocompatible[35] and the delivery of complex systems such as using hydrogels to deliver therapeutic cells.[36]