Exploiting the role of TEMPO as radical scavenger

Hydrogels are drug delivery systems which have been considered since the early 1960s. They are hydrophilic polymers with cross-links which form a polymeric network that enables them to absorb water or biological fluids from 10-20 percent up to thousands of times of their own weight. The ability of water absorption of hydrogels results in the presence of hydrophilic moieties in their polymeric structure but their resistance in water dissolution is the outcome of the presence of cross-linkers between the polymeric chains. Hydrogels can be engineered to respond to various stimuli and have demonstrated significant utility in the medical and pharmaceutical areas. [22]
The solid form of a hydrogel is a network structure of crosslinked polymer chains. At the molecular level, the most important properties to define the hydrogel structure are the mesh size, and the molecular weight of polymer chain between the crosslinks.
Hydrogels are characterized by reticular meshes with dimensions varying from a few dozen or hundreds of nanometres up to a few microns, while the global dimensions can be macroscopic (Figure 12).

The swelling of a hydrogel is mainly defined by the diffusion of water into the hydrogel. The rate of swelling depends on the concentration of polymer and the crosslinking density. The high degree of crosslinking density causes a decrease in swelling ratio, and it increases the brittleness of hydrogel. [23]
The crosslinks which bond the polymers of a hydrogel fall under two general categories: physical and chemical. Chemical hydrogels, also called permanent, have covalent cross-linking bonds, resulting in strong irreversible gels due to the covalent bonding. They attain an equilibrium swelling state which depends on the polymer-water interaction parameter and the crosslink density. Physical hydrogels, also called reversible, have non-covalent bonds. They have high biocompatibility, and are also easily reversible, by simply changing an external stimulus such as pH, ionic strength of the solution or temperature; thus they are favourable for use in medical applications.
Physical crosslinks consist of hydrogen bonds, hydrophobic interactions, and chain entanglements (among others). [24]

Hydrogels have many advantages such as their biocompatibility, hydrophilicity, flexibility, they are stimuli-responsive, and they have a soft structure. The high content of water makes them able to simulate tissue microenvironments in term of elasticity: they resemble natural fabrics more than any other class of synthetic biomaterials.
Moreover, their capability to carry drugs, growth factors and cells make hydrogels an ideal class of materials for biomedical applications, such as drug delivery and tissue engineering.

The main disadvantage of hydrogels is related to the production costs, that limit their commercialization. Problems like a too slow responsiveness of stimuli-sensitive hydrogels or rapid release of drug were solved making thinner and smaller hydrogels which are fast-acting or changing the links between the active principle and the polymer chains. Hydrogels are widely used for various biomedical applications with the possibility of implanting the device directly in site, resort to oral administration or even to injection. However, limitations are linked to the ability to deform and slide during injection and the possibility of having an unwanted dissolution of the polymer with consequent premature release of the drug in differentiated sites from the target.