Hydrogels

To address this problem, hydrogels can be used for targeted drug delivery.

Hydrogels are 3D structures in which hydrophilic, water-insoluble, polymeric chains are dispersed in water and maintain their shape due to the presence of cross-linking and strong water retention.

The hydrogels exhibit excellent biocompatibility (which could be defined as the ability of a material to be in contact with the body organs without any damages for the surrounding tissues and without triggering any undesirable response) and capability to easily encapsulate hydrophilic drugs as it has high water content which provides physical similarity to tissues. Denaturation and aggregation of the drug occurs when exposed to organic solvents. Since hydrogels are formed typically in aqueous solutions, the problem caused organic solvents is minimized. The mechanical properties of hydrogels, which makes them solid-like, is due to the crosslinked polymer network.

Preparation of hydrogels

The three integral parts of the hydrogels preparation are monomer, initiator, and cross-linker. To control the heat of polymerization and the final hydrogels properties, diluents can be used, such as water or other aqueous solutions. Then, the hydrogel mass needs to be washed to remove impurities left from the preparation process. These include non-reacted monomer, initiators, cross-linkers, and unwanted products produced via side reactions

The behavior of unswollen hydrogel can be sensitive to various external conditions such as temperature, glucose, pH, ionic strength, light, electric fields and many other conditions. The response to the stimuli is swelling which in-turn releases the drug.

Hydrogel technical features

The functional features of an ideal hydrogel material can be listed as follows:

•The highest absorption capacity (maximum equilibrium swelling) in saline.

•Desired rate of absorption (preferred particle size and porosity) depending on the application requirement.

•The highest absorbency under load (AUL).

•The lowest soluble content and residual monomer.

•The lowest price.

•The highest durability and stability in the swelling environment and during the storage.

•The highest biodegradability without formation of toxic species following the degradation.

•pH-neutrality after swelling in water.

•Colorlessness, odorlessness, and absolute non-toxic.

•Photo stability.

•Re-wetting capability (if required) the hydrogel has to be able to give back the imbibed solution or to maintain it; depending on the application requirement (e.g., in agricultural or hygienic applications).

The type and degree of cross-linking will decide the shape and strength of the hydrogel.
The first water molecules that enter the matrix will hydrate the hydrophilic group. This initial absorption of water leads to primary bound water.
Swelling of the network occurs when the polar groups are hydrated and this exposes hydrophobic groups. Secondary bound water is formed when water interacts with these hydrophobic groups.
After polar and hydrophobic sites interaction with the bound water molecules, additional water is imbibed into the network due to osmotic driving force of the network chains towards infinite dilution. Free water or bulk water is the water that is imbibed beyond the total bound water which causes additional swelling.
The physical crosslinks oppose this swelling leading to an elastic network retraction force. Hence, equilibrium swelling level is reached by the hydrogels.

The mesh size governs how drugs diffuse inside the hydrogel network.

Since, the antibiotics are one of the reason for clostridium difficile infection, the targeted drug delivery can be done using hydrogels which will not interefere with the bacterial flora present in the GI tract and hence preventing the infection.

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