Hydrogel materials are composed of long chains of homopolymers or copolymers which are connected together at various points, resulting in a three-dimensional crosslinked structure. They are characterized by ability to absorb large quantities of water and are therefore often referred to as superabsorbents. Polymer hydrogels can be divided into two groups. The first comprised physical gels, also called reversible gels, which can be converted into a polymeric mixture (e.g., agar). The three-dimensional network in a physical gel is characterized by the weak hydrogen bonds, electrostatic forces, or hydrophobic interactions which connect the polymer chains. The second group includes chemical gels; however, in a chemical gel, the polymer chains that comprise the three-dimensional network are connected by permanent covalent bonds.

Hydrogels, which are biocompatible and biodegradable, play a very important role in biomedicine, pharmacology, and tissue engineering. Controlled-release or controlled-delivery systems are required to provide the drug in a predetermined temporal or spatial way within the body in order to fulfill specific therapeutic needs. The controlled delivery of drugs can be effectively obtained using systems based on hydrogels. Many hydrogels have enhanced tissue permeability and drug residence time owing to their mucoadhesive and bioadhesive properties which make them very good vehicles for drug delivery.

In this special issue, we intend to publish and welcome papers including original research articles or reviews on synthesis, properties of hydrogels, and their application.

Potential topics include but are not limited to the following:

• Pharmaceutical applications of hydrogels
• Transdermal delivery
• Drug release from hydrogels
• Nanohydrogels
• Nanocomposite hydrogels
• Hydrogel for growth factor delivery in tissue
• Biodegradable and pH responsive hydrogels