Hydrogels are a particular class of compounds of which the major constituent is wa- ter. In fact, water is present in the hydrogel up to 90% and is contained in a scaffold which is generally polymeric and obviously hydrophilic. As a result, hydrogels re- semble each other even though obtained from different polymers. Nevertheless, the polymeric matrix gives particular characteristics to the hydrogel leading to applica- tions in different fields. Water is the main element of the human body, thus hydrogels are excellent struc- tures to favourably shelter proteins, cells etc., without altering their characteristics and properties. This is why hydrogels are mainly designed and synthesized for their usein thebiologicalfield;hence the name biohydrogels. Their propertiespoint totheir use as scaffolds for stem cells which has turned out to be a very promising technique for tissue and organ regeneration. For this reason their investigation falls within the Biomaterials Science. Paradoxically, the conceptual simplicity of hydrogelsupto nowhas led to a super- ficialstudy of their chemistry, chemical physics and mechanics preventing their wider application in the human body due to a lack of knowledge of biological component interactions. For example, it is not clear, yet, how to store hydrogels without alter- ing their characteristics. In fact, hydrogels re-hydrated after lyophylization or oven drying, generally show corrupted properties once swollen in water, in comparison with their native counterparts.
The studies on Biohydrogels have had a rapid, exponential evolution in the last
decades. Starting from one of the first applications of hydrogels in surgery,
polyvinylalcohol crosslinked with formaldehyde under the trade name of Ivalon, we
passed through PolyHema hydrogels as contact lens materials, hydrophilic
polyurethanes (HPU), biodegradable hydrogels for both reconstructive surgery and
pharmaceutical delivery systems, and finally more recently, one decade old, the
thermoreversible and transient network hydrogels. Of course all these classes of
hydrogels have been always and continuosly studied, improving their performance and
field of applications. Recently, most of them have been used as scaffolds for cells,
even stem ones, for regenerative applications (tissue engineering). Nevertheless
hydrogels are odd materials and many studies still have to be carried out to fully
understand their behaviour from mechanical, physicochemical and biological point of
view.
