Hydrogel are networked structures of polymer crosslinked to each other and surrounded by an aqueous solution. The polymer chains contain acidic or basic groups bound to them. The acidic groups on the chains deprotonate at high pH, whereas the basic groups protonate at low pH. In the presence of an aqueous solution, the polymer chains absorb water and the association, dissociation and binding of various ions to polymer chains causes the hydrogel to swell. The swelling and shrinking properties of hydrogels are currently being exploited in a number of applications including control of microfluidic flow, muscle-like actuators filtration/separation, and drug delivery. The structure and properties of hydrogels are similar to many biological tissues such as cartilage and the corneal stroma in the eye.