Hydrogels, consisting of cross-linked macromolecules and water, are soft and wet materials.As a combination of solid and liquid components, hydrogels have a high permeability to small molecules and undergo reversible volume change by exuding or absorbing water in response to a wide range of stimuli, such as light, temperature, pH, ionic strength, chemical reactions.In the past three decades,sensing and actuating properties of hydrogels have been extensively studied based on the volume changes.Owing to the similarity between hydrogels and soft tissues, recently, bulk hydrogels have drawn great attention as biomaterials, such as extra-cellular matrix, artificial bio-organs, and phantom tissues in medical device development.