A gastric device for a stomach having a pyloric valve comprising a biodegradable delivery structure disposable within the stomach configured to erode upon exposure to gastric fluid. A first volume filling agent retained by the biodegradable delivery structure and is permeable to water and/or gastric fluids. A second volume filling agent retained by the first volume filling agent, wherein the second volume filling agent is a hydrogel that expands in contact with the water and/or gastric fluids thereby expanding the first volume filling agent from a first displacement to a second displacement larger than the first displacement, and wherein the first volume filling agent does not pass through the pyloric valve in the normal digestive process.