Gelesis engineered a super-absorbent polymer – is it ironic that it’s derived from an unspecified food source? – that can be reduced to small beads about the size of a grain of sugar. These tiny polymer beads swell up more than 100 times over when introduced to water, kind of like those little sponges you used to play with in the bath. So when you down a pill with a glass of water, the capsule dissolves in your stomach and the hydro-gel beads begin to grow. In a few minutes you’re feeling pretty full, and that second Double Down from KFC is decidedly less attractive.
http://www.popsci.com/science/article/2010-04/obesity-pill-swells-your-stomach-making-you-full-you-even-start-eating