Pharmaceutical start-up Gelesis has developed a pill that is filled with tiny polymer beads. Swallow the pill, and the beads absorb water in your stomach, swelling over one hundred times in size. The idea is to partially fill up the stomach so that the patient is less hungry:
So when you down a pill with a glass of water, the capsule dissolves in your stomach and the hydrogel beads begin to grow. In a few minutes you’re feeling pretty full, and that second Double Down from KFC is decidedly less attractive.
Of course, now you have a belly full of hydrogel, and this is where the engineers at Gelesis had to be clever. The food is now mixed in with the gel, but you still need to digest that food (the object here is weight loss, not starvation). The hydrogel keeps food in the stomach longer, giving stomach acid more time to break down both the food and the hydrogel, which begins to release its water. Everything then moves to the small intestine where the gel can re-expand to some extent, slowing the absorption of fatty materials and sugars. Finally everything ends up in the lower bowels, and the rest is history.
http://www.popsci.com/science/article/2010-04/obesity-pill-swells-your-stomach-making-you-full-you-even-start-eating | Image: Gelesis