The grippers are made from a rubber balloon filled with coffee grounds. The coffee grounds are a granular medium (made from grains… small solid pieces), and when one removes the air the coffee grains pack together and become extremely rigid, which is a process called granular jamming. This enables the gripper to grasp and hold on to objects of any shape. The jamming transition is considered to be a new type of phase transition similar to the glass transition that occurs when liquid glass is cooled.
What is amazing to me is that by rapidly forcing air back into the balloon, the gripper expels the object—basically throwing it! It is pretty amazing to see such a simple gripper throwing a ball or even darts!