An international team, lead by Kwang Kim with University of Nevada, is now developing artificial muscles to give robots the ability to grasp, stretch, and squish objects. These muscles will be printed using a 3D printed electroactive polymer called Ionic Polymer-Metal Composites, which is a synthetic material that changes shape in response to electricity.

Source: http://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=138883&