An implantable medical device, comprising a body including a lumen extending therethrough at least one activatable element disposed within the body, a dimension of each of the at least one activatable elements changing in response to a predetermined stimulation between an activated state and an inactivate state to change a shape of the body to control a degree of opening of the lumen and at least one actuation element, each of the at least one actuation elements being configured to selectively supply the predetermined stimulation to a corresponding one of the at least one activatable elements to move the corresponding one of the at least one activatable elements between the activated and inactive states.