an apparatus and method are described for actuating a minimally invasive medical device using fluid pressure . the invention involves a medical device that includes a fluid source , such as a compressible bladder that may apply positive fluid pressure into a sheath to controllably deploy an end - effector from a distal end of the sheath . in some embodiments , the fluid source may apply negative fluid pressure into the sheath to controllably retract the end - effector into the distal end of the sheath . typical end - effectors for use with the medical device of the invention include biopsy devices and retrieval devices , including basket - type retrieval devices and grasper retrieval devices . generally , the fluid that is used to actuate the device may be a liquid or a gas , including air .