An actuator having a variable internal volume is mechanically coupled to an adjustable implantable band so as to effect changes in the effective internal perimeter of the band. The actuator may be directly connected to the band, or be connected through a cable. Configurations of the actuator include a series of folds and ridges and bellows. A plurality of actuators may be used in combination with a single band. A clutch mechanism may be included to hold the band in place when not acted upon by the actuators. One end of the actuator may be connected directly to a bidirectional flow device.