An improved dental appliance system, and methods for using and fabricating the improved appliance, including a polymeric overlay or shell having a teeth-receiving cavity formed therein. The dental appliance having the necessary stiffness or strength to firmly secure the appliance on the teeth and provide controlled forces required for repositioning the teeth, until such time as removal of the appliance is desired. The appliance may be configured for use with a removal mechanism. The removal mechanism provides for selective release of the appliance from the teeth as the removal mechanism undergoes a state change stimulated by an environmental stimulus or environmental switch.