range from a very short time to a very long time after the device is inserted in one embodiment the membrane 30 biodegrades upon contact with an enzymatic agent wherein the enzymatic agent is injected to degrade the membrane after the patient 39 s cerebral blood flow returns to a substantially normal level in another embodiment the membrane biodegrades when irradiated wherein the membrane is irradiated after the patient 39 s cerebral blood flow returns to a substantially normal level in one embodiment the membrane 30 is made of polyglycolide in another embodiment the membrane 30 is a thin membrane with one or more laser cut holes to allow blood flow the thin membrane can be made of poly dioxanone in the embodiment shown in fig1 the membrane 30 has a single opening 40 substantially centrally located alternatively the opening 40 can be located off center or the membrane can have multiple openings the number size and position of openings 40 in the membrane 30 are selected to achieve a desired amount of blood flow diversion to the brain in some embodiments the frame 20 includes multiple support members such as struts 50 the struts 50 can be compressible expandable or flexible the frame 20 is at least partially expandable to conform to the lumen of a vessel in one embodiment the.