There are many medical situations that require removal of fluid from a patient. There are many ways to remove the fluid but many of these situations have not been solved. Herein, inventions are disclosed that generate osmotic pressure that can be used to remove these fluids very effectively, a technique which requires no harsh suction or pumping. One embodiment involves implanting a reservoir that has a semipermeable membrane sidewall. The reservoir contains trapped osmotic solutes that can not pass out through the membrane. But fluid from the patient can flow in. The osmotic pressure from the trapped solutes in the reservoir draws fluid from the patient across the semipermeable membrane and into the device. In some versions, the membrane can be changed as desired during the lifetime of the device by pulling the membrane out through the implanted device and putting a new one back in.