Abstract: A non-thrombogenic surface is provided by forming a reservoir from which anticoagulant solution can permeate through a flexible wall (5, 6) of bio-compatible permeable material. Such reservoirs can be used in catheters one form of which includes a hollow-walled tube reservoir with outer wall (5) and inner wall (6). The reservoirs may also be used in cardiac assist devices comprising a bladder (56) with an outer permeable wall (54) and an inner impermeable wall (55), means to introduce anticoagulant solution into the reservoir defined between these walls, and means (60) for the controlled inflation and deflation of bladder (56).