We form an artificial lipid bilayer between a nanolitre aqueous droplet and a supporting hydrogel immersed in an oil/lipid solution. Manipulation of the axial position of the droplet relative to the hydrogel controls the size of the bilayer formed at the interface; this enables the surface density of integral membrane proteins to be controlled. We are able to modulate the surface density of the β-barrel pore-forming toxin α-hemolysin over a range of 4 orders of magnitude within a time frame of a few seconds. The concentration changes are fully reversible. Membrane protein function and diffusion are unaltered, as measured by single molecule microscopy and single channel electrical recording.