Early experiments measured the force between mica surfaces in air or vacuum. The technique has been extended, however, to enable an arbitrary vapor or solvent to be introduced between the two surfaces. In this way, interactions in various media can be carefully probed, and the dielectric constant of the gap between the surfaces can be tuned. Moreover, use of water as a solvent enables the measurement of interactions between biological molecules (such as lipids in biological membranes or proteins) in their native environment. In a solvent environment, SFA can even measure the oscillatory solvation and structural forces arising from the packing of individual layers of solvent molecules. It can also measure the electrostatic double layer forces between charged surfaces in an aqueous medium with electrolyte.