semipermeable

Partially permeable.

Allowing passage of certain, especially small, molecules or ions but acting as a barrier to others. Used of biological and synthetic membranes.

Capable of permitting the passage of certain molecules and of impeding that of others; said of membranes or septa which are permeable to the molecules of a solvent such as water, but which prevent the diffusion of the molecules of the dissolved substance.

Describing something that is permeable to some things and not to others, as a cell membrane which allows some molecules through but blocks others substances.

(of a membrane) selectively permeable