sem·i·per·me·a·ble 
(sĕm′ē-pûrmē-ə-bəl, sĕm′ī-)
(sĕm′ē-pûrmē-ə-bəl, sĕm′ī-)adj.
1. Partially permeable.
2. Allowing passage of certain, especially small, molecules or ions but acting as a barrier to others. Used of biological and synthetic membranes.
sem′i·per′me·a·bili·ty n.