Complex sentence: The main disadvantage to transdermal delivery systems stems from the fact that the skin is a very effective barrier; as a result, historically only medications whose molecules were thought small enough to penetrate the skin could be delivered by this method.
Split into simpler sentences: The main disadvantage to transdermal delivery systems stems from the fact that the skin is a very effective barrier. As a result, historically only medications whose molecules were thought small enough to penetrate the skin could be delivered by this method.