The present invention provides a method for forming on a medical device, preferably an ophthalmic lens, more preferably a contact lens, a diffusion-controllable coating capable of controlling the out-diffusion or release of guest materials from the medical device. The method of the invention comprises: (1) applying one layer of clay and optionally one or more layers of polyionic materials onto the medical device; or (2) applying alternatively a layer of a first polyionic material and a layer of a second polyionic material having charges opposite of the charges of the first polyionic material onto the medical device and releasing the coated medical device into a releasing medium having a composition capable of imparting a desired permeability to the diffusion-controllable coating on the medical device.