Several methods of drug delivery have been used experimentally. Included are chemical modification of a drug to enable it to penetrate membranes such as the blood-brain barrier; incorporation of microparticles in colloidal carriers made of proteins, carbohydrates, lipids, or synthetic polymers; controlled-release systems that permit a drug to be delivered for very long periods; and transdermal controlled-release systems (e.g., those currently in use for administration of scopolamine or nitroglycerin). In addition to the use of various carriers for drugs, cell transplantation could be used to provide therapeutic agents, and the possibility of inserting genes into cells to produce desired effects is being explored.