**2.6 Other covalent linkages**

Other covalent linkages that include carbamate linkage, Schiff bases, and polycyclic linkages have also been employed to form covalent linkages between the drug and delivery systems. Reaction between a diene and dienophile results to cycloaddition via the Diels-Alder chemistry which forms bicyclic compounds. This chemistry can be utilized to form polycyclic linkages between the drug and the delivery system [49]. Whereas other hydrolyzable linkages include carbamate [50, 51], oximes [52], and Schiff bases [53, 54]. These types of linkages are specifically designed to make drug delivery systems have targeting ability due to the physiological changes brought about by the diseases.
